GCC(1) GNU GCC(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. In Apple's version of GCC, both cc and gcc are actually symbolic links to gcc3, while c++ and g++ are links to g++3. Note that Apple's GCC includes a number of extensions to standard GCC (flagged below with ``APPLE ONLY''), and that not all generic GCC options are available or supported on Darwin / Mac OS X. In particu- lar, Apple does not currently support the compilation of Fortran, Ada, or Java, although there are third parties who have made these work. 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 -c -S -E -o file -pipe -pass-exit-codes -x language -ObjC (APPLE ONLY) -ObjC++ (APPLE ONLY) -arch arch (APPLE ONLY) -v -### --target-help --help C Language Options -ansi -std=standard -aux-info filename -faltivec (APPLE ONLY) -fno-asm -fno-builtin -fno-builtin-function -fhosted -ffreestand- ing -trigraphs -no-integrated-cpp -traditional -traditional-cpp -fallow-single-precision -fcond-mismatch -fconstant-cfstrings (APPLE ONLY) -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char -fwritable-strings -fshort-wchar -fpascal-strings (APPLE ONLY) -fcoalesce (APPLE ONLY) -fweak-coalesced (APPLE ONLY) -Wno-#warnings (APPLE ONLY) -Wextra-tokens (APPLE ONLY) -Wpragma- once (APPLE ONLY) -Wnewline-eof (APPLE ONLY) -Wno-altivec-long-dep- recated (APPLE ONLY) C++ Language Options -fno-access-control -fcheck-new -fconserve-space -fno-const- strings -fdollars-in-identifiers -fno-elide-constructors -fno- enforce-eh-specs -fexternal-templates -falt-external-templates -ffor-scope -fno-for-scope -fno-gnu-keywords -fno-implicit-tem- plates -fno-implicit-inline-templates -fno-implement-inlines -find- irect-virtual-calls (APPLE ONLY) -fapple-kext (APPLE ONLY) -fcoa- lesce-templates (APPLE ONLY) -fms-extensions -fno-nonansi-builtins -fno-operator-names -fno-optional-diags -fpermissive -frepo -fno- rtti -fstats -ftemplate-depth-n -fuse-cxa-atexit -fvtable-gc -fno-weak -nostdinc++ -fno-default-inline -Wctor-dtor-privacy -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 -Wno-long-double (APPLE ONLY) -Wlong-long -Wmain -Wmissing-braces -Wmissing-declarations -Wmissing-format-attribute -Wmissing-noreturn -Wmost (APPLE ONLY) -Wmultichar -Wno-format-extra-args -Wno-format-y2k -Wno-import -Wpacked -Wpadded -Wparentheses -Wpointer-arith -Wredundant- decls -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare -Wswitch -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized -Wunknown-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-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 -fprofile-arcs -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 -falign-functions=n -falign-jumps=n -falign-labels=n -falign- loops=n -fbranch-probabilities -fcaller-saves -fcprop-registers -fcse-follow-jumps -fcse-skip-blocks -fdata-sections -fdelayed- branch -fdelete-null-pointer-checks -fexpensive-optimizations -ffast-math -ffloat-store -fforce-addr -fforce-mem -ffunction- sections -fgcse -fgcse-lm -fgcse-sm -finline-functions -finline- limit=n -fkeep-inline-functions -fkeep-static-consts -fmerge-con- stants -fmerge-all-constants -fmove-all-movables -fno-default- inline -fno-defer-pop -fno-function-cse -fno-guess-branch-proba- bility -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 -frerun-loop-opt -fschedule-insns -fschedule-insns2 -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 -depen- dency-file (APPLE ONLY) -no-cpp-precomp (APPLE ONLY) --dump-pch name (APPLE ONLY) --load-pch name (APPLE ONLY) -trigraphs -undef -Umacro -Wp,option Assembler Option -Wa,option Linker Options object-file-name -llibrary -nostartfiles -nodefaultlibs -nost- dlib -no-c++filt (APPLE ONLY) -s -static -static-libgcc -shared -shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol Directory Options -Bprefix -Idir -I- -Fdir (APPLE ONLY) -Ldir -specs=file Target Options -b machine -V version Machine Dependent Options 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 -malign-mac68k (APPLE ONLY) -malign-power (APPLE ONLY) -malign-natural (APPLE ONLY) -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 -mdynamic-no-pic (APPLE ONLY) -mlong-branch (APPLE ONLY) -mcall-aix -mcall-sysv -mcall-netbsd -maix-struct- return -msvr4-struct-return -mabi=altivec -mabi=no-altivec -mproto- type -mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -G num -pthread 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 -msse-math -m3dnow -mthreads -mno-align- stringops -minline-all-stringops -mpush-args -maccumulate-outgo- ing-args -m128bit-long-double -m96bit-long-double -mregparm=num -momit-leaf-frame-pointer -mno-red-zone -m32 -m64 Code Generation Options -fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon- call-exceptions -funwind-tables -fasynchronous-unwind-tables -fin- hibit-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 -fvolatile -fvolatile-global -fvolatile-static -fverbose-asm -fpack-struct -fstack-check -fstack-limit-register=reg -fstack- limit-symbol=sym -fargument-alias -fargument-noalias -fargument- noalias-global -fleading-underscore 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 object file; linking combines all the object files (those newly com- piled, 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 C source code which must be preprocessed. 