avr-g++ (1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below for the remain-
der. g++ accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The ``overall options'' allow you to stop this
process at an intermediate stage. For example, the -c option says not
to run the linker. Then the output consists of object files output by
the assembler.
Other options are passed on to one stage of processing. Some options
control the preprocessor and others the compiler itself. Yet other
options control the assembler and linker; most of these are not docu-
mented here, since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful
for C programs; when an option is only useful with another language
(usually C++), the explanation says so explicitly. If the description
for a particular option does not mention a source language, you can use
that option with all supported languages.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dr is very different from -d -r.
You can mix options and other arguments. For the most part, the order
you use doesn't matter. Order does matter when you use several options
of the same kind; for example, if you specify -L more than once, the
directories are searched in the order specified.
Many options have long names starting with -f or with -W---for example,
-fforce-mem, -fstrength-reduce, -Wformat and so on. Most of these have
both positive and negative forms; the negative form of -ffoo would be
-fno-foo. This manual documents only one of these two forms, whichever
one is not the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations
are in the following sections.
Overall Options
-fwritable-strings
C++ Language Options
-fno-access-control -fcheck-new -fconserve-space
-fno-const-strings -fdollars-in-identifiers -fno-elide-construc-
tors -fno-enforce-eh-specs -fexternal-templates -falt-exter-
nal-templates -ffor-scope -fno-for-scope -fno-gnu-keywords
-fno-implicit-templates -fno-implicit-inline-templates -fno-imple-
ment-inlines -fms-extensions -fno-nonansi-builtins -fno-opera-
tor-names -fno-optional-diags -fpermissive -frepo -fno-rtti
-fstats -ftemplate-depth-n -fuse-cxa-atexit -fvtable-gc
-fno-weak -nostdinc++ -fno-default-inline -Wabi -Wctor-dtor-pri-
vacy -Wnon-virtual-dtor -Wreorder -Weffc++ -Wno-deprecated
-Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wsign-promo -Wsynth
Objective-C Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-gen-decls -Wno-protocol -Wselector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -W -Wall -Waggre-
gate-return -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment
-Wconversion -Wno-deprecated-declarations -Wdisabled-optimization
-Wdiv-by-zero -Werror -Wfloat-equal -Wformat -Wformat=2 -Wfor-
mat-nonliteral -Wformat-security -Wimplicit -Wimplicit-int -Wim-
plicit-function-declaration -Werror-implicit-function-declaration
-Wimport -Winline -Wlarger-than-len -Wlong-long -Wmain -Wmiss-
ing-braces -Wmissing-format-attribute -Wmissing-noreturn -Wmulti-
char -Wno-format-extra-args -Wno-format-y2k -Wno-import -Wpacked
-Wpadded -Wparentheses -Wpointer-arith -Wredundant-decls -Wre-
turn-type -Wsequence-point -Wshadow -Wsign-compare -Wswitch
-Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized -Wun-
known-pragmas -Wunreachable-code -Wunused -Wunused-function
-Wunused-label -Wunused-parameter -Wunused-value -Wunused-vari-
able -Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes
-Wnested-externs -Wstrict-prototypes -Wtraditional
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered
-fdump-translation-unit[-n] -fdump-class-hierarchy[-n]
-fdump-tree-original[-n] -fdump-tree-optimized[-n]
-fdump-tree-inlined[-n] -fmem-report -fpretend-float -fpro-
file-arcs -fsched-verbose=n -ftest-coverage -ftime-report -g
-glevel -gcoff -gdwarf -gdwarf-1 -gdwarf-1+ -gdwarf-2 -ggdb
-gstabs -gstabs+ -gvms -gxcoff -gxcoff+ -p -pg
-print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib -print-prog-name=program
-print-search-dirs -Q -save-temps -time
Optimization Options
-fno-default-inline -fno-defer-pop -fno-function-cse
-fno-guess-branch-probability -fno-inline -fno-math-errno
-fno-peephole -fno-peephole2 -funsafe-math-optimizations
-fno-trapping-math -fomit-frame-pointer -foptimize-register-move
-foptimize-sibling-calls -fprefetch-loop-arrays -freduce-all-givs
-fregmove -frename-registers -frerun-cse-after-loop -fre-
run-loop-opt -fschedule-insns -fschedule-insns2
-fno-sched-interblock -fno-sched-spec -fsched-spec-load
-fsched-spec-load-dangerous -fsingle-precision-constant -fssa
-fssa-ccp -fssa-dce -fstrength-reduce -fstrict-aliasing
-fthread-jumps -ftrapv -funroll-all-loops -funroll-loops --param
name=value -O -O0 -O1 -O2 -O3 -Os
Preprocessor Options
-$ -Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN
-Dmacro[=defn] -E -H -idirafter dir -include file -imacros file
-iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem
dir -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -remap -tri-
graphs -undef -Umacro -Wp,option
Assembler Option
-Wa,option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs -nost-
dlib -s -static -static-libgcc -shared -shared-libgcc -sym-
bolic -Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -I- -Ldir -specs=file
Target Options
-b machine -V version
Machine Dependent Options
M680x0 Options
-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060
-mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 -mfpa
-mnobitfield -mrtd -mshort -msoft-float -mpcrel -malign-int
-mstrict-align
M68hc1x Options
-m6811 -m6812 -m68hc11 -m68hc12 -mauto-incdec -mshort
-msoft-reg-count=count
VAX Options
-mg -mgnu -munix
SPARC Options
-mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -m32 -m64
-mapp-regs -mbroken-saverestore -mcypress -mfaster-structs
-mflat -mfpu -mhard-float -mhard-quad-float -mimpure-text
-mlive-g0 -mno-app-regs -mno-faster-structs -mno-flat -mno-fpu
AMD29K Options
-m29000 -m29050 -mbw -mnbw -mdw -mndw -mlarge -mnormal
-msmall -mkernel-registers -mno-reuse-arg-regs -mno-stack-check
-mno-storem-bug -mreuse-arg-regs -msoft-float -mstack-check
-mstorem-bug -muser-registers
ARM Options
-mapcs-frame -mno-apcs-frame -mapcs-26 -mapcs-32
-mapcs-stack-check -mno-apcs-stack-check -mapcs-float
-mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant -msched-pro-
log -mno-sched-prolog -mlittle-endian -mbig-endian -mwords-lit-
tle-endian -malignment-traps -mno-alignment-traps -msoft-float
-mhard-float -mfpe -mthumb-interwork -mno-thumb-interwork
-mcpu=name -march=name -mfpe=name -mstructure-size-boundary=n
-mbsd -mxopen -mno-symrename -mabort-on-noreturn -mlong-calls
-mno-long-calls -msingle-pic-base -mno-single-pic-base -mpic-reg-
ister=reg -mnop-fun-dllimport -mpoke-function-name -mthumb -marm
-mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking
-mcallee-super-interworking
MN10200 Options
-mrelax
MN10300 Options
-mmult-bug -mno-mult-bug -mam33 -mno-am33 -mno-crt0 -mrelax
M32R/D Options
-m32rx -m32r -mcode-model=model-type -msdata=sdata-type -G num
M88K Options
-m88000 -m88100 -m88110 -mbig-pic -mcheck-zero-division -mhan-
dle-large-shift -midentify-revision -mno-check-zero-division
-mno-ocs-debug-info -mno-ocs-frame-position -mno-optimize-arg-area
-mno-serialize-volatile -mno-underscores -mocs-debug-info
-mocs-frame-position -moptimize-arg-area -mserialize-volatile
-mshort-data-num -msvr3 -msvr4 -mtrap-large-shift
-muse-div-instruction -mversion-03.00 -mwarn-passed-structs
RS/6000 and PowerPC Options
-mcpu=cpu-type -mtune=cpu-type -mpower -mno-power -mpower2
-mno-power2 -mpowerpc -mpowerpc64 -mno-powerpc -maltivec
-mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt
-mno-powerpc-gfxopt -mnew-mnemonics -mold-mnemonics -mfull-toc
-mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32
-mxl-call -mno-xl-call -mpe -msoft-float -mhard-float -mmulti-
ple -mno-multiple -mstring -mno-string -mupdate -mno-update
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable -mno-relocatable
-mrelocatable-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle
-mlittle-endian -mbig -mbig-endian -mcall-aix -mcall-sysv
mum-fp-blocks -mnohc-struct-return
MIPS Options
-mabicalls -march=cpu-type -mtune=cpu=type -mcpu=cpu-type -membed-
ded-data -muninit-const-in-rodata -membedded-pic -mfp32 -mfp64
