1 This is Info file gcc.info, produced by Makeinfo version 1.68 from the
4 This file documents the use and the internals of the GNU compiler.
6 Published by the Free Software Foundation 59 Temple Place - Suite 330
7 Boston, MA 02111-1307 USA
9 Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997 Free
10 Software Foundation, Inc.
12 Permission is granted to make and distribute verbatim copies of this
13 manual provided the copyright notice and this permission notice are
14 preserved on all copies.
16 Permission is granted to copy and distribute modified versions of
17 this manual under the conditions for verbatim copying, provided also
18 that the sections entitled "GNU General Public License," "Funding for
19 Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are
20 included exactly as in the original, and provided that the entire
21 resulting derived work is distributed under the terms of a permission
22 notice identical to this one.
24 Permission is granted to copy and distribute translations of this
25 manual into another language, under the above conditions for modified
26 versions, except that the sections entitled "GNU General Public
27 License," "Funding for Free Software," and "Protect Your Freedom--Fight
28 `Look And Feel'", and this permission notice, may be included in
29 translations approved by the Free Software Foundation instead of in the
33 File: gcc.info, Node: Top, Next: G++ and GCC, Up: (DIR)
38 This manual documents how to run, install and port the GNU compiler,
39 as well as its new features and incompatibilities, and how to report
40 bugs. It corresponds to GNU CC version 2.7.2.
45 * G++ and GCC:: You can compile C or C++ programs.
46 * Invoking GCC:: Command options supported by `gcc'.
47 * Installation:: How to configure, compile and install GNU CC.
48 * C Extensions:: GNU extensions to the C language family.
49 * C++ Extensions:: GNU extensions to the C++ language.
50 * Gcov:: gcov: a GNU CC test coverage program.
51 * Trouble:: If you have trouble installing GNU CC.
52 * Bugs:: How, why and where to report bugs.
53 * Service:: How to find suppliers of support for GNU CC.
54 * Contributing:: How to contribute to testing and developing GNU CC.
55 * VMS:: Using GNU CC on VMS.
57 * Portability:: Goals of GNU CC's portability features.
58 * Interface:: Function-call interface of GNU CC output.
59 * Passes:: Order of passes, what they do, and what each file is for.
60 * RTL:: The intermediate representation that most passes work on.
61 * Machine Desc:: How to write machine description instruction patterns.
62 * Target Macros:: How to write the machine description C macros.
63 * Config:: Writing the `xm-MACHINE.h' file.
64 * Fragments:: Writing the `t-TARGET' and `x-HOST' files.
66 * Funding:: How to help assure funding for free software.
67 * Look and Feel:: Protect your freedom--fight "look and feel".
69 * Copying:: GNU General Public License says
70 how you can copy and share GNU CC.
71 * Contributors:: People who have contributed to GNU CC.
73 * Index:: Index of concepts and symbol names.
76 File: gcc.info, Node: G++ and GCC, Next: Invoking GCC, Prev: Top, Up: Top
78 Compile C, C++, or Objective C
79 ******************************
81 The C, C++, and Objective C versions of the compiler are integrated;
82 the GNU C compiler can compile programs written in C, C++, or Objective
85 "GCC" is a common shorthand term for the GNU C compiler. This is
86 both the most general name for the compiler, and the name used when the
87 emphasis is on compiling C programs.
89 When referring to C++ compilation, it is usual to call the compiler
90 "G++". Since there is only one compiler, it is also accurate to call
91 it "GCC" no matter what the language context; however, the term "G++"
92 is more useful when the emphasis is on compiling C++ programs.
94 We use the name "GNU CC" to refer to the compilation system as a
95 whole, and more specifically to the language-independent part of the
96 compiler. For example, we refer to the optimization options as
97 affecting the behavior of "GNU CC" or sometimes just "the compiler".
99 Front ends for other languages, such as Ada 9X, Fortran, Modula-3,
100 and Pascal, are under development. These front-ends, like that for
101 C++, are built in subdirectories of GNU CC and link to it. The result
102 is an integrated compiler that can compile programs written in C, C++,
103 Objective C, or any of the languages for which you have installed front
106 In this manual, we only discuss the options for the C, Objective-C,
107 and C++ compilers and those of the GNU CC core. Consult the
108 documentation of the other front ends for the options to use when
109 compiling programs written in other languages.
111 G++ is a *compiler*, not merely a preprocessor. G++ builds object
112 code directly from your C++ program source. There is no intermediate C
113 version of the program. (By contrast, for example, some other
114 implementations use a program that generates a C program from your C++
115 source.) Avoiding an intermediate C representation of the program means
116 that you get better object code, and better debugging information. The
117 GNU debugger, GDB, works with this information in the object code to
118 give you comprehensive C++ source-level editing capabilities (*note C
119 and C++: (gdb.info)C.).
122 File: gcc.info, Node: Invoking GCC, Next: Installation, Prev: G++ and GCC, Up: Top
124 GNU CC Command Options
125 **********************
127 When you invoke GNU CC, it normally does preprocessing, compilation,
128 assembly and linking. The "overall options" allow you to stop this
129 process at an intermediate stage. For example, the `-c' option says
130 not to run the linker. Then the output consists of object files output
133 Other options are passed on to one stage of processing. Some options
134 control the preprocessor and others the compiler itself. Yet other
135 options control the assembler and linker; most of these are not
136 documented here, since you rarely need to use any of them.
138 Most of the command line options that you can use with GNU CC are
139 useful for C programs; when an option is only useful with another
140 language (usually C++), the explanation says so explicitly. If the
141 description for a particular option does not mention a source language,
142 you can use that option with all supported languages.