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.mm file.M Objective-C++ source code which must be preprocessed. (APPLE ONLY) file.mii Objective-C++ source code which should not be preprocessed. (APPLE ONLY) 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. Apple's version of GCC runs the preprocessor on these files as well as those ending in .S. file.S Assembler code which must be preprocessed. other An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way. You can specify the input language explicitly with the -x option: -x language Specify explicitly the language for the following input files (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 objective-c++ (APPLE ONLY) 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). -ObjC -ObjC++ These are similar in effect to -x objective-c and -x objective-c++, but affect only the choice of compiler for files already identified as source files. (APPLE ONLY) -arch arch Compile for the specified target architecture arch. The allowable values are i386 and ppc. Multiple options work, and direct the compiler to produce ``fat'' binaries including object code for each architecture specified with -arch. This option only works if assembler and libraries are available for each architecture speci- fied. (APPLE ONLY) -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, an object file, an assembler file or preprocessed C code. 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 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. 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: -ansi In C mode, support all ISO C89 programs. In C++ mode, remove GNU extensions that conflict with ISO C++. This turns off certain features of GCC that are incompatible with ISO C89 (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 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 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 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 C89 but are in the specified version (for example, // comments and the "inline" keyword in ISO C99) are not disabled. -aux-info filename 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. -faltivec Enable the AltiVec language extensions, as defined in Motorola's AltiVec PIM. This includes the recognition of "vector" and "pixel" as (context-dependent) keywords, the definition of built-in func- tions such as "vec_add", and other extensions. Note that unlike the option -maltivec, the extensions do not require the inclusion of any special header files. (APPLE ONLY) -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: #define abs(n) __builtin_abs ((n)) #define strcpy(d, s) __builtin_strcpy ((d), (s)) -fhosted Assert that compilation takes place in a hosted environment. This implies -fbuiltin. A hosted environment is one in which the entire standard library is available, and in which "main" has a return type of "int". Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding. -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. -no-cpp-precomp By default, Apple's GCC preprocesses C and Objective-C with a spe- cial preprocessor called cpp-precomp that supports precompiled headers. This preprocessor cannot always handle every construct that GCC supports; use -no-cpp-precomp to switch to using GNU cpp instead. (APPLE ONLY) --dump-pch name Dump the state of the compiler into a directory named name, after processing all the other arguments. This is useful for creating precompiled headers. (APPLE ONLY) --load-pch name Restore the state of the compiler from the directory name before processing the other arguments. The net effect is similar to -include, but it happens much more quickly. (APPLE ONLY) So for instance if the file myprefix.c #includes various headers that are useful to all files in your program, you can do gcc --dump-pch foo -c myprefix.c gcc --load-pch foo myfile1.c gcc --load-pch foo myfile2.c gcc --load-pch foo myfile2.c ... -traditional Attempt to support some aspects of traditional C compilers. Specifically: o All "extern" declarations take effect globally even if they are written inside of a function definition. This includes implicit declarations of functions. o 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.) o Comparisons between pointers and integers are always allowed. o Integer types "unsigned short" and "unsigned char" promote to "unsigned int". o Out-of-range floating point literals are not an error. o Certain constructs which ISO regards as a single invalid pre- processing number, such as 0xe-0xd, are treated as expressions instead. o 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.) o 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. o 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. This option is deprecated and may be removed. You may wish to use -fno-builtin as well as -traditional if your program uses names that are normally GNU C built-in functions for other purposes of its own. You cannot use -traditional if you include any header files that rely on ISO C features. Some vendors are starting to ship systems 