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When you invoke GNU CC, 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 documented here, since you rarely need to use any of them.
Most of the command line options that you can use with GNU CC 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.
@xref{Invoking options for compiling C++ programs.}++,,Compiling C++ Programs}, for a summary of special
The gcc program accepts options and file names as operands. Many
options have multiletter 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.
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
-c -S -E -o file -pipe -v -x language
-ansi -fcond-mismatch -fno-asm -fno-builtin -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char -fwritable-strings -traditional -traditional-cpp -trigraphs
}++ Dialect Options,,Options Controlling C++ Dialect}. -fall-virtual -fdollars-in-identifiers -felide-constructors -fenum-int-equiv -fexternal-templates -fmemoize-lookups -fno-strict-prototype -fnonnull-objects -fthis-is-variable -nostdinc++
-fsyntax-only -pedantic -pedantic-errors -w -W -Wall -Waggregate-return -Wcast-align -Wcast-qual -Wchar-subscript -Wcomment -Wconversion -Wenum-clash -Werror -Wformat -Wid-clash-len -Wimplicit -Wimport -Winline -Wmissing-prototypes -Wnested-externs -Woverloaded-virtual -Wparentheses -Wpointer-arith -Wredundant-decls -Wreturn-type -Wshadow -Wstrict-prototypes -Wswitch -Wtemplate-debugging -Wtraditional -Wtrigraphs -Wuninitialized -Wunused -Wwrite-strings
-a -dletters -fpretend-float -g -glevel -ggdb -gdwarf -gdwarf+ -gstabs -gstabs+ -gcoff -gxcoff -gxcoff+ -p -pg -save-temps -print-libgcc-file-name
-fcaller-saves -fcse-follow-jumps -fcse-skip-blocks -fdelayed-branch -fexpensive-optimizations -ffast-math -ffloat-store -fforce-addr -fforce-mem -finline-functions -fkeep-inline-functions -fno-default-inline -fno-defer-pop -fno-function-cse -fno-inline -fno-peephole -fomit-frame-pointer -frerun-cse-after-loop -fschedule-insns -fschedule-insns2 -fstrength-reduce -fthread-jumps -funroll-all-loops -funroll-loops -O -O2
-Aassertion -C -dD -dM -dN -Dmacro[=defn] -E -H -idirafter dir -include file -imacros file -iprefix file -iwithprefix dir -iwithprefixbefore dir -M -MD -MM -MMD -nostdinc -P -trigraphs -Umacro
-Wa,option
object-file-name -llibrary -nostartfiles -nostdlib -static -shared -symbolic -Wl,option -Xlinker option -u symbol
-Bprefix -Idir -I- -Ldir
-b machine -V version
M680x0 Options -m68000 -m68020 -m68020-40 -m68030 -m68040 -m68881 -mbitfield -mc68000 -mc68020 -mfpa -mnobitfield -mrtd -mshort -msoft-float VAX Options -mg -mgnu -munix SPARC Options -mepilogue -mfpu -mhard-float -mno-fpu -mno-epilogue -msoft-float -msparclite -mv8 Convex Options -mc1 -mc2 -mc32 -mc34 -mc38 -margcount -mnoargcount -mlong32 -mlong64 -mbolatile-cache -mvolatile-nocache AMD29K Options -m29000 -m29050 -mbw -mdw -mkernel-registers -mlarge -mnbw -mnodw -mnormal -msmall -mstack-check -muser-registers 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 Options and PowerPC -mcpu=cpu type -mpower -mno-power -mpower2 -pno-power2 -mpowerpc -mno-powerpc -mpowerpcsqr -mno-powerpcsqr -mpowerpc64 -mno-powerpc64 -mnew-mnemonics -mno-new-mnemonics -mnormal-toc -mminimal-toc -mno-fop-in-toc 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 -mcpu=cpu type -mips2 -mips3 -mint64 -mlong64 -mlonglong128 -mmips-as -mgas -mrnames -mno-rnames -mgpopt -mno-gpopt -mstats -mno-stats -mmemcpy -mno-memcpy -mno-mips-tfile -mmips-tfile -msoft-float -mhard-float -mabicalls -mno-abicalls -mhalf-pic -mno-half-pic -mlong-calls -mno-long-calls -G num -nocpp i386 Options -m486 -mno-486 -msoft-float -msvr3-shlib -mieee-fp -mno-fp-ret-in-387 HPPA Options -mpa-risc-1-0 -mpa-risc-1-1 -mlong-calls -mdisable-fpregs -mdisable-indexing -mtrailing-colon Intel 960 Options -mcpu type -mnumerics -msoft-float -mcode-align -mno-code-align -mleaf-procedures -mno-leaf-procedures -mtail-call -mno-tail-call -mcomplex-addr -mno-complex-addr -mclean-linkage -mno-clean-linkage -mic-compat -mic2.0-compat -mic3.0-compat -masm-compat -mintel-asm -mstrict-align -mno-strict-align -mold-align -mno-old-align DEC Alpha Options -mfp-regs -mno-fp-regs -mno-soft-float -msoft-float Clipper Options -mc300 -mc400 System V Options -G -Qy -Qn -YP,paths -Ym,dir
-fcall-saved-reg -fcall-used-reg -ffixed-reg -finhibit-size-directive -fno-common -fno-ident -fno-gnu-linker -fpcc-struct-return -fpic -fPIC -freg-struct-return -fshared-data -fshort-enums -fshort-double -fvolatile -fvolatile-global -fverbose-asm
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 object file; linking combines all the object files (those newly compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
file.c
file.i
file.ii
file.m
file.h
file.cc
file.cxx
file.C
file.s
file.S
other
You can specify the input language explicitly with the `-x' option:
-x language
c objective-c c++ c-header cpp-output c++-cpp-output assembler assembler-with-cpp
-x none
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
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
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
Input files which don't require preprocessing are ignored.