144 *Note Compiling C++ Programs: Invoking G++, for a summary of special
145 options for compiling C++ programs.
147 The `gcc' program accepts options and file names as operands. Many
148 options have multiletter names; therefore multiple single-letter options
149 may *not* be grouped: `-dr' is very different from `-d -r'.
151 You can mix options and other arguments. For the most part, the
152 order you use doesn't matter. Order does matter when you use several
153 options of the same kind; for example, if you specify `-L' more than
154 once, the directories are searched in the order specified.
156 Many options have long names starting with `-f' or with `-W'--for
157 example, `-fforce-mem', `-fstrength-reduce', `-Wformat' and so on.
158 Most of these have both positive and negative forms; the negative form
159 of `-ffoo' would be `-fno-foo'. This manual documents only one of
160 these two forms, whichever one is not the default.
164 * Option Summary:: Brief list of all options, without explanations.
165 * Overall Options:: Controlling the kind of output:
166 an executable, object files, assembler files,
167 or preprocessed source.
168 * Invoking G++:: Compiling C++ programs.
169 * C Dialect Options:: Controlling the variant of C language compiled.
170 * C++ Dialect Options:: Variations on C++.
171 * Warning Options:: How picky should the compiler be?
172 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
173 * Optimize Options:: How much optimization?
174 * Preprocessor Options:: Controlling header files and macro definitions.
175 Also, getting dependency information for Make.
176 * Assembler Options:: Passing options to the assembler.
177 * Link Options:: Specifying libraries and so on.
178 * Directory Options:: Where to find header files and libraries.
179 Where to find the compiler executable files.
180 * Target Options:: Running a cross-compiler, or an old version of GNU CC.
181 * Submodel Options:: Specifying minor hardware or convention variations,
182 such as 68010 vs 68020.
183 * Code Gen Options:: Specifying conventions for function calls, data layout
185 * Environment Variables:: Env vars that affect GNU CC.
186 * Running Protoize:: Automatically adding or removing function prototypes.
189 File: gcc.info, Node: Option Summary, Next: Overall Options, Up: Invoking GCC
194 Here is a summary of all the options, grouped by type. Explanations
195 are in the following sections.
198 *Note Options Controlling the Kind of Output: Overall Options.
199 -c -S -E -o FILE -pipe -v -x LANGUAGE
202 *Note Options Controlling C Dialect: C Dialect Options.
203 -ansi -fallow-single-precision -fcond-mismatch -fno-asm
204 -fno-builtin -ffreestanding -fhosted -fsigned-bitfields -fsigned-char
205 -funsigned-bitfields -funsigned-char -fwritable-strings
206 -traditional -traditional-cpp -trigraphs
208 *C++ Language Options*
209 *Note Options Controlling C++ Dialect: C++ Dialect Options.
210 -fall-virtual -fdollars-in-identifiers -felide-constructors
211 -fenum-int-equiv -fexternal-templates -ffor-scope -fno-for-scope
212 -fhandle-signatures -fmemoize-lookups -fname-mangling-version-N
213 -fno-default-inline -fno-gnu-keywords -fnonnull-objects -fguiding-decls
214 -foperator-names -fstrict-prototype -fthis-is-variable
215 -ftemplate-depth-N -nostdinc++ -traditional +eN
218 *Note Options to Request or Suppress Warnings: Warning Options.
219 -fsyntax-only -pedantic -pedantic-errors
220 -w -W -Wall -Waggregate-return -Wbad-function-cast
221 -Wcast-align -Wcast-qual -Wchar-subscript -Wcomment
222 -Wconversion -Werror -Wformat
223 -Wid-clash-LEN -Wimplicit -Wimplicit-int
224 -Wimplicit-function-declarations -Wimport -Winline
225 -Wlarger-than-LEN -Wmain -Wmissing-declarations
226 -Wmissing-prototypes -Wnested-externs
227 -Wno-import -Woverloaded-virtual -Wparentheses
228 -Wpointer-arith -Wredundant-decls -Wreorder -Wreturn-type -Wshadow
229 -Wsign-compare -Wstrict-prototypes -Wswitch -Wsynth
230 -Wtemplate-debugging -Wtraditional -Wtrigraphs
231 -Wundef -Wuninitialized -Wunused -Wwrite-strings
234 *Note Options for Debugging Your Program or GCC: Debugging Options.
235 -a -ax -dLETTERS -fpretend-float
236 -fprofile-arcs -ftest-coverage
237 -g -gLEVEL -gcoff -gdwarf -gdwarf-1 -gdwarf-1+ -gdwarf-2
238 -ggdb -gstabs -gstabs+ -gxcoff -gxcoff+
239 -p -pg -print-file-name=LIBRARY -print-libgcc-file-name
240 -print-prog-name=PROGRAM -print-search-dirs -save-temps
242 *Optimization Options*
243 *Note Options that Control Optimization: Optimize Options.
244 -fbranch-probabilities
245 -fcaller-saves -fcse-follow-jumps -fcse-skip-blocks
246 -fdelayed-branch -fexpensive-optimizations
247 -ffast-math -ffloat-store -fforce-addr -fforce-mem
248 -ffunction-sections -finline-functions
249 -fkeep-inline-functions -fno-default-inline
250 -fno-defer-pop -fno-function-cse
251 -fno-inline -fno-peephole -fomit-frame-pointer
252 -frerun-cse-after-loop -fschedule-insns
253 -fschedule-insns2 -fstrength-reduce -fthread-jumps
254 -funroll-all-loops -funroll-loops
257 *Preprocessor Options*
258 *Note Options Controlling the Preprocessor: Preprocessor Options.
