1 @c Copyright (C) 1988,1989,1992,1993,1994,1996,1998,1999,2000,2001,2002 Free Software Foundation, Inc.
2 @c This is part of the GCC manual.
3 @c For copying conditions, see the file gcc.texi.
6 @chapter C Implementation-defined behavior
7 @cindex implementation-defined behavior, C language
9 A conforming implementation of ISO C is required to document its
10 choice of behavior in each of the areas that are designated
11 ``implementation defined.'' The following lists all such areas,
12 along with the section number from the ISO/IEC 9899:1999 standard.
15 * Translation implementation::
16 * Environment implementation::
17 * Identifiers implementation::
18 * Characters implementation::
19 * Integers implementation::
20 * Floating point implementation::
21 * Arrays and pointers implementation::
22 * Hints implementation::
23 * Structures unions enumerations and bit-fields implementation::
24 * Qualifiers implementation::
25 * Preprocessing directives implementation::
26 * Library functions implementation::
27 * Architecture implementation::
28 * Locale-specific behavior implementation::
31 @node Translation implementation
36 @cite{How a diagnostic is identified (3.10, 5.1.1.3).}
38 Diagnostics consist of all the output sent to stderr by GCC.
41 @cite{Whether each nonempty sequence of white-space characters other than
42 new-line is retained or replaced by one space character in translation
46 @node Environment implementation
49 The behavior of these points are dependent on the implementation
50 of the C library, and are not defined by GCC itself.
52 @node Identifiers implementation
57 @cite{Which additional multibyte characters may appear in identifiers
58 and their correspondence to universal character names (6.4.2).}
61 @cite{The number of significant initial characters in an identifier
64 For internal names, all characters are significant. For external names,
65 the number of significant characters are defined by the linker; for
66 almost all targets, all characters are significant.
70 @node Characters implementation
75 @cite{The number of bits in a byte (3.6).}
78 @cite{The values of the members of the execution character set (5.2.1).}
81 @cite{The unique value of the member of the execution character set produced
82 for each of the standard alphabetic escape sequences (5.2.2).}
85 @cite{The value of a @code{char} object into which has been stored any
86 character other than a member of the basic execution character set (6.2.5).}
89 @cite{Which of @code{signed char} or @code{unsigned char} has the same range,
90 representation, and behavior as ``plain'' @code{char} (6.2.5, 6.3.1.1).}
93 @cite{The mapping of members of the source character set (in character
94 constants and string literals) to members of the execution character
95 set (6.4.4.4, 5.1.1.2).}
98 @cite{The value of an integer character constant containing more than one
99 character or containing a character or escape sequence that does not map
100 to a single-byte execution character (6.4.4.4).}
103 @cite{The value of a wide character constant containing more than one
104 multibyte character, or containing a multibyte character or escape
105 sequence not represented in the extended execution character set (6.4.4.4).}
108 @cite{The current locale used to convert a wide character constant consisting
109 of a single multibyte character that maps to a member of the extended
110 execution character set into a corresponding wide character code (6.4.4.4).}
113 @cite{The current locale used to convert a wide string literal into
114 corresponding wide character codes (6.4.5).}
117 @cite{The value of a string literal containing a multibyte character or escape
118 sequence not represented in the execution character set (6.4.5).}
121 @node Integers implementation
126 @cite{Any extended integer types that exist in the implementation (6.2.5).}
129 @cite{Whether signed integer types are represented using sign and magnitude,
130 two's complement, or one's complement, and whether the extraordinary value
131 is a trap representation or an ordinary value (6.2.6.2).}
133 GCC supports only two's complement integer types, and all bit patterns
137 @cite{The rank of any extended integer type relative to another extended
138 integer type with the same precision (6.3.1.1).}
141 @cite{The result of, or the signal raised by, converting an integer to a
142 signed integer type when the value cannot be represented in an object of
143 that type (6.3.1.3).}
146 @cite{The results of some bitwise operations on signed integers (6.5).}
149 @node Floating point implementation
150 @section Floating point
154 @cite{The accuracy of the floating-point operations and of the library
155 functions in @code{<math.h>} and @code{<complex.h>} that return floating-point
156 results (5.2.4.2.2).}
159 @cite{The rounding behaviors characterized by non-standard values
160 of @code{FLT_ROUNDS} @gol
164 @cite{The evaluation methods characterized by non-standard negative
165 values of @code{FLT_EVAL_METHOD} (5.2.4.2.2).}
168 @cite{The direction of rounding when an integer is converted to a
169 floating-point number that cannot exactly represent the original
173 @cite{The direction of rounding when a floating-point number is
174 converted to a narrower floating-point number (6.3.1.5).}
177 @cite{How the nearest representable value or the larger or smaller
178 representable value immediately adjacent to the nearest representable
179 value is chosen for certain floating constants (6.4.4.2).}
182 @cite{Whether and how floating expressions are contracted when not
183 disallowed by the @code{FP_CONTRACT} pragma (6.5).}
186 @cite{The default state for the @code{FENV_ACCESS} pragma (7.6.1).}
189 @cite{Additional floating-point exceptions, rounding modes, environments,
190 and classifications, and their macro names (7.6, 7.12).}
193 @cite{The default state for the @code{FP_CONTRACT} pragma (7.12.2).}
196 @cite{Whether the ``inexact'' floating-point exception can be raised
197 when the rounded result actually does equal the mathematical result
198 in an IEC 60559 conformant implementation (F.9).}
201 @cite{Whether the ``underflow'' (and ``inexact'') floating-point
202 exception can be raised when a result is tiny but not inexact in an
203 IEC 60559 conformant implementation (F.9).}
207 @node Arrays and pointers implementation
208 @section Arrays and pointers
212 @cite{The result of converting a pointer to an integer or
213 vice versa (6.3.2.3).}
215 A cast from pointer to integer discards most-significant bits if the
216 pointer representation is larger than the integer type,
217 sign-extends@footnote{Future versions of GCC may zero-extend, or use
218 a target-defined @code{ptr_extend} pattern. Do not rely on sign extension.}
219 if the pointer representation is smaller than the integer type, otherwise
220 the bits are unchanged.
221 @c ??? We've always claimed that pointers were unsigned entities.
222 @c Shouldn't we therefore be doing zero-extension? If so, the bug
223 @c is in convert_to_integer, where we call type_for_size and request
224 @c a signed integral type. On the other hand, it might be most useful
225 @c for the target if we extend according to POINTERS_EXTEND_UNSIGNED.
227 A cast from integer to pointer discards most-significant bits if the
228 pointer representation is smaller than the integer type, extends according
229 to the signedness of the integer type if the pointer representation
230 is larger than the integer type, otherwise the bits are unchanged.
232 When casting from pointer to integer and back again, the resulting
233 pointer must reference the same object as the original pointer, otherwise
234 the behavior is undefined. That is, one may not use integer arithmetic to
235 avoid the undefined behavior of pointer arithmetic as proscribed in 6.5.6/8.
238 @cite{The size of the result of subtracting two pointers to elements
239 of the same array (6.5.6).}
243 @node Hints implementation
248 @cite{The extent to which suggestions made by using the @code{register}
249 storage-class specifier are effective (6.7.1).}
251 The @code{register} specifier affects code generation only in these ways:
255 When used as part of the register variable extension, see
256 @ref{Explicit Reg Vars}.
259 When @option{-O0} is in use, the compiler allocates distinct stack
260 memory for all variables that do not have the @code{register}
261 storage-class specifier; if @code{register} is specified, the variable
262 may have a shorter lifespan than the code would indicate and may never
266 On some rare x86 targets, @code{setjmp} doesn't save the registers in
267 all circumstances. In those cases, GCC doesn't allocate any variables
268 in registers unless they are marked @code{register}.
273 @cite{The extent to which suggestions made by using the inline function
274 specifier are effective (6.7.4).}
276 GCC will not inline any functions if the @option{-fno-inline} option is
277 used or if @option{-O0} is used. Otherwise, GCC may still be unable to
278 inline a function for many reasons; the @option{-Winline} option may be
279 used to determine if a function has not been inlined and why not.
283 @node Structures unions enumerations and bit-fields implementation
284 @section Structures, unions, enumerations, and bit-fields
288 @cite{Whether a ``plain'' int bit-field is treated as a @code{signed int}
289 bit-field or as an @code{unsigned int} bit-field (6.7.2, 6.7.2.1).}
292 @cite{Allowable bit-field types other than @code{_Bool}, @code{signed int},
293 and @code{unsigned int} (6.7.2.1).}
296 @cite{Whether a bit-field can straddle a storage-unit boundary (6.7.2.1).}
299 @cite{The order of allocation of bit-fields within a unit (6.7.2.1).}
302 @cite{The alignment of non-bit-field members of structures (6.7.2.1).}
305 @cite{The integer type compatible with each enumerated type (6.7.2.2).}
309 @node Qualifiers implementation
314 @cite{What constitutes an access to an object that has volatile-qualified
319 @node Preprocessing directives implementation
320 @section Preprocessing directives
324 @cite{How sequences in both forms of header names are mapped to headers
325 or external source file names (6.4.7).}
328 @cite{Whether the value of a character constant in a constant expression
329 that controls conditional inclusion matches the value of the same character
330 constant in the execution character set (6.10.1).}
333 @cite{Whether the value of a single-character character constant in a
334 constant expression that controls conditional inclusion may have a
335 negative value (6.10.1).}
338 @cite{The places that are searched for an included @samp{<>} delimited
339 header, and how the places are specified or the header is
340 identified (6.10.2).}
343 @cite{How the named source file is searched for in an included @samp{""}
344 delimited header (6.10.2).}
347 @cite{The method by which preprocessing tokens (possibly resulting from
348 macro expansion) in a @code{#include} directive are combined into a header
352 @cite{The nesting limit for @code{#include} processing (6.10.2).}
354 GCC imposes a limit of 200 nested @code{#include}s.
357 @cite{Whether the @samp{#} operator inserts a @samp{\} character before
358 the @samp{\} character that begins a universal character name in a
359 character constant or string literal (6.10.3.2).}
362 @cite{The behavior on each recognized non-@code{STDC #pragma}
366 @cite{The definitions for @code{__DATE__} and @code{__TIME__} when
367 respectively, the date and time of translation are not available (6.10.8).}
369 If the date and time are not available, @code{__DATE__} expands to
370 @code{@w{"??? ?? ????"}} and @code{__TIME__} expands to
375 @node Library functions implementation
376 @section Library functions
378 The behavior of these points are dependent on the implementation
379 of the C library, and are not defined by GCC itself.
381 @node Architecture implementation
382 @section Architecture
386 @cite{The values or expressions assigned to the macros specified in the
387 headers @code{<float.h>}, @code{<limits.h>}, and @code{<stdint.h>}
388 (5.2.4.2, 7.18.2, 7.18.3).}
391 @cite{The number, order, and encoding of bytes in any object
392 (when not explicitly specified in this International Standard) (6.2.6.1).}
395 @cite{The value of the result of the sizeof operator (6.5.3.4).}
399 @node Locale-specific behavior implementation
400 @section Locale-specific behavior
402 The behavior of these points are dependent on the implementation
403 of the C library, and are not defined by GCC itself.
406 @chapter Extensions to the C Language Family
407 @cindex extensions, C language
408 @cindex C language extensions
411 GNU C provides several language features not found in ISO standard C@.
412 (The @option{-pedantic} option directs GCC to print a warning message if
413 any of these features is used.) To test for the availability of these
414 features in conditional compilation, check for a predefined macro
415 @code{__GNUC__}, which is always defined under GCC@.
417 These extensions are available in C and Objective-C@. Most of them are
418 also available in C++. @xref{C++ Extensions,,Extensions to the
419 C++ Language}, for extensions that apply @emph{only} to C++.
421 Some features that are in ISO C99 but not C89 or C++ are also, as
422 extensions, accepted by GCC in C89 mode and in C++.
425 * Statement Exprs:: Putting statements and declarations inside expressions.
426 * Local Labels:: Labels local to a statement-expression.
427 * Labels as Values:: Getting pointers to labels, and computed gotos.
428 * Nested Functions:: As in Algol and Pascal, lexical scoping of functions.
429 * Constructing Calls:: Dispatching a call to another function.
430 * Naming Types:: Giving a name to the type of some expression.
431 * Typeof:: @code{typeof}: referring to the type of an expression.
432 * Lvalues:: Using @samp{?:}, @samp{,} and casts in lvalues.
433 * Conditionals:: Omitting the middle operand of a @samp{?:} expression.
434 * Long Long:: Double-word integers---@code{long long int}.
435 * Complex:: Data types for complex numbers.
436 * Hex Floats:: Hexadecimal floating-point constants.
437 * Zero Length:: Zero-length arrays.
438 * Variable Length:: Arrays whose length is computed at run time.
439 * Variadic Macros:: Macros with a variable number of arguments.
440 * Escaped Newlines:: Slightly looser rules for escaped newlines.
441 * Multi-line Strings:: String literals with embedded newlines.
442 * Subscripting:: Any array can be subscripted, even if not an lvalue.
443 * Pointer Arith:: Arithmetic on @code{void}-pointers and function pointers.
444 * Initializers:: Non-constant initializers.
445 * Compound Literals:: Compound literals give structures, unions
447 * Designated Inits:: Labeling elements of initializers.
448 * Cast to Union:: Casting to union type from any member of the union.
449 * Case Ranges:: `case 1 ... 9' and such.
450 * Mixed Declarations:: Mixing declarations and code.
451 * Function Attributes:: Declaring that functions have no side effects,
452 or that they can never return.
453 * Attribute Syntax:: Formal syntax for attributes.
454 * Function Prototypes:: Prototype declarations and old-style definitions.
455 * C++ Comments:: C++ comments are recognized.
456 * Dollar Signs:: Dollar sign is allowed in identifiers.
457 * Character Escapes:: @samp{\e} stands for the character @key{ESC}.
458 * Variable Attributes:: Specifying attributes of variables.
459 * Type Attributes:: Specifying attributes of types.
460 * Alignment:: Inquiring about the alignment of a type or variable.
461 * Inline:: Defining inline functions (as fast as macros).
462 * Extended Asm:: Assembler instructions with C expressions as operands.
463 (With them you can define ``built-in'' functions.)
464 * Constraints:: Constraints for asm operands
465 * Asm Labels:: Specifying the assembler name to use for a C symbol.
466 * Explicit Reg Vars:: Defining variables residing in specified registers.
467 * Alternate Keywords:: @code{__const__}, @code{__asm__}, etc., for header files.
468 * Incomplete Enums:: @code{enum foo;}, with details to follow.
469 * Function Names:: Printable strings which are the name of the current
471 * Return Address:: Getting the return or frame address of a function.
472 * Vector Extensions:: Using vector instructions through built-in functions.
473 * Other Builtins:: Other built-in functions.
474 * Target Builtins:: Built-in functions specific to particular targets.
475 * Pragmas:: Pragmas accepted by GCC.
476 * Unnamed Fields:: Unnamed struct/union fields within structs/unions.
477 * Thread-Local:: Per-thread variables.
480 @node Statement Exprs
481 @section Statements and Declarations in Expressions
482 @cindex statements inside expressions
483 @cindex declarations inside expressions
484 @cindex expressions containing statements
485 @cindex macros, statements in expressions
487 @c the above section title wrapped and causes an underfull hbox.. i
488 @c changed it from "within" to "in". --mew 4feb93
490 A compound statement enclosed in parentheses may appear as an expression
491 in GNU C@. This allows you to use loops, switches, and local variables
492 within an expression.
494 Recall that a compound statement is a sequence of statements surrounded
495 by braces; in this construct, parentheses go around the braces. For
499 (@{ int y = foo (); int z;
506 is a valid (though slightly more complex than necessary) expression
507 for the absolute value of @code{foo ()}.
509 The last thing in the compound statement should be an expression
510 followed by a semicolon; the value of this subexpression serves as the
511 value of the entire construct. (If you use some other kind of statement
512 last within the braces, the construct has type @code{void}, and thus
513 effectively no value.)
515 This feature is especially useful in making macro definitions ``safe'' (so
516 that they evaluate each operand exactly once). For example, the
517 ``maximum'' function is commonly defined as a macro in standard C as
521 #define max(a,b) ((a) > (b) ? (a) : (b))
525 @cindex side effects, macro argument
526 But this definition computes either @var{a} or @var{b} twice, with bad
527 results if the operand has side effects. In GNU C, if you know the
528 type of the operands (here let's assume @code{int}), you can define
529 the macro safely as follows:
532 #define maxint(a,b) \
533 (@{int _a = (a), _b = (b); _a > _b ? _a : _b; @})
536 Embedded statements are not allowed in constant expressions, such as
537 the value of an enumeration constant, the width of a bit-field, or
538 the initial value of a static variable.
540 If you don't know the type of the operand, you can still do this, but you
541 must use @code{typeof} (@pxref{Typeof}) or type naming (@pxref{Naming
544 Statement expressions are not supported fully in G++, and their fate
545 there is unclear. (It is possible that they will become fully supported
546 at some point, or that they will be deprecated, or that the bugs that
547 are present will continue to exist indefinitely.) Presently, statement
548 expressions do not work well as default arguments.
550 In addition, there are semantic issues with statement-expressions in
551 C++. If you try to use statement-expressions instead of inline
552 functions in C++, you may be surprised at the way object destruction is
553 handled. For example:
556 #define foo(a) (@{int b = (a); b + 3; @})
560 does not work the same way as:
563 inline int foo(int a) @{ int b = a; return b + 3; @}
567 In particular, if the expression passed into @code{foo} involves the
568 creation of temporaries, the destructors for those temporaries will be
569 run earlier in the case of the macro than in the case of the function.
571 These considerations mean that it is probably a bad idea to use
572 statement-expressions of this form in header files that are designed to
573 work with C++. (Note that some versions of the GNU C Library contained
574 header files using statement-expression that lead to precisely this
578 @section Locally Declared Labels
580 @cindex macros, local labels
582 Each statement expression is a scope in which @dfn{local labels} can be
583 declared. A local label is simply an identifier; you can jump to it
584 with an ordinary @code{goto} statement, but only from within the
585 statement expression it belongs to.
587 A local label declaration looks like this:
590 __label__ @var{label};
597 __label__ @var{label1}, @var{label2}, /* @r{@dots{}} */;
600 Local label declarations must come at the beginning of the statement
601 expression, right after the @samp{(@{}, before any ordinary
604 The label declaration defines the label @emph{name}, but does not define
605 the label itself. You must do this in the usual way, with
606 @code{@var{label}:}, within the statements of the statement expression.
608 The local label feature is useful because statement expressions are
609 often used in macros. If the macro contains nested loops, a @code{goto}
610 can be useful for breaking out of them. However, an ordinary label
611 whose scope is the whole function cannot be used: if the macro can be
612 expanded several times in one function, the label will be multiply
613 defined in that function. A local label avoids this problem. For
617 #define SEARCH(array, target) \
620 typeof (target) _SEARCH_target = (target); \
621 typeof (*(array)) *_SEARCH_array = (array); \
624 for (i = 0; i < max; i++) \
625 for (j = 0; j < max; j++) \
626 if (_SEARCH_array[i][j] == _SEARCH_target) \
627 @{ value = i; goto found; @} \
634 @node Labels as Values
635 @section Labels as Values
636 @cindex labels as values
637 @cindex computed gotos
638 @cindex goto with computed label
639 @cindex address of a label
641 You can get the address of a label defined in the current function
642 (or a containing function) with the unary operator @samp{&&}. The
643 value has type @code{void *}. This value is a constant and can be used
644 wherever a constant of that type is valid. For example:
652 To use these values, you need to be able to jump to one. This is done
653 with the computed goto statement@footnote{The analogous feature in
654 Fortran is called an assigned goto, but that name seems inappropriate in
655 C, where one can do more than simply store label addresses in label
656 variables.}, @code{goto *@var{exp};}. For example,
663 Any expression of type @code{void *} is allowed.
665 One way of using these constants is in initializing a static array that
666 will serve as a jump table:
669 static void *array[] = @{ &&foo, &&bar, &&hack @};
672 Then you can select a label with indexing, like this:
679 Note that this does not check whether the subscript is in bounds---array
680 indexing in C never does that.
682 Such an array of label values serves a purpose much like that of the
683 @code{switch} statement. The @code{switch} statement is cleaner, so
684 use that rather than an array unless the problem does not fit a
685 @code{switch} statement very well.
687 Another use of label values is in an interpreter for threaded code.
688 The labels within the interpreter function can be stored in the
689 threaded code for super-fast dispatching.
691 You may not use this mechanism to jump to code in a different function.
692 If you do that, totally unpredictable things will happen. The best way to
693 avoid this is to store the label address only in automatic variables and
694 never pass it as an argument.
696 An alternate way to write the above example is
699 static const int array[] = @{ &&foo - &&foo, &&bar - &&foo,
701 goto *(&&foo + array[i]);
705 This is more friendly to code living in shared libraries, as it reduces
706 the number of dynamic relocations that are needed, and by consequence,
707 allows the data to be read-only.
709 @node Nested Functions
710 @section Nested Functions
711 @cindex nested functions
712 @cindex downward funargs
715 A @dfn{nested function} is a function defined inside another function.
716 (Nested functions are not supported for GNU C++.) The nested function's
717 name is local to the block where it is defined. For example, here we
718 define a nested function named @code{square}, and call it twice:
722 foo (double a, double b)
724 double square (double z) @{ return z * z; @}
726 return square (a) + square (b);
731 The nested function can access all the variables of the containing
732 function that are visible at the point of its definition. This is
733 called @dfn{lexical scoping}. For example, here we show a nested
734 function which uses an inherited variable named @code{offset}:
738 bar (int *array, int offset, int size)
740 int access (int *array, int index)
741 @{ return array[index + offset]; @}
744 for (i = 0; i < size; i++)
745 /* @r{@dots{}} */ access (array, i) /* @r{@dots{}} */
750 Nested function definitions are permitted within functions in the places
751 where variable definitions are allowed; that is, in any block, before
752 the first statement in the block.
754 It is possible to call the nested function from outside the scope of its
755 name by storing its address or passing the address to another function:
758 hack (int *array, int size)
760 void store (int index, int value)
761 @{ array[index] = value; @}
763 intermediate (store, size);
767 Here, the function @code{intermediate} receives the address of
768 @code{store} as an argument. If @code{intermediate} calls @code{store},
769 the arguments given to @code{store} are used to store into @code{array}.
770 But this technique works only so long as the containing function
771 (@code{hack}, in this example) does not exit.
773 If you try to call the nested function through its address after the
774 containing function has exited, all hell will break loose. If you try
775 to call it after a containing scope level has exited, and if it refers
776 to some of the variables that are no longer in scope, you may be lucky,
777 but it's not wise to take the risk. If, however, the nested function
778 does not refer to anything that has gone out of scope, you should be
781 GCC implements taking the address of a nested function using a technique
782 called @dfn{trampolines}. A paper describing them is available as
785 @uref{http://people.debian.org/~aaronl/Usenix88-lexic.pdf}.
787 A nested function can jump to a label inherited from a containing
788 function, provided the label was explicitly declared in the containing
789 function (@pxref{Local Labels}). Such a jump returns instantly to the
790 containing function, exiting the nested function which did the
791 @code{goto} and any intermediate functions as well. Here is an example:
795 bar (int *array, int offset, int size)
798 int access (int *array, int index)
802 return array[index + offset];
806 for (i = 0; i < size; i++)
807 /* @r{@dots{}} */ access (array, i) /* @r{@dots{}} */
811 /* @r{Control comes here from @code{access}
812 if it detects an error.} */
819 A nested function always has internal linkage. Declaring one with
820 @code{extern} is erroneous. If you need to declare the nested function
821 before its definition, use @code{auto} (which is otherwise meaningless
822 for function declarations).
825 bar (int *array, int offset, int size)
828 auto int access (int *, int);
830 int access (int *array, int index)
834 return array[index + offset];
840 @node Constructing Calls
841 @section Constructing Function Calls
842 @cindex constructing calls
843 @cindex forwarding calls
845 Using the built-in functions described below, you can record
846 the arguments a function received, and call another function
847 with the same arguments, without knowing the number or types
850 You can also record the return value of that function call,
851 and later return that value, without knowing what data type
852 the function tried to return (as long as your caller expects
855 @deftypefn {Built-in Function} {void *} __builtin_apply_args ()
856 This built-in function returns a pointer to data
857 describing how to perform a call with the same arguments as were passed
858 to the current function.
860 The function saves the arg pointer register, structure value address,
861 and all registers that might be used to pass arguments to a function
862 into a block of memory allocated on the stack. Then it returns the
863 address of that block.
866 @deftypefn {Built-in Function} {void *} __builtin_apply (void (*@var{function})(), void *@var{arguments}, size_t @var{size})
867 This built-in function invokes @var{function}
868 with a copy of the parameters described by @var{arguments}
871 The value of @var{arguments} should be the value returned by
872 @code{__builtin_apply_args}. The argument @var{size} specifies the size
873 of the stack argument data, in bytes.
875 This function returns a pointer to data describing
876 how to return whatever value was returned by @var{function}. The data
877 is saved in a block of memory allocated on the stack.
879 It is not always simple to compute the proper value for @var{size}. The
880 value is used by @code{__builtin_apply} to compute the amount of data
881 that should be pushed on the stack and copied from the incoming argument
885 @deftypefn {Built-in Function} {void} __builtin_return (void *@var{result})
886 This built-in function returns the value described by @var{result} from
887 the containing function. You should specify, for @var{result}, a value
888 returned by @code{__builtin_apply}.
892 @section Naming an Expression's Type
895 You can give a name to the type of an expression using a @code{typedef}
896 declaration with an initializer. Here is how to define @var{name} as a
897 type name for the type of @var{exp}:
900 typedef @var{name} = @var{exp};
903 This is useful in conjunction with the statements-within-expressions
904 feature. Here is how the two together can be used to define a safe
905 ``maximum'' macro that operates on any arithmetic type:
909 (@{typedef _ta = (a), _tb = (b); \
910 _ta _a = (a); _tb _b = (b); \
911 _a > _b ? _a : _b; @})
914 @cindex underscores in variables in macros
915 @cindex @samp{_} in variables in macros
916 @cindex local variables in macros
917 @cindex variables, local, in macros
918 @cindex macros, local variables in
920 The reason for using names that start with underscores for the local
921 variables is to avoid conflicts with variable names that occur within the
922 expressions that are substituted for @code{a} and @code{b}. Eventually we
923 hope to design a new form of declaration syntax that allows you to declare
924 variables whose scopes start only after their initializers; this will be a
925 more reliable way to prevent such conflicts.
928 @section Referring to a Type with @code{typeof}
931 @cindex macros, types of arguments
933 Another way to refer to the type of an expression is with @code{typeof}.
934 The syntax of using of this keyword looks like @code{sizeof}, but the
935 construct acts semantically like a type name defined with @code{typedef}.
937 There are two ways of writing the argument to @code{typeof}: with an
938 expression or with a type. Here is an example with an expression:
945 This assumes that @code{x} is an array of pointers to functions;
946 the type described is that of the values of the functions.
948 Here is an example with a typename as the argument:
955 Here the type described is that of pointers to @code{int}.
957 If you are writing a header file that must work when included in ISO C
958 programs, write @code{__typeof__} instead of @code{typeof}.
959 @xref{Alternate Keywords}.
961 A @code{typeof}-construct can be used anywhere a typedef name could be
962 used. For example, you can use it in a declaration, in a cast, or inside
963 of @code{sizeof} or @code{typeof}.
967 This declares @code{y} with the type of what @code{x} points to.
974 This declares @code{y} as an array of such values.
981 This declares @code{y} as an array of pointers to characters:
984 typeof (typeof (char *)[4]) y;
988 It is equivalent to the following traditional C declaration:
994 To see the meaning of the declaration using @code{typeof}, and why it
995 might be a useful way to write, let's rewrite it with these macros:
998 #define pointer(T) typeof(T *)
999 #define array(T, N) typeof(T [N])
1003 Now the declaration can be rewritten this way:
1006 array (pointer (char), 4) y;
1010 Thus, @code{array (pointer (char), 4)} is the type of arrays of 4
1011 pointers to @code{char}.
1015 @section Generalized Lvalues
1016 @cindex compound expressions as lvalues
1017 @cindex expressions, compound, as lvalues
1018 @cindex conditional expressions as lvalues
1019 @cindex expressions, conditional, as lvalues
1020 @cindex casts as lvalues
1021 @cindex generalized lvalues
1022 @cindex lvalues, generalized
1023 @cindex extensions, @code{?:}
1024 @cindex @code{?:} extensions
1025 Compound expressions, conditional expressions and casts are allowed as
1026 lvalues provided their operands are lvalues. This means that you can take
1027 their addresses or store values into them.
1029 Standard C++ allows compound expressions and conditional expressions as
1030 lvalues, and permits casts to reference type, so use of this extension
1031 is deprecated for C++ code.
1033 For example, a compound expression can be assigned, provided the last
1034 expression in the sequence is an lvalue. These two expressions are
1042 Similarly, the address of the compound expression can be taken. These two
1043 expressions are equivalent:
1050 A conditional expression is a valid lvalue if its type is not void and the
1051 true and false branches are both valid lvalues. For example, these two
1052 expressions are equivalent:
1056 (a ? b = 5 : (c = 5))
1059 A cast is a valid lvalue if its operand is an lvalue. A simple
1060 assignment whose left-hand side is a cast works by converting the
1061 right-hand side first to the specified type, then to the type of the
1062 inner left-hand side expression. After this is stored, the value is
1063 converted back to the specified type to become the value of the
1064 assignment. Thus, if @code{a} has type @code{char *}, the following two
1065 expressions are equivalent:
1069 (int)(a = (char *)(int)5)
1072 An assignment-with-arithmetic operation such as @samp{+=} applied to a cast
1073 performs the arithmetic using the type resulting from the cast, and then
1074 continues as in the previous case. Therefore, these two expressions are
1079 (int)(a = (char *)(int) ((int)a + 5))
1082 You cannot take the address of an lvalue cast, because the use of its
1083 address would not work out coherently. Suppose that @code{&(int)f} were
1084 permitted, where @code{f} has type @code{float}. Then the following
1085 statement would try to store an integer bit-pattern where a floating
1086 point number belongs:
1092 This is quite different from what @code{(int)f = 1} would do---that
1093 would convert 1 to floating point and store it. Rather than cause this
1094 inconsistency, we think it is better to prohibit use of @samp{&} on a cast.
1096 If you really do want an @code{int *} pointer with the address of
1097 @code{f}, you can simply write @code{(int *)&f}.
1100 @section Conditionals with Omitted Operands
1101 @cindex conditional expressions, extensions
1102 @cindex omitted middle-operands
1103 @cindex middle-operands, omitted
1104 @cindex extensions, @code{?:}
1105 @cindex @code{?:} extensions
1107 The middle operand in a conditional expression may be omitted. Then
1108 if the first operand is nonzero, its value is the value of the conditional
1111 Therefore, the expression
1118 has the value of @code{x} if that is nonzero; otherwise, the value of
1121 This example is perfectly equivalent to
1127 @cindex side effect in ?:
1128 @cindex ?: side effect
1130 In this simple case, the ability to omit the middle operand is not
1131 especially useful. When it becomes useful is when the first operand does,
1132 or may (if it is a macro argument), contain a side effect. Then repeating
1133 the operand in the middle would perform the side effect twice. Omitting
1134 the middle operand uses the value already computed without the undesirable
1135 effects of recomputing it.
1138 @section Double-Word Integers
1139 @cindex @code{long long} data types
1140 @cindex double-word arithmetic
1141 @cindex multiprecision arithmetic
1142 @cindex @code{LL} integer suffix
1143 @cindex @code{ULL} integer suffix
1145 ISO C99 supports data types for integers that are at least 64 bits wide,
1146 and as an extension GCC supports them in C89 mode and in C++.
1147 Simply write @code{long long int} for a signed integer, or
1148 @code{unsigned long long int} for an unsigned integer. To make an
1149 integer constant of type @code{long long int}, add the suffix @samp{LL}
1150 to the integer. To make an integer constant of type @code{unsigned long
1151 long int}, add the suffix @samp{ULL} to the integer.
1153 You can use these types in arithmetic like any other integer types.
1154 Addition, subtraction, and bitwise boolean operations on these types
1155 are open-coded on all types of machines. Multiplication is open-coded
1156 if the machine supports fullword-to-doubleword a widening multiply
1157 instruction. Division and shifts are open-coded only on machines that
1158 provide special support. The operations that are not open-coded use
1159 special library routines that come with GCC@.
1161 There may be pitfalls when you use @code{long long} types for function
1162 arguments, unless you declare function prototypes. If a function
1163 expects type @code{int} for its argument, and you pass a value of type
1164 @code{long long int}, confusion will result because the caller and the
1165 subroutine will disagree about the number of bytes for the argument.
1166 Likewise, if the function expects @code{long long int} and you pass
1167 @code{int}. The best way to avoid such problems is to use prototypes.
1170 @section Complex Numbers
1171 @cindex complex numbers
1172 @cindex @code{_Complex} keyword
1173 @cindex @code{__complex__} keyword
1175 ISO C99 supports complex floating data types, and as an extension GCC
1176 supports them in C89 mode and in C++, and supports complex integer data
1177 types which are not part of ISO C99. You can declare complex types
1178 using the keyword @code{_Complex}. As an extension, the older GNU
1179 keyword @code{__complex__} is also supported.
