-Copyright (C) 2000 Free Software Foundation, Inc.
+Copyright (C) 2000, 2003 Free Software Foundation, Inc.
This file is intended to contain a few notes about writing C code
within GCC so that it compiles without error on the full range of
thought I'd collect it in one useful place. Please add and correct
any problems as you come across them.
-I'm going to start from a base of the ISO C89 standard, since that is
+I'm going to start from a base of the ISO C90 standard, since that is
probably what most people code to naturally. Obviously using
constructs introduced after that is not a good idea.
-The first section of this file deals strictly with portability issues,
-the second with common coding pitfalls.
-
-
- Portability Issues
- ==================
-
-Unary +
--------
-
-K+R C compilers and preprocessors have no notion of unary '+'. Thus
-the following code snippet contains 2 portability problems.
-
-int x = +2; /* int x = 2; */
-#if +1 /* #if 1 */
-#endif
-
-
-Pointers to void
-----------------
-
-K+R C compilers did not have a void pointer, and used char * as the
-pointer to anything. The macro PTR is defined as either void * or
-char * depending on whether you have a standards compliant compiler or
-a K+R one. Thus
-
- free ((void *) h->value.expansion);
-
-should be written
-
- free ((PTR) h->value.expansion);
+For the complete coding style conventions used in GCC, please read
+http://gcc.gnu.org/codingconventions.html
String literals
---------------
-K+R C did not allow concatenation of string literals like
+Irix6 "cc -n32" and OSF4 "cc" have problems with constant string
+initializers with parens around it, e.g.
- "This is a " "single string literal".
+const char string[] = ("A string");
-Moreover, some compilers like MSVC++ have fairly low limits on the
-maximum length of a string literal; 509 is the lowest we've come
-across. You may need to break up a long printf statement into many
-smaller ones.
+This is unfortunate since this is what the GNU gettext macro N_
+produces. You need to find a different way to code it.
+
+Some compilers like MSVC++ have fairly low limits on the maximum
+length of a string literal; 509 is the lowest we've come across. You
+may need to break up a long printf statement into many smaller ones.
Empty macro arguments
needs to be coded in some other way.
-signed keyword
---------------
-
-The signed keyword did not exist in K+R comilers, it was introduced in
-ISO C89, so you cannot use it. In both K+R and standard C,
-unqualified char and bitfields may be signed or unsigned. There is no
-way to portably declare signed chars or signed bitfields.
-
-All other arithmetic types are signed unless you use the 'unsigned'
-qualifier. For instance, it is safe to write
-
- short paramc;
-
-instead of
-
- signed short paramc;
-
-If you have an algorithm that depends on signed char or signed
-bitfields, you must find another way to write it before it can be
-integrated into GCC.
-
-
-Function prototypes
--------------------
-
-You need to provide a function prototype for every function before you
-use it, and functions must be defined K+R style. The function
-prototype should use the PARAMS macro, which takes a single argument.
-Therefore the parameter list must be enclosed in parentheses. For
-example,
-
-int myfunc PARAMS ((double, int *));
-
-int
-myfunc (var1, var2)
- double var1;
- int *var2;
-{
- ...
-}
-
-You also need to use PARAMS when referring to function protypes in
-other circumstances, for example see "Calling functions through
-pointers to functions" below.
-
-Variable-argument functions are best described by example:-
-
-void cpp_ice PARAMS ((cpp_reader *, const char *msgid, ...));
-
-void
-cpp_ice VPARAMS ((cpp_reader *pfile, const char *msgid, ...))
-{
-#ifndef ANSI_PROTOTYPES
- cpp_reader *pfile;
- const char *msgid;
-#endif
- va_list ap;
-
- VA_START (ap, msgid);
-
-#ifndef ANSI_PROTOTYPES
- pfile = va_arg (ap, cpp_reader *);
- msgid = va_arg (ap, const char *);
-#endif
-
- ...
- va_end (ap);
-}
-
-For the curious, here are the definitions of the above macros. See
-ansidecl.h for the definitions of the above macros and more.