-mfused-madd -mno-fused-madd -mgas -mgp32 -mgp64 -mgpopt
-mhalf-pic -mhard-float -mint64 -mips1 -mips2 -mips3 -mips4
-mlong64 -mlong32 -mlong-calls -mmemcpy -mmips-as -mmips-tfile
-mno-abicalls -mno-embedded-data -mno-uninit-const-in-rodata
-mno-embedded-pic -mno-gpopt -mno-long-calls -mno-memcpy
-mno-mips-tfile -mno-rnames -mno-stats -mrnames -msoft-float
-m4650 -msingle-float -mmad -mstats -EL -EB -G num -nocpp
-mabi=32 -mabi=n32 -mabi=64 -mabi=eabi -mfix7000 -mno-crt0
-mflush-func=func -mno-flush-func
i386 and x86-64 Options
-mcpu=cpu-type -march=cpu-type -mfpmath=unit -masm=dialect
-mno-fancy-math-387 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib
-mno-wide-multiply -mrtd -malign-double -mpreferred-stack-bound-
ary=num -mmmx -msse -msse2 -m3dnow -mthreads -mno-align-stringops
-minline-all-stringops -mpush-args -maccumulate-outgoing-args
-m128bit-long-double -m96bit-long-double -mregparm=num
-momit-leaf-frame-pointer -mno-red-zone -mcmodel=code-model -m32
-m64
HPPA Options
-march=architecture-type -mbig-switch -mdisable-fpregs -mdis-
able-indexing -mfast-indirect-calls -mgas -mjump-in-delay
-mlong-load-store -mno-big-switch -mno-disable-fpregs -mno-dis-
able-indexing -mno-fast-indirect-calls -mno-gas
-mno-jump-in-delay -mno-long-load-store -mno-portable-runtime
-mno-soft-float -mno-space-regs -msoft-float -mpa-risc-1-0
-mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -mschedule=cpu-
type -mspace-regs
Intel 960 Options
-mcpu-type -masm-compat -mclean-linkage -mcode-align -mcom-
plex-addr -mleaf-procedures -mic-compat -mic2.0-compat
-mic3.0-compat -mintel-asm -mno-clean-linkage -mno-code-align
-mno-complex-addr -mno-leaf-procedures -mno-old-align
-mno-strict-align -mno-tail-call -mnumerics -mold-align
-msoft-float -mstrict-align -mtail-call
DEC Alpha Options
-mno-fp-regs -msoft-float -malpha-as -mgas -mieee
-mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode
-mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants
-mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix
-mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data
-mlarge-data -mmemory-latency=time
DEC Alpha/VMS Options
-mrelax -mh -ms -mint32 -malign-300
SH Options
-m1 -m2 -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4
-m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu
-m5-compact -m5-compact-nofpu -mb -ml -mdalign -mrelax
-mbigtable -mfmovd -mhitachi -mnomacsave -mieee -misize -mpad-
struct -mspace -mprefergot -musermode
System V Options
-Qy -Qn -YP,paths -Ym,dir
ARC Options
-EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section -mdata=data-
section -mrodata=readonly-data-section
TMS320C3x/C4x Options
-mcpu=cpu -mbig -msmall -mregparm -mmemparm -mfast-fix -mmpyi
-mbk -mti -mdp-isr-reload -mrpts=count -mrptb -mdb
-mloop-unsigned -mparallel-insns -mparallel-mpy -mpreserve-float
V850 Options
-mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function
-mno-prolog-function -mspace -mtda=n -msda=n -mzda=n -mv850
-mbig-switch
NS32K Options
-m32032 -m32332 -m32532 -m32081 -m32381 -mmult-add -mno-
mult-add -msoft-float -mrtd -mnortd -mregparam -mnoregparam
-msb -mnosb -mbitfield -mnobitfield -mhimem -mnohimem
AVR Options
-mmcu=mcu -msize -minit-stack=n -mno-interrupts -mcall-prologues
-mno-tablejump -mtiny-stack
MCore Options
-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates
-mno-relax-immediates -mwide-bitfields -mno-wide-bitfields
-m4byte-functions -mno-4byte-functions -mcallgraph-data
-mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim
-mlittle-endian -mbig-endian -m210 -m340 -mstack-increment
MMIX Options
-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu
-mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols -melf
-mbranch-predict -mno-branch-predict -mbase-addresses
-mno-base-addresses
-mextmem -mextmemory -monchip -mno-asm-optimize -masm-optimize
-mbranch-cost=n -mcond-exec=n
S/390 and zSeries Options
-mhard-float -msoft-float -mbackchain -mno-backchain
-msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug
-mno-debug
CRIS Options
-mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -melinux-stack-
size=n -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects
-mstack-align -mdata-align -mconst-align -m32-bit -m16-bit -m8-bit
-mno-prologue-epilogue -mno-gotplt -melf -maout -melinux -mlinux
-sim -sim2
PDP-11 Options
-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy
-mbcopy-builtin -mint32 -mno-int16 -mint16 -mno-int32 -mfloat32
-mno-float64 -mfloat64 -mno-float32 -mabshi -mno-abshi
-mbranch-expensive -mbranch-cheap -msplit -mno-split -munix-asm
-mdec-asm
Xstormy16 Options
-msim
Xtensa Options
-mbig-endian -mlittle-endian -mdensity -mno-density -mmac16
-mno-mac16 -mmul16 -mno-mul16 -mmul32 -mno-mul32 -mnsa -mno-nsa
-mminmax -mno-minmax -msext -mno-sext -mbooleans -mno-booleans
-mhard-float -msoft-float -mfused-madd -mno-fused-madd -mserial-
ize-volatile -mno-serialize-volatile -mtext-section-literals
-mno-text-section-literals -mtarget-align -mno-target-align -mlong-
calls -mno-longcalls
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -funwind-tables -fasynchronous-unwind-tables
-finhibit-size-directive -finstrument-functions -fno-common
-fno-ident -fno-gnu-linker -fpcc-struct-return -fpic -fPIC
-freg-struct-return -fshared-data -fshort-enums -fshort-double
-fshort-wchar -fvolatile -fvolatile-global -fvolatile-static
-fverbose-asm -fpack-struct -fstack-check -fstack-limit-regis-
ter=reg -fstack-limit-symbol=sym -fargument-alias -fargu-
ment-noalias -fargument-noalias-global -fleading-underscore
-ftls-model=model
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order. The first three
stages apply to an individual source file, and end by producing an
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the library
libobjc.a to make an Objective-C program work.
file.mi
Objective-C source code which should not be preprocessed.
file.h
C header file (not to be compiled or linked).
file.cc
file.cp
file.cxx
file.cpp
file.c++
file.C
C++ source code which must be preprocessed. Note that in .cxx, the
last two letters must both be literally x. Likewise, .C refers to
a literal capital C.
file.f
file.for
file.FOR
Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fortran source code which must be preprocessed (with the tradi-
tional preprocessor).
file.r
Fortran source code which must be preprocessed with a RATFOR pre-
processor (not included with GCC).
file.ads
Ada source code file which contains a library unit declaration (a
declaration of a package, subprogram, or generic, or a generic
instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration). Such files are also
called specs.
file.adb
Ada source code file containing a library unit body (a subprogram
or package body). Such files are also called bodies.
file.s
Assembler code.
file.S
Assembler code which must be preprocessed.
(rather than letting the compiler choose a default based on the
file name suffix). This option applies to all following input
files until the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-cpp-output
objective-c objc-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
java
-x none
Turn off any specification of a language, so that subsequent files
are handled according to their file name suffixes (as they are if
-x has not been used at all).
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase
of the compiler returns a non-success return code. If you specify
-pass-exit-codes, the gcc program will instead return with numeri-
cally highest error produced by any phase that returned an error
indication.
If you only want some of the stages of compilation, you can use -x (or
filename suffixes) to tell gcc where to start, and one of the options
-c, -S, or -E to say where gcc is to stop. Note that some combinations
(for example, -x cpp-output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking
stage simply is not done. The ultimate output is in the form of an
object file for each source file.
By default, the object file name for a source file is made by
replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly,
are ignored.
-S Stop after the stage of compilation proper; do not assemble. The
output is in the form of an assembler code file for each non-assem-
bler input file specified.