1181 For example, @samp{_Complex double x;} declares @code{x} as a
1182 variable whose real part and imaginary part are both of type
1183 @code{double}. @samp{_Complex short int y;} declares @code{y} to
1184 have real and imaginary parts of type @code{short int}; this is not
1185 likely to be useful, but it shows that the set of complex types is
1188 To write a constant with a complex data type, use the suffix @samp{i} or
1189 @samp{j} (either one; they are equivalent). For example, @code{2.5fi}
1190 has type @code{_Complex float} and @code{3i} has type
1191 @code{_Complex int}. Such a constant always has a pure imaginary
1192 value, but you can form any complex value you like by adding one to a
1193 real constant. This is a GNU extension; if you have an ISO C99
1194 conforming C library (such as GNU libc), and want to construct complex
1195 constants of floating type, you should include @code{<complex.h>} and
1196 use the macros @code{I} or @code{_Complex_I} instead.
1198 @cindex @code{__real__} keyword
1199 @cindex @code{__imag__} keyword
1200 To extract the real part of a complex-valued expression @var{exp}, write
1201 @code{__real__ @var{exp}}. Likewise, use @code{__imag__} to
1202 extract the imaginary part. This is a GNU extension; for values of
1203 floating type, you should use the ISO C99 functions @code{crealf},
1204 @code{creal}, @code{creall}, @code{cimagf}, @code{cimag} and
1205 @code{cimagl}, declared in @code{<complex.h>} and also provided as
1206 built-in functions by GCC@.
1208 @cindex complex conjugation
1209 The operator @samp{~} performs complex conjugation when used on a value
1210 with a complex type. This is a GNU extension; for values of
1211 floating type, you should use the ISO C99 functions @code{conjf},
1212 @code{conj} and @code{conjl}, declared in @code{<complex.h>} and also
1213 provided as built-in functions by GCC@.
1215 GCC can allocate complex automatic variables in a noncontiguous
1216 fashion; it's even possible for the real part to be in a register while
1217 the imaginary part is on the stack (or vice-versa). None of the
1218 supported debugging info formats has a way to represent noncontiguous
1219 allocation like this, so GCC describes a noncontiguous complex
1220 variable as if it were two separate variables of noncomplex type.
1221 If the variable's actual name is @code{foo}, the two fictitious
1222 variables are named @code{foo$real} and @code{foo$imag}. You can
1223 examine and set these two fictitious variables with your debugger.
1225 A future version of GDB will know how to recognize such pairs and treat
1226 them as a single variable with a complex type.
1232 ISO C99 supports floating-point numbers written not only in the usual
1233 decimal notation, such as @code{1.55e1}, but also numbers such as
1234 @code{0x1.fp3} written in hexadecimal format. As a GNU extension, GCC
1235 supports this in C89 mode (except in some cases when strictly
1236 conforming) and in C++. In that format the
1237 @samp{0x} hex introducer and the @samp{p} or @samp{P} exponent field are
1238 mandatory. The exponent is a decimal number that indicates the power of
1239 2 by which the significant part will be multiplied. Thus @samp{0x1.f} is
1246 @samp{p3} multiplies it by 8, and the value of @code{0x1.fp3}
1247 is the same as @code{1.55e1}.
1249 Unlike for floating-point numbers in the decimal notation the exponent
1250 is always required in the hexadecimal notation. Otherwise the compiler
1251 would not be able to resolve the ambiguity of, e.g., @code{0x1.f}. This
1252 could mean @code{1.0f} or @code{1.9375} since @samp{f} is also the
1253 extension for floating-point constants of type @code{float}.
1256 @section Arrays of Length Zero
1257 @cindex arrays of length zero
1258 @cindex zero-length arrays
1259 @cindex length-zero arrays
1260 @cindex flexible array members
1262 Zero-length arrays are allowed in GNU C@. They are very useful as the
1263 last element of a structure which is really a header for a variable-length
1272 struct line *thisline = (struct line *)
1273 malloc (sizeof (struct line) + this_length);
1274 thisline->length = this_length;
1277 In ISO C90, you would have to give @code{contents} a length of 1, which
1278 means either you waste space or complicate the argument to @code{malloc}.
1280 In ISO C99, you would use a @dfn{flexible array member}, which is
1281 slightly different in syntax and semantics:
1285 Flexible array members are written as @code{contents[]} without
1289 Flexible array members have incomplete type, and so the @code{sizeof}
1290 operator may not be applied. As a quirk of the original implementation
1291 of zero-length arrays, @code{sizeof} evaluates to zero.
1294 Flexible array members may only appear as the last member of a
1295 @code{struct} that is otherwise non-empty.
1298 A structure containing a flexible array member, or a union containing
1299 such a structure (possibly recursively), may not be a member of a
1300 structure or an element of an array. (However, these uses are
1301 permitted by GCC as extensions.)
1304 GCC versions before 3.0 allowed zero-length arrays to be statically
1305 initialized, as if they were flexible arrays. In addition to those
1306 cases that were useful, it also allowed initializations in situations
1307 that would corrupt later data. Non-empty initialization of zero-length
1308 arrays is now treated like any case where there are more initializer
1309 elements than the array holds, in that a suitable warning about "excess
1310 elements in array" is given, and the excess elements (all of them, in
1311 this case) are ignored.
1313 Instead GCC allows static initialization of flexible array members.
1314 This is equivalent to defining a new structure containing the original
1315 structure followed by an array of sufficient size to contain the data.
1316 I.e.@: in the following, @code{f1} is constructed as if it were declared
1322 @} f1 = @{ 1, @{ 2, 3, 4 @} @};
1325 struct f1 f1; int data[3];
1326 @} f2 = @{ @{ 1 @}, @{ 2, 3, 4 @} @};
1330 The convenience of this extension is that @code{f1} has the desired
1331 type, eliminating the need to consistently refer to @code{f2.f1}.
1333 This has symmetry with normal static arrays, in that an array of
1334 unknown size is also written with @code{[]}.
1336 Of course, this extension only makes sense if the extra data comes at
1337 the end of a top-level object, as otherwise we would be overwriting
1338 data at subsequent offsets. To avoid undue complication and confusion
1339 with initialization of deeply nested arrays, we simply disallow any
1340 non-empty initialization except when the structure is the top-level
1341 object. For example:
1344 struct foo @{ int x; int y[]; @};
1345 struct bar @{ struct foo z; @};
1347 struct foo a = @{ 1, @{ 2, 3, 4 @} @}; // @r{Valid.}
1348 struct bar b = @{ @{ 1, @{ 2, 3, 4 @} @} @}; // @r{Invalid.}
1349 struct bar c = @{ @{ 1, @{ @} @} @}; // @r{Valid.}
1350 struct foo d[1] = @{ @{ 1 @{ 2, 3, 4 @} @} @}; // @r{Invalid.}
1353 @node Variable Length
1354 @section Arrays of Variable Length
1355 @cindex variable-length arrays
1356 @cindex arrays of variable length
1359 Variable-length automatic arrays are allowed in ISO C99, and as an
1360 extension GCC accepts them in C89 mode and in C++. (However, GCC's
1361 implementation of variable-length arrays does not yet conform in detail
1362 to the ISO C99 standard.) These arrays are
1363 declared like any other automatic arrays, but with a length that is not
1364 a constant expression. The storage is allocated at the point of
1365 declaration and deallocated when the brace-level is exited. For
1370 concat_fopen (char *s1, char *s2, char *mode)
1372 char str[strlen (s1) + strlen (s2) + 1];
1375 return fopen (str, mode);
1379 @cindex scope of a variable length array
1380 @cindex variable-length array scope
1381 @cindex deallocating variable length arrays
1382 Jumping or breaking out of the scope of the array name deallocates the
1383 storage. Jumping into the scope is not allowed; you get an error
1386 @cindex @code{alloca} vs variable-length arrays
1387 You can use the function @code{alloca} to get an effect much like
1388 variable-length arrays. The function @code{alloca} is available in
1389 many other C implementations (but not in all). On the other hand,
1390 variable-length arrays are more elegant.
1392 There are other differences between these two methods. Space allocated
1393 with @code{alloca} exists until the containing @emph{function} returns.
1394 The space for a variable-length array is deallocated as soon as the array
1395 name's scope ends. (If you use both variable-length arrays and
1396 @code{alloca} in the same function, deallocation of a variable-length array
1397 will also deallocate anything more recently allocated with @code{alloca}.)
1399 You can also use variable-length arrays as arguments to functions:
1403 tester (int len, char data[len][len])
1409 The length of an array is computed once when the storage is allocated
1410 and is remembered for the scope of the array in case you access it with
1413 If you want to pass the array first and the length afterward, you can
1414 use a forward declaration in the parameter list---another GNU extension.
1418 tester (int len; char data[len][len], int len)
1424 @cindex parameter forward declaration
1425 The @samp{int len} before the semicolon is a @dfn{parameter forward
1426 declaration}, and it serves the purpose of making the name @code{len}
1427 known when the declaration of @code{data} is parsed.
1429 You can write any number of such parameter forward declarations in the
1430 parameter list. They can be separated by commas or semicolons, but the
1431 last one must end with a semicolon, which is followed by the ``real''
1432 parameter declarations. Each forward declaration must match a ``real''
1433 declaration in parameter name and data type. ISO C99 does not support
1434 parameter forward declarations.
1436 @node Variadic Macros
1437 @section Macros with a Variable Number of Arguments.
1438 @cindex variable number of arguments
1439 @cindex macro with variable arguments
1440 @cindex rest argument (in macro)
1441 @cindex variadic macros
1443 In the ISO C standard of 1999, a macro can be declared to accept a
1444 variable number of arguments much as a function can. The syntax for
1445 defining the macro is similar to that of a function. Here is an
1449 #define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)
1452 Here @samp{@dots{}} is a @dfn{variable argument}. In the invocation of
1453 such a macro, it represents the zero or more tokens until the closing
1454 parenthesis that ends the invocation, including any commas. This set of
1455 tokens replaces the identifier @code{__VA_ARGS__} in the macro body
1456 wherever it appears. See the CPP manual for more information.
1458 GCC has long supported variadic macros, and used a different syntax that
1459 allowed you to give a name to the variable arguments just like any other
1460 argument. Here is an example:
1463 #define debug(format, args...) fprintf (stderr, format, args)
1466 This is in all ways equivalent to the ISO C example above, but arguably
1467 more readable and descriptive.
1469 GNU CPP has two further variadic macro extensions, and permits them to
1470 be used with either of the above forms of macro definition.
1472 In standard C, you are not allowed to leave the variable argument out
1473 entirely; but you are allowed to pass an empty argument. For example,
1474 this invocation is invalid in ISO C, because there is no comma after
1481 GNU CPP permits you to completely omit the variable arguments in this
1482 way. In the above examples, the compiler would complain, though since
1483 the expansion of the macro still has the extra comma after the format
1486 To help solve this problem, CPP behaves specially for variable arguments
1487 used with the token paste operator, @samp{##}. If instead you write
1490 #define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)
1493 and if the variable arguments are omitted or empty, the @samp{##}
1494 operator causes the preprocessor to remove the comma before it. If you
1495 do provide some variable arguments in your macro invocation, GNU CPP
1496 does not complain about the paste operation and instead places the
1497 variable arguments after the comma. Just like any other pasted macro
1498 argument, these arguments are not macro expanded.
1500 @node Escaped Newlines
1501 @section Slightly Looser Rules for Escaped Newlines
1502 @cindex escaped newlines
1503 @cindex newlines (escaped)
1505 Recently, the preprocessor has relaxed its treatment of escaped
1506 newlines. Previously, the newline had to immediately follow a
1507 backslash. The current implementation allows whitespace in the form of
1508 spaces, horizontal and vertical tabs, and form feeds between the
1509 backslash and the subsequent newline. The preprocessor issues a
1510 warning, but treats it as a valid escaped newline and combines the two
1511 lines to form a single logical line. This works within comments and
1512 tokens, including multi-line strings, as well as between tokens.
1513 Comments are @emph{not} treated as whitespace for the purposes of this
1514 relaxation, since they have not yet been replaced with spaces.
1516 @node Multi-line Strings
1517 @section String Literals with Embedded Newlines
1518 @cindex multi-line string literals
1520 As an extension, GNU CPP permits string literals to cross multiple lines
1521 without escaping the embedded newlines. Each embedded newline is
1522 replaced with a single @samp{\n} character in the resulting string
1523 literal, regardless of what form the newline took originally.
1525 CPP currently allows such strings in directives as well (other than the
1526 @samp{#include} family). This is deprecated and will eventually be
1530 @section Non-Lvalue Arrays May Have Subscripts
1531 @cindex subscripting
1532 @cindex arrays, non-lvalue
1534 @cindex subscripting and function values
1535 In ISO C99, arrays that are not lvalues still decay to pointers, and
1536 may be subscripted, although they may not be modified or used after
1537 the next sequence point and the unary @samp{&} operator may not be
1538 applied to them. As an extension, GCC allows such arrays to be
1539 subscripted in C89 mode, though otherwise they do not decay to
1540 pointers outside C99 mode. For example,
1541 this is valid in GNU C though not valid in C89:
1545 struct foo @{int a[4];@};
1551 return f().a[index];
1557 @section Arithmetic on @code{void}- and Function-Pointers
1558 @cindex void pointers, arithmetic
1559 @cindex void, size of pointer to
1560 @cindex function pointers, arithmetic
1561 @cindex function, size of pointer to
1563 In GNU C, addition and subtraction operations are supported on pointers to
1564 @code{void} and on pointers to functions. This is done by treating the
1565 size of a @code{void} or of a function as 1.
1567 A consequence of this is that @code{sizeof} is also allowed on @code{void}
1568 and on function types, and returns 1.
1570 @opindex Wpointer-arith
1571 The option @option{-Wpointer-arith} requests a warning if these extensions
1575 @section Non-Constant Initializers
1576 @cindex initializers, non-constant
1577 @cindex non-constant initializers
1579 As in standard C++ and ISO C99, the elements of an aggregate initializer for an
1580 automatic variable are not required to be constant expressions in GNU C@.
1581 Here is an example of an initializer with run-time varying elements:
1584 foo (float f, float g)
1586 float beat_freqs[2] = @{ f-g, f+g @};
1591 @node Compound Literals
1592 @section Compound Literals
1593 @cindex constructor expressions
1594 @cindex initializations in expressions
1595 @cindex structures, constructor expression
1596 @cindex expressions, constructor
1597 @cindex compound literals
1598 @c The GNU C name for what C99 calls compound literals was "constructor expressions".
1600 ISO C99 supports compound literals. A compound literal looks like
1601 a cast containing an initializer. Its value is an object of the
1602 type specified in the cast, containing the elements specified in
1603 the initializer; it is an lvalue. As an extension, GCC supports
1604 compound literals in C89 mode and in C++.
1606 Usually, the specified type is a structure. Assume that
1607 @code{struct foo} and @code{structure} are declared as shown:
1610 struct foo @{int a; char b[2];@} structure;
1614 Here is an example of constructing a @code{struct foo} with a compound literal:
1617 structure = ((struct foo) @{x + y, 'a', 0@});
1621 This is equivalent to writing the following:
1625 struct foo temp = @{x + y, 'a', 0@};
1630 You can also construct an array. If all the elements of the compound literal
1631 are (made up of) simple constant expressions, suitable for use in
1632 initializers of objects of static storage duration, then the compound
1633 literal can be coerced to a pointer to its first element and used in
1634 such an initializer, as shown here:
1637 char **foo = (char *[]) @{ "x", "y", "z" @};
1640 Compound literals for scalar types and union types are is
1641 also allowed, but then the compound literal is equivalent
1644 As a GNU extension, GCC allows initialization of objects with static storage
1645 duration by compound literals (which is not possible in ISO C99, because
1646 the initializer is not a constant).
1647 It is handled as if the object was initialized only with the bracket
1648 enclosed list if compound literal's and object types match.
1649 The initializer list of the compound literal must be constant.
1650 If the object being initialized has array type of unknown size, the size is
1651 determined by compound literal size.
1654 static struct foo x = (struct foo) @{1, 'a', 'b'@};
1655 static int y[] = (int []) @{1, 2, 3@};
1656 static int z[] = (int [3]) @{1@};
1660 The above lines are equivalent to the following:
1662 static struct foo x = @{1, 'a', 'b'@};
1663 static int y[] = @{1, 2, 3@};
1664 static int z[] = @{1, 0, 0@};
1667 @node Designated Inits
1668 @section Designated Initializers
1669 @cindex initializers with labeled elements
1670 @cindex labeled elements in initializers
1671 @cindex case labels in initializers
1672 @cindex designated initializers
1674 Standard C89 requires the elements of an initializer to appear in a fixed
1675 order, the same as the order of the elements in the array or structure
1678 In ISO C99 you can give the elements in any order, specifying the array
1679 indices or structure field names they apply to, and GNU C allows this as
1680 an extension in C89 mode as well. This extension is not
1681 implemented in GNU C++.
1683 To specify an array index, write
1684 @samp{[@var{index}] =} before the element value. For example,
1687 int a[6] = @{ [4] = 29, [2] = 15 @};
1694 int a[6] = @{ 0, 0, 15, 0, 29, 0 @};
1698 The index values must be constant expressions, even if the array being
1699 initialized is automatic.
1701 An alternative syntax for this which has been obsolete since GCC 2.5 but
1702 GCC still accepts is to write @samp{[@var{index}]} before the element
1703 value, with no @samp{=}.
1705 To initialize a range of elements to the same value, write
1706 @samp{[@var{first} ... @var{last}] = @var{value}}. This is a GNU
1707 extension. For example,
1710 int widths[] = @{ [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 @};
1714 If the value in it has side-effects, the side-effects will happen only once,
1715 not for each initialized field by the range initializer.
1718 Note that the length of the array is the highest value specified
1721 In a structure initializer, specify the name of a field to initialize
1722 with @samp{.@var{fieldname} =} before the element value. For example,
1723 given the following structure,
1726 struct point @{ int x, y; @};
1730 the following initialization
1733 struct point p = @{ .y = yvalue, .x = xvalue @};
1740 struct point p = @{ xvalue, yvalue @};
1743 Another syntax which has the same meaning, obsolete since GCC 2.5, is
1744 @samp{@var{fieldname}:}, as shown here:
1747 struct point p = @{ y: yvalue, x: xvalue @};
1751 The @samp{[@var{index}]} or @samp{.@var{fieldname}} is known as a
1752 @dfn{designator}. You can also use a designator (or the obsolete colon
1753 syntax) when initializing a union, to specify which element of the union
1754 should be used. For example,
1757 union foo @{ int i; double d; @};
1759 union foo f = @{ .d = 4 @};
1763 will convert 4 to a @code{double} to store it in the union using
1764 the second element. By contrast, casting 4 to type @code{union foo}
1765 would store it into the union as the integer @code{i}, since it is
1766 an integer. (@xref{Cast to Union}.)
1768 You can combine this technique of naming elements with ordinary C
1769 initialization of successive elements. Each initializer element that
1770 does not have a designator applies to the next consecutive element of the
1771 array or structure. For example,
1774 int a[6] = @{ [1] = v1, v2, [4] = v4 @};
1781 int a[6] = @{ 0, v1, v2, 0, v4, 0 @};
1784 Labeling the elements of an array initializer is especially useful
1785 when the indices are characters or belong to an @code{enum} type.
1790 = @{ [' '] = 1, ['\t'] = 1, ['\h'] = 1,
1791 ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 @};
1794 @cindex designator lists
1795 You can also write a series of @samp{.@var{fieldname}} and
1796 @samp{[@var{index}]} designators before an @samp{=} to specify a
1797 nested subobject to initialize; the list is taken relative to the
1798 subobject corresponding to the closest surrounding brace pair. For
1799 example, with the @samp{struct point} declaration above:
1802 struct point ptarray[10] = @{ [2].y = yv2, [2].x = xv2, [0].x = xv0 @};
1806 If the same field is initialized multiple times, it will have value from
1807 the last initialization. If any such overridden initialization has
1808 side-effect, it is unspecified whether the side-effect happens or not.
1809 Currently, gcc will discard them and issue a warning.
1812 @section Case Ranges
1814 @cindex ranges in case statements
1816 You can specify a range of consecutive values in a single @code{case} label,
1820 case @var{low} ... @var{high}:
1824 This has the same effect as the proper number of individual @code{case}
1825 labels, one for each integer value from @var{low} to @var{high}, inclusive.
1827 This feature is especially useful for ranges of ASCII character codes:
1833 @strong{Be careful:} Write spaces around the @code{...}, for otherwise
1834 it may be parsed wrong when you use it with integer values. For example,
1849 @section Cast to a Union Type
1850 @cindex cast to a union
1851 @cindex union, casting to a
1853 A cast to union type is similar to other casts, except that the type
1854 specified is a union type. You can specify the type either with
1855 @code{union @var{tag}} or with a typedef name. A cast to union is actually
1856 a constructor though, not a cast, and hence does not yield an lvalue like
1857 normal casts. (@xref{Compound Literals}.)
1859 The types that may be cast to the union type are those of the members
1860 of the union. Thus, given the following union and variables:
1863 union foo @{ int i; double d; @};
1869 both @code{x} and @code{y} can be cast to type @code{union foo}.
1871 Using the cast as the right-hand side of an assignment to a variable of
1872 union type is equivalent to storing in a member of the union:
1877 u = (union foo) x @equiv{} u.i = x
1878 u = (union foo) y @equiv{} u.d = y
1881 You can also use the union cast as a function argument:
1884 void hack (union foo);
1886 hack ((union foo) x);
1889 @node Mixed Declarations
1890 @section Mixed Declarations and Code
1891 @cindex mixed declarations and code
1892 @cindex declarations, mixed with code
1893 @cindex code, mixed with declarations
1895 ISO C99 and ISO C++ allow declarations and code to be freely mixed
1896 within compound statements. As an extension, GCC also allows this in
1897 C89 mode. For example, you could do:
1906 Each identifier is visible from where it is declared until the end of
1907 the enclosing block.
1909 @node Function Attributes
1910 @section Declaring Attributes of Functions
1911 @cindex function attributes
1912 @cindex declaring attributes of functions
1913 @cindex functions that never return
1914 @cindex functions that have no side effects
1915 @cindex functions in arbitrary sections
1916 @cindex functions that behave like malloc
1917 @cindex @code{volatile} applied to function
1918 @cindex @code{const} applied to function
1919 @cindex functions with @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon} style arguments
1920 @cindex functions with non-null pointer arguments
1921 @cindex functions that are passed arguments in registers on the 386
1922 @cindex functions that pop the argument stack on the 386
1923 @cindex functions that do not pop the argument stack on the 386
1925 In GNU C, you declare certain things about functions called in your program
1926 which help the compiler optimize function calls and check your code more
1929 The keyword @code{__attribute__} allows you to specify special
1930 attributes when making a declaration. This keyword is followed by an
1931 attribute specification inside double parentheses. The following
1932 attributes are currently defined for functions on all targets:
1933 @code{noreturn}, @code{noinline}, @code{always_inline},
1934 @code{pure}, @code{const}, @code{nothrow},
1935 @code{format}, @code{format_arg}, @code{no_instrument_function},
1936 @code{section}, @code{constructor}, @code{destructor}, @code{used},
1937 @code{unused}, @code{deprecated}, @code{weak}, @code{malloc},
1938 @code{alias}, and @code{nonnull}. Several other attributes are defined
1939 for functions on particular target systems. Other attributes, including
1940 @code{section} are supported for variables declarations
1941 (@pxref{Variable Attributes}) and for types (@pxref{Type Attributes}).
1943 You may also specify attributes with @samp{__} preceding and following
1944 each keyword. This allows you to use them in header files without
1945 being concerned about a possible macro of the same name. For example,
1946 you may use @code{__noreturn__} instead of @code{noreturn}.
1948 @xref{Attribute Syntax}, for details of the exact syntax for using
1952 @cindex @code{noreturn} function attribute
1954 A few standard library functions, such as @code{abort} and @code{exit},
1955 cannot return. GCC knows this automatically. Some programs define
1956 their own functions that never return. You can declare them
1957 @code{noreturn} to tell the compiler this fact. For example,
1961 void fatal () __attribute__ ((noreturn));
1964 fatal (/* @r{@dots{}} */)
1966 /* @r{@dots{}} */ /* @r{Print error message.} */ /* @r{@dots{}} */
1972 The @code{noreturn} keyword tells the compiler to assume that
1973 @code{fatal} cannot return. It can then optimize without regard to what
1974 would happen if @code{fatal} ever did return. This makes slightly
1975 better code. More importantly, it helps avoid spurious warnings of
1976 uninitialized variables.
1978 Do not assume that registers saved by the calling function are
1979 restored before calling the @code{noreturn} function.
1981 It does not make sense for a @code{noreturn} function to have a return
1982 type other than @code{void}.
1984 The attribute @code{noreturn} is not implemented in GCC versions
1985 earlier than 2.5. An alternative way to declare that a function does
1986 not return, which works in the current version and in some older
1987 versions, is as follows:
1990 typedef void voidfn ();
1992 volatile voidfn fatal;
1995 @cindex @code{noinline} function attribute
1997 This function attribute prevents a function from being considered for
2000 @cindex @code{always_inline} function attribute
2002 Generally, functions are not inlined unless optimization is specified.
2003 For functions declared inline, this attribute inlines the function even
2004 if no optimization level was specified.
2006 @cindex @code{pure} function attribute
2008 Many functions have no effects except the return value and their
2009 return value depends only on the parameters and/or global variables.
2010 Such a function can be subject
2011 to common subexpression elimination and loop optimization just as an
2012 arithmetic operator would be. These functions should be declared
2013 with the attribute @code{pure}. For example,
2016 int square (int) __attribute__ ((pure));
2020 says that the hypothetical function @code{square} is safe to call
2021 fewer times than the program says.
2023 Some of common examples of pure functions are @code{strlen} or @code{memcmp}.
2024 Interesting non-pure functions are functions with infinite loops or those
2025 depending on volatile memory or other system resource, that may change between
2026 two consecutive calls (such as @code{feof} in a multithreading environment).
2028 The attribute @code{pure} is not implemented in GCC versions earlier
2030 @cindex @code{const} function attribute
2032 Many functions do not examine any values except their arguments, and
2033 have no effects except the return value. Basically this is just slightly
2034 more strict class than the @code{pure} attribute above, since function is not
2035 allowed to read global memory.
2037 @cindex pointer arguments
2038 Note that a function that has pointer arguments and examines the data
2039 pointed to must @emph{not} be declared @code{const}. Likewise, a
2040 function that calls a non-@code{const} function usually must not be
2041 @code{const}. It does not make sense for a @code{const} function to
2044 The attribute @code{const} is not implemented in GCC versions earlier
2045 than 2.5. An alternative way to declare that a function has no side
2046 effects, which works in the current version and in some older versions,
2050 typedef int intfn ();
2052 extern const intfn square;
2055 This approach does not work in GNU C++ from 2.6.0 on, since the language
2056 specifies that the @samp{const} must be attached to the return value.
2058 @cindex @code{nothrow} function attribute
2060 The @code{nothrow} attribute is used to inform the compiler that a
2061 function cannot throw an exception. For example, most functions in
2062 the standard C library can be guaranteed not to throw an exception
2063 with the notable exceptions of @code{qsort} and @code{bsearch} that
2064 take function pointer arguments. The @code{nothrow} attribute is not
2065 implemented in GCC versions earlier than 3.2.
2067 @item format (@var{archetype}, @var{string-index}, @var{first-to-check})
2068 @cindex @code{format} function attribute
2070 The @code{format} attribute specifies that a function takes @code{printf},
2071 @code{scanf}, @code{strftime} or @code{strfmon} style arguments which
2072 should be type-checked against a format string. For example, the
2077 my_printf (void *my_object, const char *my_format, ...)
2078 __attribute__ ((format (printf, 2, 3)));
2082 causes the compiler to check the arguments in calls to @code{my_printf}
2083 for consistency with the @code{printf} style format string argument
2086 The parameter @var{archetype} determines how the format string is
2087 interpreted, and should be @code{printf}, @code{scanf}, @code{strftime}
2088 or @code{strfmon}. (You can also use @code{__printf__},
2089 @code{__scanf__}, @code{__strftime__} or @code{__strfmon__}.) The
2090 parameter @var{string-index} specifies which argument is the format
2091 string argument (starting from 1), while @var{first-to-check} is the
2092 number of the first argument to check against the format string. For
2093 functions where the arguments are not available to be checked (such as
2094 @code{vprintf}), specify the third parameter as zero. In this case the
2095 compiler only checks the format string for consistency. For
2096 @code{strftime} formats, the third parameter is required to be zero.
2098 In the example above, the format string (@code{my_format}) is the second
2099 argument of the function @code{my_print}, and the arguments to check
2100 start with the third argument, so the correct parameters for the format
2101 attribute are 2 and 3.
2103 @opindex ffreestanding
2104 The @code{format} attribute allows you to identify your own functions
2105 which take format strings as arguments, so that GCC can check the
2106 calls to these functions for errors. The compiler always (unless
2107 @option{-ffreestanding} is used) checks formats
2108 for the standard library functions @code{printf}, @code{fprintf},
2109 @code{sprintf}, @code{scanf}, @code{fscanf}, @code{sscanf}, @code{strftime},
2110 @code{vprintf}, @code{vfprintf} and @code{vsprintf} whenever such
2111 warnings are requested (using @option{-Wformat}), so there is no need to
2112 modify the header file @file{stdio.h}. In C99 mode, the functions
2113 @code{snprintf}, @code{vsnprintf}, @code{vscanf}, @code{vfscanf} and
2114 @code{vsscanf} are also checked. Except in strictly conforming C
2115 standard modes, the X/Open function @code{strfmon} is also checked as
2116 are @code{printf_unlocked} and @code{fprintf_unlocked}.
2117 @xref{C Dialect Options,,Options Controlling C Dialect}.
2119 @item format_arg (@var{string-index})
2120 @cindex @code{format_arg} function attribute
2121 @opindex Wformat-nonliteral
2122 The @code{format_arg} attribute specifies that a function takes a format
2123 string for a @code{printf}, @code{scanf}, @code{strftime} or
2124 @code{strfmon} style function and modifies it (for example, to translate
2125 it into another language), so the result can be passed to a
2126 @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon} style
2127 function (with the remaining arguments to the format function the same
2128 as they would have been for the unmodified string). For example, the
2133 my_dgettext (char *my_domain, const char *my_format)
2134 __attribute__ ((format_arg (2)));
2138 causes the compiler to check the arguments in calls to a @code{printf},
2139 @code{scanf}, @code{strftime} or @code{strfmon} type function, whose
2140 format string argument is a call to the @code{my_dgettext} function, for
2141 consistency with the format string argument @code{my_format}. If the
2142 @code{format_arg} attribute had not been specified, all the compiler
2143 could tell in such calls to format functions would be that the format
2144 string argument is not constant; this would generate a warning when
2145 @option{-Wformat-nonliteral} is used, but the calls could not be checked
2146 without the attribute.
2148 The parameter @var{string-index} specifies which argument is the format
2149 string argument (starting from 1).
2151 The @code{format-arg} attribute allows you to identify your own
2152 functions which modify format strings, so that GCC can check the
2153 calls to @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon}
2154 type function whose operands are a call to one of your own function.
2155 The compiler always treats @code{gettext}, @code{dgettext}, and
2156 @code{dcgettext} in this manner except when strict ISO C support is
2157 requested by @option{-ansi} or an appropriate @option{-std} option, or
2158 @option{-ffreestanding} is used. @xref{C Dialect Options,,Options
2159 Controlling C Dialect}.
2161 @item nonnull (@var{arg-index}, @dots{})
2162 @cindex @code{nonnull} function attribute
2163 The @code{nonnull} attribute specifies that some function parameters should
2164 be non-null pointers. For instance, the declaration:
2168 my_memcpy (void *dest, const void *src, size_t len)
2169 __attribute__((nonnull (1, 2)));
2173 causes the compiler to check that, in calls to @code{my_memcpy},
2174 arguments @var{dest} and @var{src} are non-null. If the compiler
2175 determines that a null pointer is passed in an argument slot marked
2176 as non-null, and the @option{-Wnonnull} option is enabled, a warning
2177 is issued. The compiler may also choose to make optimizations based
2178 on the knowledge that certain function arguments will not be null.
2180 If no argument index list is given to the @code{nonnull} attribute,
2181 all pointer arguments are marked as non-null. To illustrate, the
2182 following declaration is equivalent to the previous example:
2186 my_memcpy (void *dest, const void *src, size_t len)
2187 __attribute__((nonnull));
2190 @item no_instrument_function
2191 @cindex @code{no_instrument_function} function attribute
2192 @opindex finstrument-functions
2193 If @option{-finstrument-functions} is given, profiling function calls will
2194 be generated at entry and exit of most user-compiled functions.
2195 Functions with this attribute will not be so instrumented.
2197 @item section ("@var{section-name}")
2198 @cindex @code{section} function attribute
2199 Normally, the compiler places the code it generates in the @code{text} section.
2200 Sometimes, however, you need additional sections, or you need certain
2201 particular functions to appear in special sections. The @code{section}
2202 attribute specifies that a function lives in a particular section.
2203 For example, the declaration:
2206 extern void foobar (void) __attribute__ ((section ("bar")));
2210 puts the function @code{foobar} in the @code{bar} section.
2212 Some file formats do not support arbitrary sections so the @code{section}
2213 attribute is not available on all platforms.
2214 If you need to map the entire contents of a module to a particular
2215 section, consider using the facilities of the linker instead.
2219 @cindex @code{constructor} function attribute
2220 @cindex @code{destructor} function attribute
2221 The @code{constructor} attribute causes the function to be called
2222 automatically before execution enters @code{main ()}. Similarly, the
2223 @code{destructor} attribute causes the function to be called
2224 automatically after @code{main ()} has completed or @code{exit ()} has
2225 been called. Functions with these attributes are useful for
2226 initializing data that will be used implicitly during the execution of
2229 These attributes are not currently implemented for Objective-C@.
2231 @cindex @code{unused} attribute.