259 -AQUESTION(ANSWER) -C -dD -dM -dN
262 -include FILE -imacros FILE
263 -iprefix FILE -iwithprefix DIR
264 -iwithprefixbefore DIR -isystem DIR
265 -M -MD -MM -MMD -MG -nostdinc -P -trigraphs
266 -undef -UMACRO -Wp,OPTION
269 *Note Passing Options to the Assembler: Assembler Options.
273 *Note Options for Linking: Link Options.
274 OBJECT-FILE-NAME -lLIBRARY
275 -nostartfiles -nodefaultlibs -nostdlib
276 -s -static -shared -symbolic
277 -Wl,OPTION -Xlinker OPTION
281 *Note Options for Directory Search: Directory Options.
282 -BPREFIX -IDIR -I- -LDIR -specs=FILE
285 *Note Target Options::.
286 -b MACHINE -V VERSION
288 *Machine Dependent Options*
289 *Note Hardware Models and Configurations: Submodel Options.
291 -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040
292 -m68060 -m5200 -m68881 -mbitfield -mc68000 -mc68020 -mfpa
293 -mnobitfield -mrtd -mshort -msoft-float -malign-int
302 -malign-jumps=NUM -malign-loops=NUM
303 -malign-functions=NUM
305 -mapp-regs -mbroken-saverestore -mcypress -mepilogue
306 -mflat -mfpu -mhard-float -mhard-quad-float
307 -mimpure-text -mlive-g0 -mno-app-regs -mno-epilogue
308 -mno-flat -mno-fpu -mno-impure-text
309 -mno-stack-bias -mno-unaligned-doubles
310 -msoft-float -msoft-quad-float -msparclite -mstack-bias
311 -msupersparc -munaligned-doubles -mv8
314 -mc1 -mc2 -mc32 -mc34 -mc38
315 -margcount -mnoargcount
317 -mvolatile-cache -mvolatile-nocache
320 -m29000 -m29050 -mbw -mnbw -mdw -mndw
321 -mlarge -mnormal -msmall
322 -mkernel-registers -mno-reuse-arg-regs
323 -mno-stack-check -mno-storem-bug
324 -mreuse-arg-regs -msoft-float -mstack-check
325 -mstorem-bug -muser-registers
328 -mapcs-frame -mapcs-26 -mapcs-32
329 -mlittle-endian -mbig-endian -mwords-little-endian
330 -mshort-load-bytes -mno-short-load-bytes
331 -msoft-float -mhard-float
332 -mbsd -mxopen -mno-symrename
335 -mcode-model=MODEL TYPE -msdata=SDATA TYPE
339 -m88000 -m88100 -m88110 -mbig-pic
340 -mcheck-zero-division -mhandle-large-shift
341 -midentify-revision -mno-check-zero-division
342 -mno-ocs-debug-info -mno-ocs-frame-position
343 -mno-optimize-arg-area -mno-serialize-volatile
344 -mno-underscores -mocs-debug-info
345 -mocs-frame-position -moptimize-arg-area
346 -mserialize-volatile -mshort-data-NUM -msvr3
347 -msvr4 -mtrap-large-shift -muse-div-instruction
348 -mversion-03.00 -mwarn-passed-structs
350 *RS/6000 and PowerPC Options*
353 -mpower -mno-power -mpower2 -mno-power2
354 -mpowerpc -mno-powerpc
355 -mpowerpc-gpopt -mno-powerpc-gpopt
356 -mpowerpc-gfxopt -mno-powerpc-gfxopt
357 -mnew-mnemonics -mno-new-mnemonics
358 -mfull-toc -mminimal-toc -mno-fop-in-toc -mno-sum-in-toc
359 -mxl-call -mno-xl-call -mthreads -mpe
360 -msoft-float -mhard-float -mmultiple -mno-multiple
361 -mstring -mno-string -mupdate -mno-update
362 -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
363 -mstrict-align -mno-strict-align -mrelocatable
364 -mno-relocatable -mrelocatable-lib -mno-relocatable-lib
365 -mtoc -mno-toc -mtraceback -mno-traceback
366 -mlittle -mlittle-endian -mbig -mbig-endian
367 -mcall-aix -mcall-sysv -mprototype -mno-prototype
368 -msim -mmvme -mads -myellowknife -memb
369 -msdata -msdata=OPT -G NUM
372 -mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs
373 -mfull-fp-blocks -mhc-struct-return -min-line-mul
374 -mminimum-fp-blocks -mnohc-struct-return
377 -mabicalls -mcpu=CPU TYPE -membedded-data
378 -membedded-pic -mfp32 -mfp64 -mgas -mgp32 -mgp64
379 -mgpopt -mhalf-pic -mhard-float -mint64 -mips1
380 -mips2 -mips3 -mlong64 -mlong-calls -mmemcpy
381 -mmips-as -mmips-tfile -mno-abicalls
382 -mno-embedded-data -mno-embedded-pic
383 -mno-gpopt -mno-long-calls
384 -mno-memcpy -mno-mips-tfile -mno-rnames -mno-stats
385 -mrnames -msoft-float
386 -m4650 -msingle-float -mmad
387 -mstats -EL -EB -G NUM -nocpp
390 -m486 -m386 -mieee-fp -mno-fancy-math-387
391 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib
392 -mno-wide-multiply -mrtd -malign-double
393 -mreg-alloc=LIST -mregparm=NUM
394 -malign-jumps=NUM -malign-loops=NUM
395 -malign-functions=NUM
398 -mbig-switch -mdisable-fpregs -mdisable-indexing -mfast-indirect-calls
399 -mgas -mjump-in-delay -mlong-load-store -mno-big-switch -mno-disable-fpregs
400 -mno-disable-indexing -mno-fast-indirect-calls -mno-gas
403 -mno-portable-runtime -mno-soft-float -mno-space -mno-space-regs
405 -mpa-risc-1-0 -mpa-risc-1-1 -mportable-runtime
406 -mschedule=LIST -mspace -mspace-regs
409 -mCPU TYPE -masm-compat -mclean-linkage
410 -mcode-align -mcomplex-addr -mleaf-procedures
411 -mic-compat -mic2.0-compat -mic3.0-compat
412 -mintel-asm -mno-clean-linkage -mno-code-align
413 -mno-complex-addr -mno-leaf-procedures
