Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.
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 documented 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.
M68hc1x Options -m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 -mauto-incdec -minmax -mlong-calls -mrelax -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 -mlittle-endian -mlive-g0 -mno-app-regs -mno-faster-structs -mno-flat -mno-fpu -mno-impure-text -mno-stack-bias -mno-unaligned-doubles -msoft-float -msoft-quad-float -msparclite -mstack-bias -msupersparc -munaligned-doubles -mv8 -threads -pthreads
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-prolog -mno-sched-prolog -mlittle-endian -mbig-endian -mwords-little-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 -mabort-on-noreturn -mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base -mpic-register=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 -mhandle-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 -mmultiple -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 -mcall-netbsd -maix-struct-return -msvr4-struct-return -mabi=altivec -mabi=no-altivec -mabi=spe -mabi=no-spe -misel=yes -misel=no -mprototype -mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -mwindiss -G num -pthread
Darwin Options
-all_load -allowable_client -arch -arch_errors_fatal -arch_only -bind_at_load -bundle -bundle_loader -client_name -compatibility_version -current_version -dependency-file -dylib_file -dylinker_install_name -dynamic -dynamiclib -exported_symbols_list -filelist -flat_namespace -force_cpusubtype_ALL -force_flat_namespace -headerpad_max_install_names -image_base -init -install_name -keep_private_externs -multi_module -multiply_defined -multiply_defined_unused -noall_load -nomultidefs -noprebind -noseglinkedit -pagezero_size -prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs -sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder -seg_addr_table -seg_addr_table_filename -seglinkedit -segprot -segs_read_only_addr -segs_read_write_addr -single_module -static -sub_library -sub_umbrella -twolevel_namespace -umbrella -undefined -unexported_symbols_list -weak_reference_mismatches -whatsloaded
RT Options -mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs -mfull-fp-blocks -mhc-struct-return -min-line-mul -mminimum-fp-blocks -mnohc-struct-return
MIPS Options -mabicalls -march=cpu-type -mtune=cpu=type -mcpu=cpu-type -membedded-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 -mbranch-likely -mno-branch-likely
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-boundary=num -mmmx -msse -msse2 -msse3 -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 -mdisable-indexing -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld -mjump-in-delay -mlinker-opt -mlong-calls -mlong-load-store -mno-big-switch -mno-disable-fpregs -mno-disable-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 -msio -mwsio -nolibdld -static -threads
Intel 960 Options -mcpu-type -masm-compat -mclean-linkage -mcode-align -mcomplex-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 -mvms-return-codes
H8/300 Options -mrelax -mh -ms -mn -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 -mpadstruct -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 -mapp-regs -mno-app-regs -mdisable-callt -mno-disable-callt -mv850e -mv850 -mbig-switch
NS32K Options -m32032 -m32332 -m32532 -m32081 -m32381 -mmult-add -mnomult-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 -msingle-exit -mno-single-exit
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mb-step -mregister-names -mno-sdata -mconstant-gp -mauto-pic -minline-float-divide-min-latency -minline-float-divide-max-throughput -minline-int-divide-min-latency -minline-int-divide-max-throughput -mno-dwarf2-asm -mfixed-range=register-range
D30V Options -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-stacksize=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 -mmul-bug-workaround -mno-mul-bug-workaround
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 -mserialize-volatile -mno-serialize-volatile -mtext-section-literals -mno-text-section-literals -mtarget-align -mno-target-align -mlongcalls -mno-longcalls
FRV Options
-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64
-mhard-float -msoft-float -malloc-cc -mfixed-cc
-mdword -mno-dword -mdouble -mno-double
-mmedia -mno-media -mmuladd -mno-muladd -mlibrary-pic
-macc-4 -macc-8 -mpack -mno-pack -mno-eflags
-mcond-move -mno-cond-move -mscc -mno-scc
-mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch
-mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec
-mno-nested-cond-exec -mtomcat-stats
-mcpu=cpu
For any given input file, the file name suffix determines what kind of
compilation is done:
You can specify the input language explicitly with the -x option:
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
treelang
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.
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.
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.
Input files which don't require preprocessing are ignored.
Since only one output file can be specified, it does not make sense to use -o when compiling more than one input file, unless you are producing an executable file as output.
If -o is not specified, the default is to put an executable file
in a.out, the object file for source.suffix in
source.o, its assembler file in source.s, and
all preprocessed C source on standard output.
However, C++ programs often require class libraries as well as a compiler 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 command-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.
This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the "asm" and "typeof" keywords, and predefined macros such as "unix" and "vax" that identify the type of system 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 semantics
defined by ISO C (such as "alloca" and "ffs") are not built-in
functions with -ansi is used.