2233 This attribute, attached to a function, means that the function is meant
2234 to be possibly unused. GCC will not produce a warning for this
2235 function. GNU C++ does not currently support this attribute as
2236 definitions without parameters are valid in C++.
2238 @cindex @code{used} attribute.
2240 This attribute, attached to a function, means that code must be emitted
2241 for the function even if it appears that the function is not referenced.
2242 This is useful, for example, when the function is referenced only in
2245 @cindex @code{deprecated} attribute.
2247 The @code{deprecated} attribute results in a warning if the function
2248 is used anywhere in the source file. This is useful when identifying
2249 functions that are expected to be removed in a future version of a
2250 program. The warning also includes the location of the declaration
2251 of the deprecated function, to enable users to easily find further
2252 information about why the function is deprecated, or what they should
2253 do instead. Note that the warnings only occurs for uses:
2256 int old_fn () __attribute__ ((deprecated));
2258 int (*fn_ptr)() = old_fn;
2261 results in a warning on line 3 but not line 2.
2263 The @code{deprecated} attribute can also be used for variables and
2264 types (@pxref{Variable Attributes}, @pxref{Type Attributes}.)
2267 @cindex @code{weak} attribute
2268 The @code{weak} attribute causes the declaration to be emitted as a weak
2269 symbol rather than a global. This is primarily useful in defining
2270 library functions which can be overridden in user code, though it can
2271 also be used with non-function declarations. Weak symbols are supported
2272 for ELF targets, and also for a.out targets when using the GNU assembler
2276 @cindex @code{malloc} attribute
2277 The @code{malloc} attribute is used to tell the compiler that a function
2278 may be treated as if it were the malloc function. The compiler assumes
2279 that calls to malloc result in a pointers that cannot alias anything.
2280 This will often improve optimization.
2282 @item alias ("@var{target}")
2283 @cindex @code{alias} attribute
2284 The @code{alias} attribute causes the declaration to be emitted as an
2285 alias for another symbol, which must be specified. For instance,
2288 void __f () @{ /* @r{Do something.} */; @}
2289 void f () __attribute__ ((weak, alias ("__f")));
2292 declares @samp{f} to be a weak alias for @samp{__f}. In C++, the
2293 mangled name for the target must be used.
2295 Not all target machines support this attribute.
2297 @item visibility ("@var{visibility_type}")
2298 @cindex @code{visibility} attribute
2299 The @code{visibility} attribute on ELF targets causes the declaration
2300 to be emitted with hidden, protected or internal visibility.
2303 void __attribute__ ((visibility ("protected")))
2304 f () @{ /* @r{Do something.} */; @}
2305 int i __attribute__ ((visibility ("hidden")));
2308 See the ELF gABI for complete details, but the short story is
2312 Hidden visibility indicates that the symbol will not be placed into
2313 the dynamic symbol table, so no other @dfn{module} (executable or
2314 shared library) can reference it directly.
2317 Protected visibility indicates that the symbol will be placed in the
2318 dynamic symbol table, but that references within the defining module
2319 will bind to the local symbol. That is, the symbol cannot be overridden
2323 Internal visibility is like hidden visibility, but with additional
2324 processor specific semantics. Unless otherwise specified by the psABI,
2325 gcc defines internal visibility to mean that the function is @emph{never}
2326 called from another module. Note that hidden symbols, while then cannot
2327 be referenced directly by other modules, can be referenced indirectly via
2328 function pointers. By indicating that a symbol cannot be called from
2329 outside the module, gcc may for instance omit the load of a PIC register
2330 since it is known that the calling function loaded the correct value.
2333 Not all ELF targets support this attribute.
2335 @item regparm (@var{number})
2336 @cindex functions that are passed arguments in registers on the 386
2337 On the Intel 386, the @code{regparm} attribute causes the compiler to
2338 pass up to @var{number} integer arguments in registers EAX,
2339 EDX, and ECX instead of on the stack. Functions that take a
2340 variable number of arguments will continue to be passed all of their
2341 arguments on the stack.
2344 @cindex functions that pop the argument stack on the 386
2345 On the Intel 386, the @code{stdcall} attribute causes the compiler to
2346 assume that the called function will pop off the stack space used to
2347 pass arguments, unless it takes a variable number of arguments.
2349 The PowerPC compiler for Windows NT currently ignores the @code{stdcall}
2353 @cindex functions that do pop the argument stack on the 386
2355 On the Intel 386, the @code{cdecl} attribute causes the compiler to
2356 assume that the calling function will pop off the stack space used to
2357 pass arguments. This is
2358 useful to override the effects of the @option{-mrtd} switch.
2360 The PowerPC compiler for Windows NT currently ignores the @code{cdecl}
2363 @item longcall/shortcall
2364 @cindex functions called via pointer on the RS/6000 and PowerPC
2365 On the RS/6000 and PowerPC, the @code{longcall} attribute causes the
2366 compiler to always call this function via a pointer, just as it would if
2367 the @option{-mlongcall} option had been specified. The @code{shortcall}
2368 attribute causes the compiler not to do this. These attributes override
2369 both the @option{-mlongcall} switch and the @code{#pragma longcall}
2372 @xref{RS/6000 and PowerPC Options}, for more information on when long
2373 calls are and are not necessary.
2375 @item long_call/short_call
2376 @cindex indirect calls on ARM
2377 This attribute allows to specify how to call a particular function on
2378 ARM@. Both attributes override the @option{-mlong-calls} (@pxref{ARM Options})
2379 command line switch and @code{#pragma long_calls} settings. The
2380 @code{long_call} attribute causes the compiler to always call the
2381 function by first loading its address into a register and then using the
2382 contents of that register. The @code{short_call} attribute always places
2383 the offset to the function from the call site into the @samp{BL}
2384 instruction directly.
2387 @cindex functions which are imported from a dll on PowerPC Windows NT
2388 On the PowerPC running Windows NT, the @code{dllimport} attribute causes
2389 the compiler to call the function via a global pointer to the function
2390 pointer that is set up by the Windows NT dll library. The pointer name
2391 is formed by combining @code{__imp_} and the function name.
2394 @cindex functions which are exported from a dll on PowerPC Windows NT
2395 On the PowerPC running Windows NT, the @code{dllexport} attribute causes
2396 the compiler to provide a global pointer to the function pointer, so
2397 that it can be called with the @code{dllimport} attribute. The pointer
2398 name is formed by combining @code{__imp_} and the function name.
2400 @item exception (@var{except-func} [, @var{except-arg}])
2401 @cindex functions which specify exception handling on PowerPC Windows NT
2402 On the PowerPC running Windows NT, the @code{exception} attribute causes
2403 the compiler to modify the structured exception table entry it emits for
2404 the declared function. The string or identifier @var{except-func} is
2405 placed in the third entry of the structured exception table. It
2406 represents a function, which is called by the exception handling
2407 mechanism if an exception occurs. If it was specified, the string or
2408 identifier @var{except-arg} is placed in the fourth entry of the
2409 structured exception table.
2411 @item function_vector
2412 @cindex calling functions through the function vector on the H8/300 processors
2413 Use this attribute on the H8/300 and H8/300H to indicate that the specified
2414 function should be called through the function vector. Calling a
2415 function through the function vector will reduce code size, however;
2416 the function vector has a limited size (maximum 128 entries on the H8/300
2417 and 64 entries on the H8/300H) and shares space with the interrupt vector.
2419 You must use GAS and GLD from GNU binutils version 2.7 or later for
2420 this attribute to work correctly.
2423 @cindex interrupt handler functions
2424 Use this attribute on the ARM, AVR, M32R/D and Xstormy16 ports to indicate
2425 that the specified function is an interrupt handler. The compiler will
2426 generate function entry and exit sequences suitable for use in an
2427 interrupt handler when this attribute is present.
2429 Note, interrupt handlers for the H8/300, H8/300H and SH processors can
2430 be specified via the @code{interrupt_handler} attribute.
2432 Note, on the AVR interrupts will be enabled inside the function.
2434 Note, for the ARM you can specify the kind of interrupt to be handled by
2435 adding an optional parameter to the interrupt attribute like this:
2438 void f () __attribute__ ((interrupt ("IRQ")));
2441 Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF@.
2443 @item interrupt_handler
2444 @cindex interrupt handler functions on the H8/300 and SH processors
2445 Use this attribute on the H8/300, H8/300H and SH to indicate that the
2446 specified function is an interrupt handler. The compiler will generate
2447 function entry and exit sequences suitable for use in an interrupt
2448 handler when this attribute is present.
2451 Use this attribute on the SH to indicate an @code{interrupt_handler}
2452 function should switch to an alternate stack. It expects a string
2453 argument that names a global variable holding the address of the
2458 void f () __attribute__ ((interrupt_handler,
2459 sp_switch ("alt_stack")));
2463 Use this attribute on the SH for an @code{interrupt_handle} to return using
2464 @code{trapa} instead of @code{rte}. This attribute expects an integer
2465 argument specifying the trap number to be used.
2468 @cindex eight bit data on the H8/300 and H8/300H
2469 Use this attribute on the H8/300 and H8/300H to indicate that the specified
2470 variable should be placed into the eight bit data section.
2471 The compiler will generate more efficient code for certain operations
2472 on data in the eight bit data area. Note the eight bit data area is limited to
2475 You must use GAS and GLD from GNU binutils version 2.7 or later for
2476 this attribute to work correctly.
2479 @cindex tiny data section on the H8/300H
2480 Use this attribute on the H8/300H to indicate that the specified
2481 variable should be placed into the tiny data section.
2482 The compiler will generate more efficient code for loads and stores
2483 on data in the tiny data section. Note the tiny data area is limited to
2484 slightly under 32kbytes of data.
2487 @cindex signal handler functions on the AVR processors
2488 Use this attribute on the AVR to indicate that the specified
2489 function is an signal handler. The compiler will generate function
2490 entry and exit sequences suitable for use in an signal handler when this
2491 attribute is present. Interrupts will be disabled inside function.
2494 @cindex function without a prologue/epilogue code
2495 Use this attribute on the ARM, AVR and IP2K ports to indicate that the
2496 specified function do not need prologue/epilogue sequences generated by
2497 the compiler. It is up to the programmer to provide these sequences.
2499 @item model (@var{model-name})
2500 @cindex function addressability on the M32R/D
2501 Use this attribute on the M32R/D to set the addressability of an object,
2502 and the code generated for a function.
2503 The identifier @var{model-name} is one of @code{small}, @code{medium},
2504 or @code{large}, representing each of the code models.
2506 Small model objects live in the lower 16MB of memory (so that their
2507 addresses can be loaded with the @code{ld24} instruction), and are
2508 callable with the @code{bl} instruction.
2510 Medium model objects may live anywhere in the 32-bit address space (the
2511 compiler will generate @code{seth/add3} instructions to load their addresses),
2512 and are callable with the @code{bl} instruction.
2514 Large model objects may live anywhere in the 32-bit address space (the
2515 compiler will generate @code{seth/add3} instructions to load their addresses),
2516 and may not be reachable with the @code{bl} instruction (the compiler will
2517 generate the much slower @code{seth/add3/jl} instruction sequence).
2521 You can specify multiple attributes in a declaration by separating them
2522 by commas within the double parentheses or by immediately following an
2523 attribute declaration with another attribute declaration.
2525 @cindex @code{#pragma}, reason for not using
2526 @cindex pragma, reason for not using
2527 Some people object to the @code{__attribute__} feature, suggesting that
2528 ISO C's @code{#pragma} should be used instead. At the time
2529 @code{__attribute__} was designed, there were two reasons for not doing
2534 It is impossible to generate @code{#pragma} commands from a macro.
2537 There is no telling what the same @code{#pragma} might mean in another
2541 These two reasons applied to almost any application that might have been
2542 proposed for @code{#pragma}. It was basically a mistake to use
2543 @code{#pragma} for @emph{anything}.
2545 The ISO C99 standard includes @code{_Pragma}, which now allows pragmas
2546 to be generated from macros. In addition, a @code{#pragma GCC}
2547 namespace is now in use for GCC-specific pragmas. However, it has been
2548 found convenient to use @code{__attribute__} to achieve a natural
2549 attachment of attributes to their corresponding declarations, whereas
2550 @code{#pragma GCC} is of use for constructs that do not naturally form
2551 part of the grammar. @xref{Other Directives,,Miscellaneous
2552 Preprocessing Directives, cpp, The C Preprocessor}.
2554 @node Attribute Syntax
2555 @section Attribute Syntax
2556 @cindex attribute syntax
2558 This section describes the syntax with which @code{__attribute__} may be
2559 used, and the constructs to which attribute specifiers bind, for the C
2560 language. Some details may vary for C++ and Objective-C@. Because of
2561 infelicities in the grammar for attributes, some forms described here
2562 may not be successfully parsed in all cases.
2564 There are some problems with the semantics of attributes in C++. For
2565 example, there are no manglings for attributes, although they may affect
2566 code generation, so problems may arise when attributed types are used in
2567 conjunction with templates or overloading. Similarly, @code{typeid}
2568 does not distinguish between types with different attributes. Support
2569 for attributes in C++ may be restricted in future to attributes on
2570 declarations only, but not on nested declarators.
2572 @xref{Function Attributes}, for details of the semantics of attributes
2573 applying to functions. @xref{Variable Attributes}, for details of the
2574 semantics of attributes applying to variables. @xref{Type Attributes},
2575 for details of the semantics of attributes applying to structure, union
2576 and enumerated types.
2578 An @dfn{attribute specifier} is of the form
2579 @code{__attribute__ ((@var{attribute-list}))}. An @dfn{attribute list}
2580 is a possibly empty comma-separated sequence of @dfn{attributes}, where
2581 each attribute is one of the following:
2585 Empty. Empty attributes are ignored.
2588 A word (which may be an identifier such as @code{unused}, or a reserved
2589 word such as @code{const}).
2592 A word, followed by, in parentheses, parameters for the attribute.
2593 These parameters take one of the following forms:
2597 An identifier. For example, @code{mode} attributes use this form.
2600 An identifier followed by a comma and a non-empty comma-separated list
2601 of expressions. For example, @code{format} attributes use this form.
2604 A possibly empty comma-separated list of expressions. For example,
2605 @code{format_arg} attributes use this form with the list being a single
2606 integer constant expression, and @code{alias} attributes use this form
2607 with the list being a single string constant.
2611 An @dfn{attribute specifier list} is a sequence of one or more attribute
2612 specifiers, not separated by any other tokens.
2614 An attribute specifier list may appear after the colon following a
2615 label, other than a @code{case} or @code{default} label. The only
2616 attribute it makes sense to use after a label is @code{unused}. This
2617 feature is intended for code generated by programs which contains labels
2618 that may be unused but which is compiled with @option{-Wall}. It would
2619 not normally be appropriate to use in it human-written code, though it
2620 could be useful in cases where the code that jumps to the label is
2621 contained within an @code{#ifdef} conditional.
2623 An attribute specifier list may appear as part of a @code{struct},
2624 @code{union} or @code{enum} specifier. It may go either immediately
2625 after the @code{struct}, @code{union} or @code{enum} keyword, or after
2626 the closing brace. It is ignored if the content of the structure, union
2627 or enumerated type is not defined in the specifier in which the
2628 attribute specifier list is used---that is, in usages such as
2629 @code{struct __attribute__((foo)) bar} with no following opening brace.
2630 Where attribute specifiers follow the closing brace, they are considered
2631 to relate to the structure, union or enumerated type defined, not to any
2632 enclosing declaration the type specifier appears in, and the type
2633 defined is not complete until after the attribute specifiers.
2634 @c Otherwise, there would be the following problems: a shift/reduce
2635 @c conflict between attributes binding the struct/union/enum and
2636 @c binding to the list of specifiers/qualifiers; and "aligned"
2637 @c attributes could use sizeof for the structure, but the size could be
2638 @c changed later by "packed" attributes.
2640 Otherwise, an attribute specifier appears as part of a declaration,
2641 counting declarations of unnamed parameters and type names, and relates
2642 to that declaration (which may be nested in another declaration, for
2643 example in the case of a parameter declaration), or to a particular declarator
2644 within a declaration. Where an
2645 attribute specifier is applied to a parameter declared as a function or
2646 an array, it should apply to the function or array rather than the
2647 pointer to which the parameter is implicitly converted, but this is not
2648 yet correctly implemented.
2650 Any list of specifiers and qualifiers at the start of a declaration may
2651 contain attribute specifiers, whether or not such a list may in that
2652 context contain storage class specifiers. (Some attributes, however,
2653 are essentially in the nature of storage class specifiers, and only make
2654 sense where storage class specifiers may be used; for example,
2655 @code{section}.) There is one necessary limitation to this syntax: the
2656 first old-style parameter declaration in a function definition cannot
2657 begin with an attribute specifier, because such an attribute applies to
2658 the function instead by syntax described below (which, however, is not
2659 yet implemented in this case). In some other cases, attribute
2660 specifiers are permitted by this grammar but not yet supported by the
2661 compiler. All attribute specifiers in this place relate to the
2662 declaration as a whole. In the obsolescent usage where a type of
2663 @code{int} is implied by the absence of type specifiers, such a list of
2664 specifiers and qualifiers may be an attribute specifier list with no
2665 other specifiers or qualifiers.
2667 An attribute specifier list may appear immediately before a declarator
2668 (other than the first) in a comma-separated list of declarators in a
2669 declaration of more than one identifier using a single list of
2670 specifiers and qualifiers. Such attribute specifiers apply
2671 only to the identifier before whose declarator they appear. For
2675 __attribute__((noreturn)) void d0 (void),
2676 __attribute__((format(printf, 1, 2))) d1 (const char *, ...),
2681 the @code{noreturn} attribute applies to all the functions
2682 declared; the @code{format} attribute only applies to @code{d1}.
2684 An attribute specifier list may appear immediately before the comma,
2685 @code{=} or semicolon terminating the declaration of an identifier other
2686 than a function definition. At present, such attribute specifiers apply
2687 to the declared object or function, but in future they may attach to the
2688 outermost adjacent declarator. In simple cases there is no difference,
2689 but, for example, in
2692 void (****f)(void) __attribute__((noreturn));
2696 at present the @code{noreturn} attribute applies to @code{f}, which
2697 causes a warning since @code{f} is not a function, but in future it may
2698 apply to the function @code{****f}. The precise semantics of what
2699 attributes in such cases will apply to are not yet specified. Where an
2700 assembler name for an object or function is specified (@pxref{Asm
2701 Labels}), at present the attribute must follow the @code{asm}
2702 specification; in future, attributes before the @code{asm} specification
2703 may apply to the adjacent declarator, and those after it to the declared
2706 An attribute specifier list may, in future, be permitted to appear after
2707 the declarator in a function definition (before any old-style parameter
2708 declarations or the function body).
2710 Attribute specifiers may be mixed with type qualifiers appearing inside
2711 the @code{[]} of a parameter array declarator, in the C99 construct by
2712 which such qualifiers are applied to the pointer to which the array is
2713 implicitly converted. Such attribute specifiers apply to the pointer,
2714 not to the array, but at present this is not implemented and they are
2717 An attribute specifier list may appear at the start of a nested
2718 declarator. At present, there are some limitations in this usage: the
2719 attributes correctly apply to the declarator, but for most individual
2720 attributes the semantics this implies are not implemented.
2721 When attribute specifiers follow the @code{*} of a pointer
2722 declarator, they may be mixed with any type qualifiers present.
2723 The following describes the formal semantics of this syntax. It will make the
2724 most sense if you are familiar with the formal specification of
2725 declarators in the ISO C standard.
2727 Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration @code{T
2728 D1}, where @code{T} contains declaration specifiers that specify a type
2729 @var{Type} (such as @code{int}) and @code{D1} is a declarator that
2730 contains an identifier @var{ident}. The type specified for @var{ident}
2731 for derived declarators whose type does not include an attribute
2732 specifier is as in the ISO C standard.
2734 If @code{D1} has the form @code{( @var{attribute-specifier-list} D )},
2735 and the declaration @code{T D} specifies the type
2736 ``@var{derived-declarator-type-list} @var{Type}'' for @var{ident}, then
2737 @code{T D1} specifies the type ``@var{derived-declarator-type-list}
2738 @var{attribute-specifier-list} @var{Type}'' for @var{ident}.
2740 If @code{D1} has the form @code{*
2741 @var{type-qualifier-and-attribute-specifier-list} D}, and the
2742 declaration @code{T D} specifies the type
2743 ``@var{derived-declarator-type-list} @var{Type}'' for @var{ident}, then
2744 @code{T D1} specifies the type ``@var{derived-declarator-type-list}
2745 @var{type-qualifier-and-attribute-specifier-list} @var{Type}'' for
2751 void (__attribute__((noreturn)) ****f) (void);
2755 specifies the type ``pointer to pointer to pointer to pointer to
2756 non-returning function returning @code{void}''. As another example,
2759 char *__attribute__((aligned(8))) *f;
2763 specifies the type ``pointer to 8-byte-aligned pointer to @code{char}''.
2764 Note again that this does not work with most attributes; for example,
2765 the usage of @samp{aligned} and @samp{noreturn} attributes given above
2766 is not yet supported.
2768 For compatibility with existing code written for compiler versions that
2769 did not implement attributes on nested declarators, some laxity is
2770 allowed in the placing of attributes. If an attribute that only applies
2771 to types is applied to a declaration, it will be treated as applying to
2772 the type of that declaration. If an attribute that only applies to
2773 declarations is applied to the type of a declaration, it will be treated
2774 as applying to that declaration; and, for compatibility with code
2775 placing the attributes immediately before the identifier declared, such
2776 an attribute applied to a function return type will be treated as
2777 applying to the function type, and such an attribute applied to an array
2778 element type will be treated as applying to the array type. If an
2779 attribute that only applies to function types is applied to a
2780 pointer-to-function type, it will be treated as applying to the pointer
2781 target type; if such an attribute is applied to a function return type
2782 that is not a pointer-to-function type, it will be treated as applying
2783 to the function type.
2785 @node Function Prototypes
2786 @section Prototypes and Old-Style Function Definitions
2787 @cindex function prototype declarations
2788 @cindex old-style function definitions
2789 @cindex promotion of formal parameters
2791 GNU C extends ISO C to allow a function prototype to override a later
2792 old-style non-prototype definition. Consider the following example:
2795 /* @r{Use prototypes unless the compiler is old-fashioned.} */
2802 /* @r{Prototype function declaration.} */
2803 int isroot P((uid_t));
2805 /* @r{Old-style function definition.} */
2807 isroot (x) /* ??? lossage here ??? */
2814 Suppose the type @code{uid_t} happens to be @code{short}. ISO C does
2815 not allow this example, because subword arguments in old-style
2816 non-prototype definitions are promoted. Therefore in this example the
2817 function definition's argument is really an @code{int}, which does not
2818 match the prototype argument type of @code{short}.
2820 This restriction of ISO C makes it hard to write code that is portable
2821 to traditional C compilers, because the programmer does not know
2822 whether the @code{uid_t} type is @code{short}, @code{int}, or
2823 @code{long}. Therefore, in cases like these GNU C allows a prototype
2824 to override a later old-style definition. More precisely, in GNU C, a
2825 function prototype argument type overrides the argument type specified
2826 by a later old-style definition if the former type is the same as the
2827 latter type before promotion. Thus in GNU C the above example is
2828 equivalent to the following:
2841 GNU C++ does not support old-style function definitions, so this
2842 extension is irrelevant.
2845 @section C++ Style Comments
2847 @cindex C++ comments
2848 @cindex comments, C++ style
2850 In GNU C, you may use C++ style comments, which start with @samp{//} and
2851 continue until the end of the line. Many other C implementations allow
2852 such comments, and they are included in the 1999 C standard. However,
2853 C++ style comments are not recognized if you specify an @option{-std}
2854 option specifying a version of ISO C before C99, or @option{-ansi}
2855 (equivalent to @option{-std=c89}).
2858 @section Dollar Signs in Identifier Names
2860 @cindex dollar signs in identifier names
2861 @cindex identifier names, dollar signs in
2863 In GNU C, you may normally use dollar signs in identifier names.
2864 This is because many traditional C implementations allow such identifiers.
2865 However, dollar signs in identifiers are not supported on a few target
2866 machines, typically because the target assembler does not allow them.
2868 @node Character Escapes
2869 @section The Character @key{ESC} in Constants
2871 You can use the sequence @samp{\e} in a string or character constant to
2872 stand for the ASCII character @key{ESC}.
2875 @section Inquiring on Alignment of Types or Variables
2877 @cindex type alignment
2878 @cindex variable alignment
2880 The keyword @code{__alignof__} allows you to inquire about how an object
2881 is aligned, or the minimum alignment usually required by a type. Its
2882 syntax is just like @code{sizeof}.
2884 For example, if the target machine requires a @code{double} value to be
2885 aligned on an 8-byte boundary, then @code{__alignof__ (double)} is 8.
2886 This is true on many RISC machines. On more traditional machine
2887 designs, @code{__alignof__ (double)} is 4 or even 2.
2889 Some machines never actually require alignment; they allow reference to any
2890 data type even at an odd addresses. For these machines, @code{__alignof__}
2891 reports the @emph{recommended} alignment of a type.
2893 If the operand of @code{__alignof__} is an lvalue rather than a type,
2894 its value is the required alignment for its type, taking into account
2895 any minimum alignment specified with GCC's @code{__attribute__}
2896 extension (@pxref{Variable Attributes}). For example, after this
2900 struct foo @{ int x; char y; @} foo1;
2904 the value of @code{__alignof__ (foo1.y)} is 1, even though its actual
2905 alignment is probably 2 or 4, the same as @code{__alignof__ (int)}.
2907 It is an error to ask for the alignment of an incomplete type.
2909 @node Variable Attributes
2910 @section Specifying Attributes of Variables
2911 @cindex attribute of variables
2912 @cindex variable attributes
2914 The keyword @code{__attribute__} allows you to specify special
2915 attributes of variables or structure fields. This keyword is followed
2916 by an attribute specification inside double parentheses. Ten
2917 attributes are currently defined for variables: @code{aligned},
2918 @code{mode}, @code{nocommon}, @code{packed}, @code{section},
2919 @code{transparent_union}, @code{unused}, @code{deprecated},
2920 @code{vector_size}, and @code{weak}. Some other attributes are defined
2921 for variables on particular target systems. Other attributes are
2922 available for functions (@pxref{Function Attributes}) and for types
2923 (@pxref{Type Attributes}). Other front ends might define more
2924 attributes (@pxref{C++ Extensions,,Extensions to the C++ Language}).
2926 You may also specify attributes with @samp{__} preceding and following
2927 each keyword. This allows you to use them in header files without
2928 being concerned about a possible macro of the same name. For example,
2929 you may use @code{__aligned__} instead of @code{aligned}.
2931 @xref{Attribute Syntax}, for details of the exact syntax for using
2935 @cindex @code{aligned} attribute
2936 @item aligned (@var{alignment})
2937 This attribute specifies a minimum alignment for the variable or
2938 structure field, measured in bytes. For example, the declaration:
2941 int x __attribute__ ((aligned (16))) = 0;
2945 causes the compiler to allocate the global variable @code{x} on a
2946 16-byte boundary. On a 68040, this could be used in conjunction with
2947 an @code{asm} expression to access the @code{move16} instruction which
2948 requires 16-byte aligned operands.
2950 You can also specify the alignment of structure fields. For example, to
2951 create a double-word aligned @code{int} pair, you could write:
2954 struct foo @{ int x[2] __attribute__ ((aligned (8))); @};
2958 This is an alternative to creating a union with a @code{double} member
2959 that forces the union to be double-word aligned.
2961 As in the preceding examples, you can explicitly specify the alignment
2962 (in bytes) that you wish the compiler to use for a given variable or
2963 structure field. Alternatively, you can leave out the alignment factor
2964 and just ask the compiler to align a variable or field to the maximum
2965 useful alignment for the target machine you are compiling for. For
2966 example, you could write:
2969 short array[3] __attribute__ ((aligned));
2972 Whenever you leave out the alignment factor in an @code{aligned} attribute
2973 specification, the compiler automatically sets the alignment for the declared
2974 variable or field to the largest alignment which is ever used for any data
2975 type on the target machine you are compiling for. Doing this can often make
2976 copy operations more efficient, because the compiler can use whatever
2977 instructions copy the biggest chunks of memory when performing copies to
2978 or from the variables or fields that you have aligned this way.
2980 The @code{aligned} attribute can only increase the alignment; but you
2981 can decrease it by specifying @code{packed} as well. See below.
2983 Note that the effectiveness of @code{aligned} attributes may be limited
2984 by inherent limitations in your linker. On many systems, the linker is
2985 only able to arrange for variables to be aligned up to a certain maximum
2986 alignment. (For some linkers, the maximum supported alignment may
2987 be very very small.) If your linker is only able to align variables
2988 up to a maximum of 8 byte alignment, then specifying @code{aligned(16)}
2989 in an @code{__attribute__} will still only provide you with 8 byte
2990 alignment. See your linker documentation for further information.
2992 @item mode (@var{mode})
2993 @cindex @code{mode} attribute
2994 This attribute specifies the data type for the declaration---whichever
2995 type corresponds to the mode @var{mode}. This in effect lets you
2996 request an integer or floating point type according to its width.
2998 You may also specify a mode of @samp{byte} or @samp{__byte__} to
2999 indicate the mode corresponding to a one-byte integer, @samp{word} or
3000 @samp{__word__} for the mode of a one-word integer, and @samp{pointer}
3001 or @samp{__pointer__} for the mode used to represent pointers.
3004 @cindex @code{nocommon} attribute
3006 This attribute specifies requests GCC not to place a variable
3007 ``common'' but instead to allocate space for it directly. If you
3008 specify the @option{-fno-common} flag, GCC will do this for all
3011 Specifying the @code{nocommon} attribute for a variable provides an
3012 initialization of zeros. A variable may only be initialized in one
3016 @cindex @code{packed} attribute
3017 The @code{packed} attribute specifies that a variable or structure field
3018 should have the smallest possible alignment---one byte for a variable,
3019 and one bit for a field, unless you specify a larger value with the
3020 @code{aligned} attribute.
3022 Here is a structure in which the field @code{x} is packed, so that it
3023 immediately follows @code{a}:
3029 int x[2] __attribute__ ((packed));
3033 @item section ("@var{section-name}")
3034 @cindex @code{section} variable attribute
3035 Normally, the compiler places the objects it generates in sections like
3036 @code{data} and @code{bss}. Sometimes, however, you need additional sections,
3037 or you need certain particular variables to appear in special sections,
3038 for example to map to special hardware. The @code{section}
3039 attribute specifies that a variable (or function) lives in a particular
3040 section. For example, this small program uses several specific section names:
3043 struct duart a __attribute__ ((section ("DUART_A"))) = @{ 0 @};
3044 struct duart b __attribute__ ((section ("DUART_B"))) = @{ 0 @};
3045 char stack[10000] __attribute__ ((section ("STACK"))) = @{ 0 @};
3046 int init_data __attribute__ ((section ("INITDATA"))) = 0;
3050 /* Initialize stack pointer */
3051 init_sp (stack + sizeof (stack));
3053 /* Initialize initialized data */
3054 memcpy (&init_data, &data, &edata - &data);
3056 /* Turn on the serial ports */
3063 Use the @code{section} attribute with an @emph{initialized} definition
3064 of a @emph{global} variable, as shown in the example. GCC issues
3065 a warning and otherwise ignores the @code{section} attribute in
3066 uninitialized variable declarations.
3068 You may only use the @code{section} attribute with a fully initialized
3069 global definition because of the way linkers work. The linker requires
3070 each object be defined once, with the exception that uninitialized
3071 variables tentatively go in the @code{common} (or @code{bss}) section
3072 and can be multiply ``defined''. You can force a variable to be
3073 initialized with the @option{-fno-common} flag or the @code{nocommon}
3076 Some file formats do not support arbitrary sections so the @code{section}
3077 attribute is not available on all platforms.
3078 If you need to map the entire contents of a module to a particular
3079 section, consider using the facilities of the linker instead.
3082 @cindex @code{shared} variable attribute
3083 On Windows NT, in addition to putting variable definitions in a named
3084 section, the section can also be shared among all running copies of an
3085 executable or DLL@. For example, this small program defines shared data
3086 by putting it in a named section @code{shared} and marking the section
3090 int foo __attribute__((section ("shared"), shared)) = 0;
3095 /* Read and write foo. All running
3096 copies see the same value. */
3102 You may only use the @code{shared} attribute along with @code{section}
3103 attribute with a fully initialized global definition because of the way
3104 linkers work. See @code{section} attribute for more information.
3106 The @code{shared} attribute is only available on Windows NT@.
3108 @item transparent_union
3109 This attribute, attached to a function parameter which is a union, means
3110 that the corresponding argument may have the type of any union member,
3111 but the argument is passed as if its type were that of the first union
3112 member. For more details see @xref{Type Attributes}. You can also use
3113 this attribute on a @code{typedef} for a union data type; then it
3114 applies to all function parameters with that type.
3117 This attribute, attached to a variable, means that the variable is meant
3118 to be possibly unused. GCC will not produce a warning for this
3122 The @code{deprecated} attribute results in a warning if the variable
3123 is used anywhere in the source file. This is useful when identifying
3124 variables that are expected to be removed in a future version of a
3125 program. The warning also includes the location of the declaration
3126 of the deprecated variable, to enable users to easily find further
3127 information about why the variable is deprecated, or what they should
3128 do instead. Note that the warnings only occurs for uses:
3131 extern int old_var __attribute__ ((deprecated));
3133 int new_fn () @{ return old_var; @}
3136 results in a warning on line 3 but not line 2.