414 -mno-old-align -mno-strict-align -mno-tail-call
415 -mnumerics -mold-align -msoft-float -mstrict-align
419 -mfp-regs -mno-fp-regs
420 -mno-soft-float -msoft-float
421 -mieee -mieee-with-inexact -mieee-conformant
422 -mfp-trap-mode -mfp-rounding-mode -mtrap-precision
429 -mrelax -mh -ms -mint32 -malign-300
432 -m1 -m2 -m3 -m3e -mb -ml -mrelax
435 -Qy -Qn -YP,PATHS -Ym,DIR
438 -mlong-calls -mno-long-calls -mep -mno-ep
439 -mprolog-function -mno-prolog-function -mspace
440 -mtda=N -msda=N -mzda=N
443 *Code Generation Options*
444 *Note Options for Code Generation Conventions: Code Gen Options.
445 -fcall-saved-REG -fcall-used-REG
446 -ffixed-REG -finhibit-size-directive
447 -fcheck-memory-usage -fprefix-function-name
448 -fno-common -fno-ident -fno-gnu-linker
449 -fpcc-struct-return -fpic -fPIC
450 -freg-struct-return -fshared-data -fshort-enums
451 -fshort-double -fvolatile -fvolatile-global
452 -fverbose-asm -fpack-struct -fstack-check +e0 +e1
456 * Overall Options:: Controlling the kind of output:
457 an executable, object files, assembler files,
458 or preprocessed source.
459 * C Dialect Options:: Controlling the variant of C language compiled.
460 * C++ Dialect Options:: Variations on C++.
461 * Warning Options:: How picky should the compiler be?
462 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
463 * Optimize Options:: How much optimization?
464 * Preprocessor Options:: Controlling header files and macro definitions.
465 Also, getting dependency information for Make.
466 * Assembler Options:: Passing options to the assembler.
467 * Link Options:: Specifying libraries and so on.
468 * Directory Options:: Where to find header files and libraries.
469 Where to find the compiler executable files.
470 * Target Options:: Running a cross-compiler, or an old version of GNU CC.
473 File: gcc.info, Node: Overall Options, Next: Invoking G++, Prev: Option Summary, Up: Invoking GCC
475 Options Controlling the Kind of Output
476 ======================================
478 Compilation can involve up to four stages: preprocessing, compilation
479 proper, assembly and linking, always in that order. The first three
480 stages apply to an individual source file, and end by producing an
481 object file; linking combines all the object files (those newly
482 compiled, and those specified as input) into an executable file.
484 For any given input file, the file name suffix determines what kind
485 of compilation is done:
488 C source code which must be preprocessed.
491 C source code which should not be preprocessed.
494 C++ source code which should not be preprocessed.
497 Objective-C source code. Note that you must link with the library
498 `libobjc.a' to make an Objective-C program work.
501 C header file (not to be compiled or linked).
507 C++ source code which must be preprocessed. Note that in `.cxx',
508 the last two letters must both be literally `x'. Likewise, `.C'
509 refers to a literal capital C.
515 Assembler code which must be preprocessed.
518 An object file to be fed straight into linking. Any file name
519 with no recognized suffix is treated this way.
521 You can specify the input language explicitly with the `-x' option:
524 Specify explicitly the LANGUAGE for the following input files
525 (rather than letting the compiler choose a default based on the
526 file name suffix). This option applies to all following input
527 files until the next `-x' option. Possible values for LANGUAGE
530 c-header cpp-output c++-cpp-output
531 assembler assembler-with-cpp
534 Turn off any specification of a language, so that subsequent files
535 are handled according to their file name suffixes (as they are if
536 `-x' has not been used at all).
538 If you only want some of the stages of compilation, you can use `-x'
539 (or filename suffixes) to tell `gcc' where to start, and one of the
540 options `-c', `-S', or `-E' to say where `gcc' is to stop. Note that
541 some combinations (for example, `-x cpp-output -E' instruct `gcc' to do
545 Compile or assemble the source files, but do not link. The linking
546 stage simply is not done. The ultimate output is in the form of an
547 object file for each source file.
549 By default, the object file name for a source file is made by
550 replacing the suffix `.c', `.i', `.s', etc., with `.o'.
552 Unrecognized input files, not requiring compilation or assembly,
556 Stop after the stage of compilation proper; do not assemble. The
557 output is in the form of an assembler code file for each
558 non-assembler input file specified.
560 By default, the assembler file name for a source file is made by
561 replacing the suffix `.c', `.i', etc., with `.s'.