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 In Apple's version of GCC, this means to use GNU cpp instead of cpp-precomp to preprocess. This meaning of the option is depre- cated, and will eventually revert to its standard meaning. -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. Writing into string constants is a very bad idea; ``constants'' should be constant. -fconstant-cfstrings Enable the automatic creation of a CoreFoundation-type constant string whenever a special builtin "__builtin__CFStringMakeCon- stantString" is called on a literal string. (APPLE ONLY) -fallow-single-precision Do not promote single precision math operations to double preci- sion, even when compiling with -traditional. Traditional K&R C promotes all floating point operations to double precision, regardless of the sizes of the operands. On the archi- 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). -fshort-wchar Override the underlying type for wchar_t to be short unsigned int instead of the default for the target. This option is useful for building programs to run under WINE. -fpascal-strings Allow Pascal-style string literals to be constructed. (APPLE ONLY) -fcoalesce Coalesce duplicated functions and data. The linker will discard all but one, saving space. Enabled by default. (APPLE ONLY) -fweak-coalesced Use the new OS X "weak_definitions" section attribute for coalesced items. A single "normal" definition will be chosen by the linker over any number of weakly-coalesced ones. (APPLE ONLY) 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 throw(), 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 *". -fdollars-in-identifiers Accept $ in identifiers. You can also explicitly prohibit use of $ with the option -fno-dollars-in-identifiers. (GNU C allows $ by default on most target systems, but there are a few exceptions.) Traditional C allowed the character $ to form part of identifiers. However, ISO C and C++ forbid $ in identifiers. -fno-elide-constructors The C++ standard allows an implementation to omit creating a tempo- 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 instantiations. -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. -findirect-virtual-calls Do not make direct calls to virtual functions; instead, always go through the vtable. (APPLE ONLY) -fapple-kext Alter vtables, destructors, and other implementation details to more closely resemble the GCC 2.95 ABI. This is to make kernel extensions loadable by Darwin kernels built using older compilers, and is required to build any Darwin kernel extension. (APPLE ONLY) -fcoalesce-templates Mark instantiated templates as "coalesced": the linker will discard all but one, thus saving space. (APPLE ONLY) -fms-extensions Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-stan- dard syntax. -fno-nonansi-builtins Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include "ffs", "alloca", "_exit", "index", "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". This option is not supported on Mac OS X. -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- port this option. -Wl,--gc-sections is ignored without -static. -fno-weak Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak symbols if they are avail- able. This option exists only for testing, and should not be used by end-users; it will result in inferior code and has no benefits. This option may be removed in a future release of G++. -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. -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 public static member functions. -Wnon-virtual-dtor (C++ only) Warn when a class declares a non-virtual destructor that should probably be virtual, because it looks like the class will be used polymorphically. -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: 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: o Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated memory. o Item 12: Prefer initialization to assignment in constructors. o Item 14: Make destructors virtual in base classes. o Item 15: Have "operator=" return a reference to "*this". o 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: o Item 6: Distinguish between prefix and postfix forms of incre- ment and decrement operators. o 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-template- friend allows the compiler to check existing code for potential trouble spots, and is on by default. This new compiler behavior can be turned off with -Wno-non-template-friend which keeps the conformant compiler code but disables the helpful warning. -Wold-style-cast (C++ only) Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (static_cast, reinter- pret_cast, and const_cast) are less vulnerable to unintended effects, and much easier to grep for. -Woverloaded-virtual (C++ only) 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- ported by GCC. Here is a list of options that are only for compiling Objective-C pro- grams: -fconstant-string-class=class-name Use class-name as the name of the class to instantiate for each literal string specified with the syntax "@"..."". The default class name is "NXConstantString". -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. 