3138 The @code{deprecated} attribute can also be used for functions and
3139 types (@pxref{Function Attributes}, @pxref{Type Attributes}.)
3141 @item vector_size (@var{bytes})
3142 This attribute specifies the vector size for the variable, measured in
3143 bytes. For example, the declaration:
3146 int foo __attribute__ ((vector_size (16)));
3150 causes the compiler to set the mode for @code{foo}, to be 16 bytes,
3151 divided into @code{int} sized units. Assuming a 32-bit int (a vector of
3152 4 units of 4 bytes), the corresponding mode of @code{foo} will be V4SI@.
3154 This attribute is only applicable to integral and float scalars,
3155 although arrays, pointers, and function return values are allowed in
3156 conjunction with this construct.
3158 Aggregates with this attribute are invalid, even if they are of the same
3159 size as a corresponding scalar. For example, the declaration:
3162 struct S @{ int a; @};
3163 struct S __attribute__ ((vector_size (16))) foo;
3167 is invalid even if the size of the structure is the same as the size of
3171 The @code{weak} attribute is described in @xref{Function Attributes}.
3173 @item model (@var{model-name})
3174 @cindex variable addressability on the M32R/D
3175 Use this attribute on the M32R/D to set the addressability of an object.
3176 The identifier @var{model-name} is one of @code{small}, @code{medium},
3177 or @code{large}, representing each of the code models.
3179 Small model objects live in the lower 16MB of memory (so that their
3180 addresses can be loaded with the @code{ld24} instruction).
3182 Medium and large model objects may live anywhere in the 32-bit address space
3183 (the compiler will generate @code{seth/add3} instructions to load their
3188 To specify multiple attributes, separate them by commas within the
3189 double parentheses: for example, @samp{__attribute__ ((aligned (16),
3192 @node Type Attributes
3193 @section Specifying Attributes of Types
3194 @cindex attribute of types
3195 @cindex type attributes
3197 The keyword @code{__attribute__} allows you to specify special
3198 attributes of @code{struct} and @code{union} types when you define such
3199 types. This keyword is followed by an attribute specification inside
3200 double parentheses. Six attributes are currently defined for types:
3201 @code{aligned}, @code{packed}, @code{transparent_union}, @code{unused},
3202 @code{deprecated} and @code{may_alias}. Other attributes are defined for
3203 functions (@pxref{Function Attributes}) and for variables
3204 (@pxref{Variable Attributes}).
3206 You may also specify any one of these attributes with @samp{__}
3207 preceding and following its keyword. This allows you to use these
3208 attributes in header files without being concerned about a possible
3209 macro of the same name. For example, you may use @code{__aligned__}
3210 instead of @code{aligned}.
3212 You may specify the @code{aligned} and @code{transparent_union}
3213 attributes either in a @code{typedef} declaration or just past the
3214 closing curly brace of a complete enum, struct or union type
3215 @emph{definition} and the @code{packed} attribute only past the closing
3216 brace of a definition.
3218 You may also specify attributes between the enum, struct or union
3219 tag and the name of the type rather than after the closing brace.
3221 @xref{Attribute Syntax}, for details of the exact syntax for using
3225 @cindex @code{aligned} attribute
3226 @item aligned (@var{alignment})
3227 This attribute specifies a minimum alignment (in bytes) for variables
3228 of the specified type. For example, the declarations:
3231 struct S @{ short f[3]; @} __attribute__ ((aligned (8)));
3232 typedef int more_aligned_int __attribute__ ((aligned (8)));
3236 force the compiler to insure (as far as it can) that each variable whose
3237 type is @code{struct S} or @code{more_aligned_int} will be allocated and
3238 aligned @emph{at least} on a 8-byte boundary. On a SPARC, having all
3239 variables of type @code{struct S} aligned to 8-byte boundaries allows
3240 the compiler to use the @code{ldd} and @code{std} (doubleword load and
3241 store) instructions when copying one variable of type @code{struct S} to
3242 another, thus improving run-time efficiency.
3244 Note that the alignment of any given @code{struct} or @code{union} type
3245 is required by the ISO C standard to be at least a perfect multiple of
3246 the lowest common multiple of the alignments of all of the members of
3247 the @code{struct} or @code{union} in question. This means that you @emph{can}
3248 effectively adjust the alignment of a @code{struct} or @code{union}
3249 type by attaching an @code{aligned} attribute to any one of the members
3250 of such a type, but the notation illustrated in the example above is a
3251 more obvious, intuitive, and readable way to request the compiler to
3252 adjust the alignment of an entire @code{struct} or @code{union} type.
3254 As in the preceding example, you can explicitly specify the alignment
3255 (in bytes) that you wish the compiler to use for a given @code{struct}
3256 or @code{union} type. Alternatively, you can leave out the alignment factor
3257 and just ask the compiler to align a type to the maximum
3258 useful alignment for the target machine you are compiling for. For
3259 example, you could write:
3262 struct S @{ short f[3]; @} __attribute__ ((aligned));
3265 Whenever you leave out the alignment factor in an @code{aligned}
3266 attribute specification, the compiler automatically sets the alignment
3267 for the type to the largest alignment which is ever used for any data
3268 type on the target machine you are compiling for. Doing this can often
3269 make copy operations more efficient, because the compiler can use
3270 whatever instructions copy the biggest chunks of memory when performing
3271 copies to or from the variables which have types that you have aligned
3274 In the example above, if the size of each @code{short} is 2 bytes, then
3275 the size of the entire @code{struct S} type is 6 bytes. The smallest
3276 power of two which is greater than or equal to that is 8, so the
3277 compiler sets the alignment for the entire @code{struct S} type to 8
3280 Note that although you can ask the compiler to select a time-efficient
3281 alignment for a given type and then declare only individual stand-alone
3282 objects of that type, the compiler's ability to select a time-efficient
3283 alignment is primarily useful only when you plan to create arrays of
3284 variables having the relevant (efficiently aligned) type. If you
3285 declare or use arrays of variables of an efficiently-aligned type, then
3286 it is likely that your program will also be doing pointer arithmetic (or
3287 subscripting, which amounts to the same thing) on pointers to the
3288 relevant type, and the code that the compiler generates for these
3289 pointer arithmetic operations will often be more efficient for
3290 efficiently-aligned types than for other types.
3292 The @code{aligned} attribute can only increase the alignment; but you
3293 can decrease it by specifying @code{packed} as well. See below.
3295 Note that the effectiveness of @code{aligned} attributes may be limited
3296 by inherent limitations in your linker. On many systems, the linker is
3297 only able to arrange for variables to be aligned up to a certain maximum
3298 alignment. (For some linkers, the maximum supported alignment may
3299 be very very small.) If your linker is only able to align variables
3300 up to a maximum of 8 byte alignment, then specifying @code{aligned(16)}
3301 in an @code{__attribute__} will still only provide you with 8 byte
3302 alignment. See your linker documentation for further information.
3305 This attribute, attached to an @code{enum}, @code{struct}, or
3306 @code{union} type definition, specified that the minimum required memory
3307 be used to represent the type.
3309 @opindex fshort-enums
3310 Specifying this attribute for @code{struct} and @code{union} types is
3311 equivalent to specifying the @code{packed} attribute on each of the
3312 structure or union members. Specifying the @option{-fshort-enums}
3313 flag on the line is equivalent to specifying the @code{packed}
3314 attribute on all @code{enum} definitions.
3316 You may only specify this attribute after a closing curly brace on an
3317 @code{enum} definition, not in a @code{typedef} declaration, unless that
3318 declaration also contains the definition of the @code{enum}.
3320 @item transparent_union
3321 This attribute, attached to a @code{union} type definition, indicates
3322 that any function parameter having that union type causes calls to that
3323 function to be treated in a special way.
3325 First, the argument corresponding to a transparent union type can be of
3326 any type in the union; no cast is required. Also, if the union contains
3327 a pointer type, the corresponding argument can be a null pointer
3328 constant or a void pointer expression; and if the union contains a void
3329 pointer type, the corresponding argument can be any pointer expression.
3330 If the union member type is a pointer, qualifiers like @code{const} on
3331 the referenced type must be respected, just as with normal pointer
3334 Second, the argument is passed to the function using the calling
3335 conventions of first member of the transparent union, not the calling
3336 conventions of the union itself. All members of the union must have the
3337 same machine representation; this is necessary for this argument passing
3340 Transparent unions are designed for library functions that have multiple
3341 interfaces for compatibility reasons. For example, suppose the
3342 @code{wait} function must accept either a value of type @code{int *} to
3343 comply with Posix, or a value of type @code{union wait *} to comply with
3344 the 4.1BSD interface. If @code{wait}'s parameter were @code{void *},
3345 @code{wait} would accept both kinds of arguments, but it would also
3346 accept any other pointer type and this would make argument type checking
3347 less useful. Instead, @code{<sys/wait.h>} might define the interface
3355 @} wait_status_ptr_t __attribute__ ((__transparent_union__));
3357 pid_t wait (wait_status_ptr_t);
3360 This interface allows either @code{int *} or @code{union wait *}
3361 arguments to be passed, using the @code{int *} calling convention.
3362 The program can call @code{wait} with arguments of either type:
3365 int w1 () @{ int w; return wait (&w); @}
3366 int w2 () @{ union wait w; return wait (&w); @}
3369 With this interface, @code{wait}'s implementation might look like this:
3372 pid_t wait (wait_status_ptr_t p)
3374 return waitpid (-1, p.__ip, 0);
3379 When attached to a type (including a @code{union} or a @code{struct}),
3380 this attribute means that variables of that type are meant to appear
3381 possibly unused. GCC will not produce a warning for any variables of
3382 that type, even if the variable appears to do nothing. This is often
3383 the case with lock or thread classes, which are usually defined and then
3384 not referenced, but contain constructors and destructors that have
3385 nontrivial bookkeeping functions.
3388 The @code{deprecated} attribute results in a warning if the type
3389 is used anywhere in the source file. This is useful when identifying
3390 types that are expected to be removed in a future version of a program.
3391 If possible, the warning also includes the location of the declaration
3392 of the deprecated type, to enable users to easily find further
3393 information about why the type is deprecated, or what they should do
3394 instead. Note that the warnings only occur for uses and then only
3395 if the type is being applied to an identifier that itself is not being
3396 declared as deprecated.
3399 typedef int T1 __attribute__ ((deprecated));
3403 typedef T1 T3 __attribute__ ((deprecated));
3404 T3 z __attribute__ ((deprecated));
3407 results in a warning on line 2 and 3 but not lines 4, 5, or 6. No
3408 warning is issued for line 4 because T2 is not explicitly
3409 deprecated. Line 5 has no warning because T3 is explicitly
3410 deprecated. Similarly for line 6.
3412 The @code{deprecated} attribute can also be used for functions and
3413 variables (@pxref{Function Attributes}, @pxref{Variable Attributes}.)
3416 Accesses to objects with types with this attribute are not subjected to
3417 type-based alias analysis, but are instead assumed to be able to alias
3418 any other type of objects, just like the @code{char} type. See
3419 @option{-fstrict-aliasing} for more information on aliasing issues.
3424 typedef short __attribute__((__may_alias__)) short_a;
3430 short_a *b = (short_a *) &a;
3434 if (a == 0x12345678)
3441 If you replaced @code{short_a} with @code{short} in the variable
3442 declaration, the above program would abort when compiled with
3443 @option{-fstrict-aliasing}, which is on by default at @option{-O2} or
3444 above in recent GCC versions.
3447 To specify multiple attributes, separate them by commas within the
3448 double parentheses: for example, @samp{__attribute__ ((aligned (16),
3452 @section An Inline Function is As Fast As a Macro
3453 @cindex inline functions
3454 @cindex integrating function code
3456 @cindex macros, inline alternative
3458 By declaring a function @code{inline}, you can direct GCC to
3459 integrate that function's code into the code for its callers. This
3460 makes execution faster by eliminating the function-call overhead; in
3461 addition, if any of the actual argument values are constant, their known
3462 values may permit simplifications at compile time so that not all of the
3463 inline function's code needs to be included. The effect on code size is
3464 less predictable; object code may be larger or smaller with function
3465 inlining, depending on the particular case. Inlining of functions is an
3466 optimization and it really ``works'' only in optimizing compilation. If
3467 you don't use @option{-O}, no function is really inline.
3469 Inline functions are included in the ISO C99 standard, but there are
3470 currently substantial differences between what GCC implements and what
3471 the ISO C99 standard requires.
3473 To declare a function inline, use the @code{inline} keyword in its
3474 declaration, like this:
3484 (If you are writing a header file to be included in ISO C programs, write
3485 @code{__inline__} instead of @code{inline}. @xref{Alternate Keywords}.)
3486 You can also make all ``simple enough'' functions inline with the option
3487 @option{-finline-functions}.
3490 Note that certain usages in a function definition can make it unsuitable
3491 for inline substitution. Among these usages are: use of varargs, use of
3492 alloca, use of variable sized data types (@pxref{Variable Length}),
3493 use of computed goto (@pxref{Labels as Values}), use of nonlocal goto,
3494 and nested functions (@pxref{Nested Functions}). Using @option{-Winline}
3495 will warn when a function marked @code{inline} could not be substituted,
3496 and will give the reason for the failure.
3498 Note that in C and Objective-C, unlike C++, the @code{inline} keyword
3499 does not affect the linkage of the function.
3501 @cindex automatic @code{inline} for C++ member fns
3502 @cindex @code{inline} automatic for C++ member fns
3503 @cindex member fns, automatically @code{inline}
3504 @cindex C++ member fns, automatically @code{inline}
3505 @opindex fno-default-inline
3506 GCC automatically inlines member functions defined within the class
3507 body of C++ programs even if they are not explicitly declared
3508 @code{inline}. (You can override this with @option{-fno-default-inline};
3509 @pxref{C++ Dialect Options,,Options Controlling C++ Dialect}.)
3511 @cindex inline functions, omission of
3512 @opindex fkeep-inline-functions
3513 When a function is both inline and @code{static}, if all calls to the
3514 function are integrated into the caller, and the function's address is
3515 never used, then the function's own assembler code is never referenced.
3516 In this case, GCC does not actually output assembler code for the
3517 function, unless you specify the option @option{-fkeep-inline-functions}.
3518 Some calls cannot be integrated for various reasons (in particular,
3519 calls that precede the function's definition cannot be integrated, and
3520 neither can recursive calls within the definition). If there is a
3521 nonintegrated call, then the function is compiled to assembler code as
3522 usual. The function must also be compiled as usual if the program
3523 refers to its address, because that can't be inlined.
3525 @cindex non-static inline function
3526 When an inline function is not @code{static}, then the compiler must assume
3527 that there may be calls from other source files; since a global symbol can
3528 be defined only once in any program, the function must not be defined in
3529 the other source files, so the calls therein cannot be integrated.
3530 Therefore, a non-@code{static} inline function is always compiled on its
3531 own in the usual fashion.
3533 If you specify both @code{inline} and @code{extern} in the function
3534 definition, then the definition is used only for inlining. In no case
3535 is the function compiled on its own, not even if you refer to its
3536 address explicitly. Such an address becomes an external reference, as
3537 if you had only declared the function, and had not defined it.
3539 This combination of @code{inline} and @code{extern} has almost the
3540 effect of a macro. The way to use it is to put a function definition in
3541 a header file with these keywords, and put another copy of the
3542 definition (lacking @code{inline} and @code{extern}) in a library file.
3543 The definition in the header file will cause most calls to the function
3544 to be inlined. If any uses of the function remain, they will refer to
3545 the single copy in the library.
3547 For future compatibility with when GCC implements ISO C99 semantics for
3548 inline functions, it is best to use @code{static inline} only. (The
3549 existing semantics will remain available when @option{-std=gnu89} is
3550 specified, but eventually the default will be @option{-std=gnu99} and
3551 that will implement the C99 semantics, though it does not do so yet.)
3553 GCC does not inline any functions when not optimizing unless you specify
3554 the @samp{always_inline} attribute for the function, like this:
3558 inline void foo (const char) __attribute__((always_inline));
3562 @section Assembler Instructions with C Expression Operands
3563 @cindex extended @code{asm}
3564 @cindex @code{asm} expressions
3565 @cindex assembler instructions
3568 In an assembler instruction using @code{asm}, you can specify the
3569 operands of the instruction using C expressions. This means you need not
3570 guess which registers or memory locations will contain the data you want
3573 You must specify an assembler instruction template much like what
3574 appears in a machine description, plus an operand constraint string for
3577 For example, here is how to use the 68881's @code{fsinx} instruction:
3580 asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
3584 Here @code{angle} is the C expression for the input operand while
3585 @code{result} is that of the output operand. Each has @samp{"f"} as its
3586 operand constraint, saying that a floating point register is required.
3587 The @samp{=} in @samp{=f} indicates that the operand is an output; all
3588 output operands' constraints must use @samp{=}. The constraints use the
3589 same language used in the machine description (@pxref{Constraints}).
3591 Each operand is described by an operand-constraint string followed by
3592 the C expression in parentheses. A colon separates the assembler
3593 template from the first output operand and another separates the last
3594 output operand from the first input, if any. Commas separate the
3595 operands within each group. The total number of operands is currently
3596 limited to 30; this limitation may be lifted in some future version of
3599 If there are no output operands but there are input operands, you must
3600 place two consecutive colons surrounding the place where the output
3603 As of GCC version 3.1, it is also possible to specify input and output
3604 operands using symbolic names which can be referenced within the
3605 assembler code. These names are specified inside square brackets
3606 preceding the constraint string, and can be referenced inside the
3607 assembler code using @code{%[@var{name}]} instead of a percentage sign
3608 followed by the operand number. Using named operands the above example
3612 asm ("fsinx %[angle],%[output]"
3613 : [output] "=f" (result)
3614 : [angle] "f" (angle));
3618 Note that the symbolic operand names have no relation whatsoever to
3619 other C identifiers. You may use any name you like, even those of
3620 existing C symbols, but must ensure that no two operands within the same
3621 assembler construct use the same symbolic name.
3623 Output operand expressions must be lvalues; the compiler can check this.
3624 The input operands need not be lvalues. The compiler cannot check
3625 whether the operands have data types that are reasonable for the
3626 instruction being executed. It does not parse the assembler instruction
3627 template and does not know what it means or even whether it is valid
3628 assembler input. The extended @code{asm} feature is most often used for
3629 machine instructions the compiler itself does not know exist. If
3630 the output expression cannot be directly addressed (for example, it is a
3631 bit-field), your constraint must allow a register. In that case, GCC
3632 will use the register as the output of the @code{asm}, and then store
3633 that register into the output.
3635 The ordinary output operands must be write-only; GCC will assume that
3636 the values in these operands before the instruction are dead and need
3637 not be generated. Extended asm supports input-output or read-write
3638 operands. Use the constraint character @samp{+} to indicate such an
3639 operand and list it with the output operands.
3641 When the constraints for the read-write operand (or the operand in which
3642 only some of the bits are to be changed) allows a register, you may, as
3643 an alternative, logically split its function into two separate operands,
3644 one input operand and one write-only output operand. The connection
3645 between them is expressed by constraints which say they need to be in
3646 the same location when the instruction executes. You can use the same C
3647 expression for both operands, or different expressions. For example,
3648 here we write the (fictitious) @samp{combine} instruction with
3649 @code{bar} as its read-only source operand and @code{foo} as its
3650 read-write destination:
3653 asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));
3657 The constraint @samp{"0"} for operand 1 says that it must occupy the
3658 same location as operand 0. A number in constraint is allowed only in
3659 an input operand and it must refer to an output operand.
3661 Only a number in the constraint can guarantee that one operand will be in
3662 the same place as another. The mere fact that @code{foo} is the value
3663 of both operands is not enough to guarantee that they will be in the
3664 same place in the generated assembler code. The following would not
3668 asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));
3671 Various optimizations or reloading could cause operands 0 and 1 to be in
3672 different registers; GCC knows no reason not to do so. For example, the
3673 compiler might find a copy of the value of @code{foo} in one register and
3674 use it for operand 1, but generate the output operand 0 in a different
3675 register (copying it afterward to @code{foo}'s own address). Of course,
3676 since the register for operand 1 is not even mentioned in the assembler
3677 code, the result will not work, but GCC can't tell that.
3679 As of GCC version 3.1, one may write @code{[@var{name}]} instead of
3680 the operand number for a matching constraint. For example:
3683 asm ("cmoveq %1,%2,%[result]"
3684 : [result] "=r"(result)
3685 : "r" (test), "r"(new), "[result]"(old));
3688 Some instructions clobber specific hard registers. To describe this,
3689 write a third colon after the input operands, followed by the names of
3690 the clobbered hard registers (given as strings). Here is a realistic
3691 example for the VAX:
3694 asm volatile ("movc3 %0,%1,%2"
3696 : "g" (from), "g" (to), "g" (count)
3697 : "r0", "r1", "r2", "r3", "r4", "r5");
3700 You may not write a clobber description in a way that overlaps with an
3701 input or output operand. For example, you may not have an operand
3702 describing a register class with one member if you mention that register
3703 in the clobber list. Variables declared to live in specific registers
3704 (@pxref{Explicit Reg Vars}), and used as asm input or output operands must
3705 have no part mentioned in the clobber description.
3706 There is no way for you to specify that an input
3707 operand is modified without also specifying it as an output
3708 operand. Note that if all the output operands you specify are for this
3709 purpose (and hence unused), you will then also need to specify
3710 @code{volatile} for the @code{asm} construct, as described below, to
3711 prevent GCC from deleting the @code{asm} statement as unused.
3713 If you refer to a particular hardware register from the assembler code,
3714 you will probably have to list the register after the third colon to
3715 tell the compiler the register's value is modified. In some assemblers,
3716 the register names begin with @samp{%}; to produce one @samp{%} in the
3717 assembler code, you must write @samp{%%} in the input.
3719 If your assembler instruction can alter the condition code register, add
3720 @samp{cc} to the list of clobbered registers. GCC on some machines
3721 represents the condition codes as a specific hardware register;
3722 @samp{cc} serves to name this register. On other machines, the
3723 condition code is handled differently, and specifying @samp{cc} has no
3724 effect. But it is valid no matter what the machine.
3726 If your assembler instruction modifies memory in an unpredictable
3727 fashion, add @samp{memory} to the list of clobbered registers. This
3728 will cause GCC to not keep memory values cached in registers across
3729 the assembler instruction. You will also want to add the
3730 @code{volatile} keyword if the memory affected is not listed in the
3731 inputs or outputs of the @code{asm}, as the @samp{memory} clobber does
3732 not count as a side-effect of the @code{asm}.
3734 You can put multiple assembler instructions together in a single
3735 @code{asm} template, separated by the characters normally used in assembly
3736 code for the system. A combination that works in most places is a newline
3737 to break the line, plus a tab character to move to the instruction field
3738 (written as @samp{\n\t}). Sometimes semicolons can be used, if the
3739 assembler allows semicolons as a line-breaking character. Note that some
3740 assembler dialects use semicolons to start a comment.
3741 The input operands are guaranteed not to use any of the clobbered
3742 registers, and neither will the output operands' addresses, so you can
3743 read and write the clobbered registers as many times as you like. Here
3744 is an example of multiple instructions in a template; it assumes the
3745 subroutine @code{_foo} accepts arguments in registers 9 and 10:
3748 asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo"
3750 : "g" (from), "g" (to)
3754 Unless an output operand has the @samp{&} constraint modifier, GCC
3755 may allocate it in the same register as an unrelated input operand, on
3756 the assumption the inputs are consumed before the outputs are produced.
3757 This assumption may be false if the assembler code actually consists of
3758 more than one instruction. In such a case, use @samp{&} for each output
3759 operand that may not overlap an input. @xref{Modifiers}.
3761 If you want to test the condition code produced by an assembler
3762 instruction, you must include a branch and a label in the @code{asm}
3763 construct, as follows:
3766 asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:"
3772 This assumes your assembler supports local labels, as the GNU assembler
3773 and most Unix assemblers do.
3775 Speaking of labels, jumps from one @code{asm} to another are not
3776 supported. The compiler's optimizers do not know about these jumps, and
3777 therefore they cannot take account of them when deciding how to
3780 @cindex macros containing @code{asm}
3781 Usually the most convenient way to use these @code{asm} instructions is to
3782 encapsulate them in macros that look like functions. For example,
3786 (@{ double __value, __arg = (x); \
3787 asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \
3792 Here the variable @code{__arg} is used to make sure that the instruction
3793 operates on a proper @code{double} value, and to accept only those
3794 arguments @code{x} which can convert automatically to a @code{double}.
3796 Another way to make sure the instruction operates on the correct data
3797 type is to use a cast in the @code{asm}. This is different from using a
3798 variable @code{__arg} in that it converts more different types. For
3799 example, if the desired type were @code{int}, casting the argument to
3800 @code{int} would accept a pointer with no complaint, while assigning the
3801 argument to an @code{int} variable named @code{__arg} would warn about
3802 using a pointer unless the caller explicitly casts it.
3804 If an @code{asm} has output operands, GCC assumes for optimization
3805 purposes the instruction has no side effects except to change the output
3806 operands. This does not mean instructions with a side effect cannot be
3807 used, but you must be careful, because the compiler may eliminate them
3808 if the output operands aren't used, or move them out of loops, or
3809 replace two with one if they constitute a common subexpression. Also,
3810 if your instruction does have a side effect on a variable that otherwise
3811 appears not to change, the old value of the variable may be reused later
3812 if it happens to be found in a register.
3814 You can prevent an @code{asm} instruction from being deleted, moved
3815 significantly, or combined, by writing the keyword @code{volatile} after
3816 the @code{asm}. For example:
3819 #define get_and_set_priority(new) \
3821 asm volatile ("get_and_set_priority %0, %1" \
3822 : "=g" (__old) : "g" (new)); \
3827 If you write an @code{asm} instruction with no outputs, GCC will know
3828 the instruction has side-effects and will not delete the instruction or
3829 move it outside of loops.
3831 The @code{volatile} keyword indicates that the instruction has
3832 important side-effects. GCC will not delete a volatile @code{asm} if
3833 it is reachable. (The instruction can still be deleted if GCC can
3834 prove that control-flow will never reach the location of the
3835 instruction.) In addition, GCC will not reschedule instructions
3836 across a volatile @code{asm} instruction. For example:
3839 *(volatile int *)addr = foo;
3840 asm volatile ("eieio" : : );
3844 Assume @code{addr} contains the address of a memory mapped device
3845 register. The PowerPC @code{eieio} instruction (Enforce In-order
3846 Execution of I/O) tells the CPU to make sure that the store to that
3847 device register happens before it issues any other I/O@.
3849 Note that even a volatile @code{asm} instruction can be moved in ways
3850 that appear insignificant to the compiler, such as across jump
3851 instructions. You can't expect a sequence of volatile @code{asm}
3852 instructions to remain perfectly consecutive. If you want consecutive
3853 output, use a single @code{asm}. Also, GCC will perform some
3854 optimizations across a volatile @code{asm} instruction; GCC does not
3855 ``forget everything'' when it encounters a volatile @code{asm}
3856 instruction the way some other compilers do.
3858 An @code{asm} instruction without any operands or clobbers (an ``old
3859 style'' @code{asm}) will be treated identically to a volatile
3860 @code{asm} instruction.
3862 It is a natural idea to look for a way to give access to the condition
3863 code left by the assembler instruction. However, when we attempted to
3864 implement this, we found no way to make it work reliably. The problem
3865 is that output operands might need reloading, which would result in
3866 additional following ``store'' instructions. On most machines, these
3867 instructions would alter the condition code before there was time to
3868 test it. This problem doesn't arise for ordinary ``test'' and
3869 ``compare'' instructions because they don't have any output operands.
3871 For reasons similar to those described above, it is not possible to give
3872 an assembler instruction access to the condition code left by previous
3875 If you are writing a header file that should be includable in ISO C
3876 programs, write @code{__asm__} instead of @code{asm}. @xref{Alternate
3879 @subsection i386 floating point asm operands
3881 There are several rules on the usage of stack-like regs in
3882 asm_operands insns. These rules apply only to the operands that are
3887 Given a set of input regs that die in an asm_operands, it is
3888 necessary to know which are implicitly popped by the asm, and
3889 which must be explicitly popped by gcc.
3891 An input reg that is implicitly popped by the asm must be
3892 explicitly clobbered, unless it is constrained to match an
3896 For any input reg that is implicitly popped by an asm, it is
3897 necessary to know how to adjust the stack to compensate for the pop.
3898 If any non-popped input is closer to the top of the reg-stack than
3899 the implicitly popped reg, it would not be possible to know what the
3900 stack looked like---it's not clear how the rest of the stack ``slides
3903 All implicitly popped input regs must be closer to the top of
3904 the reg-stack than any input that is not implicitly popped.
3906 It is possible that if an input dies in an insn, reload might
3907 use the input reg for an output reload. Consider this example:
3910 asm ("foo" : "=t" (a) : "f" (b));
3913 This asm says that input B is not popped by the asm, and that
3914 the asm pushes a result onto the reg-stack, i.e., the stack is one
3915 deeper after the asm than it was before. But, it is possible that
3916 reload will think that it can use the same reg for both the input and
3917 the output, if input B dies in this insn.
3919 If any input operand uses the @code{f} constraint, all output reg
3920 constraints must use the @code{&} earlyclobber.
3922 The asm above would be written as
3925 asm ("foo" : "=&t" (a) : "f" (b));
3929 Some operands need to be in particular places on the stack. All
3930 output operands fall in this category---there is no other way to
3931 know which regs the outputs appear in unless the user indicates
3932 this in the constraints.
3934 Output operands must specifically indicate which reg an output
3935 appears in after an asm. @code{=f} is not allowed: the operand
3936 constraints must select a class with a single reg.
3939 Output operands may not be ``inserted'' between existing stack regs.
3940 Since no 387 opcode uses a read/write operand, all output operands
3941 are dead before the asm_operands, and are pushed by the asm_operands.
3942 It makes no sense to push anywhere but the top of the reg-stack.
3944 Output operands must start at the top of the reg-stack: output
3945 operands may not ``skip'' a reg.
3948 Some asm statements may need extra stack space for internal
3949 calculations. This can be guaranteed by clobbering stack registers
3950 unrelated to the inputs and outputs.
3954 Here are a couple of reasonable asms to want to write. This asm
3955 takes one input, which is internally popped, and produces two outputs.
3958 asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));
3961 This asm takes two inputs, which are popped by the @code{fyl2xp1} opcode,
3962 and replaces them with one output. The user must code the @code{st(1)}
3963 clobber for reg-stack.c to know that @code{fyl2xp1} pops both inputs.
3966 asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");
3972 @section Controlling Names Used in Assembler Code
3973 @cindex assembler names for identifiers
3974 @cindex names used in assembler code
3975 @cindex identifiers, names in assembler code
3977 You can specify the name to be used in the assembler code for a C
3978 function or variable by writing the @code{asm} (or @code{__asm__})
3979 keyword after the declarator as follows:
3982 int foo asm ("myfoo") = 2;
3986 This specifies that the name to be used for the variable @code{foo} in
3987 the assembler code should be @samp{myfoo} rather than the usual
3990 On systems where an underscore is normally prepended to the name of a C
3991 function or variable, this feature allows you to define names for the
3992 linker that do not start with an underscore.
3994 It does not make sense to use this feature with a non-static local
3995 variable since such variables do not have assembler names. If you are
3996 trying to put the variable in a particular register, see @ref{Explicit
3997 Reg Vars}. GCC presently accepts such code with a warning, but will
3998 probably be changed to issue an error, rather than a warning, in the
4001 You cannot use @code{asm} in this way in a function @emph{definition}; but
4002 you can get the same effect by writing a declaration for the function
4003 before its definition and putting @code{asm} there, like this:
4006 extern func () asm ("FUNC");
4013 It is up to you to make sure that the assembler names you choose do not
4014 conflict with any other assembler symbols. Also, you must not use a
4015 register name; that would produce completely invalid assembler code. GCC
4016 does not as yet have the ability to store static variables in registers.
4017 Perhaps that will be added.
4019 @node Explicit Reg Vars
4020 @section Variables in Specified Registers
4021 @cindex explicit register variables
4022 @cindex variables in specified registers
4023 @cindex specified registers
4024 @cindex registers, global allocation
4026 GNU C allows you to put a few global variables into specified hardware
4027 registers. You can also specify the register in which an ordinary
4028 register variable should be allocated.
4032 Global register variables reserve registers throughout the program.
4033 This may be useful in programs such as programming language
4034 interpreters which have a couple of global variables that are accessed
4038 Local register variables in specific registers do not reserve the
4039 registers. The compiler's data flow analysis is capable of determining
4040 where the specified registers contain live values, and where they are
4041 available for other uses. Stores into local register variables may be deleted
4042 when they appear to be dead according to dataflow analysis. References
4043 to local register variables may be deleted or moved or simplified.
4045 These local variables are sometimes convenient for use with the extended
4046 @code{asm} feature (@pxref{Extended Asm}), if you want to write one
4047 output of the assembler instruction directly into a particular register.
4048 (This will work provided the register you specify fits the constraints
4049 specified for that operand in the @code{asm}.)
4057 @node Global Reg Vars
4058 @subsection Defining Global Register Variables
4059 @cindex global register variables
4060 @cindex registers, global variables in
4062 You can define a global register variable in GNU C like this:
4065 register int *foo asm ("a5");
4069 Here @code{a5} is the name of the register which should be used. Choose a
4070 register which is normally saved and restored by function calls on your
4071 machine, so that library routines will not clobber it.