-o file
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.
-v
-pipe
C++ source files conventionally use one of the suffixes `.C',
`.cc', or `.cxx'; preprocessed C++ files use the suffix
`.ii'. GNU CC 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
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 shell script that calls GNU CC with the default language
set to C++, and automatically specifies linking against the GNU class
library libg++.
(1) On many systems, the script 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. See section Options Controlling C Dialect, for explanations of options for languages related to C. @xref{ explanations of options that are meaningful only for C++ programs.}++ Dialect Options,,Options Controlling C++ Dialect}, for
The following options control the dialect of C (or languages derived from C, such as C++ and Objective C) that the compiler accepts:
-ansi
This turns off certain features of GNU C that are incompatible with ANSI
C, such as the asm, inline 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 ANSI trigraph feature, and disallows `$' as part of
identifiers.
The alternate keywords __asm__, __extension__,
__inline__ and __typeof__ continue to work despite
`-ansi'. You would not want to use them in an ANSI C program, of
course, but it 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-ANSI programs to be rejected gratuitously. For that, `-pedantic' is required in addition to `-ansi'. See section Options to Request or Suppress Warnings.
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
ANSI standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
The functions alloca, abort, exit, and
_exit are not builtin functions when `-ansi' is used.
-fno-asm
asm, inline or typeof as a
keyword. These words may then be used as identifiers. You can use the
keywords __asm__, __inline__ and __typeof__
instead. `-ansi' implies `-fno-asm'.
-fno-builtin
abort,
abs, alloca, cos, exit, fabs,
ffs, labs, memcmp, memcpy, sin,
sqrt, strcmp, strcpy, and strlen.
GCC normally generates special code to handle certain builtin 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.
The `-ansi' option prevents alloca and ffs from being
builtin functions, since these functions do not have an ANSI standard
meaning.
-trigraphs
-traditional
extern declarations take effect globally even if they
are written inside of a function definition. This includes implicit
declarations of functions.
typeof, inline, signed, const
and volatile are not recognized. (You can still use the
alternative keywords such as __typeof__, __inline__, and
so on.)
unsigned short and unsigned char promote
to unsigned int.
register are preserved by
longjmp. Ordinarily, GNU C follows ANSI C: automatic variables
not declared volatile may be clobbered.
__STDC__ is not defined when you use
`-traditional', but __GNUC__ is (since the GNU extensions
which __GNUC__ indicates are not affected by
`-traditional'). If you need to write header files that work
differently depending on whether `-traditional' is in use, by
testing both of these predefined macros you can distinguish four
situations: GNU C, traditional GNU C, other ANSI C compilers, and other
old C compilers. See section 'Standard Predefined Macros' in The C Preprocessor, for more discussion of these and other
predefined macros.
this is permitted with
`-traditional'. (The option `-fthis-is-variable' also has
this effect.)
You may wish to use `-fno-builtin' as well as `-traditional' if your program uses names that are normally GNU C builtin functions for other purposes of its own.
-traditional-cpp
-fcond-mismatch
-funsigned-char
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
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
signed or unsigned. By
default, such a bitfield is signed, because this is consistent: the
basic integer types such as int are signed types.
However, when `-traditional' is used, bitfields are all unsigned no matter what.