By default, the assembler file name for a source file is made by
replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper.
The output is in the form of preprocessed source code, which is
sent to the standard output.
Input files which don't require preprocessing are ignored.
-o file
Place output in file file. This applies regardless to whatever
sort of output is being produced, whether it be an executable file,
-v Print (on standard error output) the commands executed to run the
stages of compilation. Also print the version number of the com-
piler driver program and of the preprocessor and the compiler
proper.
-###
Like -v except the commands are not executed and all command argu-
ments are quoted. This is useful for shell scripts to capture the
driver-generated command lines.
-pipe
Use pipes rather than temporary files for communication between the
various stages of compilation. This fails to work on some systems
where the assembler is unable to read from a pipe; but the GNU
assembler has no trouble.
--help
Print (on the standard output) a description of the command line
options understood by gcc. If the -v option is also specified then
--help will also be passed on to the various processes invoked by
gcc, so that they can display the command line options they accept.
If the -W option is also specified then command line options which
have no documentation associated with them will also be displayed.
--target-help
Print (on the standard output) a description of target specific
command line options for each tool.
--version
Display the version number and copyrights of the invoked GCC.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp,
.c++, .cp, or .cxx; preprocessed C++ files use the suffix .ii. GCC
recognizes files with these names and compiles them as C++ programs
even if you call the compiler the same way as for compiling C programs
(usually with the name gcc).
However, C++ programs often require class libraries as well as a com-
piler that understands the C++ language---and under some circumstances,
you might want to compile programs from standard input, or otherwise
without a suffix that flags them as C++ programs. g++ is a program
that calls GCC with the default language set to C++, and automatically
specifies linking against the C++ library. On many systems, g++ is
also installed with the name c++.
When you compile C++ programs, you may specify many of the same com-
mand-line options that you use for compiling programs in any language;
or command-line options meaningful for C and related languages; or
options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived
from C, such as C++ and Objective-C) that the compiler accepts:
you are using. It also enables the undesirable and rarely used ISO
trigraph feature. For the C compiler, it disables recognition of
C++ style // comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and
"__typeof__" continue to work despite -ansi. You would not want to
use them in an ISO C program, of course, but it is useful to put
them in header files that might be included in compilations done
with -ansi. Alternate predefined macros such as "__unix__" and
"__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected
gratuitously. For that, -pedantic is required in addition to
-ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is
used. Some header files may notice this macro and refrain from
declaring certain functions or defining certain macros that the ISO
standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions which would normally be built in but do not have seman-
tics defined by ISO C (such as "alloca" and "ffs") are not built-in
functions with -ansi is used.
-std=
Determine the language standard. This option is currently only
supported when compiling C. A value for this option must be pro-
vided; possible values are
c89
iso9899:1990
ISO C89 (same as -ansi).
iso9899:199409
ISO C89 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported;
see <http://gcc.gnu.org/gcc-3.1/c99status.html> for more infor-
mation. The names c9x and iso9899:199x are deprecated.
gnu89
Default, ISO C89 plus GNU extensions (including some C99 fea-
tures).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented
in GCC, this will become the default. The name gnu9x is depre-
cated.
Even when this option is not specified, you can still use some of
the features of newer standards in so far as they do not conflict
Output to the given filename prototyped declarations for all func-
tions declared and/or defined in a translation unit, including
those in header files. This option is silently ignored in any lan-
guage other than C.
Besides declarations, the file indicates, in comments, the origin
of each declaration (source file and line), whether the declaration
was implicit, prototyped or unprototyped (I, N for new or O for
old, respectively, in the first character after the line number and
the colon), and whether it came from a declaration or a definition
(C or F, respectively, in the following character). In the case of
function definitions, a K&R-style list of arguments followed by
their declarations is also provided, inside comments, after the
declaration.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that
code can use these words as identifiers. You can use the keywords
"__asm__", "__inline__" and "__typeof__" instead. -ansi implies
-fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm"
and "inline" are standard keywords. You may want to use the
-fno-gnu-keywords flag instead, which has the same effect. In C99
mode (-std=c99 or -std=gnu99), this switch only affects the "asm"
and "typeof" keywords, since "inline" is a standard keyword in ISO
C99.
-fno-builtin
-fno-builtin-function (C and Objective-C only)
Don't recognize built-in functions that do not begin with
__builtin_ as prefix.
GCC normally generates special code to handle certain built-in
functions more efficiently; for instance, calls to "alloca" may
become single instructions that adjust the stack directly, and
calls to "memcpy" may become inline copy loops. The resulting code
is often both smaller and faster, but since the function calls no
longer appear as such, you cannot set a breakpoint on those calls,
nor can you change the behavior of the functions by linking with a
different library.
In C++, -fno-builtin is always in effect. The -fbuiltin option has
no effect. Therefore, in C++, the only way to get the optimization
benefits of built-in functions is to call the function using the
__builtin_ prefix. The GNU C++ Standard Library uses built-in
functions to implement many functions (like "std::strchr"), so that
you automatically get efficient code.
With the -fno-builtin-function option, not available when compiling
C++, only the built-in function function is disabled. function
must not begin with __builtin_. If a function is named this is not
built-in in this version of GCC, this option is ignored. There is
no corresponding -fbuiltin-function option; if you wish to enable
built-in functions selectively when using -fno-builtin or -ffree-
standing, you may define macros such as:
-ffreestanding
Assert that compilation takes place in a freestanding environment.
This implies -fno-builtin. A freestanding environment is one in
which the standard library may not exist, and program startup may
not necessarily be at "main". The most obvious example is an OS
kernel. This is equivalent to -fno-hosted.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for
strict ISO C conformance) implies -trigraphs.
-no-integrated-cpp
Invoke the external cpp during compilation. The default is to use
the integrated cpp (internal cpp). This option also allows a user-
supplied cpp via the -B option. This flag is applicable in both C
and C++ modes.
We do not guarantee to retain this option in future, and we may
change its semantics.
-traditional
Attempt to support some aspects of traditional C compilers.
Specifically:
· All "extern" declarations take effect globally even if they are
written inside of a function definition. This includes
implicit declarations of functions.
· The newer keywords "typeof", "inline", "signed", "const" and
"volatile" are not recognized. (You can still use the alterna-
tive keywords such as "__typeof__", "__inline__", and so on.)
· Comparisons between pointers and integers are always allowed.
· Integer types "unsigned short" and "unsigned char" promote to
"unsigned int".
· Out-of-range floating point literals are not an error.
· Certain constructs which ISO regards as a single invalid pre-
processing number, such as 0xe-0xd, are treated as expressions
instead.
· String ``constants'' are not necessarily constant; they are
stored in writable space, and identical looking constants are
allocated separately. (This is the same as the effect of
-fwritable-strings.)
· All automatic variables not declared "register" are preserved
by "longjmp". Ordinarily, GNU C follows ISO C: automatic vari-
ables not declared "volatile" may be clobbered.
· The character escape sequences \x and \a evaluate as the lit-
eral characters x and a respectively. Without -traditional, \x
is a prefix for the hexadecimal representation of a character,
and \a produces a bell.
with ISO C header files and you cannot use -traditional on such
systems to compile files that include any system headers.
The -traditional option also enables -traditional-cpp.
-traditional-cpp
Attempt to support some aspects of traditional C preprocessors.
See the GNU CPP manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second
and third arguments. The value of such an expression is void.
This option is not supported for C++.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It
is either like "unsigned char" by default or like "signed char" by
default.
Ideally, a portable program should always use "signed char" or
"unsigned char" when it depends on the signedness of an object.
But many programs have been written to use plain "char" and expect
it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let
you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed
char" or "unsigned char", even though its behavior is always just
like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the
negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned,
when the declaration does not use either "signed" or "unsigned".
By default, such a bit-field is signed, because this is consistent:
the basic integer types such as "int" are signed types.
However, when -traditional is used, bit-fields are all unsigned no
matter what.
-fwritable-strings
Store string constants in the writable data segment and don't
uniquize them. This is for compatibility with old programs which
assume they can write into string constants. The option -tradi-
tional also has this effect.
tecture for which you are compiling, single precision may be faster
than double precision. If you must use -traditional, but want to
use single precision operations when the operands are single preci-
sion, use this option. This option has no effect when compiling
with ISO or GNU C conventions (the default).