4073 Naturally the register name is cpu-dependent, so you would need to
4074 conditionalize your program according to cpu type. The register
4075 @code{a5} would be a good choice on a 68000 for a variable of pointer
4076 type. On machines with register windows, be sure to choose a ``global''
4077 register that is not affected magically by the function call mechanism.
4079 In addition, operating systems on one type of cpu may differ in how they
4080 name the registers; then you would need additional conditionals. For
4081 example, some 68000 operating systems call this register @code{%a5}.
4083 Eventually there may be a way of asking the compiler to choose a register
4084 automatically, but first we need to figure out how it should choose and
4085 how to enable you to guide the choice. No solution is evident.
4087 Defining a global register variable in a certain register reserves that
4088 register entirely for this use, at least within the current compilation.
4089 The register will not be allocated for any other purpose in the functions
4090 in the current compilation. The register will not be saved and restored by
4091 these functions. Stores into this register are never deleted even if they
4092 would appear to be dead, but references may be deleted or moved or
4095 It is not safe to access the global register variables from signal
4096 handlers, or from more than one thread of control, because the system
4097 library routines may temporarily use the register for other things (unless
4098 you recompile them specially for the task at hand).
4100 @cindex @code{qsort}, and global register variables
4101 It is not safe for one function that uses a global register variable to
4102 call another such function @code{foo} by way of a third function
4103 @code{lose} that was compiled without knowledge of this variable (i.e.@: in a
4104 different source file in which the variable wasn't declared). This is
4105 because @code{lose} might save the register and put some other value there.
4106 For example, you can't expect a global register variable to be available in
4107 the comparison-function that you pass to @code{qsort}, since @code{qsort}
4108 might have put something else in that register. (If you are prepared to
4109 recompile @code{qsort} with the same global register variable, you can
4110 solve this problem.)
4112 If you want to recompile @code{qsort} or other source files which do not
4113 actually use your global register variable, so that they will not use that
4114 register for any other purpose, then it suffices to specify the compiler
4115 option @option{-ffixed-@var{reg}}. You need not actually add a global
4116 register declaration to their source code.
4118 A function which can alter the value of a global register variable cannot
4119 safely be called from a function compiled without this variable, because it
4120 could clobber the value the caller expects to find there on return.
4121 Therefore, the function which is the entry point into the part of the
4122 program that uses the global register variable must explicitly save and
4123 restore the value which belongs to its caller.
4125 @cindex register variable after @code{longjmp}
4126 @cindex global register after @code{longjmp}
4127 @cindex value after @code{longjmp}
4130 On most machines, @code{longjmp} will restore to each global register
4131 variable the value it had at the time of the @code{setjmp}. On some
4132 machines, however, @code{longjmp} will not change the value of global
4133 register variables. To be portable, the function that called @code{setjmp}
4134 should make other arrangements to save the values of the global register
4135 variables, and to restore them in a @code{longjmp}. This way, the same
4136 thing will happen regardless of what @code{longjmp} does.
4138 All global register variable declarations must precede all function
4139 definitions. If such a declaration could appear after function
4140 definitions, the declaration would be too late to prevent the register from
4141 being used for other purposes in the preceding functions.
4143 Global register variables may not have initial values, because an
4144 executable file has no means to supply initial contents for a register.
4146 On the SPARC, there are reports that g3 @dots{} g7 are suitable
4147 registers, but certain library functions, such as @code{getwd}, as well
4148 as the subroutines for division and remainder, modify g3 and g4. g1 and
4149 g2 are local temporaries.
4151 On the 68000, a2 @dots{} a5 should be suitable, as should d2 @dots{} d7.
4152 Of course, it will not do to use more than a few of those.
4154 @node Local Reg Vars
4155 @subsection Specifying Registers for Local Variables
4156 @cindex local variables, specifying registers
4157 @cindex specifying registers for local variables
4158 @cindex registers for local variables
4160 You can define a local register variable with a specified register
4164 register int *foo asm ("a5");
4168 Here @code{a5} is the name of the register which should be used. Note
4169 that this is the same syntax used for defining global register
4170 variables, but for a local variable it would appear within a function.
4172 Naturally the register name is cpu-dependent, but this is not a
4173 problem, since specific registers are most often useful with explicit
4174 assembler instructions (@pxref{Extended Asm}). Both of these things
4175 generally require that you conditionalize your program according to
4178 In addition, operating systems on one type of cpu may differ in how they
4179 name the registers; then you would need additional conditionals. For
4180 example, some 68000 operating systems call this register @code{%a5}.
4182 Defining such a register variable does not reserve the register; it
4183 remains available for other uses in places where flow control determines
4184 the variable's value is not live. However, these registers are made
4185 unavailable for use in the reload pass; excessive use of this feature
4186 leaves the compiler too few available registers to compile certain
4189 This option does not guarantee that GCC will generate code that has
4190 this variable in the register you specify at all times. You may not
4191 code an explicit reference to this register in an @code{asm} statement
4192 and assume it will always refer to this variable.
4194 Stores into local register variables may be deleted when they appear to be dead
4195 according to dataflow analysis. References to local register variables may
4196 be deleted or moved or simplified.
4198 @node Alternate Keywords
4199 @section Alternate Keywords
4200 @cindex alternate keywords
4201 @cindex keywords, alternate
4203 @option{-ansi} and the various @option{-std} options disable certain
4204 keywords. This causes trouble when you want to use GNU C extensions, or
4205 a general-purpose header file that should be usable by all programs,
4206 including ISO C programs. The keywords @code{asm}, @code{typeof} and
4207 @code{inline} are not available in programs compiled with
4208 @option{-ansi} or @option{-std} (although @code{inline} can be used in a
4209 program compiled with @option{-std=c99}). The ISO C99 keyword
4210 @code{restrict} is only available when @option{-std=gnu99} (which will
4211 eventually be the default) or @option{-std=c99} (or the equivalent
4212 @option{-std=iso9899:1999}) is used.
4214 The way to solve these problems is to put @samp{__} at the beginning and
4215 end of each problematical keyword. For example, use @code{__asm__}
4216 instead of @code{asm}, and @code{__inline__} instead of @code{inline}.
4218 Other C compilers won't accept these alternative keywords; if you want to
4219 compile with another compiler, you can define the alternate keywords as
4220 macros to replace them with the customary keywords. It looks like this:
4228 @findex __extension__
4230 @option{-pedantic} and other options cause warnings for many GNU C extensions.
4232 prevent such warnings within one expression by writing
4233 @code{__extension__} before the expression. @code{__extension__} has no
4234 effect aside from this.
4236 @node Incomplete Enums
4237 @section Incomplete @code{enum} Types
4239 You can define an @code{enum} tag without specifying its possible values.
4240 This results in an incomplete type, much like what you get if you write
4241 @code{struct foo} without describing the elements. A later declaration
4242 which does specify the possible values completes the type.
4244 You can't allocate variables or storage using the type while it is
4245 incomplete. However, you can work with pointers to that type.
4247 This extension may not be very useful, but it makes the handling of
4248 @code{enum} more consistent with the way @code{struct} and @code{union}
4251 This extension is not supported by GNU C++.
4253 @node Function Names
4254 @section Function Names as Strings
4255 @cindex @code{__FUNCTION__} identifier
4256 @cindex @code{__PRETTY_FUNCTION__} identifier
4257 @cindex @code{__func__} identifier
4259 GCC predefines two magic identifiers to hold the name of the current
4260 function. The identifier @code{__FUNCTION__} holds the name of the function
4261 as it appears in the source. The identifier @code{__PRETTY_FUNCTION__}
4262 holds the name of the function pretty printed in a language specific
4265 These names are always the same in a C function, but in a C++ function
4266 they may be different. For example, this program:
4270 extern int printf (char *, ...);
4277 printf ("__FUNCTION__ = %s\n", __FUNCTION__);
4278 printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
4296 __PRETTY_FUNCTION__ = int a::sub (int)
4299 The compiler automagically replaces the identifiers with a string
4300 literal containing the appropriate name. Thus, they are neither
4301 preprocessor macros, like @code{__FILE__} and @code{__LINE__}, nor
4302 variables. This means that they catenate with other string literals, and
4303 that they can be used to initialize char arrays. For example
4306 char here[] = "Function " __FUNCTION__ " in " __FILE__;
4309 On the other hand, @samp{#ifdef __FUNCTION__} does not have any special
4310 meaning inside a function, since the preprocessor does not do anything
4311 special with the identifier @code{__FUNCTION__}.
4313 Note that these semantics are deprecated, and that GCC 3.2 will handle
4314 @code{__FUNCTION__} and @code{__PRETTY_FUNCTION__} the same way as
4315 @code{__func__}. @code{__func__} is defined by the ISO standard C99:
4318 The identifier @code{__func__} is implicitly declared by the translator
4319 as if, immediately following the opening brace of each function
4320 definition, the declaration
4323 static const char __func__[] = "function-name";
4326 appeared, where function-name is the name of the lexically-enclosing
4327 function. This name is the unadorned name of the function.
4330 By this definition, @code{__func__} is a variable, not a string literal.
4331 In particular, @code{__func__} does not catenate with other string
4334 In @code{C++}, @code{__FUNCTION__} and @code{__PRETTY_FUNCTION__} are
4335 variables, declared in the same way as @code{__func__}.
4337 @node Return Address
4338 @section Getting the Return or Frame Address of a Function
4340 These functions may be used to get information about the callers of a
4343 @deftypefn {Built-in Function} {void *} __builtin_return_address (unsigned int @var{level})
4344 This function returns the return address of the current function, or of
4345 one of its callers. The @var{level} argument is number of frames to
4346 scan up the call stack. A value of @code{0} yields the return address
4347 of the current function, a value of @code{1} yields the return address
4348 of the caller of the current function, and so forth. When inlining
4349 the expected behavior is that the function will return the address of
4350 the function that will be returned to. To work around this behavior use
4351 the @code{noinline} function attribute.
4353 The @var{level} argument must be a constant integer.
4355 On some machines it may be impossible to determine the return address of
4356 any function other than the current one; in such cases, or when the top
4357 of the stack has been reached, this function will return @code{0} or a
4358 random value. In addition, @code{__builtin_frame_address} may be used
4359 to determine if the top of the stack has been reached.
4361 This function should only be used with a nonzero argument for debugging
4365 @deftypefn {Built-in Function} {void *} __builtin_frame_address (unsigned int @var{level})
4366 This function is similar to @code{__builtin_return_address}, but it
4367 returns the address of the function frame rather than the return address
4368 of the function. Calling @code{__builtin_frame_address} with a value of
4369 @code{0} yields the frame address of the current function, a value of
4370 @code{1} yields the frame address of the caller of the current function,
4373 The frame is the area on the stack which holds local variables and saved
4374 registers. The frame address is normally the address of the first word
4375 pushed on to the stack by the function. However, the exact definition
4376 depends upon the processor and the calling convention. If the processor
4377 has a dedicated frame pointer register, and the function has a frame,
4378 then @code{__builtin_frame_address} will return the value of the frame
4381 On some machines it may be impossible to determine the frame address of
4382 any function other than the current one; in such cases, or when the top
4383 of the stack has been reached, this function will return @code{0} if
4384 the first frame pointer is properly initialized by the startup code.
4386 This function should only be used with a nonzero argument for debugging
4390 @node Vector Extensions
4391 @section Using vector instructions through built-in functions
4393 On some targets, the instruction set contains SIMD vector instructions that
4394 operate on multiple values contained in one large register at the same time.
4395 For example, on the i386 the MMX, 3Dnow! and SSE extensions can be used
4398 The first step in using these extensions is to provide the necessary data
4399 types. This should be done using an appropriate @code{typedef}:
4402 typedef int v4si __attribute__ ((mode(V4SI)));
4405 The base type @code{int} is effectively ignored by the compiler, the
4406 actual properties of the new type @code{v4si} are defined by the
4407 @code{__attribute__}. It defines the machine mode to be used; for vector
4408 types these have the form @code{V@var{n}@var{B}}; @var{n} should be the
4409 number of elements in the vector, and @var{B} should be the base mode of the
4410 individual elements. The following can be used as base modes:
4414 An integer that is as wide as the smallest addressable unit, usually 8 bits.
4416 An integer, twice as wide as a QI mode integer, usually 16 bits.
4418 An integer, four times as wide as a QI mode integer, usually 32 bits.
4420 An integer, eight times as wide as a QI mode integer, usually 64 bits.
4422 A floating point value, as wide as a SI mode integer, usually 32 bits.
4424 A floating point value, as wide as a DI mode integer, usually 64 bits.
4427 There are no @code{V1xx} vector modes - they would be identical to the
4428 corresponding base mode.
4430 Specifying a combination that is not valid for the current architecture
4431 will cause gcc to synthesize the instructions using a narrower mode.
4432 For example, if you specify a variable of type @code{V4SI} and your
4433 architecture does not allow for this specific SIMD type, gcc will
4434 produce code that uses 4 @code{SIs}.
4436 The types defined in this manner can be used with a subset of normal C
4437 operations. Currently, gcc will allow using the following operators on
4438 these types: @code{+, -, *, /, unary minus}@.
4440 The operations behave like C++ @code{valarrays}. Addition is defined as
4441 the addition of the corresponding elements of the operands. For
4442 example, in the code below, each of the 4 elements in @var{a} will be
4443 added to the corresponding 4 elements in @var{b} and the resulting
4444 vector will be stored in @var{c}.
4447 typedef int v4si __attribute__ ((mode(V4SI)));
4454 Subtraction, multiplication, and division operate in a similar manner.
4455 Likewise, the result of using the unary minus operator on a vector type
4456 is a vector whose elements are the negative value of the corresponding
4457 elements in the operand.
4459 You can declare variables and use them in function calls and returns, as
4460 well as in assignments and some casts. You can specify a vector type as
4461 a return type for a function. Vector types can also be used as function
4462 arguments. It is possible to cast from one vector type to another,
4463 provided they are of the same size (in fact, you can also cast vectors
4464 to and from other datatypes of the same size).
4466 You cannot operate between vectors of different lengths or different
4467 signness without a cast.
4469 A port that supports hardware vector operations, usually provides a set
4470 of built-in functions that can be used to operate on vectors. For
4471 example, a function to add two vectors and multiply the result by a
4472 third could look like this:
4475 v4si f (v4si a, v4si b, v4si c)
4477 v4si tmp = __builtin_addv4si (a, b);
4478 return __builtin_mulv4si (tmp, c);
4483 @node Other Builtins
4484 @section Other built-in functions provided by GCC
4485 @cindex built-in functions
4486 @findex __builtin_isgreater
4487 @findex __builtin_isgreaterequal
4488 @findex __builtin_isless
4489 @findex __builtin_islessequal
4490 @findex __builtin_islessgreater
4491 @findex __builtin_isunordered
4520 @findex fprintf_unlocked
4522 @findex fputs_unlocked
4534 @findex printf_unlocked
4556 GCC provides a large number of built-in functions other than the ones
4557 mentioned above. Some of these are for internal use in the processing
4558 of exceptions or variable-length argument lists and will not be
4559 documented here because they may change from time to time; we do not
4560 recommend general use of these functions.
4562 The remaining functions are provided for optimization purposes.
4564 @opindex fno-builtin
4565 GCC includes built-in versions of many of the functions in the standard
4566 C library. The versions prefixed with @code{__builtin_} will always be
4567 treated as having the same meaning as the C library function even if you
4568 specify the @option{-fno-builtin} option. (@pxref{C Dialect Options})
4569 Many of these functions are only optimized in certain cases; if they are
4570 not optimized in a particular case, a call to the library function will
4575 The functions @code{abort}, @code{exit}, @code{_Exit} and @code{_exit}
4576 are recognized and presumed not to return, but otherwise are not built
4577 in. @code{_exit} is not recognized in strict ISO C mode (@option{-ansi},
4578 @option{-std=c89} or @option{-std=c99}). @code{_Exit} is not recognized in
4579 strict C89 mode (@option{-ansi} or @option{-std=c89}). All these functions
4580 have corresponding versions prefixed with @code{__builtin_}, which may be
4581 used even in strict C89 mode.
4583 Outside strict ISO C mode, the functions @code{alloca}, @code{bcmp},
4584 @code{bzero}, @code{index}, @code{rindex}, @code{ffs}, @code{fputs_unlocked},
4585 @code{printf_unlocked} and @code{fprintf_unlocked} may be handled as
4586 built-in functions. All these functions have corresponding versions
4587 prefixed with @code{__builtin_}, which may be used even in strict C89
4590 The ISO C99 functions @code{conj}, @code{conjf}, @code{conjl},
4591 @code{creal}, @code{crealf}, @code{creall}, @code{cimag}, @code{cimagf},
4592 @code{cimagl}, @code{llabs} and @code{imaxabs} are handled as built-in
4593 functions except in strict ISO C90 mode. There are also built-in
4594 versions of the ISO C99 functions @code{cosf}, @code{cosl},
4595 @code{expf}, @code{expl}, @code{fabsf}, @code{fabsl},
4596 @code{logf}, @code{logl}, @code{sinf}, @code{sinl}, @code{sqrtf}, and
4597 @code{sqrtl}, that are recognized in any mode since ISO C90 reserves
4598 these names for the purpose to which ISO C99 puts them. All these
4599 functions have corresponding versions prefixed with @code{__builtin_}.
4601 The ISO C90 functions @code{abs}, @code{cos}, @code{exp}, @code{fabs},
4602 @code{fprintf}, @code{fputs}, @code{labs}, @code{log},
4603 @code{memcmp}, @code{memcpy},
4604 @code{memset}, @code{printf}, @code{sin}, @code{sqrt}, @code{strcat},
4605 @code{strchr}, @code{strcmp}, @code{strcpy}, @code{strcspn},
4606 @code{strlen}, @code{strncat}, @code{strncmp}, @code{strncpy},
4607 @code{strpbrk}, @code{strrchr}, @code{strspn}, and @code{strstr} are all
4608 recognized as built-in functions unless @option{-fno-builtin} is
4609 specified (or @option{-fno-builtin-@var{function}} is specified for an
4610 individual function). All of these functions have corresponding
4611 versions prefixed with @code{__builtin_}.
4613 GCC provides built-in versions of the ISO C99 floating point comparison
4614 macros that avoid raising exceptions for unordered operands. They have
4615 the same names as the standard macros ( @code{isgreater},
4616 @code{isgreaterequal}, @code{isless}, @code{islessequal},
4617 @code{islessgreater}, and @code{isunordered}) , with @code{__builtin_}
4618 prefixed. We intend for a library implementor to be able to simply
4619 @code{#define} each standard macro to its built-in equivalent.
4621 @deftypefn {Built-in Function} int __builtin_types_compatible_p (@var{type1}, @var{type2})
4623 You can use the built-in function @code{__builtin_types_compatible_p} to
4624 determine whether two types are the same.
4626 This built-in function returns 1 if the unqualified versions of the
4627 types @var{type1} and @var{type2} (which are types, not expressions) are
4628 compatible, 0 otherwise. The result of this built-in function can be
4629 used in integer constant expressions.
4631 This built-in function ignores top level qualifiers (e.g., @code{const},
4632 @code{volatile}). For example, @code{int} is equivalent to @code{const
4635 The type @code{int[]} and @code{int[5]} are compatible. On the other
4636 hand, @code{int} and @code{char *} are not compatible, even if the size
4637 of their types, on the particular architecture are the same. Also, the
4638 amount of pointer indirection is taken into account when determining
4639 similarity. Consequently, @code{short *} is not similar to
4640 @code{short **}. Furthermore, two types that are typedefed are
4641 considered compatible if their underlying types are compatible.
4643 An @code{enum} type is considered to be compatible with another
4644 @code{enum} type. For example, @code{enum @{foo, bar@}} is similar to
4645 @code{enum @{hot, dog@}}.
4647 You would typically use this function in code whose execution varies
4648 depending on the arguments' types. For example:
4654 if (__builtin_types_compatible_p (typeof (x), long double)) \
4655 tmp = foo_long_double (tmp); \
4656 else if (__builtin_types_compatible_p (typeof (x), double)) \
4657 tmp = foo_double (tmp); \
4658 else if (__builtin_types_compatible_p (typeof (x), float)) \
4659 tmp = foo_float (tmp); \
4666 @emph{Note:} This construct is only available for C.
4670 @deftypefn {Built-in Function} @var{type} __builtin_choose_expr (@var{const_exp}, @var{exp1}, @var{exp2})
4672 You can use the built-in function @code{__builtin_choose_expr} to
4673 evaluate code depending on the value of a constant expression. This
4674 built-in function returns @var{exp1} if @var{const_exp}, which is a
4675 constant expression that must be able to be determined at compile time,
4676 is nonzero. Otherwise it returns 0.
4678 This built-in function is analogous to the @samp{? :} operator in C,
4679 except that the expression returned has its type unaltered by promotion
4680 rules. Also, the built-in function does not evaluate the expression
4681 that was not chosen. For example, if @var{const_exp} evaluates to true,
4682 @var{exp2} is not evaluated even if it has side-effects.
4684 This built-in function can return an lvalue if the chosen argument is an
4687 If @var{exp1} is returned, the return type is the same as @var{exp1}'s
4688 type. Similarly, if @var{exp2} is returned, its return type is the same
4695 __builtin_choose_expr ( \
4696 __builtin_types_compatible_p (typeof (x), double), \
4698 __builtin_choose_expr ( \
4699 __builtin_types_compatible_p (typeof (x), float), \
4701 /* @r{The void expression results in a compile-time error} \
4702 @r{when assigning the result to something.} */ \
4706 @emph{Note:} This construct is only available for C. Furthermore, the
4707 unused expression (@var{exp1} or @var{exp2} depending on the value of
4708 @var{const_exp}) may still generate syntax errors. This may change in
4713 @deftypefn {Built-in Function} int __builtin_constant_p (@var{exp})
4714 You can use the built-in function @code{__builtin_constant_p} to
4715 determine if a value is known to be constant at compile-time and hence
4716 that GCC can perform constant-folding on expressions involving that
4717 value. The argument of the function is the value to test. The function
4718 returns the integer 1 if the argument is known to be a compile-time
4719 constant and 0 if it is not known to be a compile-time constant. A
4720 return of 0 does not indicate that the value is @emph{not} a constant,
4721 but merely that GCC cannot prove it is a constant with the specified
4722 value of the @option{-O} option.
4724 You would typically use this function in an embedded application where
4725 memory was a critical resource. If you have some complex calculation,
4726 you may want it to be folded if it involves constants, but need to call
4727 a function if it does not. For example:
4730 #define Scale_Value(X) \
4731 (__builtin_constant_p (X) \
4732 ? ((X) * SCALE + OFFSET) : Scale (X))
4735 You may use this built-in function in either a macro or an inline
4736 function. However, if you use it in an inlined function and pass an
4737 argument of the function as the argument to the built-in, GCC will
4738 never return 1 when you call the inline function with a string constant
4739 or compound literal (@pxref{Compound Literals}) and will not return 1
4740 when you pass a constant numeric value to the inline function unless you
4741 specify the @option{-O} option.
4743 You may also use @code{__builtin_constant_p} in initializers for static
4744 data. For instance, you can write
4747 static const int table[] = @{
4748 __builtin_constant_p (EXPRESSION) ? (EXPRESSION) : -1,
4754 This is an acceptable initializer even if @var{EXPRESSION} is not a
4755 constant expression. GCC must be more conservative about evaluating the
4756 built-in in this case, because it has no opportunity to perform
4759 Previous versions of GCC did not accept this built-in in data
4760 initializers. The earliest version where it is completely safe is
4764 @deftypefn {Built-in Function} long __builtin_expect (long @var{exp}, long @var{c})
4765 @opindex fprofile-arcs
4766 You may use @code{__builtin_expect} to provide the compiler with
4767 branch prediction information. In general, you should prefer to
4768 use actual profile feedback for this (@option{-fprofile-arcs}), as
4769 programmers are notoriously bad at predicting how their programs
4770 actually perform. However, there are applications in which this
4771 data is hard to collect.
4773 The return value is the value of @var{exp}, which should be an
4774 integral expression. The value of @var{c} must be a compile-time
4775 constant. The semantics of the built-in are that it is expected
4776 that @var{exp} == @var{c}. For example:
4779 if (__builtin_expect (x, 0))
4784 would indicate that we do not expect to call @code{foo}, since
4785 we expect @code{x} to be zero. Since you are limited to integral
4786 expressions for @var{exp}, you should use constructions such as
4789 if (__builtin_expect (ptr != NULL, 1))
4794 when testing pointer or floating-point values.
4797 @deftypefn {Built-in Function} void __builtin_prefetch (const void *@var{addr}, ...)
4798 This function is used to minimize cache-miss latency by moving data into
4799 a cache before it is accessed.
4800 You can insert calls to @code{__builtin_prefetch} into code for which
4801 you know addresses of data in memory that is likely to be accessed soon.
4802 If the target supports them, data prefetch instructions will be generated.
4803 If the prefetch is done early enough before the access then the data will
4804 be in the cache by the time it is accessed.
4806 The value of @var{addr} is the address of the memory to prefetch.
4807 There are two optional arguments, @var{rw} and @var{locality}.
4808 The value of @var{rw} is a compile-time constant one or zero; one
4809 means that the prefetch is preparing for a write to the memory address
4810 and zero, the default, means that the prefetch is preparing for a read.
4811 The value @var{locality} must be a compile-time constant integer between
4812 zero and three. A value of zero means that the data has no temporal
4813 locality, so it need not be left in the cache after the access. A value
4814 of three means that the data has a high degree of temporal locality and
4815 should be left in all levels of cache possible. Values of one and two
4816 mean, respectively, a low or moderate degree of temporal locality. The
4820 for (i = 0; i < n; i++)
4823 __builtin_prefetch (&a[i+j], 1, 1);
4824 __builtin_prefetch (&b[i+j], 0, 1);
4829 Data prefetch does not generate faults if @var{addr} is invalid, but
4830 the address expression itself must be valid. For example, a prefetch
4831 of @code{p->next} will not fault if @code{p->next} is not a valid
4832 address, but evaluation will fault if @code{p} is not a valid address.
4834 If the target does not support data prefetch, the address expression
4835 is evaluated if it includes side effects but no other code is generated
4836 and GCC does not issue a warning.
4839 @deftypefn {Built-in Function} double __builtin_huge_val (void)
4840 Returns a positive infinity, if supported by the floating-point format,
4841 else @code{DBL_MAX}. This function is suitable for implementing the
4842 ISO C macro @code{HUGE_VAL}.
4845 @deftypefn {Built-in Function} float __builtin_huge_valf (void)
4846 Similar to @code{__builtin_huge_val}, except the return type is @code{float}.
4849 @deftypefn {Built-in Function} long double __builtin_huge_vall (void)
4850 Similar to @code{__builtin_huge_val}, except the return
4851 type is @code{long double}.
4854 @deftypefn {Built-in Function} double __builtin_inf (void)
4855 Similar to @code{__builtin_huge_val}, except a warning is generated
4856 if the target floating-point format does not support infinities.
4857 This function is suitable for implementing the ISO C99 macro @code{INFINITY}.
4860 @deftypefn {Built-in Function} float __builtin_inff (void)
4861 Similar to @code{__builtin_inf}, except the return type is @code{float}.
4864 @deftypefn {Built-in Function} long double __builtin_infl (void)
4865 Similar to @code{__builtin_inf}, except the return
4866 type is @code{long double}.
4869 @deftypefn {Built-in Function} double __builtin_nan (const char *str)
4870 This is an implementation of the ISO C99 function @code{nan}.
4872 Since ISO C99 defines this function in terms of @code{strtod}, which we
4873 do not implement, a desription of the parsing is in order. The string
4874 is parsed as by @code{strtol}; that is, the base is recognized by
4875 leading @samp{0} or @samp{0x} prefixes. The number parsed is placed
4876 in the significand such that the least significant bit of the number
4877 is at the least significant bit of the significand. The number is
4878 truncated to fit the significand field provided. The significand is
4879 forced to be a quiet NaN.
4881 This function, if given a string literal, is evaluated early enough
4882 that it is considered a compile-time constant.
4885 @deftypefn {Built-in Function} float __builtin_nanf (const char *str)
4886 Similar to @code{__builtin_nan}, except the return type is @code{float}.
4889 @deftypefn {Built-in Function} long double __builtin_nanl (const char *str)
4890 Similar to @code{__builtin_nan}, except the return type is @code{long double}.
4893 @deftypefn {Built-in Function} double __builtin_nans (const char *str)
4894 Similar to @code{__builtin_nan}, except the significand is forced
4895 to be a signaling NaN. The @code{nans} function is proposed by
4896 @uref{http://std.dkuug.dk/JTC1/SC22/WG14/www/docs/n965.htm,,WG14 N965}.
4899 @deftypefn {Built-in Function} float __builtin_nansf (const char *str)
4900 Similar to @code{__builtin_nans}, except the return type is @code{float}.
4903 @deftypefn {Built-in Function} long double __builtin_nansl (const char *str)
4904 Similar to @code{__builtin_nans}, except the return type is @code{long double}.
4907 @node Target Builtins
4908 @section Built-in Functions Specific to Particular Target Machines
4910 On some target machines, GCC supports many built-in functions specific
4911 to those machines. Generally these generate calls to specific machine
4912 instructions, but allow the compiler to schedule those calls.
4915 * Alpha Built-in Functions::
4916 * X86 Built-in Functions::
4917 * PowerPC AltiVec Built-in Functions::
4920 @node Alpha Built-in Functions
4921 @subsection Alpha Built-in Functions
4923 These built-in functions are available for the Alpha family of
4924 processors, depending on the command-line switches used.
4926 The following built-in functions are always available. They
4927 all generate the machine instruction that is part of the name.
4930 long __builtin_alpha_implver (void)
4931 long __builtin_alpha_rpcc (void)
4932 long __builtin_alpha_amask (long)
4933 long __builtin_alpha_cmpbge (long, long)
4934 long __builtin_alpha_extbl (long, long)
4935 long __builtin_alpha_extwl (long, long)
4936 long __builtin_alpha_extll (long, long)
4937 long __builtin_alpha_extql (long, long)
4938 long __builtin_alpha_extwh (long, long)
4939 long __builtin_alpha_extlh (long, long)
4940 long __builtin_alpha_extqh (long, long)
4941 long __builtin_alpha_insbl (long, long)
4942 long __builtin_alpha_inswl (long, long)
4943 long __builtin_alpha_insll (long, long)
4944 long __builtin_alpha_insql (long, long)
4945 long __builtin_alpha_inswh (long, long)
4946 long __builtin_alpha_inslh (long, long)
4947 long __builtin_alpha_insqh (long, long)
4948 long __builtin_alpha_mskbl (long, long)
4949 long __builtin_alpha_mskwl (long, long)
4950 long __builtin_alpha_mskll (long, long)
4951 long __builtin_alpha_mskql (long, long)
4952 long __builtin_alpha_mskwh (long, long)
4953 long __builtin_alpha_msklh (long, long)
4954 long __builtin_alpha_mskqh (long, long)
4955 long __builtin_alpha_umulh (long, long)
4956 long __builtin_alpha_zap (long, long)
4957 long __builtin_alpha_zapnot (long, long)
4960 The following built-in functions are always with @option{-mmax}
4961 or @option{-mcpu=@var{cpu}} where @var{cpu} is @code{pca56} or
4962 later. They all generate the machine instruction that is part
4966 long __builtin_alpha_pklb (long)
4967 long __builtin_alpha_pkwb (long)
4968 long __builtin_alpha_unpkbl (long)
4969 long __builtin_alpha_unpkbw (long)
4970 long __builtin_alpha_minub8 (long, long)
4971 long __builtin_alpha_minsb8 (long, long)
4972 long __builtin_alpha_minuw4 (long, long)
4973 long __builtin_alpha_minsw4 (long, long)
4974 long __builtin_alpha_maxub8 (long, long)
4975 long __builtin_alpha_maxsb8 (long, long)
4976 long __builtin_alpha_maxuw4 (long, long)
4977 long __builtin_alpha_maxsw4 (long, long)
4978 long __builtin_alpha_perr (long, long)
4981 The following built-in functions are always with @option{-mcix}
4982 or @option{-mcpu=@var{cpu}} where @var{cpu} is @code{ev67} or
4983 later. They all generate the machine instruction that is part
4987 long __builtin_alpha_cttz (long)
4988 long __builtin_alpha_ctlz (long)
4989 long __builtin_alpha_ctpop (long)
4992 The following builtins are available on systems that use the OSF/1
4993 PALcode. Normally they invoke the @code{rduniq} and @code{wruniq}
4994 PAL calls, but when invoked with @option{-mtls-kernel}, they invoke
4995 @code{rdval} and @code{wrval}.
4998 void *__builtin_thread_pointer (void)
4999 void __builtin_set_thread_pointer (void *)
5002 @node X86 Built-in Functions
5003 @subsection X86 Built-in Functions
5005 These built-in functions are available for the i386 and x86-64 family
5006 of computers, depending on the command-line switches used.
5008 The following machine modes are available for use with MMX built-in functions
5009 (@pxref{Vector Extensions}): @code{V2SI} for a vector of two 32-bit integers,
5010 @code{V4HI} for a vector of four 16-bit integers, and @code{V8QI} for a
5011 vector of eight 8-bit integers. Some of the built-in functions operate on
5012 MMX registers as a whole 64-bit entity, these use @code{DI} as their mode.
5014 If 3Dnow extensions are enabled, @code{V2SF} is used as a mode for a vector
5015 of two 32-bit floating point values.