563 Input files that don't require compilation are ignored.
566 Stop after the preprocessing stage; do not run the compiler
567 proper. The output is in the form of preprocessed source code,
568 which is sent to the standard output.
570 Input files which don't require preprocessing are ignored.
573 Place output in file FILE. This applies regardless to whatever
574 sort of output is being produced, whether it be an executable file,
575 an object file, an assembler file or preprocessed C code.
577 Since only one output file can be specified, it does not make
578 sense to use `-o' when compiling more than one input file, unless
579 you are producing an executable file as output.
581 If `-o' is not specified, the default is to put an executable file
582 in `a.out', the object file for `SOURCE.SUFFIX' in `SOURCE.o', its
583 assembler file in `SOURCE.s', and all preprocessed C source on
587 Print (on standard error output) the commands executed to run the
588 stages of compilation. Also print the version number of the
589 compiler driver program and of the preprocessor and the compiler
593 Use pipes rather than temporary files for communication between the
594 various stages of compilation. This fails to work on some systems
595 where the assembler is unable to read from a pipe; but the GNU
596 assembler has no trouble.
599 File: gcc.info, Node: Invoking G++, Next: C Dialect Options, Prev: Overall Options, Up: Invoking GCC
601 Compiling C++ Programs
602 ======================
604 C++ source files conventionally use one of the suffixes `.C', `.cc',
605 `cpp', or `.cxx'; preprocessed C++ files use the suffix `.ii'. GNU CC
606 recognizes files with these names and compiles them as C++ programs
607 even if you call the compiler the same way as for compiling C programs
608 (usually with the name `gcc').
610 However, C++ programs often require class libraries as well as a
611 compiler that understands the C++ language--and under some
612 circumstances, you might want to compile programs from standard input,
613 or otherwise without a suffix that flags them as C++ programs. `g++'
614 is a program that calls GNU CC with the default language set to C++,
615 and automatically specifies linking against the GNU class library
616 libg++. (1) On many systems, the script `g++' is also installed with
619 When you compile C++ programs, you may specify many of the same
620 command-line options that you use for compiling programs in any
621 language; or command-line options meaningful for C and related
622 languages; or options that are meaningful only for C++ programs. *Note
623 Options Controlling C Dialect: C Dialect Options, for explanations of
624 options for languages related to C. *Note Options Controlling C++
625 Dialect: C++ Dialect Options, for explanations of options that are
626 meaningful only for C++ programs.
628 ---------- Footnotes ----------
630 (1) Prior to release 2 of the compiler, there was a separate `g++'
631 compiler. That version was based on GNU CC, but not integrated with
632 it. Versions of `g++' with a `1.XX' version number--for example, `g++'
633 version 1.37 or 1.42--are much less reliable than the versions
634 integrated with GCC 2. Moreover, combining G++ `1.XX' with a version 2
635 GCC will simply not work.
638 File: gcc.info, Node: C Dialect Options, Next: C++ Dialect Options, Prev: Invoking G++, Up: Invoking GCC
640 Options Controlling C Dialect
641 =============================
643 The following options control the dialect of C (or languages derived
644 from C, such as C++ and Objective C) that the compiler accepts:
647 Support all ANSI standard C programs.
649 This turns off certain features of GNU C that are incompatible
650 with ANSI C, such as the `asm', `inline' and `typeof' keywords, and
651 predefined macros such as `unix' and `vax' that identify the type
652 of system you are using. It also enables the undesirable and
653 rarely used ANSI trigraph feature, and it disables recognition of
654 C++ style `//' comments.
656 The alternate keywords `__asm__', `__extension__', `__inline__'
657 and `__typeof__' continue to work despite `-ansi'. You would not
658 want to use them in an ANSI C program, of course, but it is useful
659 to put them in header files that might be included in compilations
660 done with `-ansi'. Alternate predefined macros such as `__unix__'
661 and `__vax__' are also available, with or without `-ansi'.
663 The `-ansi' option does not cause non-ANSI programs to be rejected
664 gratuitously. For that, `-pedantic' is required in addition to
665 `-ansi'. *Note Warning Options::.
667 The macro `__STRICT_ANSI__' is predefined when the `-ansi' option
668 is used. Some header files may notice this macro and refrain from
669 declaring certain functions or defining certain macros that the
670 ANSI standard doesn't call for; this is to avoid interfering with
671 any programs that might use these names for other things.
673 The functions `alloca', `abort', `exit', and `_exit' are not
674 builtin functions when `-ansi' is used.
677 Do not recognize `asm', `inline' or `typeof' as a keyword, so that
678 code can use these words as identifiers. You can use the keywords
679 `__asm__', `__inline__' and `__typeof__' instead. `-ansi' implies
682 In C++, this switch only affects the `typeof' keyword, since `asm'
683 and `inline' are standard keywords. You may want to use the
684 `-fno-gnu-keywords' flag instead, as it also disables the other,
685 C++-specific, extension keywords such as `headof'.
688 Don't recognize builtin functions that do not begin with two
689 leading underscores. Currently, the functions affected include
690 `abort', `abs', `alloca', `cos', `exit', `fabs', `ffs', `labs',
691 `memcmp', `memcpy', `sin', `sqrt', `strcmp', `strcpy', and
694 GCC normally generates special code to handle certain builtin
695 functions more efficiently; for instance, calls to `alloca' may
696 become single instructions that adjust the stack directly, and
697 calls to `memcpy' may become inline copy loops. The resulting
698 code is often both smaller and faster, but since the function
699 calls no longer appear as such, you cannot set a breakpoint on
700 those calls, nor can you change the behavior of the functions by
701 linking with a different library.