Options Controlling C++ Dialect
This section describes the command-line options that are only meaning-
ful for C++ programs; but you can also use most of the GNU compiler
options regardless of what language your program is in. For example,
you might compile a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs;
you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fno-access-control
Turn off all access checking. This switch is mainly useful for
working around bugs in the access control code.
-fcheck-new
Check that the pointer returned by "operator new" is non-null
before attempting to modify the storage allocated. The current
Working Paper requires that "operator new" never return a null
pointer, so this check is normally unnecessary.
An alternative to using this option is to specify that your "opera-
tor new" does not throw any exceptions; if you declare it tthhrrooww(()),
G++ will check the return value. See also new (nothrow).
-fconserve-space
Put uninitialized or runtime-initialized global variables into the
common segment, as C does. This saves space in the executable at
the cost of not diagnosing duplicate definitions. If you compile
with this flag and your program mysteriously crashes after "main()"
has completed, you may have an object that is being destroyed twice
because two definitions were merged.
This option is no longer useful on most targets, now that support
has been added for putting variables into BSS without making them
common.
-fno-const-strings
Give string constants type "char *" instead of type "const char *".
By default, G++ uses type "const char *" as required by the stan-
dard. Even if you use -fno-const-strings, you cannot actually mod-
ify the value of a string constant, unless you also use
-fwritable-strings.
This option might be removed in a future release of G++. For maxi-
mum portability, you should structure your code so that it works
with string constants that have type "const char *".
rary which is only used to initialize another object of the same
type. Specifying this option disables that optimization, and
forces G++ to call the copy constructor in all cases.
-fno-enforce-eh-specs
Don't check for violation of exception specifications at runtime.
This option violates the C++ standard, but may be useful for reduc-
ing code size in production builds, much like defining NDEBUG. The
compiler will still optimize based on the exception specifications.
-fexternal-templates
Cause #pragma interface and implementation to apply to template
instantiation; template instances are emitted or not according to
the location of the template definition.
This option is deprecated.
-falt-external-templates
Similar to -fexternal-templates, but template instances are emitted
or not according to the place where they are first instantiated.
This option is deprecated.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a
for-init-statement is limited to the for loop itself, as specified
by the C++ standard. If -fno-for-scope is specified, the scope of
variables declared in a for-init-statement extends to the end of
the enclosing scope, as was the case in old versions of G++, and
other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to
allow and give a warning for old-style code that would otherwise be
invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this
word as an identifier. You can use the keyword "__typeof__"
instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated
implicitly (i.e. by use); only emit code for explicit instantia-
tions.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates,
either. The default is to handle inlines differently so that com-
piles with and without optimization will need the same set of
explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
controlled by #pragma implementation. This will cause linker
errors if these functions are not inlined everywhere they are
called.
"bzero", "conjf", and other related functions.
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor",
"compl", "not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need
to issue. Currently, the only such diagnostic issued by G++ is the
one for a name having multiple meanings within a class.
-fpermissive
Downgrade messages about nonconformant code from errors to warn-
ings. By default, G++ effectively sets -pedantic-errors without
-pedantic; this option reverses that. This behavior and this
option are superseded by -pedantic, which works as it does for GNU
C.
-frepo
Enable automatic template instantiation at link time. This option
also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual
functions for use by the C++ runtime type identification features
(dynamic_cast and typeid). If you don't use those parts of the
language, you can save some space by using this flag. Note that
exception handling uses the same information, but it will generate
it as needed.
-fstats
Emit statistics about front-end processing at the end of the compi-
lation. This information is generally only useful to the G++
development team.
-ftemplate-depth-n
Set the maximum instantiation depth for template classes to n. A
limit on the template instantiation depth is needed to detect end-
less recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than
17.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with
the "__cxa_atexit" function rather than the "atexit" function.
This option is required for fully standards-compliant handling of
static destructors, but will only work if your C library supports
"__cxa_atexit".
-fvtable-gc
Emit special relocations for vtables and virtual function refer-
ences so that the linker can identify unused virtual functions and
zero out vtable slots that refer to them. This is most useful with
-ffunction-sections and -Wl,--gc-sections, in order to also discard
the functions themselves.
This optimization requires GNU as and GNU ld. Not all systems sup-
-nostdinc++
Do not search for header files in the standard directories specific
to C++, but do still search the other standard directories. (This
option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline func-
tions; they just won't be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with
the vendor-neutral C++ ABI. Although an effort has been made to
warn about all such cases, there are probably some cases that are
not warned about, even though G++ is generating incompatible code.
There may also be cases where warnings are emitted even though the
code that is generated will be compatible.
You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be
binary compatible with code generated by other compilers.
The known incompatibilites at this point include:
· Incorrect handling of tail-padding for bit-fields. G++ may
attempt to pack data into the same byte as a base class. For
example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte
as"A::f1"; other compilers will not. You can avoid this prob-
lem by explicitly padding "A" so that its size is a multiple of
the byte size on your platform; that will cause G++ and other
compilers to layout "B" identically.
· Incorrect handling of tail-padding for virtual bases. G++ does
not use tail padding when laying out virtual bases. For exam-
ple:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for
"A"; other compilers will. You can avoid this problem by
explicitly padding "A" so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++
and other compilers to layout "C" identically.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable, because all the constructors or
destructors in a class are private and the class has no friends or
not match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
Here the compiler will warn that the member initializers for i and
j will be rearranged to match the declaration order of the members.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)
Warn about violations of the following style guidelines from Scott
Meyers' Effective C++ book:
· Item 11: Define a copy constructor and an assignment operator
for classes with dynamically allocated memory.
· Item 12: Prefer initialization to assignment in constructors.
· Item 14: Make destructors virtual in base classes.
· Item 15: Have "operator=" return a reference to *this.
· Item 23: Don't try to return a reference when you must return
an object.
and about violations of the following style guidelines from Scott
Meyers' More Effective C++ book:
· Item 6: Distinguish between prefix and postfix forms of incre-
ment and decrement operators.
· Item 7: Never overload "&&", "||", or ",".
If you use this option, you should be aware that the standard
library headers do not obey all of these guidelines; you can use
grep -v to filter out those warnings.
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared
within a template. With the advent of explicit template specifica-
tion support in G++, if the name of the friend is an unqualified-id
(i.e., friend foo(int)), the C++ language specification demands
that the friend declare or define an ordinary, nontemplate func-
tion. (Section 14.5.3). Before G++ implemented explicit specifi-
cation, unqualified-ids could be interpreted as a particular spe-
cialization of a templatized function. Because this non-conforming
behavior is no longer the default behavior for G++, -Wnon-tem-
plate-friend allows the compiler to check existing code for poten-
tial trouble spots, and is on by default. This new compiler behav-
ior can be turned off with -Wno-non-template-friend which keeps the
Warn when a function declaration hides virtual functions from a
base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like this:
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
-Wsign-promo (C++ only)
Warn when overload resolution chooses a promotion from unsigned or
enumeral type to a signed type over a conversion to an unsigned
type of the same size. Previous versions of G++ would try to pre-
serve unsignedness, but the standard mandates the current behavior.
-Wsynth (C++ only)
Warn when G++'s synthesis behavior does not match that of cfront.
For instance:
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator = (const
A&);, while cfront will use the user-defined operator =.
Options Controlling Objective-C Dialect
This section describes the command-line options that are only meaning-
ful for Objective-C programs; but you can also use most of the GNU com-
piler options regardless of what language your program is in. For
example, you might compile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, only -fgnu-runtime is an option meant only for Objec-
tive-C programs; you can use the other options with any language sup-
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C
runtime. This is the default for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the
default for NeXT-based systems, including Darwin and Mac OS X.
-gen-decls
Dump interface declarations for all classes seen in the source file
to a file named sourcename.decl.
-Wno-protocol
Do not warn if methods required by a protocol are not implemented
in the class adopting it.
-Wselector
Warn if a selector has multiple methods of different types defined.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of
the output device's aspect (e.g. its width, ...). The options
described below can be used to control the diagnostic messages format-
ting algorithm, e.g. how many characters per line, how often source
location information should be reported. Right now, only the C++ front
end can honor these options. However it is expected, in the near
future, that the remaining front ends would be able to digest them cor-
rectly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n
characters. The default is 72 characters for g++ and 0 for the
rest of the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a
single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit once source location information; that
is, in case the message is too long to fit on a single physical
line and has to be wrapped, the source location won't be emitted
(as prefix) again, over and over, in subsequent continuation lines.