5017 If SSE extensions are enabled, @code{V4SF} is used for a vector of four 32-bit
5018 floating point values. Some instructions use a vector of four 32-bit
5019 integers, these use @code{V4SI}. Finally, some instructions operate on an
5020 entire vector register, interpreting it as a 128-bit integer, these use mode
5023 The following built-in functions are made available by @option{-mmmx}.
5024 All of them generate the machine instruction that is part of the name.
5027 v8qi __builtin_ia32_paddb (v8qi, v8qi)
5028 v4hi __builtin_ia32_paddw (v4hi, v4hi)
5029 v2si __builtin_ia32_paddd (v2si, v2si)
5030 v8qi __builtin_ia32_psubb (v8qi, v8qi)
5031 v4hi __builtin_ia32_psubw (v4hi, v4hi)
5032 v2si __builtin_ia32_psubd (v2si, v2si)
5033 v8qi __builtin_ia32_paddsb (v8qi, v8qi)
5034 v4hi __builtin_ia32_paddsw (v4hi, v4hi)
5035 v8qi __builtin_ia32_psubsb (v8qi, v8qi)
5036 v4hi __builtin_ia32_psubsw (v4hi, v4hi)
5037 v8qi __builtin_ia32_paddusb (v8qi, v8qi)
5038 v4hi __builtin_ia32_paddusw (v4hi, v4hi)
5039 v8qi __builtin_ia32_psubusb (v8qi, v8qi)
5040 v4hi __builtin_ia32_psubusw (v4hi, v4hi)
5041 v4hi __builtin_ia32_pmullw (v4hi, v4hi)
5042 v4hi __builtin_ia32_pmulhw (v4hi, v4hi)
5043 di __builtin_ia32_pand (di, di)
5044 di __builtin_ia32_pandn (di,di)
5045 di __builtin_ia32_por (di, di)
5046 di __builtin_ia32_pxor (di, di)
5047 v8qi __builtin_ia32_pcmpeqb (v8qi, v8qi)
5048 v4hi __builtin_ia32_pcmpeqw (v4hi, v4hi)
5049 v2si __builtin_ia32_pcmpeqd (v2si, v2si)
5050 v8qi __builtin_ia32_pcmpgtb (v8qi, v8qi)
5051 v4hi __builtin_ia32_pcmpgtw (v4hi, v4hi)
5052 v2si __builtin_ia32_pcmpgtd (v2si, v2si)
5053 v8qi __builtin_ia32_punpckhbw (v8qi, v8qi)
5054 v4hi __builtin_ia32_punpckhwd (v4hi, v4hi)
5055 v2si __builtin_ia32_punpckhdq (v2si, v2si)
5056 v8qi __builtin_ia32_punpcklbw (v8qi, v8qi)
5057 v4hi __builtin_ia32_punpcklwd (v4hi, v4hi)
5058 v2si __builtin_ia32_punpckldq (v2si, v2si)
5059 v8qi __builtin_ia32_packsswb (v4hi, v4hi)
5060 v4hi __builtin_ia32_packssdw (v2si, v2si)
5061 v8qi __builtin_ia32_packuswb (v4hi, v4hi)
5064 The following built-in functions are made available either with
5065 @option{-msse}, or with a combination of @option{-m3dnow} and
5066 @option{-march=athlon}. All of them generate the machine
5067 instruction that is part of the name.
5070 v4hi __builtin_ia32_pmulhuw (v4hi, v4hi)
5071 v8qi __builtin_ia32_pavgb (v8qi, v8qi)
5072 v4hi __builtin_ia32_pavgw (v4hi, v4hi)
5073 v4hi __builtin_ia32_psadbw (v8qi, v8qi)
5074 v8qi __builtin_ia32_pmaxub (v8qi, v8qi)
5075 v4hi __builtin_ia32_pmaxsw (v4hi, v4hi)
5076 v8qi __builtin_ia32_pminub (v8qi, v8qi)
5077 v4hi __builtin_ia32_pminsw (v4hi, v4hi)
5078 int __builtin_ia32_pextrw (v4hi, int)
5079 v4hi __builtin_ia32_pinsrw (v4hi, int, int)
5080 int __builtin_ia32_pmovmskb (v8qi)
5081 void __builtin_ia32_maskmovq (v8qi, v8qi, char *)
5082 void __builtin_ia32_movntq (di *, di)
5083 void __builtin_ia32_sfence (void)
5086 The following built-in functions are available when @option{-msse} is used.
5087 All of them generate the machine instruction that is part of the name.
5090 int __builtin_ia32_comieq (v4sf, v4sf)
5091 int __builtin_ia32_comineq (v4sf, v4sf)
5092 int __builtin_ia32_comilt (v4sf, v4sf)
5093 int __builtin_ia32_comile (v4sf, v4sf)
5094 int __builtin_ia32_comigt (v4sf, v4sf)
5095 int __builtin_ia32_comige (v4sf, v4sf)
5096 int __builtin_ia32_ucomieq (v4sf, v4sf)
5097 int __builtin_ia32_ucomineq (v4sf, v4sf)
5098 int __builtin_ia32_ucomilt (v4sf, v4sf)
5099 int __builtin_ia32_ucomile (v4sf, v4sf)
5100 int __builtin_ia32_ucomigt (v4sf, v4sf)
5101 int __builtin_ia32_ucomige (v4sf, v4sf)
5102 v4sf __builtin_ia32_addps (v4sf, v4sf)
5103 v4sf __builtin_ia32_subps (v4sf, v4sf)
5104 v4sf __builtin_ia32_mulps (v4sf, v4sf)
5105 v4sf __builtin_ia32_divps (v4sf, v4sf)
5106 v4sf __builtin_ia32_addss (v4sf, v4sf)
5107 v4sf __builtin_ia32_subss (v4sf, v4sf)
5108 v4sf __builtin_ia32_mulss (v4sf, v4sf)
5109 v4sf __builtin_ia32_divss (v4sf, v4sf)
5110 v4si __builtin_ia32_cmpeqps (v4sf, v4sf)
5111 v4si __builtin_ia32_cmpltps (v4sf, v4sf)
5112 v4si __builtin_ia32_cmpleps (v4sf, v4sf)
5113 v4si __builtin_ia32_cmpgtps (v4sf, v4sf)
5114 v4si __builtin_ia32_cmpgeps (v4sf, v4sf)
5115 v4si __builtin_ia32_cmpunordps (v4sf, v4sf)
5116 v4si __builtin_ia32_cmpneqps (v4sf, v4sf)
5117 v4si __builtin_ia32_cmpnltps (v4sf, v4sf)
5118 v4si __builtin_ia32_cmpnleps (v4sf, v4sf)
5119 v4si __builtin_ia32_cmpngtps (v4sf, v4sf)
5120 v4si __builtin_ia32_cmpngeps (v4sf, v4sf)
5121 v4si __builtin_ia32_cmpordps (v4sf, v4sf)
5122 v4si __builtin_ia32_cmpeqss (v4sf, v4sf)
5123 v4si __builtin_ia32_cmpltss (v4sf, v4sf)
5124 v4si __builtin_ia32_cmpless (v4sf, v4sf)
5125 v4si __builtin_ia32_cmpgtss (v4sf, v4sf)
5126 v4si __builtin_ia32_cmpgess (v4sf, v4sf)
5127 v4si __builtin_ia32_cmpunordss (v4sf, v4sf)
5128 v4si __builtin_ia32_cmpneqss (v4sf, v4sf)
5129 v4si __builtin_ia32_cmpnlts (v4sf, v4sf)
5130 v4si __builtin_ia32_cmpnless (v4sf, v4sf)
5131 v4si __builtin_ia32_cmpngtss (v4sf, v4sf)
5132 v4si __builtin_ia32_cmpngess (v4sf, v4sf)
5133 v4si __builtin_ia32_cmpordss (v4sf, v4sf)
5134 v4sf __builtin_ia32_maxps (v4sf, v4sf)
5135 v4sf __builtin_ia32_maxss (v4sf, v4sf)
5136 v4sf __builtin_ia32_minps (v4sf, v4sf)
5137 v4sf __builtin_ia32_minss (v4sf, v4sf)
5138 v4sf __builtin_ia32_andps (v4sf, v4sf)
5139 v4sf __builtin_ia32_andnps (v4sf, v4sf)
5140 v4sf __builtin_ia32_orps (v4sf, v4sf)
5141 v4sf __builtin_ia32_xorps (v4sf, v4sf)
5142 v4sf __builtin_ia32_movss (v4sf, v4sf)
5143 v4sf __builtin_ia32_movhlps (v4sf, v4sf)
5144 v4sf __builtin_ia32_movlhps (v4sf, v4sf)
5145 v4sf __builtin_ia32_unpckhps (v4sf, v4sf)
5146 v4sf __builtin_ia32_unpcklps (v4sf, v4sf)
5147 v4sf __builtin_ia32_cvtpi2ps (v4sf, v2si)
5148 v4sf __builtin_ia32_cvtsi2ss (v4sf, int)
5149 v2si __builtin_ia32_cvtps2pi (v4sf)
5150 int __builtin_ia32_cvtss2si (v4sf)
5151 v2si __builtin_ia32_cvttps2pi (v4sf)
5152 int __builtin_ia32_cvttss2si (v4sf)
5153 v4sf __builtin_ia32_rcpps (v4sf)
5154 v4sf __builtin_ia32_rsqrtps (v4sf)
5155 v4sf __builtin_ia32_sqrtps (v4sf)
5156 v4sf __builtin_ia32_rcpss (v4sf)
5157 v4sf __builtin_ia32_rsqrtss (v4sf)
5158 v4sf __builtin_ia32_sqrtss (v4sf)
5159 v4sf __builtin_ia32_shufps (v4sf, v4sf, int)
5160 void __builtin_ia32_movntps (float *, v4sf)
5161 int __builtin_ia32_movmskps (v4sf)
5164 The following built-in functions are available when @option{-msse} is used.
5167 @item v4sf __builtin_ia32_loadaps (float *)
5168 Generates the @code{movaps} machine instruction as a load from memory.
5169 @item void __builtin_ia32_storeaps (float *, v4sf)
5170 Generates the @code{movaps} machine instruction as a store to memory.
5171 @item v4sf __builtin_ia32_loadups (float *)
5172 Generates the @code{movups} machine instruction as a load from memory.
5173 @item void __builtin_ia32_storeups (float *, v4sf)
5174 Generates the @code{movups} machine instruction as a store to memory.
5175 @item v4sf __builtin_ia32_loadsss (float *)
5176 Generates the @code{movss} machine instruction as a load from memory.
5177 @item void __builtin_ia32_storess (float *, v4sf)
5178 Generates the @code{movss} machine instruction as a store to memory.
5179 @item v4sf __builtin_ia32_loadhps (v4sf, v2si *)
5180 Generates the @code{movhps} machine instruction as a load from memory.
5181 @item v4sf __builtin_ia32_loadlps (v4sf, v2si *)
5182 Generates the @code{movlps} machine instruction as a load from memory
5183 @item void __builtin_ia32_storehps (v4sf, v2si *)
5184 Generates the @code{movhps} machine instruction as a store to memory.
5185 @item void __builtin_ia32_storelps (v4sf, v2si *)
5186 Generates the @code{movlps} machine instruction as a store to memory.
5189 The following built-in functions are available when @option{-m3dnow} is used.
5190 All of them generate the machine instruction that is part of the name.
5193 void __builtin_ia32_femms (void)
5194 v8qi __builtin_ia32_pavgusb (v8qi, v8qi)
5195 v2si __builtin_ia32_pf2id (v2sf)
5196 v2sf __builtin_ia32_pfacc (v2sf, v2sf)
5197 v2sf __builtin_ia32_pfadd (v2sf, v2sf)
5198 v2si __builtin_ia32_pfcmpeq (v2sf, v2sf)
5199 v2si __builtin_ia32_pfcmpge (v2sf, v2sf)
5200 v2si __builtin_ia32_pfcmpgt (v2sf, v2sf)
5201 v2sf __builtin_ia32_pfmax (v2sf, v2sf)
5202 v2sf __builtin_ia32_pfmin (v2sf, v2sf)
5203 v2sf __builtin_ia32_pfmul (v2sf, v2sf)
5204 v2sf __builtin_ia32_pfrcp (v2sf)
5205 v2sf __builtin_ia32_pfrcpit1 (v2sf, v2sf)
5206 v2sf __builtin_ia32_pfrcpit2 (v2sf, v2sf)
5207 v2sf __builtin_ia32_pfrsqrt (v2sf)
5208 v2sf __builtin_ia32_pfrsqrtit1 (v2sf, v2sf)
5209 v2sf __builtin_ia32_pfsub (v2sf, v2sf)
5210 v2sf __builtin_ia32_pfsubr (v2sf, v2sf)
5211 v2sf __builtin_ia32_pi2fd (v2si)
5212 v4hi __builtin_ia32_pmulhrw (v4hi, v4hi)
5215 The following built-in functions are available when both @option{-m3dnow}
5216 and @option{-march=athlon} are used. All of them generate the machine
5217 instruction that is part of the name.
5220 v2si __builtin_ia32_pf2iw (v2sf)
5221 v2sf __builtin_ia32_pfnacc (v2sf, v2sf)
5222 v2sf __builtin_ia32_pfpnacc (v2sf, v2sf)
5223 v2sf __builtin_ia32_pi2fw (v2si)
5224 v2sf __builtin_ia32_pswapdsf (v2sf)
5225 v2si __builtin_ia32_pswapdsi (v2si)
5228 @node PowerPC AltiVec Built-in Functions
5229 @subsection PowerPC AltiVec Built-in Functions
5231 These built-in functions are available for the PowerPC family
5232 of computers, depending on the command-line switches used.
5234 The following machine modes are available for use with AltiVec built-in
5235 functions (@pxref{Vector Extensions}): @code{V4SI} for a vector of four
5236 32-bit integers, @code{V4SF} for a vector of four 32-bit floating point
5237 numbers, @code{V8HI} for a vector of eight 16-bit integers, and
5238 @code{V16QI} for a vector of sixteen 8-bit integers.
5240 The following functions are made available by including
5241 @code{<altivec.h>} and using @option{-maltivec} and
5242 @option{-mabi=altivec}. The functions implement the functionality
5243 described in Motorola's AltiVec Programming Interface Manual.
5245 There are a few differences from Motorola's documentation and GCC's
5246 implementation. Vector constants are done with curly braces (not
5247 parentheses). Vector initializers require no casts if the vector
5248 constant is of the same type as the variable it is initializing. The
5249 @code{vector bool} type is deprecated and will be discontinued in
5250 further revisions. Use @code{vector signed} instead. If @code{signed}
5251 or @code{unsigned} is omitted, the vector type will default to
5252 @code{signed}. Lastly, all overloaded functions are implemented with macros
5253 for the C implementation. So code the following example will not work:
5256 vec_add ((vector signed int)@{1, 2, 3, 4@}, foo);
5259 Since vec_add is a macro, the vector constant in the above example will
5260 be treated as four different arguments. Wrap the entire argument in
5261 parentheses for this to work. The C++ implementation does not use
5264 @emph{Note:} Only the @code{<altivec.h>} interface is supported.
5265 Internally, GCC uses built-in functions to achieve the functionality in
5266 the aforementioned header file, but they are not supported and are
5267 subject to change without notice.
5270 vector signed char vec_abs (vector signed char, vector signed char);
5271 vector signed short vec_abs (vector signed short, vector signed short);
5272 vector signed int vec_abs (vector signed int, vector signed int);
5273 vector signed float vec_abs (vector signed float, vector signed float);
5275 vector signed char vec_abss (vector signed char, vector signed char);
5276 vector signed short vec_abss (vector signed short, vector signed short);
5278 vector signed char vec_add (vector signed char, vector signed char);
5279 vector unsigned char vec_add (vector signed char, vector unsigned char);
5281 vector unsigned char vec_add (vector unsigned char, vector signed char);
5283 vector unsigned char vec_add (vector unsigned char,
5284 vector unsigned char);
5285 vector signed short vec_add (vector signed short, vector signed short);
5286 vector unsigned short vec_add (vector signed short,
5287 vector unsigned short);
5288 vector unsigned short vec_add (vector unsigned short,
5289 vector signed short);
5290 vector unsigned short vec_add (vector unsigned short,
5291 vector unsigned short);
5292 vector signed int vec_add (vector signed int, vector signed int);
5293 vector unsigned int vec_add (vector signed int, vector unsigned int);
5294 vector unsigned int vec_add (vector unsigned int, vector signed int);
5295 vector unsigned int vec_add (vector unsigned int, vector unsigned int);
5296 vector float vec_add (vector float, vector float);
5298 vector unsigned int vec_addc (vector unsigned int, vector unsigned int);
5300 vector unsigned char vec_adds (vector signed char,
5301 vector unsigned char);
5302 vector unsigned char vec_adds (vector unsigned char,
5303 vector signed char);
5304 vector unsigned char vec_adds (vector unsigned char,
5305 vector unsigned char);
5306 vector signed char vec_adds (vector signed char, vector signed char);
5307 vector unsigned short vec_adds (vector signed short,
5308 vector unsigned short);
5309 vector unsigned short vec_adds (vector unsigned short,
5310 vector signed short);
5311 vector unsigned short vec_adds (vector unsigned short,
5312 vector unsigned short);
5313 vector signed short vec_adds (vector signed short, vector signed short);
5315 vector unsigned int vec_adds (vector signed int, vector unsigned int);
5316 vector unsigned int vec_adds (vector unsigned int, vector signed int);
5317 vector unsigned int vec_adds (vector unsigned int, vector unsigned int);
5319 vector signed int vec_adds (vector signed int, vector signed int);
5321 vector float vec_and (vector float, vector float);
5322 vector float vec_and (vector float, vector signed int);
5323 vector float vec_and (vector signed int, vector float);
5324 vector signed int vec_and (vector signed int, vector signed int);
5325 vector unsigned int vec_and (vector signed int, vector unsigned int);
5326 vector unsigned int vec_and (vector unsigned int, vector signed int);
5327 vector unsigned int vec_and (vector unsigned int, vector unsigned int);
5328 vector signed short vec_and (vector signed short, vector signed short);
5329 vector unsigned short vec_and (vector signed short,
5330 vector unsigned short);
5331 vector unsigned short vec_and (vector unsigned short,
5332 vector signed short);
5333 vector unsigned short vec_and (vector unsigned short,
5334 vector unsigned short);
5335 vector signed char vec_and (vector signed char, vector signed char);
5336 vector unsigned char vec_and (vector signed char, vector unsigned char);
5338 vector unsigned char vec_and (vector unsigned char, vector signed char);
5340 vector unsigned char vec_and (vector unsigned char,
5341 vector unsigned char);
5343 vector float vec_andc (vector float, vector float);
5344 vector float vec_andc (vector float, vector signed int);
5345 vector float vec_andc (vector signed int, vector float);
5346 vector signed int vec_andc (vector signed int, vector signed int);
5347 vector unsigned int vec_andc (vector signed int, vector unsigned int);
5348 vector unsigned int vec_andc (vector unsigned int, vector signed int);
5349 vector unsigned int vec_andc (vector unsigned int, vector unsigned int);
5351 vector signed short vec_andc (vector signed short, vector signed short);
5353 vector unsigned short vec_andc (vector signed short,
5354 vector unsigned short);
5355 vector unsigned short vec_andc (vector unsigned short,
5356 vector signed short);
5357 vector unsigned short vec_andc (vector unsigned short,
5358 vector unsigned short);
5359 vector signed char vec_andc (vector signed char, vector signed char);
5360 vector unsigned char vec_andc (vector signed char,
5361 vector unsigned char);
5362 vector unsigned char vec_andc (vector unsigned char,
5363 vector signed char);
5364 vector unsigned char vec_andc (vector unsigned char,
5365 vector unsigned char);
5367 vector unsigned char vec_avg (vector unsigned char,
5368 vector unsigned char);
5369 vector signed char vec_avg (vector signed char, vector signed char);
5370 vector unsigned short vec_avg (vector unsigned short,
5371 vector unsigned short);
5372 vector signed short vec_avg (vector signed short, vector signed short);
5373 vector unsigned int vec_avg (vector unsigned int, vector unsigned int);
5374 vector signed int vec_avg (vector signed int, vector signed int);
5376 vector float vec_ceil (vector float);
5378 vector signed int vec_cmpb (vector float, vector float);
5380 vector signed char vec_cmpeq (vector signed char, vector signed char);
5381 vector signed char vec_cmpeq (vector unsigned char,
5382 vector unsigned char);
5383 vector signed short vec_cmpeq (vector signed short,
5384 vector signed short);
5385 vector signed short vec_cmpeq (vector unsigned short,
5386 vector unsigned short);
5387 vector signed int vec_cmpeq (vector signed int, vector signed int);
5388 vector signed int vec_cmpeq (vector unsigned int, vector unsigned int);
5389 vector signed int vec_cmpeq (vector float, vector float);
5391 vector signed int vec_cmpge (vector float, vector float);
5393 vector signed char vec_cmpgt (vector unsigned char,
5394 vector unsigned char);
5395 vector signed char vec_cmpgt (vector signed char, vector signed char);
5396 vector signed short vec_cmpgt (vector unsigned short,
5397 vector unsigned short);
5398 vector signed short vec_cmpgt (vector signed short,
5399 vector signed short);
5400 vector signed int vec_cmpgt (vector unsigned int, vector unsigned int);
5401 vector signed int vec_cmpgt (vector signed int, vector signed int);
5402 vector signed int vec_cmpgt (vector float, vector float);
5404 vector signed int vec_cmple (vector float, vector float);
5406 vector signed char vec_cmplt (vector unsigned char,
5407 vector unsigned char);
5408 vector signed char vec_cmplt (vector signed char, vector signed char);
5409 vector signed short vec_cmplt (vector unsigned short,
5410 vector unsigned short);
5411 vector signed short vec_cmplt (vector signed short,
5412 vector signed short);
5413 vector signed int vec_cmplt (vector unsigned int, vector unsigned int);
5414 vector signed int vec_cmplt (vector signed int, vector signed int);
5415 vector signed int vec_cmplt (vector float, vector float);
5417 vector float vec_ctf (vector unsigned int, const char);
5418 vector float vec_ctf (vector signed int, const char);
5420 vector signed int vec_cts (vector float, const char);
5422 vector unsigned int vec_ctu (vector float, const char);
5424 void vec_dss (const char);
5426 void vec_dssall (void);
5428 void vec_dst (void *, int, const char);
5430 void vec_dstst (void *, int, const char);
5432 void vec_dststt (void *, int, const char);
5434 void vec_dstt (void *, int, const char);
5436 vector float vec_expte (vector float, vector float);
5438 vector float vec_floor (vector float, vector float);
5440 vector float vec_ld (int, vector float *);
5441 vector float vec_ld (int, float *):
5442 vector signed int vec_ld (int, int *);
5443 vector signed int vec_ld (int, vector signed int *);
5444 vector unsigned int vec_ld (int, vector unsigned int *);
5445 vector unsigned int vec_ld (int, unsigned int *);
5446 vector signed short vec_ld (int, short *, vector signed short *);
5447 vector unsigned short vec_ld (int, unsigned short *,
5448 vector unsigned short *);
5449 vector signed char vec_ld (int, signed char *);
5450 vector signed char vec_ld (int, vector signed char *);
5451 vector unsigned char vec_ld (int, unsigned char *);
5452 vector unsigned char vec_ld (int, vector unsigned char *);
5454 vector signed char vec_lde (int, signed char *);
5455 vector unsigned char vec_lde (int, unsigned char *);
5456 vector signed short vec_lde (int, short *);
5457 vector unsigned short vec_lde (int, unsigned short *);
5458 vector float vec_lde (int, float *);
5459 vector signed int vec_lde (int, int *);
5460 vector unsigned int vec_lde (int, unsigned int *);
5462 void float vec_ldl (int, float *);
5463 void float vec_ldl (int, vector float *);
5464 void signed int vec_ldl (int, vector signed int *);
5465 void signed int vec_ldl (int, int *);
5466 void unsigned int vec_ldl (int, unsigned int *);
5467 void unsigned int vec_ldl (int, vector unsigned int *);
5468 void signed short vec_ldl (int, vector signed short *);
5469 void signed short vec_ldl (int, short *);
5470 void unsigned short vec_ldl (int, vector unsigned short *);
5471 void unsigned short vec_ldl (int, unsigned short *);
5472 void signed char vec_ldl (int, vector signed char *);
5473 void signed char vec_ldl (int, signed char *);
5474 void unsigned char vec_ldl (int, vector unsigned char *);
5475 void unsigned char vec_ldl (int, unsigned char *);
5477 vector float vec_loge (vector float);
5479 vector unsigned char vec_lvsl (int, void *, int *);
5481 vector unsigned char vec_lvsr (int, void *, int *);
5483 vector float vec_madd (vector float, vector float, vector float);
5485 vector signed short vec_madds (vector signed short, vector signed short,
5486 vector signed short);
5488 vector unsigned char vec_max (vector signed char, vector unsigned char);
5490 vector unsigned char vec_max (vector unsigned char, vector signed char);
5492 vector unsigned char vec_max (vector unsigned char,
5493 vector unsigned char);
5494 vector signed char vec_max (vector signed char, vector signed char);
5495 vector unsigned short vec_max (vector signed short,
5496 vector unsigned short);
5497 vector unsigned short vec_max (vector unsigned short,
5498 vector signed short);
5499 vector unsigned short vec_max (vector unsigned short,
5500 vector unsigned short);
5501 vector signed short vec_max (vector signed short, vector signed short);
5502 vector unsigned int vec_max (vector signed int, vector unsigned int);
5503 vector unsigned int vec_max (vector unsigned int, vector signed int);
5504 vector unsigned int vec_max (vector unsigned int, vector unsigned int);
5505 vector signed int vec_max (vector signed int, vector signed int);
5506 vector float vec_max (vector float, vector float);
5508 vector signed char vec_mergeh (vector signed char, vector signed char);
5509 vector unsigned char vec_mergeh (vector unsigned char,
5510 vector unsigned char);
5511 vector signed short vec_mergeh (vector signed short,
5512 vector signed short);
5513 vector unsigned short vec_mergeh (vector unsigned short,
5514 vector unsigned short);
5515 vector float vec_mergeh (vector float, vector float);
5516 vector signed int vec_mergeh (vector signed int, vector signed int);
5517 vector unsigned int vec_mergeh (vector unsigned int,
5518 vector unsigned int);
5520 vector signed char vec_mergel (vector signed char, vector signed char);
5521 vector unsigned char vec_mergel (vector unsigned char,
5522 vector unsigned char);
5523 vector signed short vec_mergel (vector signed short,
5524 vector signed short);
5525 vector unsigned short vec_mergel (vector unsigned short,
5526 vector unsigned short);
5527 vector float vec_mergel (vector float, vector float);
5528 vector signed int vec_mergel (vector signed int, vector signed int);
5529 vector unsigned int vec_mergel (vector unsigned int,
5530 vector unsigned int);
5532 vector unsigned short vec_mfvscr (void);
5534 vector unsigned char vec_min (vector signed char, vector unsigned char);
5536 vector unsigned char vec_min (vector unsigned char, vector signed char);
5538 vector unsigned char vec_min (vector unsigned char,
5539 vector unsigned char);
5540 vector signed char vec_min (vector signed char, vector signed char);
5541 vector unsigned short vec_min (vector signed short,
5542 vector unsigned short);
5543 vector unsigned short vec_min (vector unsigned short,
5544 vector signed short);
5545 vector unsigned short vec_min (vector unsigned short,
5546 vector unsigned short);
5547 vector signed short vec_min (vector signed short, vector signed short);
5548 vector unsigned int vec_min (vector signed int, vector unsigned int);
5549 vector unsigned int vec_min (vector unsigned int, vector signed int);
5550 vector unsigned int vec_min (vector unsigned int, vector unsigned int);
5551 vector signed int vec_min (vector signed int, vector signed int);
5552 vector float vec_min (vector float, vector float);
5554 vector signed short vec_mladd (vector signed short, vector signed short,
5555 vector signed short);
5556 vector signed short vec_mladd (vector signed short,
5557 vector unsigned short,
5558 vector unsigned short);
5559 vector signed short vec_mladd (vector unsigned short,
5560 vector signed short,
5561 vector signed short);
5562 vector unsigned short vec_mladd (vector unsigned short,
5563 vector unsigned short,
5564 vector unsigned short);
5566 vector signed short vec_mradds (vector signed short,
5567 vector signed short,
5568 vector signed short);
5570 vector unsigned int vec_msum (vector unsigned char,
5571 vector unsigned char,
5572 vector unsigned int);
5573 vector signed int vec_msum (vector signed char, vector unsigned char,
5575 vector unsigned int vec_msum (vector unsigned short,
5576 vector unsigned short,
5577 vector unsigned int);
5578 vector signed int vec_msum (vector signed short, vector signed short,
5581 vector unsigned int vec_msums (vector unsigned short,
5582 vector unsigned short,
5583 vector unsigned int);
5584 vector signed int vec_msums (vector signed short, vector signed short,
5587 void vec_mtvscr (vector signed int);
5588 void vec_mtvscr (vector unsigned int);
5589 void vec_mtvscr (vector signed short);
5590 void vec_mtvscr (vector unsigned short);
5591 void vec_mtvscr (vector signed char);
5592 void vec_mtvscr (vector unsigned char);
5594 vector unsigned short vec_mule (vector unsigned char,
5595 vector unsigned char);
5596 vector signed short vec_mule (vector signed char, vector signed char);
5597 vector unsigned int vec_mule (vector unsigned short,
5598 vector unsigned short);
5599 vector signed int vec_mule (vector signed short, vector signed short);
5601 vector unsigned short vec_mulo (vector unsigned char,
5602 vector unsigned char);
5603 vector signed short vec_mulo (vector signed char, vector signed char);
5604 vector unsigned int vec_mulo (vector unsigned short,
5605 vector unsigned short);
5606 vector signed int vec_mulo (vector signed short, vector signed short);
5608 vector float vec_nmsub (vector float, vector float, vector float);
5610 vector float vec_nor (vector float, vector float);
5611 vector signed int vec_nor (vector signed int, vector signed int);
5612 vector unsigned int vec_nor (vector unsigned int, vector unsigned int);
5613 vector signed short vec_nor (vector signed short, vector signed short);
5614 vector unsigned short vec_nor (vector unsigned short,
5615 vector unsigned short);
5616 vector signed char vec_nor (vector signed char, vector signed char);
5617 vector unsigned char vec_nor (vector unsigned char,
5618 vector unsigned char);
5620 vector float vec_or (vector float, vector float);
5621 vector float vec_or (vector float, vector signed int);
5622 vector float vec_or (vector signed int, vector float);
5623 vector signed int vec_or (vector signed int, vector signed int);
5624 vector unsigned int vec_or (vector signed int, vector unsigned int);
5625 vector unsigned int vec_or (vector unsigned int, vector signed int);
5626 vector unsigned int vec_or (vector unsigned int, vector unsigned int);
5627 vector signed short vec_or (vector signed short, vector signed short);
5628 vector unsigned short vec_or (vector signed short,
5629 vector unsigned short);
5630 vector unsigned short vec_or (vector unsigned short,
5631 vector signed short);
5632 vector unsigned short vec_or (vector unsigned short,
5633 vector unsigned short);
5634 vector signed char vec_or (vector signed char, vector signed char);
5635 vector unsigned char vec_or (vector signed char, vector unsigned char);
5636 vector unsigned char vec_or (vector unsigned char, vector signed char);
5637 vector unsigned char vec_or (vector unsigned char,
5638 vector unsigned char);
5640 vector signed char vec_pack (vector signed short, vector signed short);
5641 vector unsigned char vec_pack (vector unsigned short,
5642 vector unsigned short);
5643 vector signed short vec_pack (vector signed int, vector signed int);
5644 vector unsigned short vec_pack (vector unsigned int,
5645 vector unsigned int);
5647 vector signed short vec_packpx (vector unsigned int,
5648 vector unsigned int);
5650 vector unsigned char vec_packs (vector unsigned short,
5651 vector unsigned short);
5652 vector signed char vec_packs (vector signed short, vector signed short);
5654 vector unsigned short vec_packs (vector unsigned int,
5655 vector unsigned int);
5656 vector signed short vec_packs (vector signed int, vector signed int);
5658 vector unsigned char vec_packsu (vector unsigned short,
5659 vector unsigned short);
5660 vector unsigned char vec_packsu (vector signed short,
5661 vector signed short);
5662 vector unsigned short vec_packsu (vector unsigned int,
5663 vector unsigned int);
5664 vector unsigned short vec_packsu (vector signed int, vector signed int);
5666 vector float vec_perm (vector float, vector float,
5667 vector unsigned char);
5668 vector signed int vec_perm (vector signed int, vector signed int,
5669 vector unsigned char);
5670 vector unsigned int vec_perm (vector unsigned int, vector unsigned int,
5671 vector unsigned char);
5672 vector signed short vec_perm (vector signed short, vector signed short,
5673 vector unsigned char);
5674 vector unsigned short vec_perm (vector unsigned short,
5675 vector unsigned short,
5676 vector unsigned char);
5677 vector signed char vec_perm (vector signed char, vector signed char,
5678 vector unsigned char);
5679 vector unsigned char vec_perm (vector unsigned char,
5680 vector unsigned char,
5681 vector unsigned char);
5683 vector float vec_re (vector float);
5685 vector signed char vec_rl (vector signed char, vector unsigned char);
5686 vector unsigned char vec_rl (vector unsigned char,
5687 vector unsigned char);
5688 vector signed short vec_rl (vector signed short, vector unsigned short);
5690 vector unsigned short vec_rl (vector unsigned short,
5691 vector unsigned short);
5692 vector signed int vec_rl (vector signed int, vector unsigned int);
5693 vector unsigned int vec_rl (vector unsigned int, vector unsigned int);
5695 vector float vec_round (vector float);
5697 vector float vec_rsqrte (vector float);
5699 vector float vec_sel (vector float, vector float, vector signed int);
5700 vector float vec_sel (vector float, vector float, vector unsigned int);
5701 vector signed int vec_sel (vector signed int, vector signed int,
5703 vector signed int vec_sel (vector signed int, vector signed int,
5704 vector unsigned int);
5705 vector unsigned int vec_sel (vector unsigned int, vector unsigned int,
5707 vector unsigned int vec_sel (vector unsigned int, vector unsigned int,
5708 vector unsigned int);
5709 vector signed short vec_sel (vector signed short, vector signed short,
5710 vector signed short);
5711 vector signed short vec_sel (vector signed short, vector signed short,
5712 vector unsigned short);
5713 vector unsigned short vec_sel (vector unsigned short,
5714 vector unsigned short,
5715 vector signed short);
5716 vector unsigned short vec_sel (vector unsigned short,
5717 vector unsigned short,
5718 vector unsigned short);
5719 vector signed char vec_sel (vector signed char, vector signed char,
5720 vector signed char);
5721 vector signed char vec_sel (vector signed char, vector signed char,
5722 vector unsigned char);
5723 vector unsigned char vec_sel (vector unsigned char,
5724 vector unsigned char,
5725 vector signed char);
5726 vector unsigned char vec_sel (vector unsigned char,
5727 vector unsigned char,
5728 vector unsigned char);
5730 vector signed char vec_sl (vector signed char, vector unsigned char);
5731 vector unsigned char vec_sl (vector unsigned char,
5732 vector unsigned char);
5733 vector signed short vec_sl (vector signed short, vector unsigned short);
5735 vector unsigned short vec_sl (vector unsigned short,
5736 vector unsigned short);
5737 vector signed int vec_sl (vector signed int, vector unsigned int);
5738 vector unsigned int vec_sl (vector unsigned int, vector unsigned int);
5740 vector float vec_sld (vector float, vector float, const char);
5741 vector signed int vec_sld (vector signed int, vector signed int,
5743 vector unsigned int vec_sld (vector unsigned int, vector unsigned int,