703 The `-ansi' option prevents `alloca' and `ffs' from being builtin
704 functions, since these functions do not have an ANSI standard
708 Assert that compilation takes place in a hosted environment. This
709 implies `-fbuiltin'. A hosted environment is one in which the
710 entire standard library is available, and in which `main' has a
711 return type of `int'. Examples are nearly everything except a
712 kernel. This is equivalent to `-fno-freestanding'.
715 Assert that compilation takes place in a freestanding environment.
716 This implies `-fno-builtin'. A freestanding environment is one
717 in which the standard library may not exist, and program startup
718 may not necessarily be at `main'. The most obvious example is an
719 OS kernel. This is equivalent to `-fno-hosted'.
722 Support ANSI C trigraphs. You don't want to know about this
723 brain-damage. The `-ansi' option implies `-trigraphs'.
726 Attempt to support some aspects of traditional C compilers.
729 * All `extern' declarations take effect globally even if they
730 are written inside of a function definition. This includes
731 implicit declarations of functions.
733 * The newer keywords `typeof', `inline', `signed', `const' and
734 `volatile' are not recognized. (You can still use the
735 alternative keywords such as `__typeof__', `__inline__', and
738 * Comparisons between pointers and integers are always allowed.
740 * Integer types `unsigned short' and `unsigned char' promote to
743 * Out-of-range floating point literals are not an error.
745 * Certain constructs which ANSI regards as a single invalid
746 preprocessing number, such as `0xe-0xd', are treated as
749 * String "constants" are not necessarily constant; they are
750 stored in writable space, and identical looking constants are
751 allocated separately. (This is the same as the effect of
752 `-fwritable-strings'.)
754 * All automatic variables not declared `register' are preserved
755 by `longjmp'. Ordinarily, GNU C follows ANSI C: automatic
756 variables not declared `volatile' may be clobbered.
758 * The character escape sequences `\x' and `\a' evaluate as the
759 literal characters `x' and `a' respectively. Without
760 `-traditional', `\x' is a prefix for the hexadecimal
761 representation of a character, and `\a' produces a bell.
763 * In C++ programs, assignment to `this' is permitted with
764 `-traditional'. (The option `-fthis-is-variable' also has
767 You may wish to use `-fno-builtin' as well as `-traditional' if
768 your program uses names that are normally GNU C builtin functions
769 for other purposes of its own.
771 You cannot use `-traditional' if you include any header files that
772 rely on ANSI C features. Some vendors are starting to ship
773 systems with ANSI C header files and you cannot use `-traditional'
774 on such systems to compile files that include any system headers.
776 The `-traditional' option also enables the `-traditional-cpp'
777 option, which is described next.
780 Attempt to support some aspects of traditional C preprocessors.
783 * Comments convert to nothing at all, rather than to a space.
784 This allows traditional token concatenation.
786 * In a preprocessing directive, the `#' symbol must appear as
787 the first character of a line.
789 * Macro arguments are recognized within string constants in a
790 macro definition (and their values are stringified, though
791 without additional quote marks, when they appear in such a
792 context). The preprocessor always considers a string
793 constant to end at a newline.
795 * The predefined macro `__STDC__' is not defined when you use
796 `-traditional', but `__GNUC__' is (since the GNU extensions
797 which `__GNUC__' indicates are not affected by
798 `-traditional'). If you need to write header files that work
799 differently depending on whether `-traditional' is in use, by
800 testing both of these predefined macros you can distinguish
801 four situations: GNU C, traditional GNU C, other ANSI C
802 compilers, and other old C compilers. The predefined macro
803 `__STDC_VERSION__' is also not defined when you use
804 `-traditional'. *Note Standard Predefined Macros:
805 (cpp.info)Standard Predefined, for more discussion of these
806 and other predefined macros.
808 * The preprocessor considers a string constant to end at a
809 newline (unless the newline is escaped with `\'). (Without
810 `-traditional', string constants can contain the newline
814 Allow conditional expressions with mismatched types in the second
815 and third arguments. The value of such an expression is void.
818 Let the type `char' be unsigned, like `unsigned char'.
820 Each kind of machine has a default for what `char' should be. It
821 is either like `unsigned char' by default or like `signed char' by
824 Ideally, a portable program should always use `signed char' or
825 `unsigned char' when it depends on the signedness of an object.
826 But many programs have been written to use plain `char' and expect
827 it to be signed, or expect it to be unsigned, depending on the
828 machines they were written for. This option, and its inverse, let
829 you make such a program work with the opposite default.
831 The type `char' is always a distinct type from each of `signed
832 char' or `unsigned char', even though its behavior is always just
833 like one of those two.
836 Let the type `char' be signed, like `signed char'.
838 Note that this is equivalent to `-fno-unsigned-char', which is the
839 negative form of `-funsigned-char'. Likewise, the option
840 `-fno-signed-char' is equivalent to `-funsigned-char'.
842 You may wish to use `-fno-builtin' as well as `-traditional' if
843 your program uses names that are normally GNU C builtin functions
844 for other purposes of its own.
846 You cannot use `-traditional' if you include any header files that
847 rely on ANSI C features. Some vendors are starting to ship
848 systems with ANSI C header files and you cannot use `-traditional'
849 on such systems to compile files that include any system headers.
852 `-funsigned-bitfields'
853 `-fno-signed-bitfields'
854 `-fno-unsigned-bitfields'
855 These options control whether a bitfield is signed or unsigned,
856 when the declaration does not use either `signed' or `unsigned'.