This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location information (as
prefix) for physical lines that result from the process of breaking
a message which is too long to fit on a single line.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are
not inherently erroneous but which are risky or suggest there may have
been an error.
Dialect Options} and @ref{Objective-C Dialect Options}.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond
that.
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject
all programs that use forbidden extensions, and some other programs
that do not follow ISO C and ISO C++. For ISO C, follows the ver-
sion of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or
without this option (though a rare few will require -ansi or a -std
option specifying the required version of ISO C). However, without
this option, certain GNU extensions and traditional C and C++ fea-
tures are supported as well. With this option, they are rejected.
-pedantic does not cause warning messages for use of the alternate
keywords whose names begin and end with __. Pedantic warnings are
also disabled in the expression that follows "__extension__". How-
ever, only system header files should use these escape routes;
application programs should avoid them.
Some users try to use -pedantic to check programs for strict ISO C
conformance. They soon find that it does not do quite what they
want: it finds some non-ISO practices, but not all---only those for
which ISO C requires a diagnostic, and some others for which diag-
nostics have been added.
A feature to report any failure to conform to ISO C might be useful
in some instances, but would require considerable additional work
and would be quite different from -pedantic. We don't have plans
to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended
dialect of C, such as gnu89 or gnu99, there is a corresponding base
standard, the version of ISO C on which the GNU extended dialect is
based. Warnings from -pedantic are given where they are required
by the base standard. (It would not make sense for such warnings
to be given only for features not in the specified GNU C dialect,
since by definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be nothing
to warn about.)
-pedantic-errors
Like -pedantic, except that errors are produced rather than warn-
ings.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause
of error, as programmers often forget that this type is signed on
specified, and that the conversions specified in the format string
make sense. This includes standard functions, and others specified
by format attributes, in the "printf", "scanf", "strftime" and
"strfmon" (an X/Open extension, not in the C standard) families.
The formats are checked against the format features supported by
GNU libc version 2.2. These include all ISO C89 and C99 features,
as well as features from the Single Unix Specification and some BSD
and GNU extensions. Other library implementations may not support
all these features; GCC does not support warning about features
that go beyond a particular library's limitations. However, if
-pedantic is used with -Wformat, warnings will be given about for-
mat features not in the selected standard version (but not for
"strfmon" formats, since those are not in any version of the C
standard).
-Wformat is included in -Wall. For more control over some aspects
of format checking, the options -Wno-format-y2k, -Wno-for-
mat-extra-args, -Wformat-nonliteral, -Wformat-security and -Wfor-
mat=2 are available, but are not included in -Wall.
-Wno-format-y2k
If -Wformat is specified, do not warn about "strftime" formats
which may yield only a two-digit year.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a
"printf" or "scanf" format function. The C standard specifies that
such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with $ operand number specifications, normally warnings
are still given, since the implementation could not know what type
to pass to "va_arg" to skip the unused arguments. However, in the
case of "scanf" formats, this option will suppress the warning if
the unused arguments are all pointers, since the Single Unix Speci-
fication says that such unused arguments are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a
string literal and so cannot be checked, unless the format function
takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions
that represent possible security problems. At present, this warns
about calls to "printf" and "scanf" functions where the format
string is not a string literal and there are no format arguments,
as in "printf (foo);". This may be a security hole if the format
string came from untrusted input and contains %n. (This is cur-
rently a subset of what -Wformat-nonliteral warns about, but in
future warnings may be added to -Wformat-security that are not
included in -Wformat-nonliteral.)
-Wformat=2
Enable -Wformat plus format checks not included in -Wformat. Cur-
rently equivalent to -Wformat -Wformat-nonliteral
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration.
-Wmain
Warn if the type of main is suspicious. main should be a function
with external linkage, returning int, taking either zero arguments,
two, or three arguments of appropriate types.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed.
In the following example, the initializer for a is not fully brack-
eted, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is
expected, or when operators are nested whose precedence people
often get confused about.
Also warn about constructions where there may be confusion to which
"if" statement an "else" branch belongs. Here is an example of
such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every "else" branch belongs to the innermost possible "if"
statement, which in this example is "if (b)". This is often not
what the programmer expected, as illustrated in the above example
by indentation the programmer chose. When there is the potential
for this confusion, GCC will issue a warning when this flag is
specified. To eliminate the warning, add explicit braces around
the innermost "if" statement so there is no way the "else" could
belong to the enclosing "if". The resulting code would look like
this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
-Wsequence-point
Warn about code that may have undefined semantics because of viola-
before a function is called (but after the evaluation of its argu-
ments and the expression denoting the called function), and in cer-
tain other places. Other than as expressed by the sequence point
rules, the order of evaluation of subexpressions of an expression
is not specified. All these rules describe only a partial order
rather than a total order, since, for example, if two functions are
called within one expression with no sequence point between them,
the order in which the functions are called is not specified. How-
ever, the standards committee have ruled that function calls do not
overlap.
It is not specified when between sequence points modifications to
the values of objects take effect. Programs whose behavior depends
on this have undefined behavior; the C standard specifies that
``Between the previous and next sequence point an object shall have
its stored value modified at most once by the evaluation of an
expression. Furthermore, the prior value shall be read only to
determine the value to be stored.''. If a program breaks these
rules, the results on any particular implementation are entirely
unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] =
b[n++]" and "a[i++] = i;". Some more complicated cases are not
diagnosed by this option, and it may give an occasional false posi-
tive result, but in general it has been found fairly effective at
detecting this sort of problem in programs.
The present implementation of this option only works for C pro-
grams. A future implementation may also work for C++ programs.
The C standard is worded confusingly, therefore there is some
debate over the precise meaning of the sequence point rules in sub-
tle cases. Links to discussions of the problem, including proposed
formal definitions, may be found on our readings page, at
<http://gcc.gnu.org/readings.html>.
-Wreturn-type
Warn whenever a function is defined with a return-type that
defaults to "int". Also warn about any "return" statement with no
return-value in a function whose return-type is not "void".
For C++, a function without return type always produces a diagnos-
tic message, even when -Wno-return-type is specified. The only
exceptions are main and functions defined in system headers.
-Wswitch
Warn whenever a "switch" statement has an index of enumeral type
and lacks a "case" for one or more of the named codes of that enu-
meration. (The presence of a "default" label prevents this warn-
ing.) "case" labels outside the enumeration range also provoke
warnings when this option is used.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning
of the program (trigraphs within comments are not warned about).
-Wunused-function
Warn whenever a function parameter is unused aside from its decla-
ration.
To suppress this warning use the unused attribute.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is
unused aside from its declaration
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not
used.
To suppress this warning cast the expression to void.
-Wunused
All all the above -Wunused options combined.
In order to get a warning about an unused function parameter, you
must either specify -W -Wunused or separately specify
-Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being initial-
ized or if a variable may be clobbered by a "setjmp" call.
These warnings are possible only in optimizing compilation, because
they require data flow information that is computed only when opti-
mizing. If you don't specify -O, you simply won't get these warn-
ings.
These warnings occur only for variables that are candidates for
register allocation. Therefore, they do not occur for a variable
that is declared "volatile", or whose address is taken, or whose
size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
structures, unions or arrays, even when they are in registers.
Note that there may be no warning about a variable that is used
only to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the warn-
ings are printed.
These warnings are made optional because GCC is not smart enough to
see all the reasons why the code might be correct despite appearing
to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because "save_y" is used only if it is set.
This option also warns when a non-volatile automatic variable might
be changed by a call to "longjmp". These warnings as well are
possible only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know where
"longjmp" will be called; in fact, a signal handler could call it
at any point in the code. As a result, you may get a warning even
when there is in fact no problem because "longjmp" cannot in fact
be called at the place which would cause a problem.
Some spurious warnings can be avoided if you declare all the func-
tions you use that never return as "noreturn".
-Wreorder (C++ only)
Warn when the order of member initializers given in the code does
not match the order in which they must be executed. For instance:
-Wunknown-pragmas
Warn when a #pragma directive is encountered which is not under-
stood by GCC. If this command line option is used, warnings will
even be issued for unknown pragmas in system header files. This is
not the case if the warnings were only enabled by the -Wall command
line option.
-Wall
All of the above -W options combined. This enables all the warn-
ings about constructions that some users consider questionable, and
that are easy to avoid (or modify to prevent the warning), even in
conjunction with macros.