5745 vector signed short vec_sld (vector signed short, vector signed short,
5747 vector unsigned short vec_sld (vector unsigned short,
5748 vector unsigned short, const char);
5749 vector signed char vec_sld (vector signed char, vector signed char,
5751 vector unsigned char vec_sld (vector unsigned char,
5752 vector unsigned char,
5755 vector signed int vec_sll (vector signed int, vector unsigned int);
5756 vector signed int vec_sll (vector signed int, vector unsigned short);
5757 vector signed int vec_sll (vector signed int, vector unsigned char);
5758 vector unsigned int vec_sll (vector unsigned int, vector unsigned int);
5759 vector unsigned int vec_sll (vector unsigned int,
5760 vector unsigned short);
5761 vector unsigned int vec_sll (vector unsigned int, vector unsigned char);
5763 vector signed short vec_sll (vector signed short, vector unsigned int);
5764 vector signed short vec_sll (vector signed short,
5765 vector unsigned short);
5766 vector signed short vec_sll (vector signed short, vector unsigned char);
5768 vector unsigned short vec_sll (vector unsigned short,
5769 vector unsigned int);
5770 vector unsigned short vec_sll (vector unsigned short,
5771 vector unsigned short);
5772 vector unsigned short vec_sll (vector unsigned short,
5773 vector unsigned char);
5774 vector signed char vec_sll (vector signed char, vector unsigned int);
5775 vector signed char vec_sll (vector signed char, vector unsigned short);
5776 vector signed char vec_sll (vector signed char, vector unsigned char);
5777 vector unsigned char vec_sll (vector unsigned char,
5778 vector unsigned int);
5779 vector unsigned char vec_sll (vector unsigned char,
5780 vector unsigned short);
5781 vector unsigned char vec_sll (vector unsigned char,
5782 vector unsigned char);
5784 vector float vec_slo (vector float, vector signed char);
5785 vector float vec_slo (vector float, vector unsigned char);
5786 vector signed int vec_slo (vector signed int, vector signed char);
5787 vector signed int vec_slo (vector signed int, vector unsigned char);
5788 vector unsigned int vec_slo (vector unsigned int, vector signed char);
5789 vector unsigned int vec_slo (vector unsigned int, vector unsigned char);
5791 vector signed short vec_slo (vector signed short, vector signed char);
5792 vector signed short vec_slo (vector signed short, vector unsigned char);
5794 vector unsigned short vec_slo (vector unsigned short,
5795 vector signed char);
5796 vector unsigned short vec_slo (vector unsigned short,
5797 vector unsigned char);
5798 vector signed char vec_slo (vector signed char, vector signed char);
5799 vector signed char vec_slo (vector signed char, vector unsigned char);
5800 vector unsigned char vec_slo (vector unsigned char, vector signed char);
5802 vector unsigned char vec_slo (vector unsigned char,
5803 vector unsigned char);
5805 vector signed char vec_splat (vector signed char, const char);
5806 vector unsigned char vec_splat (vector unsigned char, const char);
5807 vector signed short vec_splat (vector signed short, const char);
5808 vector unsigned short vec_splat (vector unsigned short, const char);
5809 vector float vec_splat (vector float, const char);
5810 vector signed int vec_splat (vector signed int, const char);
5811 vector unsigned int vec_splat (vector unsigned int, const char);
5813 vector signed char vec_splat_s8 (const char);
5815 vector signed short vec_splat_s16 (const char);
5817 vector signed int vec_splat_s32 (const char);
5819 vector unsigned char vec_splat_u8 (const char);
5821 vector unsigned short vec_splat_u16 (const char);
5823 vector unsigned int vec_splat_u32 (const char);
5825 vector signed char vec_sr (vector signed char, vector unsigned char);
5826 vector unsigned char vec_sr (vector unsigned char,
5827 vector unsigned char);
5828 vector signed short vec_sr (vector signed short, vector unsigned short);
5830 vector unsigned short vec_sr (vector unsigned short,
5831 vector unsigned short);
5832 vector signed int vec_sr (vector signed int, vector unsigned int);
5833 vector unsigned int vec_sr (vector unsigned int, vector unsigned int);
5835 vector signed char vec_sra (vector signed char, vector unsigned char);
5836 vector unsigned char vec_sra (vector unsigned char,
5837 vector unsigned char);
5838 vector signed short vec_sra (vector signed short,
5839 vector unsigned short);
5840 vector unsigned short vec_sra (vector unsigned short,
5841 vector unsigned short);
5842 vector signed int vec_sra (vector signed int, vector unsigned int);
5843 vector unsigned int vec_sra (vector unsigned int, vector unsigned int);
5845 vector signed int vec_srl (vector signed int, vector unsigned int);
5846 vector signed int vec_srl (vector signed int, vector unsigned short);
5847 vector signed int vec_srl (vector signed int, vector unsigned char);
5848 vector unsigned int vec_srl (vector unsigned int, vector unsigned int);
5849 vector unsigned int vec_srl (vector unsigned int,
5850 vector unsigned short);
5851 vector unsigned int vec_srl (vector unsigned int, vector unsigned char);
5853 vector signed short vec_srl (vector signed short, vector unsigned int);
5854 vector signed short vec_srl (vector signed short,
5855 vector unsigned short);
5856 vector signed short vec_srl (vector signed short, vector unsigned char);
5858 vector unsigned short vec_srl (vector unsigned short,
5859 vector unsigned int);
5860 vector unsigned short vec_srl (vector unsigned short,
5861 vector unsigned short);
5862 vector unsigned short vec_srl (vector unsigned short,
5863 vector unsigned char);
5864 vector signed char vec_srl (vector signed char, vector unsigned int);
5865 vector signed char vec_srl (vector signed char, vector unsigned short);
5866 vector signed char vec_srl (vector signed char, vector unsigned char);
5867 vector unsigned char vec_srl (vector unsigned char,
5868 vector unsigned int);
5869 vector unsigned char vec_srl (vector unsigned char,
5870 vector unsigned short);
5871 vector unsigned char vec_srl (vector unsigned char,
5872 vector unsigned char);
5874 vector float vec_sro (vector float, vector signed char);
5875 vector float vec_sro (vector float, vector unsigned char);
5876 vector signed int vec_sro (vector signed int, vector signed char);
5877 vector signed int vec_sro (vector signed int, vector unsigned char);
5878 vector unsigned int vec_sro (vector unsigned int, vector signed char);
5879 vector unsigned int vec_sro (vector unsigned int, vector unsigned char);
5881 vector signed short vec_sro (vector signed short, vector signed char);
5882 vector signed short vec_sro (vector signed short, vector unsigned char);
5884 vector unsigned short vec_sro (vector unsigned short,
5885 vector signed char);
5886 vector unsigned short vec_sro (vector unsigned short,
5887 vector unsigned char);
5888 vector signed char vec_sro (vector signed char, vector signed char);
5889 vector signed char vec_sro (vector signed char, vector unsigned char);
5890 vector unsigned char vec_sro (vector unsigned char, vector signed char);
5892 vector unsigned char vec_sro (vector unsigned char,
5893 vector unsigned char);
5895 void vec_st (vector float, int, float *);
5896 void vec_st (vector float, int, vector float *);
5897 void vec_st (vector signed int, int, int *);
5898 void vec_st (vector signed int, int, unsigned int *);
5899 void vec_st (vector unsigned int, int, unsigned int *);
5900 void vec_st (vector unsigned int, int, vector unsigned int *);
5901 void vec_st (vector signed short, int, short *);
5902 void vec_st (vector signed short, int, vector unsigned short *);
5903 void vec_st (vector signed short, int, vector signed short *);
5904 void vec_st (vector unsigned short, int, unsigned short *);
5905 void vec_st (vector unsigned short, int, vector unsigned short *);
5906 void vec_st (vector signed char, int, signed char *);
5907 void vec_st (vector signed char, int, unsigned char *);
5908 void vec_st (vector signed char, int, vector signed char *);
5909 void vec_st (vector unsigned char, int, unsigned char *);
5910 void vec_st (vector unsigned char, int, vector unsigned char *);
5912 void vec_ste (vector signed char, int, unsigned char *);
5913 void vec_ste (vector signed char, int, signed char *);
5914 void vec_ste (vector unsigned char, int, unsigned char *);
5915 void vec_ste (vector signed short, int, short *);
5916 void vec_ste (vector signed short, int, unsigned short *);
5917 void vec_ste (vector unsigned short, int, void *);
5918 void vec_ste (vector signed int, int, unsigned int *);
5919 void vec_ste (vector signed int, int, int *);
5920 void vec_ste (vector unsigned int, int, unsigned int *);
5921 void vec_ste (vector float, int, float *);
5923 void vec_stl (vector float, int, vector float *);
5924 void vec_stl (vector float, int, float *);
5925 void vec_stl (vector signed int, int, vector signed int *);
5926 void vec_stl (vector signed int, int, int *);
5927 void vec_stl (vector signed int, int, unsigned int *);
5928 void vec_stl (vector unsigned int, int, vector unsigned int *);
5929 void vec_stl (vector unsigned int, int, unsigned int *);
5930 void vec_stl (vector signed short, int, short *);
5931 void vec_stl (vector signed short, int, unsigned short *);
5932 void vec_stl (vector signed short, int, vector signed short *);
5933 void vec_stl (vector unsigned short, int, unsigned short *);
5934 void vec_stl (vector unsigned short, int, vector signed short *);
5935 void vec_stl (vector signed char, int, signed char *);
5936 void vec_stl (vector signed char, int, unsigned char *);
5937 void vec_stl (vector signed char, int, vector signed char *);
5938 void vec_stl (vector unsigned char, int, unsigned char *);
5939 void vec_stl (vector unsigned char, int, vector unsigned char *);
5941 vector signed char vec_sub (vector signed char, vector signed char);
5942 vector unsigned char vec_sub (vector signed char, vector unsigned char);
5944 vector unsigned char vec_sub (vector unsigned char, vector signed char);
5946 vector unsigned char vec_sub (vector unsigned char,
5947 vector unsigned char);
5948 vector signed short vec_sub (vector signed short, vector signed short);
5949 vector unsigned short vec_sub (vector signed short,
5950 vector unsigned short);
5951 vector unsigned short vec_sub (vector unsigned short,
5952 vector signed short);
5953 vector unsigned short vec_sub (vector unsigned short,
5954 vector unsigned short);
5955 vector signed int vec_sub (vector signed int, vector signed int);
5956 vector unsigned int vec_sub (vector signed int, vector unsigned int);
5957 vector unsigned int vec_sub (vector unsigned int, vector signed int);
5958 vector unsigned int vec_sub (vector unsigned int, vector unsigned int);
5959 vector float vec_sub (vector float, vector float);
5961 vector unsigned int vec_subc (vector unsigned int, vector unsigned int);
5963 vector unsigned char vec_subs (vector signed char,
5964 vector unsigned char);
5965 vector unsigned char vec_subs (vector unsigned char,
5966 vector signed char);
5967 vector unsigned char vec_subs (vector unsigned char,
5968 vector unsigned char);
5969 vector signed char vec_subs (vector signed char, vector signed char);
5970 vector unsigned short vec_subs (vector signed short,
5971 vector unsigned short);
5972 vector unsigned short vec_subs (vector unsigned short,
5973 vector signed short);
5974 vector unsigned short vec_subs (vector unsigned short,
5975 vector unsigned short);
5976 vector signed short vec_subs (vector signed short, vector signed short);
5978 vector unsigned int vec_subs (vector signed int, vector unsigned int);
5979 vector unsigned int vec_subs (vector unsigned int, vector signed int);
5980 vector unsigned int vec_subs (vector unsigned int, vector unsigned int);
5982 vector signed int vec_subs (vector signed int, vector signed int);
5984 vector unsigned int vec_sum4s (vector unsigned char,
5985 vector unsigned int);
5986 vector signed int vec_sum4s (vector signed char, vector signed int);
5987 vector signed int vec_sum4s (vector signed short, vector signed int);
5989 vector signed int vec_sum2s (vector signed int, vector signed int);
5991 vector signed int vec_sums (vector signed int, vector signed int);
5993 vector float vec_trunc (vector float);
5995 vector signed short vec_unpackh (vector signed char);
5996 vector unsigned int vec_unpackh (vector signed short);
5997 vector signed int vec_unpackh (vector signed short);
5999 vector signed short vec_unpackl (vector signed char);
6000 vector unsigned int vec_unpackl (vector signed short);
6001 vector signed int vec_unpackl (vector signed short);
6003 vector float vec_xor (vector float, vector float);
6004 vector float vec_xor (vector float, vector signed int);
6005 vector float vec_xor (vector signed int, vector float);
6006 vector signed int vec_xor (vector signed int, vector signed int);
6007 vector unsigned int vec_xor (vector signed int, vector unsigned int);
6008 vector unsigned int vec_xor (vector unsigned int, vector signed int);
6009 vector unsigned int vec_xor (vector unsigned int, vector unsigned int);
6010 vector signed short vec_xor (vector signed short, vector signed short);
6011 vector unsigned short vec_xor (vector signed short,
6012 vector unsigned short);
6013 vector unsigned short vec_xor (vector unsigned short,
6014 vector signed short);
6015 vector unsigned short vec_xor (vector unsigned short,
6016 vector unsigned short);
6017 vector signed char vec_xor (vector signed char, vector signed char);
6018 vector unsigned char vec_xor (vector signed char, vector unsigned char);
6020 vector unsigned char vec_xor (vector unsigned char, vector signed char);
6022 vector unsigned char vec_xor (vector unsigned char,
6023 vector unsigned char);
6025 vector signed int vec_all_eq (vector signed char, vector unsigned char);
6027 vector signed int vec_all_eq (vector signed char, vector signed char);
6028 vector signed int vec_all_eq (vector unsigned char, vector signed char);
6030 vector signed int vec_all_eq (vector unsigned char,
6031 vector unsigned char);
6032 vector signed int vec_all_eq (vector signed short,
6033 vector unsigned short);
6034 vector signed int vec_all_eq (vector signed short, vector signed short);
6036 vector signed int vec_all_eq (vector unsigned short,
6037 vector signed short);
6038 vector signed int vec_all_eq (vector unsigned short,
6039 vector unsigned short);
6040 vector signed int vec_all_eq (vector signed int, vector unsigned int);
6041 vector signed int vec_all_eq (vector signed int, vector signed int);
6042 vector signed int vec_all_eq (vector unsigned int, vector signed int);
6043 vector signed int vec_all_eq (vector unsigned int, vector unsigned int);
6045 vector signed int vec_all_eq (vector float, vector float);
6047 vector signed int vec_all_ge (vector signed char, vector unsigned char);
6049 vector signed int vec_all_ge (vector unsigned char, vector signed char);
6051 vector signed int vec_all_ge (vector unsigned char,
6052 vector unsigned char);
6053 vector signed int vec_all_ge (vector signed char, vector signed char);
6054 vector signed int vec_all_ge (vector signed short,
6055 vector unsigned short);
6056 vector signed int vec_all_ge (vector unsigned short,
6057 vector signed short);
6058 vector signed int vec_all_ge (vector unsigned short,
6059 vector unsigned short);
6060 vector signed int vec_all_ge (vector signed short, vector signed short);
6062 vector signed int vec_all_ge (vector signed int, vector unsigned int);
6063 vector signed int vec_all_ge (vector unsigned int, vector signed int);
6064 vector signed int vec_all_ge (vector unsigned int, vector unsigned int);
6066 vector signed int vec_all_ge (vector signed int, vector signed int);
6067 vector signed int vec_all_ge (vector float, vector float);
6069 vector signed int vec_all_gt (vector signed char, vector unsigned char);
6071 vector signed int vec_all_gt (vector unsigned char, vector signed char);
6073 vector signed int vec_all_gt (vector unsigned char,
6074 vector unsigned char);
6075 vector signed int vec_all_gt (vector signed char, vector signed char);
6076 vector signed int vec_all_gt (vector signed short,
6077 vector unsigned short);
6078 vector signed int vec_all_gt (vector unsigned short,
6079 vector signed short);
6080 vector signed int vec_all_gt (vector unsigned short,
6081 vector unsigned short);
6082 vector signed int vec_all_gt (vector signed short, vector signed short);
6084 vector signed int vec_all_gt (vector signed int, vector unsigned int);
6085 vector signed int vec_all_gt (vector unsigned int, vector signed int);
6086 vector signed int vec_all_gt (vector unsigned int, vector unsigned int);
6088 vector signed int vec_all_gt (vector signed int, vector signed int);
6089 vector signed int vec_all_gt (vector float, vector float);
6091 vector signed int vec_all_in (vector float, vector float);
6093 vector signed int vec_all_le (vector signed char, vector unsigned char);
6095 vector signed int vec_all_le (vector unsigned char, vector signed char);
6097 vector signed int vec_all_le (vector unsigned char,
6098 vector unsigned char);
6099 vector signed int vec_all_le (vector signed char, vector signed char);
6100 vector signed int vec_all_le (vector signed short,
6101 vector unsigned short);
6102 vector signed int vec_all_le (vector unsigned short,
6103 vector signed short);
6104 vector signed int vec_all_le (vector unsigned short,
6105 vector unsigned short);
6106 vector signed int vec_all_le (vector signed short, vector signed short);
6108 vector signed int vec_all_le (vector signed int, vector unsigned int);
6109 vector signed int vec_all_le (vector unsigned int, vector signed int);
6110 vector signed int vec_all_le (vector unsigned int, vector unsigned int);
6112 vector signed int vec_all_le (vector signed int, vector signed int);
6113 vector signed int vec_all_le (vector float, vector float);
6115 vector signed int vec_all_lt (vector signed char, vector unsigned char);
6117 vector signed int vec_all_lt (vector unsigned char, vector signed char);
6119 vector signed int vec_all_lt (vector unsigned char,
6120 vector unsigned char);
6121 vector signed int vec_all_lt (vector signed char, vector signed char);
6122 vector signed int vec_all_lt (vector signed short,
6123 vector unsigned short);
6124 vector signed int vec_all_lt (vector unsigned short,
6125 vector signed short);
6126 vector signed int vec_all_lt (vector unsigned short,
6127 vector unsigned short);
6128 vector signed int vec_all_lt (vector signed short, vector signed short);
6130 vector signed int vec_all_lt (vector signed int, vector unsigned int);
6131 vector signed int vec_all_lt (vector unsigned int, vector signed int);
6132 vector signed int vec_all_lt (vector unsigned int, vector unsigned int);
6134 vector signed int vec_all_lt (vector signed int, vector signed int);
6135 vector signed int vec_all_lt (vector float, vector float);
6137 vector signed int vec_all_nan (vector float);
6139 vector signed int vec_all_ne (vector signed char, vector unsigned char);
6141 vector signed int vec_all_ne (vector signed char, vector signed char);
6142 vector signed int vec_all_ne (vector unsigned char, vector signed char);
6144 vector signed int vec_all_ne (vector unsigned char,
6145 vector unsigned char);
6146 vector signed int vec_all_ne (vector signed short,
6147 vector unsigned short);
6148 vector signed int vec_all_ne (vector signed short, vector signed short);
6150 vector signed int vec_all_ne (vector unsigned short,
6151 vector signed short);
6152 vector signed int vec_all_ne (vector unsigned short,
6153 vector unsigned short);
6154 vector signed int vec_all_ne (vector signed int, vector unsigned int);
6155 vector signed int vec_all_ne (vector signed int, vector signed int);
6156 vector signed int vec_all_ne (vector unsigned int, vector signed int);
6157 vector signed int vec_all_ne (vector unsigned int, vector unsigned int);
6159 vector signed int vec_all_ne (vector float, vector float);
6161 vector signed int vec_all_nge (vector float, vector float);
6163 vector signed int vec_all_ngt (vector float, vector float);
6165 vector signed int vec_all_nle (vector float, vector float);
6167 vector signed int vec_all_nlt (vector float, vector float);
6169 vector signed int vec_all_numeric (vector float);
6171 vector signed int vec_any_eq (vector signed char, vector unsigned char);
6173 vector signed int vec_any_eq (vector signed char, vector signed char);
6174 vector signed int vec_any_eq (vector unsigned char, vector signed char);
6176 vector signed int vec_any_eq (vector unsigned char,
6177 vector unsigned char);
6178 vector signed int vec_any_eq (vector signed short,
6179 vector unsigned short);
6180 vector signed int vec_any_eq (vector signed short, vector signed short);
6182 vector signed int vec_any_eq (vector unsigned short,
6183 vector signed short);
6184 vector signed int vec_any_eq (vector unsigned short,
6185 vector unsigned short);
6186 vector signed int vec_any_eq (vector signed int, vector unsigned int);
6187 vector signed int vec_any_eq (vector signed int, vector signed int);
6188 vector signed int vec_any_eq (vector unsigned int, vector signed int);
6189 vector signed int vec_any_eq (vector unsigned int, vector unsigned int);
6191 vector signed int vec_any_eq (vector float, vector float);
6193 vector signed int vec_any_ge (vector signed char, vector unsigned char);
6195 vector signed int vec_any_ge (vector unsigned char, vector signed char);
6197 vector signed int vec_any_ge (vector unsigned char,
6198 vector unsigned char);
6199 vector signed int vec_any_ge (vector signed char, vector signed char);
6200 vector signed int vec_any_ge (vector signed short,
6201 vector unsigned short);
6202 vector signed int vec_any_ge (vector unsigned short,
6203 vector signed short);
6204 vector signed int vec_any_ge (vector unsigned short,
6205 vector unsigned short);
6206 vector signed int vec_any_ge (vector signed short, vector signed short);
6208 vector signed int vec_any_ge (vector signed int, vector unsigned int);
6209 vector signed int vec_any_ge (vector unsigned int, vector signed int);
6210 vector signed int vec_any_ge (vector unsigned int, vector unsigned int);
6212 vector signed int vec_any_ge (vector signed int, vector signed int);
6213 vector signed int vec_any_ge (vector float, vector float);
6215 vector signed int vec_any_gt (vector signed char, vector unsigned char);
6217 vector signed int vec_any_gt (vector unsigned char, vector signed char);
6219 vector signed int vec_any_gt (vector unsigned char,
6220 vector unsigned char);
6221 vector signed int vec_any_gt (vector signed char, vector signed char);
6222 vector signed int vec_any_gt (vector signed short,
6223 vector unsigned short);
6224 vector signed int vec_any_gt (vector unsigned short,
6225 vector signed short);
6226 vector signed int vec_any_gt (vector unsigned short,
6227 vector unsigned short);
6228 vector signed int vec_any_gt (vector signed short, vector signed short);
6230 vector signed int vec_any_gt (vector signed int, vector unsigned int);
6231 vector signed int vec_any_gt (vector unsigned int, vector signed int);
6232 vector signed int vec_any_gt (vector unsigned int, vector unsigned int);
6234 vector signed int vec_any_gt (vector signed int, vector signed int);
6235 vector signed int vec_any_gt (vector float, vector float);
6237 vector signed int vec_any_le (vector signed char, vector unsigned char);
6239 vector signed int vec_any_le (vector unsigned char, vector signed char);
6241 vector signed int vec_any_le (vector unsigned char,
6242 vector unsigned char);
6243 vector signed int vec_any_le (vector signed char, vector signed char);
6244 vector signed int vec_any_le (vector signed short,
6245 vector unsigned short);
6246 vector signed int vec_any_le (vector unsigned short,
6247 vector signed short);
6248 vector signed int vec_any_le (vector unsigned short,
6249 vector unsigned short);
6250 vector signed int vec_any_le (vector signed short, vector signed short);
6252 vector signed int vec_any_le (vector signed int, vector unsigned int);
6253 vector signed int vec_any_le (vector unsigned int, vector signed int);
6254 vector signed int vec_any_le (vector unsigned int, vector unsigned int);
6256 vector signed int vec_any_le (vector signed int, vector signed int);
6257 vector signed int vec_any_le (vector float, vector float);
6259 vector signed int vec_any_lt (vector signed char, vector unsigned char);
6261 vector signed int vec_any_lt (vector unsigned char, vector signed char);
6263 vector signed int vec_any_lt (vector unsigned char,
6264 vector unsigned char);
6265 vector signed int vec_any_lt (vector signed char, vector signed char);
6266 vector signed int vec_any_lt (vector signed short,
6267 vector unsigned short);
6268 vector signed int vec_any_lt (vector unsigned short,
6269 vector signed short);
6270 vector signed int vec_any_lt (vector unsigned short,
6271 vector unsigned short);
6272 vector signed int vec_any_lt (vector signed short, vector signed short);
6274 vector signed int vec_any_lt (vector signed int, vector unsigned int);
6275 vector signed int vec_any_lt (vector unsigned int, vector signed int);
6276 vector signed int vec_any_lt (vector unsigned int, vector unsigned int);
6278 vector signed int vec_any_lt (vector signed int, vector signed int);
6279 vector signed int vec_any_lt (vector float, vector float);
6281 vector signed int vec_any_nan (vector float);
6283 vector signed int vec_any_ne (vector signed char, vector unsigned char);
6285 vector signed int vec_any_ne (vector signed char, vector signed char);
6286 vector signed int vec_any_ne (vector unsigned char, vector signed char);
6288 vector signed int vec_any_ne (vector unsigned char,
6289 vector unsigned char);
6290 vector signed int vec_any_ne (vector signed short,
6291 vector unsigned short);
6292 vector signed int vec_any_ne (vector signed short, vector signed short);
6294 vector signed int vec_any_ne (vector unsigned short,
6295 vector signed short);
6296 vector signed int vec_any_ne (vector unsigned short,
6297 vector unsigned short);
6298 vector signed int vec_any_ne (vector signed int, vector unsigned int);
6299 vector signed int vec_any_ne (vector signed int, vector signed int);
6300 vector signed int vec_any_ne (vector unsigned int, vector signed int);
6301 vector signed int vec_any_ne (vector unsigned int, vector unsigned int);
6303 vector signed int vec_any_ne (vector float, vector float);
6305 vector signed int vec_any_nge (vector float, vector float);
6307 vector signed int vec_any_ngt (vector float, vector float);
6309 vector signed int vec_any_nle (vector float, vector float);
6311 vector signed int vec_any_nlt (vector float, vector float);
6313 vector signed int vec_any_numeric (vector float);
6315 vector signed int vec_any_out (vector float, vector float);
6319 @section Pragmas Accepted by GCC
6323 GCC supports several types of pragmas, primarily in order to compile
6324 code originally written for other compilers. Note that in general
6325 we do not recommend the use of pragmas; @xref{Function Attributes},
6326 for further explanation.
6330 * RS/6000 and PowerPC Pragmas::
6337 @subsection ARM Pragmas
6339 The ARM target defines pragmas for controlling the default addition of
6340 @code{long_call} and @code{short_call} attributes to functions.
6341 @xref{Function Attributes}, for information about the effects of these
6346 @cindex pragma, long_calls
6347 Set all subsequent functions to have the @code{long_call} attribute.
6350 @cindex pragma, no_long_calls
6351 Set all subsequent functions to have the @code{short_call} attribute.
6353 @item long_calls_off
6354 @cindex pragma, long_calls_off
6355 Do not affect the @code{long_call} or @code{short_call} attributes of
6356 subsequent functions.
6359 @node RS/6000 and PowerPC Pragmas
6360 @subsection RS/6000 and PowerPC Pragmas
6362 The RS/6000 and PowerPC targets define one pragma for controlling
6363 whether or not the @code{longcall} attribute is added to function
6364 declarations by default. This pragma overrides the @option{-mlongcall}
6365 option, but not the @code{longcall} and @code{shortcall} attributes.
6366 @xref{RS/6000 and PowerPC Options}, for more information about when long
6367 calls are and are not necessary.
6371 @cindex pragma, longcall
6372 Apply the @code{longcall} attribute to all subsequent function
6376 Do not apply the @code{longcall} attribute to subsequent function
6380 @c Describe c4x pragmas here.
6381 @c Describe h8300 pragmas here.
6382 @c Describe i370 pragmas here.
6383 @c Describe i960 pragmas here.
6384 @c Describe sh pragmas here.
6385 @c Describe v850 pragmas here.
6387 @node Darwin Pragmas
6388 @subsection Darwin Pragmas
6390 The following pragmas are available for all architectures running the
6391 Darwin operating system. These are useful for compatibility with other
6395 @item mark @var{tokens}@dots{}
6396 @cindex pragma, mark
6397 This pragma is accepted, but has no effect.
6399 @item options align=@var{alignment}
6400 @cindex pragma, options align
6401 This pragma sets the alignment of fields in structures. The values of
6402 @var{alignment} may be @code{mac68k}, to emulate m68k alignment, or
6403 @code{power}, to emulate PowerPC alignment. Uses of this pragma nest
6404 properly; to restore the previous setting, use @code{reset} for the
6407 @item segment @var{tokens}@dots{}
6408 @cindex pragma, segment
6409 This pragma is accepted, but has no effect.
6411 @item unused (@var{var} [, @var{var}]@dots{})
6412 @cindex pragma, unused
6413 This pragma declares variables to be possibly unused. GCC will not
6414 produce warnings for the listed variables. The effect is similar to
6415 that of the @code{unused} attribute, except that this pragma may appear
6416 anywhere within the variables' scopes.
6419 @node Solaris Pragmas
6420 @subsection Solaris Pragmas
6422 For compatibility with the SunPRO compiler, the following pragma
6426 @item redefine_extname @var{oldname} @var{newname}
6427 @cindex pragma, redefine_extname
6429 This pragma gives the C function @var{oldname} the assembler label
6430 @var{newname}. The pragma must appear before the function declaration.
6431 This pragma is equivalent to the asm labels extension (@pxref{Asm
6432 Labels}). The preprocessor defines @code{__PRAGMA_REDEFINE_EXTNAME}
6433 if the pragma is available.
6437 @subsection Tru64 Pragmas
6439 For compatibility with the Compaq C compiler, the following pragma
6443 @item extern_prefix @var{string}
6444 @cindex pragma, extern_prefix
6446 This pragma renames all subsequent function and variable declarations
6447 such that @var{string} is prepended to the name. This effect may be
6448 terminated by using another @code{extern_prefix} pragma with the
6451 This pragma is similar in intent to to the asm labels extension
6452 (@pxref{Asm Labels}) in that the system programmer wants to change
6453 the assembly-level ABI without changing the source-level API. The
6454 preprocessor defines @code{__EXTERN_PREFIX} if the pragma is available.
6457 @node Unnamed Fields
6458 @section Unnamed struct/union fields within structs/unions.
6462 For compatibility with other compilers, GCC allows you to define
6463 a structure or union that contains, as fields, structures and unions
6464 without names. For example:
6477 In this example, the user would be able to access members of the unnamed
6478 union with code like @samp{foo.b}. Note that only unnamed structs and
6479 unions are allowed, you may not have, for example, an unnamed
6482 You must never create such structures that cause ambiguous field definitions.
6483 For example, this structure:
6494 It is ambiguous which @code{a} is being referred to with @samp{foo.a}.
6495 Such constructs are not supported and must be avoided. In the future,
6496 such constructs may be detected and treated as compilation errors.
6499 @section Thread-Local Storage
6500 @cindex Thread-Local Storage
6501 @cindex @acronym{TLS}
6504 Thread-local storage (@acronym{TLS}) is a mechanism by which variables
6505 are allocated such that there is one instance of the variable per extant
6506 thread. The run-time model GCC uses to implement this originates
6507 in the IA-64 processor-specific ABI, but has since been migrated
6508 to other processors as well. It requires significant support from
6509 the linker (@command{ld}), dynamic linker (@command{ld.so}), and
6510 system libraries (@file{libc.so} and @file{libpthread.so}), so it
6511 is not available everywhere.