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 with previous C standards. For example, you may use "__restrict__" even when -std=c99 is not specified.
The -std options specifying some version of ISO C have the same effects as -ansi, except that features that were not in ISO C90 but are in the specified version (for example, // comments and the "inline" keyword in ISO C99) are not disabled.
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.
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.
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.
With the -fno-builtin-function option 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 -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
The semantics of this option will change if ``cc1'', ``cc1plus'', and
``cc1obj'' are merged.
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.
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.
Writing into string constants is a very bad idea; ``constants'' should be constant.
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:
The default is version 1.
This option is no longer useful on most targets, now that support has
been added for putting variables into BSS without making them common.
This option might be removed in a future release of G++. For maximum
portability, you should structure your code so that it works with
string constants that have type "const char *".
This option is deprecated.
This option is deprecated.
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.
This optimization requires GNU as and GNU ld. Not all systems support
this option. -Wl,--gc-sections is ignored without -static.
In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:
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 incompatibilities at this point include:
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 problem
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.
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.
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make the
union too small by the number of bits in an "int".
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes that the
"A" data member of "B" is already at offset zero.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler will rearrange the member initializers for i
and j to match the declaration order of the members, emitting
a warning to that effect. This warning is enabled by -Wall.
The following -W... options are not affected by -Wall.
Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:
When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings.
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:
B* b;
b->f();
will fail to compile.
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 =.
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for
Objective-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 Objective-C
programs:
You can request many specific warnings with options beginning -W, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the default.
The following options control the amount and kinds of warnings produced
by GCC; for further, language-specific options also refer to
C++ Dialect Options and Objective-C Dialect Options.
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++ features 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__". However, 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 diagnostics 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.)
The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 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 format features not in the selected standard version (but not for "strfmon" formats, since those are not in any version of the C standard).
Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.
-Wformat is included in -Wall. For more control over some
aspects of format checking, the options -Wno-format-y2k,
-Wno-format-extra-args, -Wno-format-zero-length,
-Wformat-nonliteral, -Wformat-security, and
-Wformat=2 are available, but are not included in -Wall.
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 Specification says that such unused arguments are allowed.
-Wnonnull is included in -Wall and -Wformat. It
can be disabled with the -Wno-nonnull option.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
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 ();
}
}
The C standard defines the order in which expressions in a C program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a "&&", "||", "? :" or "," (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain 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. However, 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 positive 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 programs. 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 subtle 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>.
For C++, a function without return type always produces a diagnostic
message, even when -Wno-return-type is specified. The only
exceptions are main and functions defined in system headers.
To suppress this warning use the unused attribute.
To suppress this warning use the unused attribute.
To suppress this warning use the unused attribute.
To suppress this warning cast the expression to void.
In order to get a warning about an unused function parameter, you must
either specify -W -Wunused or separately specify
-Wunused-parameter.
These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don't specify -O, you simply won't get these warnings.
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 warnings 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;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is
always initialized, but GCC doesn't know this. Here is
another common case:
{
int save_y;
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 functions
you use that never return as "noreturn".
The following -W... options are not implied by -Wall.
Some of them warn about constructions that users generally do not
consider questionable, but which occasionally you might wish to check
for; others warn about constructions that are necessary or hard to avoid
in some cases, and there is no simple way to modify the code to suppress
the warning.
foo (a)
{
if (a > 0)
return a;
}
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
struct s { int f, g, h; };
struct s x = { 3, 4 };
The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers. If you are doing this, then you need
to compute (by analyzing the code, or in some other way) the maximum or
likely maximum error that the computation introduces, and allow for it
when performing comparisons (and when producing 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.
Also, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the assignment
"x = -1" if "x" is unsigned. But do not warn about explicit
casts like "(unsigned) -1".
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
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 procedure 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
version of a program there is often substantial code which checks
correct 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.
The compiler uses a variety of heuristics to determine whether or not
to inline a function. For example, the compiler takes into account
the size of the function being inlined and the the amount of inlining
that has already been done in the current function. Therefore,
seemingly insignificant changes in the source program can cause the
warnings produced by -Winline to appear or disappear.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program. If you want to control 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 surprising 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.
This option is deprecated.
This option is deprecated.
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 definitions present in the program. Some debuggers support macro expansion 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.
For profile-directed block ordering, compile the program with -fprofile-arcs plus optimization and code generation options, generate the arc profile information by running the program on a selected workload, and then compile the program again with the same optimization and code generation options plus -fbranch-probabilities.
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 instrumented: 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.
Use -ftest-coverage with -fprofile-arcs; the latter option adds instrumentation to the program, which then writes execution counts to another data file:
Coverage data will map better to the source files if -ftest-coverage is used without optimization.