-fwritable-strings
Writing into string constants is a very bad idea; "constants" should be constant.
-fallow-single-precision
Traditional K&R C promotes all floating point operations to double precision, regardless of the sizes of the operands. On the architecture 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 precision, use this option. This option has no effect when compiling with ANSI or GNU C conventions (the default).
This section describes the command-line options that are only meaningful
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 -felide-constructors -O -c firstClass.C
In this example, only `-felide-constructors' is an option meant only for C++ programs; you can use the other options with any language supported by GNU CC.
Here is a list of options that are only for compiling C++ programs:
-fall-virtual
new or
delete member operators) are treated as virtual functions of the
class where they appear.
This does not mean that all calls to these member functions will be made through the internal table of virtual functions. Under some circumstances, the compiler can determine that a call to a given virtual function can be made directly; in these cases the calls are direct in any case.
-fdollars-in-identifiers
-felide-constructors
y directly from the call to foo without going
through a temporary in the following code:
A foo (); A y = foo ();
Without this option, GNU C++ (1) initializes y by calling the
appropriate constructor for type A; (2) assigns the result of
foo to a temporary; and, finally, (3) replaces the initial value of
y with the temporary.
The default behavior (`-fno-elide-constructors') is specified by the draft ANSI C++ standard. If your program's constructors have side effects, `-felide-constructors' can change your program's behavior, since some constructor calls may be omitted.
-fenum-int-equiv
int to enumeration types. Normally
GNU C++ allows conversion of enum to int, but not the
other way around.
-fexternal-templates
When your code is compiled with `-fexternal-templates', all
template instantiations are external. You must arrange for all
necessary instantiations to appear in the implementation file; you can
do this with a typedef that references each instantiation needed.
Conversely, when you compile using the default option
`-fno-external-templates', all template instantiations are
explicitly internal.
You do not need to specify `-fexternal-templates' when compiling a file that does not define and instantiate templates used in other files, even if your file uses templates defined in other files that are compiled with `-fexternal-templates'. The only side effect is an increase in object size for each file that you compile without `-fexternal-templates'.
-fmemoize-lookups
-fsave-memoized
The first time the compiler must build a call to a member function (or reference to a data member), it must (1) determine whether the class implements member functions of that name; (2) resolve which member function to call (which involves figuring out what sorts of type conversions need to be made); and (3) check the visibility of the member function to the caller. All of this adds up to slower compilation. Normally, the second time a call is made to that member function (or reference to that data member), it must go through the same lengthy process again. This means that code like this:
cout << "This " << p << " has " << n << " legs.\n";
makes six passes through all three steps. By using a software cache, a "hit" significantly reduces this cost. Unfortunately, using the cache introduces another layer of mechanisms which must be implemented, and so incurs its own overhead. `-fmemoize-lookups' enables the software cache.
Because access privileges (visibility) to members and member functions may differ from one function context to the next, G++ may need to flush the cache. With the `-fmemoize-lookups' flag, the cache is flushed after every function that is compiled. The `-fsave-memoized' flag enables the same software cache, but when the compiler determines that the context of the last function compiled would yield the same access privileges of the next function to compile, it preserves the cache. This is most helpful when defining many member functions for the same class: with the exception of member functions which are friends of other classes, each member function has exactly the same access privileges as every other, and the cache need not be flushed.
-fno-strict-prototype
foo takes no arguments.
-fnonnull-objects
Normally, GNU C++ makes conservative assumptions about objects reached
through references. For example, the compiler must check that a
is not null in code like the following:
obj &a = g (); a.f (2);
Checking that references of this sort have non-null values requires extra code, however, and it is unnecessary for many programs. You can use `-fnonnull-objects' to omit the checks for null, if your program doesn't require checking.
-fthis-is-variable
this. The incorporation of
user-defined free store management into C++ has made assignment to
`this' an anachronism. Therefore, by default it is invalid to
assign to this within a class member function; that is, GNU C++
treats the type of `this' in a member function of class X
to be `X *const'. However, for backwards compatibility, you can
make it valid with `-fthis-is-variable'.
-nostdinc++
-traditional
In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-fno-default-inline
-Wenum-clash
-Woverloaded-virtual
-Wtemplate-debugging
+en
cfront 1.x. See section Options for Code Generation Conventions.
Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error.
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.
These options control the amount and kinds of warnings produced by GNU CC:
-fsyntax-only
-w
-Wno-import
-pedantic
Valid ANSI standard C programs should compile properly with or without this option (though a rare few will require `-ansi'). However, without this option, certain GNU extensions and traditional 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