857 By default, such a bitfield is signed, because this is consistent:
858 the basic integer types such as `int' are signed types.
860 However, when `-traditional' is used, bitfields are all unsigned
864 Store string constants in the writable data segment and don't
865 uniquize them. This is for compatibility with old programs which
866 assume they can write into string constants. The option
867 `-traditional' also has this effect.
869 Writing into string constants is a very bad idea; "constants"
872 `-fallow-single-precision'
873 Do not promote single precision math operations to double
874 precision, even when compiling with `-traditional'.
876 Traditional K&R C promotes all floating point operations to double
877 precision, regardless of the sizes of the operands. On the
878 architecture for which you are compiling, single precision may be
879 faster than double precision. If you must use `-traditional',
880 but want to use single precision operations when the operands are
881 single precision, use this option. This option has no effect
882 when compiling with ANSI or GNU C conventions (the default).
885 File: gcc.info, Node: C++ Dialect Options, Next: Warning Options, Prev: C Dialect Options, Up: Invoking GCC
887 Options Controlling C++ Dialect
888 ===============================
890 This section describes the command-line options that are only
891 meaningful for C++ programs; but you can also use most of the GNU
892 compiler options regardless of what language your program is in. For
893 example, you might compile a file `firstClass.C' like this:
895 g++ -g -felide-constructors -O -c firstClass.C
897 In this example, only `-felide-constructors' is an option meant only
898 for C++ programs; you can use the other options with any language
901 Here is a list of options that are *only* for compiling C++ programs:
903 `-fno-access-control'
904 Turn off all access checking. This switch is mainly useful for
905 working around bugs in the access control code.
908 Treat all possible member functions as virtual, implicitly. All
909 member functions (except for constructor functions and `new' or
910 `delete' member operators) are treated as virtual functions of the
911 class where they appear.
913 This does not mean that all calls to these member functions will
914 be made through the internal table of virtual functions. Under
915 some circumstances, the compiler can determine that a call to a
916 given virtual function can be made directly; in these cases the
917 calls are direct in any case.
920 Check that the pointer returned by `operator new' is non-null
921 before attempting to modify the storage allocated. The current
922 Working Paper requires that `operator new' never return a null
923 pointer, so this check is normally unnecessary.
926 Put uninitialized or runtime-initialized global variables into the
927 common segment, as C does. This saves space in the executable at
928 the cost of not diagnosing duplicate definitions. If you compile
929 with this flag and your program mysteriously crashes after
930 `main()' has completed, you may have an object that is being
931 destroyed twice because two definitions were merged.
933 `-fdollars-in-identifiers'
934 Accept `$' in identifiers. You can also explicitly prohibit use of
935 `$' with the option `-fno-dollars-in-identifiers'. (GNU C allows
936 `$' by default on most target systems, but there are a few
937 exceptions.) Traditional C allowed the character `$' to form part
938 of identifiers. However, ANSI C and C++ forbid `$' in identifiers.
941 Anachronistically permit implicit conversion of `int' to
942 enumeration types. Current C++ allows conversion of `enum' to
943 `int', but not the other way around.
945 `-fexternal-templates'
946 Cause template instantiations to obey `#pragma interface' and
947 `implementation'; template instances are emitted or not according
948 to the location of the template definition. *Note Template
949 Instantiation::, for more information.
951 This option is deprecated.
953 `-falt-external-templates'
954 Similar to -fexternal-templates, but template instances are
955 emitted or not according to the place where they are first
956 instantiated. *Note Template Instantiation::, for more
959 This option is deprecated.
963 If -ffor-scope is specified, the scope of variables declared in a
964 for-init-statement is limited to the `for' loop itself, as
965 specified by the draft C++ standard. If -fno-for-scope is
966 specified, the scope of variables declared in a for-init-statement
967 extends to the end of the enclosing scope, as was the case in old
968 versions of gcc, and other (traditional) implementations of C++.
970 The default if neither flag is given to follow the standard, but
971 to allow and give a warning for old-style code that would
972 otherwise be invalid, or have different behavior.
975 Do not recognize `classof', `headof', `signature', `sigof' or
976 `typeof' as a keyword, so that code can use these words as
977 identifiers. You can use the keywords `__classof__',
978 `__headof__', `__signature__', `__sigof__', and `__typeof__'
979 instead. `-ansi' implies `-fno-gnu-keywords'.
982 Treat a function declaration with the same type as a potential
983 function template instantiation as though it declares that
984 instantiation, not a normal function. If a definition is given
985 for the function later in the translation unit (or another
986 translation unit if the target supports weak symbols), that
987 definition will be used; otherwise the template will be
988 instantiated. This behavior reflects the C++ language prior to
989 September 1996, when guiding declarations were removed.
991 This option implies `-fname-mangling-version-0', and will not work
992 with other name mangling versions.
994 `-fno-implicit-templates'
995 Never emit code for templates which are instantiated implicitly
996 (i.e. by use); only emit code for explicit instantiations. *Note
997 Template Instantiation::, for more information.
999 `-fhandle-signatures'
1000 Recognize the `signature' and `sigof' keywords for specifying
1001 abstract types. The default (`-fno-handle-signatures') is not to
1002 recognize them. *Note Type Abstraction using Signatures: C++
1006 Support virtual function calls for objects that exceed the size
1007 representable by a `short int'. Users should not use this flag by
1008 default; if you need to use it, the compiler will tell you so. If
1009 you compile any of your code with this flag, you must compile
1010 *all* of your code with this flag (including libg++, if you use
1013 This flag is not useful when compiling with -fvtable-thunks.
1015 `-fno-implement-inlines'
1016 To save space, do not emit out-of-line copies of inline functions
1017 controlled by `#pragma implementation'. This will cause linker
1018 errors if these functions are not inlined everywhere they are
1023 Use heuristics to compile faster. These heuristics are not
1024 enabled by default, since they are only effective for certain
1025 input files. Other input files compile more slowly.