-Wdiv-by-zero
Warn about compile-time integer division by zero. This is default.
To inhibit the warning messages, use -Wno-div-by-zero. Floating
point division by zero is not warned about, as it can be a legiti-
mate way of obtaining infinities and NaNs.
-Wmultichar
Warn if a multicharacter constant ('FOOF') is used. This is
default. To inhibit the warning messages, use -Wno-multichar.
Usually they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in portable
code.
-Wsystem-headers
Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the
assumption that they usually do not indicate real problems and
would only make the compiler output harder to read. Using this
command line option tells GCC to emit warnings from system headers
as if they occurred in user code. However, note that using -Wall
-W Print extra warning messages for these events:
· A function can return either with or without a value. (Falling
off the end of the function body is considered returning with-
out a value.) For example, this function would evoke such a
warning:
foo (a)
{
if (a > 0)
return a;
}
· An expression-statement or the left-hand side of a comma
expression contains no side effects. To suppress the warning,
cast the unused expression to void. For example, an expression
such as x[i,j] will cause a warning, but x[(void)i,j] will not.
· An unsigned value is compared against zero with < or <=.
· A comparison like x<=y<=z appears; this is equivalent to (x<=y
? 1 : 0) <= z, which is a different interpretation from that of
ordinary mathematical notation.
· Storage-class specifiers like "static" are not the first things
in a declaration. According to the C Standard, this usage is
obsolescent.
· The return type of a function has a type qualifier such as
"const". Such a type qualifier has no effect, since the value
returned by a function is not an lvalue. (But don't warn about
the GNU extension of "volatile void" return types. That exten-
sion will be warned about if -pedantic is specified.)
· If -Wall or -Wunused is also specified, warn about unused argu-
ments.
· A comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to
unsigned. (But don't warn if -Wno-sign-compare is also speci-
fied.)
· An aggregate has a partly bracketed initializer. For example,
the following code would evoke such a warning, because braces
are missing around the initializer for "x.h":
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
· An aggregate has an initializer which does not initialize all
members. For example, the following code would cause such a
warning, because "x.h" would be implicitly initialized to zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
output, but that's a different problem). In particular, instead of
testing for equality, you would check to see whether the two values
have ranges that overlap; and this is done with the relational
operators, so equality comparisons are probably mistaken.
-Wtraditional (C only)
Warn about certain constructs that behave differently in tradi-
tional and ISO C. Also warn about ISO C constructs that have no
traditional C equivalent, and/or problematic constructs which
should be avoided.
· Macro parameters that appear within string literals in the
macro body. In traditional C macro replacement takes place
within string literals, but does not in ISO C.
· In traditional C, some preprocessor directives did not exist.
Traditional preprocessors would only consider a line to be a
directive if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C under-
stands but would ignore because the # does not appear as the
first character on the line. It also suggests you hide
directives like #pragma not understood by traditional C by
indenting them. Some traditional implementations would not
recognize #elif, so it suggests avoiding it altogether.
· A function-like macro that appears without arguments.
· The unary plus operator.
· The U integer constant suffix, or the F or L floating point
constant suffixes. (Traditional C does support the L suffix on
integer constants.) Note, these suffixes appear in macros
defined in the system headers of most modern systems, e.g. the
_MIN/_MAX macros in "<limits.h>". Use of these macros in user
code might normally lead to spurious warnings, however gcc's
integrated preprocessor has enough context to avoid warning in
these cases.
· A function declared external in one block and then used after
the end of the block.
· A "switch" statement has an operand of type "long".
· A non-"static" function declaration follows a "static" one.
This construct is not accepted by some traditional C compilers.
· The ISO type of an integer constant has a different width or
signedness from its traditional type. This warning is only
issued if the base of the constant is ten. I.e. hexadecimal or
octal values, which typically represent bit patterns, are not
warned about.
· Usage of ISO string concatenation is detected.
· Initialization of automatic aggregates.
· Identifier conflicts with labels. Traditional C lacks a sepa-
and vice versa. The absence of these prototypes when compiling
with traditional C would cause serious problems. This is a
subset of the possible conversion warnings, for the full set
use -Wconversion.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wshadow
Warn whenever a local variable shadows another local variable,
parameter or global variable or whenever a built-in function is
shadowed.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
-Wpointer-arith
Warn about anything that depends on the ``size of'' a function type
or of "void". GNU C assigns these types a size of 1, for conve-
nience in calculations with "void *" pointers and pointers to func-
tions.
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type. For
example, warn if "int malloc()" is cast to "anything *".
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if a "const char *" is
cast to an ordinary "char *".
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of
the target is increased. For example, warn if a "char *" is cast
to an "int *" on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const
char[length]" so that copying the address of one into a non-"const"
"char *" pointer will get a warning; when compiling C++, warn about
the deprecated conversion from string constants to "char *". These
warnings will help you find at compile time code that can try to
write into a string constant, but only if you have been very care-
ful about using "const" in declarations and prototypes. Otherwise,
it will just be a nuisance; this is why we did not make -Wall
request these warnings.
-Wconversion
Warn if a prototype causes a type conversion that is different from
what would happen to the same argument in the absence of a proto-
type. This includes conversions of fixed point to floating and
vice versa, and conversions changing the width or signedness of a
fixed point argument except when the same as the default promotion.
Also, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the assign-
-Waggregate-return
Warn if any functions that return structures or unions are defined
or called. (In languages where you can return an array, this also
elicits a warning.)
-Wstrict-prototypes (C only)
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration which specifies the
argument types.)
-Wmissing-prototypes (C only)
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. The aim is to detect global functions that
fail to be declared in header files.
-Wmissing-declarations
Warn if a global function is defined without a previous declara-
tion. Do so even if the definition itself provides a prototype.
Use this option to detect global functions that are not declared in
header files.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute "nore-
turn". Note these are only possible candidates, not absolute ones.
Care should be taken to manually verify functions actually do not
ever return before adding the "noreturn" attribute, otherwise sub-
tle code generation bugs could be introduced. You will not get a
warning for "main" in hosted C environments.
-Wmissing-format-attribute
If -Wformat is enabled, also warn about functions which might be
candidates for "format" attributes. Note these are only possible
candidates, not absolute ones. GCC will guess that "format"
attributes might be appropriate for any function that calls a func-
tion like "vprintf" or "vscanf", but this might not always be the
case, and some functions for which "format" attributes are appro-
priate may not be detected. This option has no effect unless
-Wformat is enabled (possibly by -Wall).
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as
deprecated by using the "deprecated" attribute. (@pxref{Function
Attributes}, @pxref{Variable Attributes}, @pxref{Type Attributes}.)
-Wpacked
Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable "f.x" in "struct bar" will be
misaligned even though "struct bar" does not itself have the packed
attribute:
struct foo {
int x;
char a, b, c, d;
when this happens it is possible to rearrange the fields of the
structure to reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even
in cases where multiple declaration is valid and changes nothing.
-Wnested-externs (C only)
Warn if an "extern" declaration is encountered within a function.
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at
least a whole line of source code will never be executed, because
some condition is never satisfied or because it is after a proce-
dure that never returns.
It is possible for this option to produce a warning even though
there are circumstances under which part of the affected line can
be executed, so care should be taken when removing apparently-
unreachable code.
For instance, when a function is inlined, a warning may mean that
the line is unreachable in only one inlined copy of the function.
This option is not made part of -Wall because in a debugging ver-
sion of a program there is often substantial code which checks cor-
rect functioning of the program and is, hopefully, unreachable
because the program does work. Another common use of unreachable
code is to provide behavior which is selectable at compile-time.
-Winline
Warn if a function can not be inlined and it was declared as
inline.
-Wlong-long
Warn if long long type is used. This is default. To inhibit the
warning messages, use -Wno-long-long. Flags -Wlong-long and
-Wno-long-long are taken into account only when -pedantic flag is
used.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning
does not generally indicate that there is anything wrong with your
code; it merely indicates that GCC's optimizers were unable to han-
dle the code effectively. Often, the problem is that your code is
too big or too complex; GCC will refuse to optimize programs when
the optimization itself is likely to take inordinate amounts of
time.
-Werror
Make all warnings into errors.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your
debuggers crash or refuse to read the program. If you want to con-
trol for certain whether to generate the extra information, use
-gstabs+, -gstabs, -gxcoff+, -gxcoff, -gdwarf-1+, -gdwarf-1, or
-gvms (see below).