6513 At the user level, the extension is visible with a new storage
6514 class keyword: @code{__thread}. For example:
6518 extern __thread struct state s;
6519 static __thread char *p;
6522 The @code{__thread} specifier may be used alone, with the @code{extern}
6523 or @code{static} specifiers, but with no other storage class specifier.
6524 When used with @code{extern} or @code{static}, @code{__thread} must appear
6525 immediately after the other storage class specifier.
6527 The @code{__thread} specifier may be applied to any global, file-scoped
6528 static, function-scoped static, or static data member of a class. It may
6529 not be applied to block-scoped automatic or non-static data member.
6531 When the address-of operator is applied to a thread-local variable, it is
6532 evaluated at run-time and returns the address of the current thread's
6533 instance of that variable. An address so obtained may be used by any
6534 thread. When a thread terminates, any pointers to thread-local variables
6535 in that thread become invalid.
6537 No static initialization may refer to the address of a thread-local variable.
6539 In C++, if an initializer is present for a thread-local variable, it must
6540 be a @var{constant-expression}, as defined in 5.19.2 of the ANSI/ISO C++
6543 See @uref{http://people.redhat.com/drepper/tls.pdf,
6544 ELF Handling For Thread-Local Storage} for a detailed explanation of
6545 the four thread-local storage addressing models, and how the run-time
6546 is expected to function.
6549 * C99 Thread-Local Edits::
6550 * C++98 Thread-Local Edits::
6553 @node C99 Thread-Local Edits
6554 @subsection ISO/IEC 9899:1999 Edits for Thread-Local Storage
6556 The following are a set of changes to ISO/IEC 9899:1999 (aka C99)
6557 that document the exact semantics of the language extension.
6561 @cite{5.1.2 Execution environments}
6563 Add new text after paragraph 1
6566 Within either execution environment, a @dfn{thread} is a flow of
6567 control within a program. It is implementation defined whether
6568 or not there may be more than one thread associated with a program.
6569 It is implementation defined how threads beyond the first are
6570 created, the name and type of the function called at thread
6571 startup, and how threads may be terminated. However, objects
6572 with thread storage duration shall be initialized before thread
6577 @cite{6.2.4 Storage durations of objects}
6579 Add new text before paragraph 3
6582 An object whose identifier is declared with the storage-class
6583 specifier @w{@code{__thread}} has @dfn{thread storage duration}.
6584 Its lifetime is the entire execution of the thread, and its
6585 stored value is initialized only once, prior to thread startup.
6589 @cite{6.4.1 Keywords}
6591 Add @code{__thread}.
6594 @cite{6.7.1 Storage-class specifiers}
6596 Add @code{__thread} to the list of storage class specifiers in
6599 Change paragraph 2 to
6602 With the exception of @code{__thread}, at most one storage-class
6603 specifier may be given [@dots{}]. The @code{__thread} specifier may
6604 be used alone, or immediately following @code{extern} or
6608 Add new text after paragraph 6
6611 The declaration of an identifier for a variable that has
6612 block scope that specifies @code{__thread} shall also
6613 specify either @code{extern} or @code{static}.
6615 The @code{__thread} specifier shall be used only with
6620 @node C++98 Thread-Local Edits
6621 @subsection ISO/IEC 14882:1998 Edits for Thread-Local Storage
6623 The following are a set of changes to ISO/IEC 14882:1998 (aka C++98)
6624 that document the exact semantics of the language extension.
6627 @b{[intro.execution]}
6629 New text after paragraph 4
6632 A @dfn{thread} is a flow of control within the abstract machine.
6633 It is implementation defined whether or not there may be more than
6637 New text after paragraph 7
6640 It is unspecified whether additional action must be taken to
6641 ensure when and whether side effects are visible to other threads.
6647 Add @code{__thread}.
6650 @b{[basic.start.main]}
6652 Add after paragraph 5
6655 The thread that begins execution at the @code{main} function is called
6656 the @dfn{main thread}. It is implementation defined how functions
6657 beginning threads other than the main thread are designated or typed.
6658 A function so designated, as well as the @code{main} function, is called
6659 a @dfn{thread startup function}. It is implementation defined what
6660 happens if a thread startup function returns. It is implementation
6661 defined what happens to other threads when any thread calls @code{exit}.
6665 @b{[basic.start.init]}
6667 Add after paragraph 4
6670 The storage for an object of thread storage duration shall be
6671 staticly initialized before the first statement of the thread startup
6672 function. An object of thread storage duration shall not require
6673 dynamic initialization.
6677 @b{[basic.start.term]}
6679 Add after paragraph 3
6682 The type of an object with thread storage duration shall not have a
6683 non-trivial destructor, nor shall it be an array type whose elements
6684 (directly or indirectly) have non-trivial destructors.
6690 Add ``thread storage duration'' to the list in paragraph 1.
6695 Thread, static, and automatic storage durations are associated with
6696 objects introduced by declarations [@dots{}].
6699 Add @code{__thread} to the list of specifiers in paragraph 3.
6702 @b{[basic.stc.thread]}
6704 New section before @b{[basic.stc.static]}
6707 The keyword @code{__thread} applied to an non-local object gives the
6708 object thread storage duration.
6710 A local variable or class data member declared both @code{static}
6711 and @code{__thread} gives the variable or member thread storage
6716 @b{[basic.stc.static]}
6721 All objects which have neither thread storage duration, dynamic
6722 storage duration nor are local [@dots{}].
6728 Add @code{__thread} to the list in paragraph 1.
6733 With the exception of @code{__thread}, at most one
6734 @var{storage-class-specifier} shall appear in a given
6735 @var{decl-specifier-seq}. The @code{__thread} specifier may
6736 be used alone, or immediately following the @code{extern} or
6737 @code{static} specifiers. [@dots{}]
6740 Add after paragraph 5
6743 The @code{__thread} specifier can be applied only to the names of objects
6744 and to anonymous unions.
6750 Add after paragraph 6
6753 Non-@code{static} members shall not be @code{__thread}.
6757 @node C++ Extensions
6758 @chapter Extensions to the C++ Language
6759 @cindex extensions, C++ language
6760 @cindex C++ language extensions
6762 The GNU compiler provides these extensions to the C++ language (and you
6763 can also use most of the C language extensions in your C++ programs). If you
6764 want to write code that checks whether these features are available, you can
6765 test for the GNU compiler the same way as for C programs: check for a
6766 predefined macro @code{__GNUC__}. You can also use @code{__GNUG__} to
6767 test specifically for GNU C++ (@pxref{Standard Predefined,,Standard
6768 Predefined Macros,cpp.info,The C Preprocessor}).
6771 * Min and Max:: C++ Minimum and maximum operators.
6772 * Volatiles:: What constitutes an access to a volatile object.
6773 * Restricted Pointers:: C99 restricted pointers and references.
6774 * Vague Linkage:: Where G++ puts inlines, vtables and such.
6775 * C++ Interface:: You can use a single C++ header file for both
6776 declarations and definitions.
6777 * Template Instantiation:: Methods for ensuring that exactly one copy of
6778 each needed template instantiation is emitted.
6779 * Bound member functions:: You can extract a function pointer to the
6780 method denoted by a @samp{->*} or @samp{.*} expression.
6781 * C++ Attributes:: Variable, function, and type attributes for C++ only.
6782 * Java Exceptions:: Tweaking exception handling to work with Java.
6783 * Deprecated Features:: Things might disappear from g++.
6784 * Backwards Compatibility:: Compatibilities with earlier definitions of C++.
6788 @section Minimum and Maximum Operators in C++
6790 It is very convenient to have operators which return the ``minimum'' or the
6791 ``maximum'' of two arguments. In GNU C++ (but not in GNU C),
6794 @item @var{a} <? @var{b}
6796 @cindex minimum operator
6797 is the @dfn{minimum}, returning the smaller of the numeric values
6798 @var{a} and @var{b};
6800 @item @var{a} >? @var{b}
6802 @cindex maximum operator
6803 is the @dfn{maximum}, returning the larger of the numeric values @var{a}
6807 These operations are not primitive in ordinary C++, since you can
6808 use a macro to return the minimum of two things in C++, as in the
6812 #define MIN(X,Y) ((X) < (Y) ? : (X) : (Y))
6816 You might then use @w{@samp{int min = MIN (i, j);}} to set @var{min} to
6817 the minimum value of variables @var{i} and @var{j}.
6819 However, side effects in @code{X} or @code{Y} may cause unintended
6820 behavior. For example, @code{MIN (i++, j++)} will fail, incrementing
6821 the smaller counter twice. A GNU C extension allows you to write safe
6822 macros that avoid this kind of problem (@pxref{Naming Types,,Naming an
6823 Expression's Type}). However, writing @code{MIN} and @code{MAX} as
6824 macros also forces you to use function-call notation for a
6825 fundamental arithmetic operation. Using GNU C++ extensions, you can
6826 write @w{@samp{int min = i <? j;}} instead.
6828 Since @code{<?} and @code{>?} are built into the compiler, they properly
6829 handle expressions with side-effects; @w{@samp{int min = i++ <? j++;}}
6833 @section When is a Volatile Object Accessed?
6834 @cindex accessing volatiles
6835 @cindex volatile read
6836 @cindex volatile write
6837 @cindex volatile access
6839 Both the C and C++ standard have the concept of volatile objects. These
6840 are normally accessed by pointers and used for accessing hardware. The
6841 standards encourage compilers to refrain from optimizations
6842 concerning accesses to volatile objects that it might perform on
6843 non-volatile objects. The C standard leaves it implementation defined
6844 as to what constitutes a volatile access. The C++ standard omits to
6845 specify this, except to say that C++ should behave in a similar manner
6846 to C with respect to volatiles, where possible. The minimum either
6847 standard specifies is that at a sequence point all previous accesses to
6848 volatile objects have stabilized and no subsequent accesses have
6849 occurred. Thus an implementation is free to reorder and combine
6850 volatile accesses which occur between sequence points, but cannot do so
6851 for accesses across a sequence point. The use of volatiles does not
6852 allow you to violate the restriction on updating objects multiple times
6853 within a sequence point.
6855 In most expressions, it is intuitively obvious what is a read and what is
6856 a write. For instance
6859 volatile int *dst = @var{somevalue};
6860 volatile int *src = @var{someothervalue};
6865 will cause a read of the volatile object pointed to by @var{src} and stores the
6866 value into the volatile object pointed to by @var{dst}. There is no
6867 guarantee that these reads and writes are atomic, especially for objects
6868 larger than @code{int}.
6870 Less obvious expressions are where something which looks like an access
6871 is used in a void context. An example would be,
6874 volatile int *src = @var{somevalue};
6878 With C, such expressions are rvalues, and as rvalues cause a read of
6879 the object, GCC interprets this as a read of the volatile being pointed
6880 to. The C++ standard specifies that such expressions do not undergo
6881 lvalue to rvalue conversion, and that the type of the dereferenced
6882 object may be incomplete. The C++ standard does not specify explicitly
6883 that it is this lvalue to rvalue conversion which is responsible for
6884 causing an access. However, there is reason to believe that it is,
6885 because otherwise certain simple expressions become undefined. However,
6886 because it would surprise most programmers, G++ treats dereferencing a
6887 pointer to volatile object of complete type in a void context as a read
6888 of the object. When the object has incomplete type, G++ issues a
6893 struct T @{int m;@};
6894 volatile S *ptr1 = @var{somevalue};
6895 volatile T *ptr2 = @var{somevalue};
6900 In this example, a warning is issued for @code{*ptr1}, and @code{*ptr2}
6901 causes a read of the object pointed to. If you wish to force an error on
6902 the first case, you must force a conversion to rvalue with, for instance
6903 a static cast, @code{static_cast<S>(*ptr1)}.
6905 When using a reference to volatile, G++ does not treat equivalent
6906 expressions as accesses to volatiles, but instead issues a warning that
6907 no volatile is accessed. The rationale for this is that otherwise it
6908 becomes difficult to determine where volatile access occur, and not
6909 possible to ignore the return value from functions returning volatile
6910 references. Again, if you wish to force a read, cast the reference to
6913 @node Restricted Pointers
6914 @section Restricting Pointer Aliasing
6915 @cindex restricted pointers
6916 @cindex restricted references
6917 @cindex restricted this pointer
6919 As with gcc, g++ understands the C99 feature of restricted pointers,
6920 specified with the @code{__restrict__}, or @code{__restrict} type
6921 qualifier. Because you cannot compile C++ by specifying the @option{-std=c99}
6922 language flag, @code{restrict} is not a keyword in C++.
6924 In addition to allowing restricted pointers, you can specify restricted
6925 references, which indicate that the reference is not aliased in the local
6929 void fn (int *__restrict__ rptr, int &__restrict__ rref)
6936 In the body of @code{fn}, @var{rptr} points to an unaliased integer and
6937 @var{rref} refers to a (different) unaliased integer.
6939 You may also specify whether a member function's @var{this} pointer is
6940 unaliased by using @code{__restrict__} as a member function qualifier.
6943 void T::fn () __restrict__
6950 Within the body of @code{T::fn}, @var{this} will have the effective
6951 definition @code{T *__restrict__ const this}. Notice that the
6952 interpretation of a @code{__restrict__} member function qualifier is
6953 different to that of @code{const} or @code{volatile} qualifier, in that it
6954 is applied to the pointer rather than the object. This is consistent with
6955 other compilers which implement restricted pointers.
6957 As with all outermost parameter qualifiers, @code{__restrict__} is
6958 ignored in function definition matching. This means you only need to
6959 specify @code{__restrict__} in a function definition, rather than
6960 in a function prototype as well.
6963 @section Vague Linkage
6964 @cindex vague linkage
6966 There are several constructs in C++ which require space in the object
6967 file but are not clearly tied to a single translation unit. We say that
6968 these constructs have ``vague linkage''. Typically such constructs are
6969 emitted wherever they are needed, though sometimes we can be more
6973 @item Inline Functions
6974 Inline functions are typically defined in a header file which can be
6975 included in many different compilations. Hopefully they can usually be
6976 inlined, but sometimes an out-of-line copy is necessary, if the address
6977 of the function is taken or if inlining fails. In general, we emit an
6978 out-of-line copy in all translation units where one is needed. As an
6979 exception, we only emit inline virtual functions with the vtable, since
6980 it will always require a copy.
6982 Local static variables and string constants used in an inline function
6983 are also considered to have vague linkage, since they must be shared
6984 between all inlined and out-of-line instances of the function.
6988 C++ virtual functions are implemented in most compilers using a lookup
6989 table, known as a vtable. The vtable contains pointers to the virtual
6990 functions provided by a class, and each object of the class contains a
6991 pointer to its vtable (or vtables, in some multiple-inheritance
6992 situations). If the class declares any non-inline, non-pure virtual
6993 functions, the first one is chosen as the ``key method'' for the class,
6994 and the vtable is only emitted in the translation unit where the key
6997 @emph{Note:} If the chosen key method is later defined as inline, the
6998 vtable will still be emitted in every translation unit which defines it.
6999 Make sure that any inline virtuals are declared inline in the class
7000 body, even if they are not defined there.
7002 @item type_info objects
7005 C++ requires information about types to be written out in order to
7006 implement @samp{dynamic_cast}, @samp{typeid} and exception handling.
7007 For polymorphic classes (classes with virtual functions), the type_info
7008 object is written out along with the vtable so that @samp{dynamic_cast}
7009 can determine the dynamic type of a class object at runtime. For all
7010 other types, we write out the type_info object when it is used: when
7011 applying @samp{typeid} to an expression, throwing an object, or
7012 referring to a type in a catch clause or exception specification.
7014 @item Template Instantiations
7015 Most everything in this section also applies to template instantiations,
7016 but there are other options as well.
7017 @xref{Template Instantiation,,Where's the Template?}.
7021 When used with GNU ld version 2.8 or later on an ELF system such as
7022 Linux/GNU or Solaris 2, or on Microsoft Windows, duplicate copies of
7023 these constructs will be discarded at link time. This is known as
7026 On targets that don't support COMDAT, but do support weak symbols, GCC
7027 will use them. This way one copy will override all the others, but
7028 the unused copies will still take up space in the executable.
7030 For targets which do not support either COMDAT or weak symbols,
7031 most entities with vague linkage will be emitted as local symbols to
7032 avoid duplicate definition errors from the linker. This will not happen
7033 for local statics in inlines, however, as having multiple copies will
7034 almost certainly break things.
7036 @xref{C++ Interface,,Declarations and Definitions in One Header}, for
7037 another way to control placement of these constructs.
7040 @section Declarations and Definitions in One Header
7042 @cindex interface and implementation headers, C++
7043 @cindex C++ interface and implementation headers
7044 C++ object definitions can be quite complex. In principle, your source
7045 code will need two kinds of things for each object that you use across
7046 more than one source file. First, you need an @dfn{interface}
7047 specification, describing its structure with type declarations and
7048 function prototypes. Second, you need the @dfn{implementation} itself.
7049 It can be tedious to maintain a separate interface description in a
7050 header file, in parallel to the actual implementation. It is also
7051 dangerous, since separate interface and implementation definitions may
7052 not remain parallel.
7054 @cindex pragmas, interface and implementation
7055 With GNU C++, you can use a single header file for both purposes.
7058 @emph{Warning:} The mechanism to specify this is in transition. For the
7059 nonce, you must use one of two @code{#pragma} commands; in a future
7060 release of GNU C++, an alternative mechanism will make these
7061 @code{#pragma} commands unnecessary.
7064 The header file contains the full definitions, but is marked with
7065 @samp{#pragma interface} in the source code. This allows the compiler
7066 to use the header file only as an interface specification when ordinary
7067 source files incorporate it with @code{#include}. In the single source
7068 file where the full implementation belongs, you can use either a naming
7069 convention or @samp{#pragma implementation} to indicate this alternate
7070 use of the header file.
7073 @item #pragma interface
7074 @itemx #pragma interface "@var{subdir}/@var{objects}.h"
7075 @kindex #pragma interface
7076 Use this directive in @emph{header files} that define object classes, to save
7077 space in most of the object files that use those classes. Normally,
7078 local copies of certain information (backup copies of inline member
7079 functions, debugging information, and the internal tables that implement
7080 virtual functions) must be kept in each object file that includes class
7081 definitions. You can use this pragma to avoid such duplication. When a
7082 header file containing @samp{#pragma interface} is included in a
7083 compilation, this auxiliary information will not be generated (unless
7084 the main input source file itself uses @samp{#pragma implementation}).
7085 Instead, the object files will contain references to be resolved at link
7088 The second form of this directive is useful for the case where you have
7089 multiple headers with the same name in different directories. If you
7090 use this form, you must specify the same string to @samp{#pragma
7093 @item #pragma implementation
7094 @itemx #pragma implementation "@var{objects}.h"
7095 @kindex #pragma implementation
7096 Use this pragma in a @emph{main input file}, when you want full output from
7097 included header files to be generated (and made globally visible). The
7098 included header file, in turn, should use @samp{#pragma interface}.
7099 Backup copies of inline member functions, debugging information, and the
7100 internal tables used to implement virtual functions are all generated in
7101 implementation files.
7103 @cindex implied @code{#pragma implementation}
7104 @cindex @code{#pragma implementation}, implied
7105 @cindex naming convention, implementation headers
7106 If you use @samp{#pragma implementation} with no argument, it applies to
7107 an include file with the same basename@footnote{A file's @dfn{basename}
7108 was the name stripped of all leading path information and of trailing
7109 suffixes, such as @samp{.h} or @samp{.C} or @samp{.cc}.} as your source
7110 file. For example, in @file{allclass.cc}, giving just
7111 @samp{#pragma implementation}
7112 by itself is equivalent to @samp{#pragma implementation "allclass.h"}.
7114 In versions of GNU C++ prior to 2.6.0 @file{allclass.h} was treated as
7115 an implementation file whenever you would include it from
7116 @file{allclass.cc} even if you never specified @samp{#pragma
7117 implementation}. This was deemed to be more trouble than it was worth,
7118 however, and disabled.
7120 If you use an explicit @samp{#pragma implementation}, it must appear in
7121 your source file @emph{before} you include the affected header files.
7123 Use the string argument if you want a single implementation file to
7124 include code from multiple header files. (You must also use
7125 @samp{#include} to include the header file; @samp{#pragma
7126 implementation} only specifies how to use the file---it doesn't actually
7129 There is no way to split up the contents of a single header file into
7130 multiple implementation files.
7133 @cindex inlining and C++ pragmas
7134 @cindex C++ pragmas, effect on inlining
7135 @cindex pragmas in C++, effect on inlining
7136 @samp{#pragma implementation} and @samp{#pragma interface} also have an
7137 effect on function inlining.
7139 If you define a class in a header file marked with @samp{#pragma
7140 interface}, the effect on a function defined in that class is similar to
7141 an explicit @code{extern} declaration---the compiler emits no code at
7142 all to define an independent version of the function. Its definition
7143 is used only for inlining with its callers.
7145 @opindex fno-implement-inlines
7146 Conversely, when you include the same header file in a main source file
7147 that declares it as @samp{#pragma implementation}, the compiler emits
7148 code for the function itself; this defines a version of the function
7149 that can be found via pointers (or by callers compiled without
7150 inlining). If all calls to the function can be inlined, you can avoid
7151 emitting the function by compiling with @option{-fno-implement-inlines}.
7152 If any calls were not inlined, you will get linker errors.
7154 @node Template Instantiation
7155 @section Where's the Template?
7157 @cindex template instantiation
7159 C++ templates are the first language feature to require more
7160 intelligence from the environment than one usually finds on a UNIX
7161 system. Somehow the compiler and linker have to make sure that each
7162 template instance occurs exactly once in the executable if it is needed,
7163 and not at all otherwise. There are two basic approaches to this
7164 problem, which I will refer to as the Borland model and the Cfront model.
7168 Borland C++ solved the template instantiation problem by adding the code
7169 equivalent of common blocks to their linker; the compiler emits template
7170 instances in each translation unit that uses them, and the linker
7171 collapses them together. The advantage of this model is that the linker
7172 only has to consider the object files themselves; there is no external
7173 complexity to worry about. This disadvantage is that compilation time
7174 is increased because the template code is being compiled repeatedly.
7175 Code written for this model tends to include definitions of all
7176 templates in the header file, since they must be seen to be
7180 The AT&T C++ translator, Cfront, solved the template instantiation
7181 problem by creating the notion of a template repository, an
7182 automatically maintained place where template instances are stored. A
7183 more modern version of the repository works as follows: As individual
7184 object files are built, the compiler places any template definitions and
7185 instantiations encountered in the repository. At link time, the link
7186 wrapper adds in the objects in the repository and compiles any needed
7187 instances that were not previously emitted. The advantages of this
7188 model are more optimal compilation speed and the ability to use the
7189 system linker; to implement the Borland model a compiler vendor also
7190 needs to replace the linker. The disadvantages are vastly increased
7191 complexity, and thus potential for error; for some code this can be
7192 just as transparent, but in practice it can been very difficult to build
7193 multiple programs in one directory and one program in multiple
7194 directories. Code written for this model tends to separate definitions
7195 of non-inline member templates into a separate file, which should be
7196 compiled separately.
7199 When used with GNU ld version 2.8 or later on an ELF system such as
7200 Linux/GNU or Solaris 2, or on Microsoft Windows, g++ supports the
7201 Borland model. On other systems, g++ implements neither automatic
7204 A future version of g++ will support a hybrid model whereby the compiler
7205 will emit any instantiations for which the template definition is
7206 included in the compile, and store template definitions and
7207 instantiation context information into the object file for the rest.
7208 The link wrapper will extract that information as necessary and invoke
7209 the compiler to produce the remaining instantiations. The linker will
7210 then combine duplicate instantiations.
7212 In the mean time, you have the following options for dealing with
7213 template instantiations:
7218 Compile your template-using code with @option{-frepo}. The compiler will
7219 generate files with the extension @samp{.rpo} listing all of the
7220 template instantiations used in the corresponding object files which
7221 could be instantiated there; the link wrapper, @samp{collect2}, will
7222 then update the @samp{.rpo} files to tell the compiler where to place
7223 those instantiations and rebuild any affected object files. The
7224 link-time overhead is negligible after the first pass, as the compiler
7225 will continue to place the instantiations in the same files.
7227 This is your best option for application code written for the Borland
7228 model, as it will just work. Code written for the Cfront model will
7229 need to be modified so that the template definitions are available at
7230 one or more points of instantiation; usually this is as simple as adding
7231 @code{#include <tmethods.cc>} to the end of each template header.
7233 For library code, if you want the library to provide all of the template
7234 instantiations it needs, just try to link all of its object files
7235 together; the link will fail, but cause the instantiations to be
7236 generated as a side effect. Be warned, however, that this may cause
7237 conflicts if multiple libraries try to provide the same instantiations.
7238 For greater control, use explicit instantiation as described in the next
7242 @opindex fno-implicit-templates
7243 Compile your code with @option{-fno-implicit-templates} to disable the
7244 implicit generation of template instances, and explicitly instantiate
7245 all the ones you use. This approach requires more knowledge of exactly
7246 which instances you need than do the others, but it's less
7247 mysterious and allows greater control. You can scatter the explicit
7248 instantiations throughout your program, perhaps putting them in the
7249 translation units where the instances are used or the translation units
7250 that define the templates themselves; you can put all of the explicit
7251 instantiations you need into one big file; or you can create small files
7258 template class Foo<int>;
7259 template ostream& operator <<
7260 (ostream&, const Foo<int>&);
7263 for each of the instances you need, and create a template instantiation
7266 If you are using Cfront-model code, you can probably get away with not
7267 using @option{-fno-implicit-templates} when compiling files that don't
7268 @samp{#include} the member template definitions.
7270 If you use one big file to do the instantiations, you may want to
7271 compile it without @option{-fno-implicit-templates} so you get all of the
7272 instances required by your explicit instantiations (but not by any
7273 other files) without having to specify them as well.
7275 g++ has extended the template instantiation syntax outlined in the
7276 Working Paper to allow forward declaration of explicit instantiations
7277 (with @code{extern}), instantiation of the compiler support data for a
7278 template class (i.e.@: the vtable) without instantiating any of its
7279 members (with @code{inline}), and instantiation of only the static data
7280 members of a template class, without the support data or member
7281 functions (with (@code{static}):
7284 extern template int max (int, int);
7285 inline template class Foo<int>;
7286 static template class Foo<int>;
7290 Do nothing. Pretend g++ does implement automatic instantiation
7291 management. Code written for the Borland model will work fine, but
7292 each translation unit will contain instances of each of the templates it
7293 uses. In a large program, this can lead to an unacceptable amount of code
7296 @xref{C++ Interface,,Declarations and Definitions in One Header}, for
7297 more discussion of these pragmas.
7300 @node Bound member functions
7301 @section Extracting the function pointer from a bound pointer to member function
7304 @cindex pointer to member function
7305 @cindex bound pointer to member function
7307 In C++, pointer to member functions (PMFs) are implemented using a wide
7308 pointer of sorts to handle all the possible call mechanisms; the PMF
7309 needs to store information about how to adjust the @samp{this} pointer,
7310 and if the function pointed to is virtual, where to find the vtable, and
7311 where in the vtable to look for the member function. If you are using
7312 PMFs in an inner loop, you should really reconsider that decision. If
7313 that is not an option, you can extract the pointer to the function that
7314 would be called for a given object/PMF pair and call it directly inside
7315 the inner loop, to save a bit of time.
7317 Note that you will still be paying the penalty for the call through a
7318 function pointer; on most modern architectures, such a call defeats the
7319 branch prediction features of the CPU@. This is also true of normal
7320 virtual function calls.
7322 The syntax for this extension is
7326 extern int (A::*fp)();
7327 typedef int (*fptr)(A *);
7329 fptr p = (fptr)(a.*fp);
7332 For PMF constants (i.e.@: expressions of the form @samp{&Klasse::Member}),
7333 no object is needed to obtain the address of the function. They can be
7334 converted to function pointers directly:
7337 fptr p1 = (fptr)(&A::foo);
7340 @opindex Wno-pmf-conversions
7341 You must specify @option{-Wno-pmf-conversions} to use this extension.
7343 @node C++ Attributes
7344 @section C++-Specific Variable, Function, and Type Attributes
7346 Some attributes only make sense for C++ programs.
7349 @item init_priority (@var{priority})
7350 @cindex init_priority attribute
7353 In Standard C++, objects defined at namespace scope are guaranteed to be
7354 initialized in an order in strict accordance with that of their definitions
7355 @emph{in a given translation unit}. No guarantee is made for initializations
7356 across translation units. However, GNU C++ allows users to control the
7357 order of initialization of objects defined at namespace scope with the
7358 @code{init_priority} attribute by specifying a relative @var{priority},
7359 a constant integral expression currently bounded between 101 and 65535
7360 inclusive. Lower numbers indicate a higher priority.
7362 In the following example, @code{A} would normally be created before
7363 @code{B}, but the @code{init_priority} attribute has reversed that order:
7366 Some_Class A __attribute__ ((init_priority (2000)));
7367 Some_Class B __attribute__ ((init_priority (543)));
7371 Note that the particular values of @var{priority} do not matter; only their
7374 @item java_interface
7375 @cindex java_interface attribute
7377 This type attribute informs C++ that the class is a Java interface. It may
7378 only be applied to classes declared within an @code{extern "Java"} block.
7379 Calls to methods declared in this interface will be dispatched using GCJ's
7380 interface table mechanism, instead of regular virtual table dispatch.
7384 @node Java Exceptions
7385 @section Java Exceptions
7387 The Java language uses a slightly different exception handling model
7388 from C++. Normally, GNU C++ will automatically detect when you are
7389 writing C++ code that uses Java exceptions, and handle them
7390 appropriately. However, if C++ code only needs to execute destructors
7391 when Java exceptions are thrown through it, GCC will guess incorrectly.
7392 Sample problematic code is:
7395 struct S @{ ~S(); @};
7396 extern void bar(); // is written in Java, and may throw exceptions
7405 The usual effect of an incorrect guess is a link failure, complaining of
7406 a missing routine called @samp{__gxx_personality_v0}.
7408 You can inform the compiler that Java exceptions are to be used in a
7409 translation unit, irrespective of what it might think, by writing
7410 @samp{@w{#pragma GCC java_exceptions}} at the head of the file. This
7411 @samp{#pragma} must appear before any functions that throw or catch
7412 exceptions, or run destructors when exceptions are thrown through them.
7414 You cannot mix Java and C++ exceptions in the same translation unit. It
7415 is believed to be safe to throw a C++ exception from one file through
7416 another file compiled for the Java exception model, or vice versa, but
7417 there may be bugs in this area.
7419 @node Deprecated Features
7420 @section Deprecated Features
7422 In the past, the GNU C++ compiler was extended to experiment with new
7423 features, at a time when the C++ language was still evolving. Now that
7424 the C++ standard is complete, some of those features are superseded by
7425 superior alternatives. Using the old features might cause a warning in
7426 some cases that the feature will be dropped in the future. In other
7427 cases, the feature might be gone already.
7429 While the list below is not exhaustive, it documents some of the options
7430 that are now deprecated:
7433 @item -fexternal-templates
7434 @itemx -falt-external-templates
7435 These are two of the many ways for g++ to implement template
7436 instantiation. @xref{Template Instantiation}. The C++ standard clearly
7437 defines how template definitions have to be organized across
7438 implementation units. g++ has an implicit instantiation mechanism that
7439 should work just fine for standard-conforming code.
7441 @item -fstrict-prototype
7442 @itemx -fno-strict-prototype
7443 Previously it was possible to use an empty prototype parameter list to
7444 indicate an unspecified number of parameters (like C), rather than no
7445 parameters, as C++ demands. This feature has been removed, except where
7446 it is required for backwards compatibility @xref{Backwards Compatibility}.
7449 The named return value extension has been deprecated, and is now
7452 The use of initializer lists with new expressions has been deprecated,
7453 and is now removed from g++.
7455 Floating and complex non-type template parameters have been deprecated,
7456 and are now removed from g++.
7458 The implicit typename extension has been deprecated and will be removed
7459 from g++ at some point. In some cases g++ determines that a dependent
7460 type such as @code{TPL<T>::X} is a type without needing a
7461 @code{typename} keyword, contrary to the standard.
7463 @node Backwards Compatibility
7464 @section Backwards Compatibility
7465 @cindex Backwards Compatibility
7466 @cindex ARM [Annotated C++ Reference Manual]
7468 Now that there is a definitive ISO standard C++, G++ has a specification
7469 to adhere to. The C++ language evolved over time, and features that
7470 used to be acceptable in previous drafts of the standard, such as the ARM
7471 [Annotated C++ Reference Manual], are no longer accepted. In order to allow
7472 compilation of C++ written to such drafts, G++ contains some backwards
7473 compatibilities. @emph{All such backwards compatibility features are
7474 liable to disappear in future versions of G++.} They should be considered
7475 deprecated @xref{Deprecated Features}.
7479 If a variable is declared at for scope, it used to remain in scope until
7480 the end of the scope which contained the for statement (rather than just
7481 within the for scope). G++ retains this, but issues a warning, if such a
7482 variable is accessed outside the for scope.
7484 @item Implicit C language
7485 Old C system header files did not contain an @code{extern "C" @{@dots{}@}}
7486 scope to set the language. On such systems, all header files are
7487 implicitly scoped inside a C language scope. Also, an empty prototype
7488 @code{()} will be treated as an unspecified number of arguments, rather
7489 than no arguments, as C++ demands.