-
-#define PARAMS(paramlist) paramlist /* ISO C. */
-#define VPARAMS(args) args
+Avoid unnecessary test before free
+----------------------------------
-#define PARAMS(paramlist) () /* K+R C. */
-#define VPARAMS(args) (va_alist) va_dcl
+Since SunOS 4 stopped being a reasonable portability target,
+(which happened around 2007) there has been no need to guard
+against "free (NULL)". Thus, any guard like the following
+constitutes a redundant test:
+ if (P)
+ free (P);
-Calling functions through pointers to functions
------------------------------------------------
+It is better to avoid the test.[*]
+Instead, simply free P, regardless of whether it is NULL.
-K+R C compilers require brackets around the dereferenced pointer
-variable. For example
-
-typedef void (* cl_directive_handler) PARAMS ((cpp_reader *, const char *));
- p->handler (pfile, p->arg);
-
-needs to become
-
- (p->handler) (pfile, p->arg);
-
-
-Macros
-------
-
-The rules under K+R C and ISO C for achieving stringification and
-token pasting are quite different. Therefore some macros have been
-defined which will get it right depending upon the compiler.
-
- CONCAT2(a,b) CONCAT3(a,b,c) and CONCAT4(a,b,c,d)
-
-will paste the tokens passed as arguments. You must not leave any
-space around the commas. Also,
-
- STRINGX(x)
-
-will stringify an argument; to get the same result on K+R and ISO
-compilers x should not have spaces around it.
-
-
-Enums
------
-
-In K+R C, you have to cast enum types to use them as integers, and
-some compilers in particular give lots of warnings for using an enum
-as an array index.
-
-
-Bitfields
----------
-
-See also "signed keyword" above. In K+R C only unsigned int bitfields
-were defined (i.e. unsigned char, unsigned short, unsigned long.
-Using plain int/short/long was not allowed).
-
-
-free and realloc
-----------------
-
-Some implementations crash upon attempts to free or realloc the null
-pointer. Thus if mem might be null, you need to write
-
- if (mem)
- free (mem);
-
-
-Reserved Keywords
------------------
-
-K+R C has "entry" as a reserved keyword, so you should not use it for
-your variable names.
-
-
-Type promotions
----------------
+[*] However, if your profiling exposes a test like this in a
+performance-critical loop, say where P is nearly always NULL, and
+the cost of calling free on a NULL pointer would be prohibitively
+high, consider using __builtin_expect, e.g., like this:
-K+R used unsigned-preserving rules for arithmetic expresssions, while
-ISO uses value-preserving. This means an unsigned char compared to an
-int is done as an unsigned comparison in K+R (since unsigned char
-promotes to unsigned) while it is signed in ISO (since all of the
-values in unsigned char fit in an int, it promotes to int).
+ if (__builtin_expect (ptr != NULL, 0))
+ free (ptr);
-** Not having any argument whose type is a short type (char, short,
-float of any flavor) and subject to promotion. **
Trigraphs
---------
-You weren't going to use them anyway, but trigraphs were not defined
-in K+R C, and some otherwise ISO C compliant compilers do not accept
-them.
+You weren't going to use them anyway, but some otherwise ISO C
+compliant compilers do not accept trigraphs.
Suffixes on Integer Constants
-----------------------------
-**Using a 'u' suffix on integer constants.**
+You should never use a 'l' suffix on integer constants ('L' is fine),
+since it can easily be confused with the number '1'.
Common Coding Pitfalls
Second, if you write a function definition with no return type at
all:
- operate(a, b)
- int a, b;
+ operate (int a, int b)
{
...
}
that function is expected to return int, *not* void. GCC will warn
-about this. K+R C has no problem with 'void' as a return type, so you
-need not worry about that.
+about this.
Implicit function declarations always have return type int. So if you
correct the above definition to
void
- operate(a, b)
- int a, b;
+ operate (int a, int b)
...
but operate() is called above its definition, you will get an error
Other common pitfalls
---------------------
-o Expecting 'plain' char to be either sign or unsigned extending
+o Expecting 'plain' char to be either sign or unsigned extending.
o Shifting an item by a negative amount or by greater than or equal to
the number of bits in a type (expecting shifts by 32 to be sensible
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