Unlike most other C compilers, GCC allows you to use -g with -O.
The shortcuts taken by optimized code may occasionally produce sur-
prising results: some variables you declared may not exist at all;
flow of control may briefly move where you did not expect it; some
statements may not be executed because they compute constant
results or their values were already at hand; some statements may
execute in different places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This
makes it reasonable to use the optimizer for programs that might
have bugs.
The following options are useful when GCC is generated with the
capability for more than one debugging format.
-ggdb
Produce debugging information for use by GDB. This means to use
the most expressive format available (DWARF 2, stabs, or the native
format if neither of those are supported), including GDB extensions
if at all possible.
-gstabs
Produce debugging information in stabs format (if that is sup-
ported), without GDB extensions. This is the format used by DBX on
most BSD systems. On MIPS, Alpha and System V Release 4 systems
this option produces stabs debugging output which is not understood
by DBX or SDB. On System V Release 4 systems this option requires
the GNU assembler.
-gstabs+
Produce debugging information in stabs format (if that is sup-
ported), using GNU extensions understood only by the GNU debugger
(GDB). The use of these extensions is likely to make other debug-
gers crash or refuse to read the program.
-gcoff
Produce debugging information in COFF format (if that is sup-
ported). This is the format used by SDB on most System V systems
prior to System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that is sup-
ported). This is the format used by the DBX debugger on IBM
RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is sup-
ported), using GNU extensions understood only by the GNU debugger
(GDB). The use of these extensions is likely to make other debug-
gers crash or refuse to read the program, and may cause assemblers
other than the GNU assembler (GAS) to fail with an error.
debuggers crash or refuse to read the program.
-gdwarf-2
Produce debugging information in DWARF version 2 format (if that is
supported). This is the format used by DBX on IRIX 6.
-gvms
Produce debugging information in VMS debug format (if that is sup-
ported). This is the format used by DEBUG on VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how
much information. The default level is 2.
Level 1 produces minimal information, enough for making backtraces
in parts of the program that you don't plan to debug. This
includes descriptions of functions and external variables, but no
information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro defini-
tions present in the program. Some debuggers support macro expan-
sion when you use -g3.
Note that in order to avoid confusion between DWARF1 debug level 2,
and DWARF2, neither -gdwarf nor -gdwarf-2 accept a concatenated
debug level. Instead use an additional -glevel option to change
the debug level for DWARF1 or DWARF2.
-p Generate extra code to write profile information suitable for the
analysis program "prof". You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-pg Generate extra code to write profile information suitable for the
analysis program "gprof". You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-Q Makes the compiler print out each function name as it is compiled,
and print some statistics about each pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by
each pass when it finishes.
-fmem-report
Makes the compiler print some statistics about permanent memory
allocation when it finishes.
-fprofile-arcs
Instrument arcs during compilation to generate coverage data or for
profile-directed block ordering. During execution the program
ties.
The other use of -fprofile-arcs is for use with "gcov", when it is
used with the -ftest-coverage option.
With -fprofile-arcs, for each function of your program GCC creates
a program flow graph, then finds a spanning tree for the graph.
Only arcs that are not on the spanning tree have to be instru-
mented: the compiler adds code to count the number of times that
these arcs are executed. When an arc is the only exit or only
entrance to a block, the instrumentation code can be added to the
block; otherwise, a new basic block must be created to hold the
instrumentation code.
-ftest-coverage
Create data files for the gcov code-coverage utility. The data
file names begin with the name of your source file:
sourcename.bb
A mapping from basic blocks to line numbers, which "gcov" uses
to associate basic block execution counts with line numbers.
sourcename.bbg
A list of all arcs in the program flow graph. This allows
"gcov" to reconstruct the program flow graph, so that it can
compute all basic block and arc execution counts from the
information in the "sourcename.da" file.
Use -ftest-coverage with -fprofile-arcs; the latter option adds
instrumentation to the program, which then writes execution counts
to another data file:
sourcename.da
Runtime arc execution counts, used in conjunction with the arc
information in the file "sourcename.bbg".
Coverage data will map better to the source files if -ftest-cover-
age is used without optimization.
-dletters
Says to make debugging dumps during compilation at times specified
by letters. This is used for debugging the compiler. The file
names for most of the dumps are made by appending a pass number and
a word to the source file name (e.g. foo.c.00.rtl or foo.c.01.sib-
ling). Here are the possible letters for use in letters, and their
meanings:
A Annotate the assembler output with miscellaneous debugging
information.
b Dump after computing branch probabilities, to file.14.bp.
B Dump after block reordering, to file.29.bbro.
c Dump after instruction combination, to the file file.16.com-
bine.
E Dump after the second if conversion, to file.26.ce2.
f Dump after life analysis, to file.15.life.
F Dump after purging "ADDRESSOF" codes, to file.09.addressof.
g Dump after global register allocation, to file.21.greg.
h Dump after finalization of EH handling code, to file.02.eh.
k Dump after reg-to-stack conversion, to file.28.stack.
o Dump after post-reload optimizations, to file.22.postreload.
G Dump after GCSE, to file.10.gcse.
i Dump after sibling call optimizations, to file.01.sibling.
j Dump after the first jump optimization, to file.03.jump.
k Dump after conversion from registers to stack, to
file.32.stack.
l Dump after local register allocation, to file.20.lreg.
L Dump after loop optimization, to file.11.loop.
M Dump after performing the machine dependent reorganisation
pass, to file.30.mach.
n Dump after register renumbering, to file.25.rnreg.
N Dump after the register move pass, to file.18.regmove.
r Dump after RTL generation, to file.00.rtl.
R Dump after the second scheduling pass, to file.27.sched2.
s Dump after CSE (including the jump optimization that sometimes
follows CSE), to file.08.cse.
S Dump after the first scheduling pass, to file.19.sched.
t Dump after the second CSE pass (including the jump optimization
that sometimes follows CSE), to file.12.cse2.
w Dump after the second flow pass, to file.23.flow2.
X Dump after SSA dead code elimination, to file.06.ssadce.
z Dump after the peephole pass, to file.24.peephole2.
a Produce all the dumps listed above.
m Print statistics on memory usage, at the end of the run, to
standard error.
suitable for viewing with VCG to file.pass.vcg.
x Just generate RTL for a function instead of compiling it. Usu-
ally used with r.
y Dump debugging information during parsing, to standard error.
-fdump-unnumbered
When doing debugging dumps (see -d option above), suppress instruc-
tion numbers and line number note output. This makes it more fea-
sible to use diff on debugging dumps for compiler invocations with
different options, in particular with and without -g.
-fdump-translation-unit (C and C++ only)
-fdump-translation-unit-options (C and C++ only)
Dump a representation of the tree structure for the entire transla-
tion unit to a file. The file name is made by appending .tu to the
source file name. If the -options form is used, options controls
the details of the dump as described for the -fdump-tree options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class's hierarchy and virtual func-
tion table layout to a file. The file name is made by appending
.class to the source file name. If the -options form is used,
options controls the details of the dump as described for the
-fdump-tree options.
-fdump-tree-switch (C++ only)
-fdump-tree-switch-options (C++ only)
Control the dumping at various stages of processing the intermedi-
ate language tree to a file. The file name is generated by append-
ing a switch specific suffix to the source file name. If the
-options form is used, options is a list of - separated options
that control the details of the dump. Not all options are applica-
ble to all dumps, those which are not meaningful will be ignored.
The following options are available
address
Print the address of each node. Usually this is not meaningful
as it changes according to the environment and source file. Its
primary use is for tying up a dump file with a debug environ-
ment.
slim
Inhibit dumping of members of a scope or body of a function
merely because that scope has been reached. Only dump such
items when they are directly reachable by some other path.
all Turn on all options.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
in which case it is output to the usual dump listing file, .sched
or .sched2 respectively. However for n greater than nine, the out-
put is always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same informa-
tion as -dRS. For n greater than one, it also output basic block
probabilities, detailed ready list information and unit/insn info.
For n greater than two, it includes RTL at abort point, control-
flow and regions info. And for n over four, -fsched-verbose also
includes dependence info.
-fpretend-float
When running a cross-compiler, pretend that the target machine uses
the same floating point format as the host machine. This causes
incorrect output of the actual floating constants, but the actual
instruction sequence will probably be the same as GCC would make
when running on the target machine.