1027 The first time the compiler must build a call to a member function
1028 (or reference to a data member), it must (1) determine whether the
1029 class implements member functions of that name; (2) resolve which
1030 member function to call (which involves figuring out what sorts of
1031 type conversions need to be made); and (3) check the visibility of
1032 the member function to the caller. All of this adds up to slower
1033 compilation. Normally, the second time a call is made to that
1034 member function (or reference to that data member), it must go
1035 through the same lengthy process again. This means that code like
1038 cout << "This " << p << " has " << n << " legs.\n";
1040 makes six passes through all three steps. By using a software
1041 cache, a "hit" significantly reduces this cost. Unfortunately,
1042 using the cache introduces another layer of mechanisms which must
1043 be implemented, and so incurs its own overhead.
1044 `-fmemoize-lookups' enables the software cache.
1046 Because access privileges (visibility) to members and member
1047 functions may differ from one function context to the next, G++
1048 may need to flush the cache. With the `-fmemoize-lookups' flag,
1049 the cache is flushed after every function that is compiled. The
1050 `-fsave-memoized' flag enables the same software cache, but when
1051 the compiler determines that the context of the last function
1052 compiled would yield the same access privileges of the next
1053 function to compile, it preserves the cache. This is most helpful
1054 when defining many member functions for the same class: with the
1055 exception of member functions which are friends of other classes,
1056 each member function has exactly the same access privileges as
1057 every other, and the cache need not be flushed.
1059 The code that implements these flags has rotted; you should
1060 probably avoid using them.
1062 `-fstrict-prototype'
1063 Within an `extern "C"' linkage specification, treat a function
1064 declaration with no arguments, such as `int foo ();', as declaring
1065 the function to take no arguments. Normally, such a declaration
1066 means that the function `foo' can take any combination of
1067 arguments, as in C. `-pedantic' implies `-fstrict-prototype'
1068 unless overridden with `-fno-strict-prototype'.
1070 This flag no longer affects declarations with C++ linkage.
1072 `-fname-mangling-version-N'
1073 Control the way in which names are mangled. Version 0 is
1074 compatible with versions of g++ before 2.8. Version 1 is the
1075 default. Version 1 will allow correct mangling of function
1076 templates. For example, version 0 mangling does not mangle
1077 foo<int, double> and foo<int, char> given this declaration:
1079 template <class T, class U> void foo(T t);
1081 `-fno-nonnull-objects'
1082 Don't assume that a reference is initialized to refer to a valid
1083 object. Although the current C++ Working Paper prohibits null
1084 references, some old code may rely on them, and you can use
1085 `-fno-nonnull-objects' to turn on checking.
1087 At the moment, the compiler only does this checking for
1088 conversions to virtual base classes.
1091 Recognize the operator name keywords `and', `bitand', `bitor',
1092 `compl', `not', `or' and `xor' as synonyms for the symbols they
1093 refer to. `-ansi' implies `-foperator-names'.
1095 `-fthis-is-variable'
1096 Permit assignment to `this'. The incorporation of user-defined
1097 free store management into C++ has made assignment to `this' an
1098 anachronism. Therefore, by default it is invalid to assign to
1099 `this' within a class member function; that is, GNU C++ treats
1100 `this' in a member function of class `X' as a non-lvalue of type
1101 `X *'. However, for backwards compatibility, you can make it
1102 valid with `-fthis-is-variable'.
1105 Use `thunks' to implement the virtual function dispatch table
1106 (`vtable'). The traditional (cfront-style) approach to
1107 implementing vtables was to store a pointer to the function and two
1108 offsets for adjusting the `this' pointer at the call site. Newer
1109 implementations store a single pointer to a `thunk' function which
1110 does any necessary adjustment and then calls the target function.
1112 This option also enables a heuristic for controlling emission of
1113 vtables; if a class has any non-inline virtual functions, the
1114 vtable will be emitted in the translation unit containing the
1117 `-ftemplate-depth-N'
1118 Set the maximum instantiation depth for template classes to N. A
1119 limit on the template instantiation depth is needed to detect
1120 endless recursions during template class instantiation. ANSI/ISO
1121 C++ conforming programs must not rely on a maximum depth greater
1125 Do not search for header files in the standard directories
1126 specific to C++, but do still search the other standard
1127 directories. (This option is used when building libg++.)
1130 For C++ programs (in addition to the effects that apply to both C
1131 and C++), this has the same effect as `-fthis-is-variable'. *Note
1132 Options Controlling C Dialect: C Dialect Options.
1134 In addition, these optimization, warning, and code generation options
1135 have meanings only for C++ programs:
1137 `-fno-default-inline'
1138 Do not assume `inline' for functions defined inside a class scope.
1139 *Note Options That Control Optimization: Optimize Options.
1141 `-Woverloaded-virtual'
1142 `-Wtemplate-debugging'
1143 Warnings that apply only to C++ programs. *Note Options to
1144 Request or Suppress Warnings: Warning Options.
1147 Warn about violation of some style rules from Effective C++ by
1151 Control how virtual function definitions are used, in a fashion
1152 compatible with `cfront' 1.x. *Note Options for Code Generation
1153 Conventions: Code Gen Options.