-save-temps
Store the usual ``temporary'' intermediate files permanently; place
them in the current directory and name them based on the source
file. Thus, compiling foo.c with -c -save-temps would produce
files foo.i and foo.s, as well as foo.o. This creates a prepro-
cessed foo.i output file even though the compiler now normally uses
an integrated preprocessor.
-time
Report the CPU time taken by each subprocess in the compilation
sequence. For C source files, this is the compiler proper and
assembler (plus the linker if linking is done). The output looks
like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the ``user time,'' that is time
spent executing the program itself. The second number is ``system
time,'' time spent executing operating system routines on behalf of
the program. Both numbers are in seconds.
-print-file-name=library
Print the full absolute name of the library file library that would
be used when linking---and don't do anything else. With this
option, GCC does not compile or link anything; it just prints the
file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by
any other switches present in the command line. This directory is
supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler
switches that enable them. The directory name is separated from
the switches by ;, and each switch starts with an @} instead of the
@samp{-, without spaces between multiple switches. This is sup-
posed to ease shell-processing.
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a list
of program and library directories gcc will search---and don't do
anything else.
This is useful when gcc prints the error message installation prob-
lem, cannot exec cpp0: No such file or directory. To resolve this
you either need to put cpp0 and the other compiler components where
gcc expects to find them, or you can set the environment variable
GCC_EXEC_PREFIX to the directory where you installed them. Don't
forget the trailing '/'.
-dumpmachine
Print the compiler's target machine (for example,
i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0)---and don't do any-
thing else.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else.
(This is used when GCC itself is being built.)
Options That Control Optimization
These options control various sorts of optimizations:
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a large function.
Without -O, the compiler's goal is to reduce the cost of compila-
tion and to make debugging produce the expected results. State-
ments are independent: if you stop the program with a breakpoint
between statements, you can then assign a new value to any variable
or change the program counter to any other statement in the func-
tion and get exactly the results you would expect from the source
code.
With -O, the compiler tries to reduce code size and execution time,
without performing any optimizations that take a great deal of com-
pilation time.
-O2 Optimize even more. GCC performs nearly all supported optimiza-
tions that do not involve a space-speed tradeoff. The compiler
does not perform loop unrolling or function inlining when you spec-
ify -O2. As compared to -O, this option increases both compilation
time and the performance of the generated code.
-O2 turns on all optional optimizations except for loop unrolling,
function inlining, and register renaming. It also turns on the
-fforce-mem option on all machines and frame pointer elimination on
machines where doing so does not interfere with debugging.
-Os Optimize for size. -Os enables all -O2 optimizations that do not
typically increase code size. It also performs further optimiza-
tions designed to reduce code size.
If you use multiple -O options, with or without level numbers, the
last such option is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most
flags have both positive and negative forms; the negative form of -ffoo
would be -fno-foo. In the table below, only one of the forms is
listed---the one which is not the default. You can figure out the
other form by either removing no- or adding it.
-ffloat-store
Do not store floating point variables in registers, and inhibit
other options that might change whether a floating point value is
taken from a register or memory.
This option prevents undesirable excess precision on machines such
as the 68000 where the floating registers (of the 68881) keep more
precision than a "double" is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does
only good, but a few programs rely on the precise definition of
IEEE floating point. Use -ffloat-store for such programs, after
modifying them to store all pertinent intermediate computations
into variables.
-fno-default-inline
Do not make member functions inline by default merely because they
are defined inside the class scope (C++ only). Otherwise, when you
specify -O, member functions defined inside class scope are com-
piled inline by default; i.e., you don't need to add inline in
front of the member function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that
function returns. For machines which must pop arguments after a
function call, the compiler normally lets arguments accumulate on
the stack for several function calls and pops them all at once.
-fforce-mem
Force memory operands to be copied into registers before doing
arithmetic on them. This produces better code by making all memory
references potential common subexpressions. When they are not com-
mon subexpressions, instruction combination should eliminate the
separate register-load. The -O2 option turns on this option.
-fforce-addr
Force memory address constants to be copied into registers before
doing arithmetic on them. This may produce better code just as
-fforce-mem may.
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that don't
need one. This avoids the instructions to save, set up and restore
frame pointers; it also makes an extra register available in many
Optimize sibling and tail recursive calls.
-ftrapv
This option generates traps for signed overflow on addition, sub-
traction, multiplication operations.
-fno-inline
Don't pay attention to the "inline" keyword. Normally this option
is used to keep the compiler from expanding any functions inline.
Note that if you are not optimizing, no functions can be expanded
inline.
-finline-functions
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be worth
integrating in this way.
If all calls to a given function are integrated, and the function
is declared "static", then the function is normally not output as
assembler code in its own right.
-finline-limit=n
By default, gcc limits the size of functions that can be inlined.
This flag allows the control of this limit for functions that are
explicitly marked as inline (ie marked with the inline keyword or
defined within the class definition in c++). n is the size of
functions that can be inlined in number of pseudo instructions (not
counting parameter handling). The default value of n is 600.
Increasing this value can result in more inlined code at the cost
of compilation time and memory consumption. Decreasing usually
makes the compilation faster and less code will be inlined (which
presumably means slower programs). This option is particularly
useful for programs that use inlining heavily such as those based
on recursive templates with C++.
Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way, it represents
a count of assembly instructions and as such its exact meaning
might change from one release to an another.
-fkeep-inline-functions
Even if all calls to a given function are integrated, and the func-
tion is declared "static", nevertheless output a separate run-time
callable version of the function. This switch does not affect
"extern inline" functions.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't
turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the com-
piler to check if the variable was referenced, regardless of
whether or not optimization is turned on, use the
-fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating
This option implies -fmerge-constants. In addition to -fmerge-con-
stants this considers e.g. even constant initialized arrays or ini-
tialized constant variables with integral or floating point types.
Languages like C or C++ require each non-automatic variable to have
distinct location, so using this option will result in non-conform-
ing behavior.
-fno-branch-count-reg
Do not use ``decrement and branch'' instructions on a count regis-
ter, but instead generate a sequence of instructions that decrement
a register, compare it against zero, then branch based upon the
result. This option is only meaningful on architectures that sup-
port such instructions, which include x86, PowerPC, IA-64 and
S/390.
-fno-function-cse
Do not put function addresses in registers; make each instruction
that calls a constant function contain the function's address
explicitly.
This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the optimiza-
tions performed when this option is not used.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, and -fno-trap-
ping-math.
This option causes the preprocessor macro "__FAST_MATH__" to be
defined.
This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math func-
tions.
-fno-math-errno
Do not set ERRNO after calling math functions that are executed
with a single instruction, e.g., sqrt. A program that relies on
IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math func-
tions.
The default is -fmath-errno.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may violate IEEE or
ANSI standards. When used at link-time, it may include libraries
or startup files that change the default FPU control word or other
similar optimizations.
This option should never be turned on by any -O option since it can
if one relies on ``non-stop'' IEEE arithmetic, for example.
This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math func-
tions.
The default is -ftrapping-math.
-fbounds-check
For front-ends that support it, generate additional code to check
that indices used to access arrays are within the declared range.
This is currenly only supported by the Java and Fortran 77
front-ends, where this option defaults to true and false respec-
tively.
The following options control specific optimizations. The -O2 option
turns on all of these optimizations except -funroll-loops and -fun-
roll-all-loops. On most machines, the -O option turns on the
-fthread-jumps and -fdelayed-branch options, but specific machines may
handle it differently.
You can use the following flags in the rare cases when ``fine-tuning''
of optimizations to be performed is desired.
Not all of the optimizations performed by GCC have -f options to con-
trol them.
-fstrength-reduce
Perform the optimizations of loop strength reduction and elimina-
tion of iteration variables.
-fthread-jumps
Perform optimizations where we check to see if a jump branches to a
location where another comparison subsumed by the first is found.
If so, the first branch is redirected to either the destination of
the second branch or a point immediately following it, depending on
whether the condition is known to be true or false.
-fcse-follow-jumps
In common subexpression elimination, scan through jump instructions
when the target of the jump is not reached by any other path. For
example, when CSE encounters an "if" statement with an "else"
clause, CSE will follow the jump when the condition tested is
false.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow
jumps which conditionally skip over blocks. When CSE encounters a
simple "if" statement with no else clause, -fcse-skip-blocks causes
CSE to follow the jump around the body of the "if".