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 @ref{C++ 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. -Wno-#warnings Inhibit warning messages issued by #warning. -Wpragma-once Warn about the use of #pragma once. (APPLE ONLY) -Wextra-tokens Warn about extra tokens at the end of prepreprocessor directives. (APPLE ONLY) -Wnewline-eof Warn about files missing a newline at the end of the file. (APPLE ONLY) -Wno-altivec-long-deprecated Do not warn about the use of the deprecated 'long' keyword in AltiVec data types. (APPLE ONLY) -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 some machines. -Wcomment Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline appears in a // comment. -Wformat Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types appropriate to the format string 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-format-extra- args, -Wformat-nonliteral, -Wformat-security and -Wformat=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 -Wformat-secu- rity. -Wimplicit-int Warn when a declaration does not specify a type. -Wimplicit-function-declaration -Werror-implicit-function-declaration Give a warning (or error) whenever a function is used before being declared. -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- tions of sequence point rules in the C standard. The C standard defines the order in which expressions in a C pro- gram 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 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 . -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 static function is declared but not defined or a non\-inline static function is unused. -Wunused-label Warn whenever a label is declared but not used. To suppress this warning use the unused attribute. -Wunused-parameter 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; } foo (x); } If the value of "y" is always 1, 2 or 3, then "x" is always ini- tialized, 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 pos- sible 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. -Wmost This is equivalent to -Wall -Wno-parentheses. (APPLE ONLY) -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 in conjunction with this option will not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used. The following -W... options are not implied by -Wall. Some of them warn about constructions that users generally do not consider question- able, 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. -W Print extra warning messages for these events: o 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; } o 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. o An unsigned value is compared against zero with < or <=. o 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. o Storage-class specifiers like "static" are not the first things in a declaration. According to the C Standard, this usage is obsolescent. o 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.) o If -Wall or -Wunused is also specified, warn about unused argu- ments. o 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.) o 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 }; o 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 }; -Wfloat-equal Warn if floating point values are used in equality comparisons. 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 analysing 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. -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. o 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. o 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 direc- tives like #pragma not understood by traditional C by indenting them. Some traditional implementations would not recognize #elif, so it suggests avoiding it altogether. o A function-like macro that appears without arguments. o The unary plus operator. o 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 "". 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. o A function declared external in one block and then used after the end of the block. o A "switch" statement has an operand of type "long". o A non-"static" function declaration follows a "static" one. This construct is not accepted by some traditional C compilers. o 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. o Usage of ISO string concatenation is detected. o Initialization of automatic aggregates. o Identifier conflicts with labels. Traditional C lacks a sepa- rate namespace for labels. o Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. "__STDC__" to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case. o Conversions by prototypes between fixed/floating point values 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- ment "x = -1" if "x" is unsigned. But do not warn about explicit casts like "(unsigned) -1". -Wsign-compare Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. This warning is also enabled by -W; to get the other warnings of -W without this warning, use -W -Wno-sign-compare. -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; } __attribute__((packed)); struct bar { char z; struct foo f; }; -Wpadded Warn if padding is included in a structure, either to align an ele- ment of the structure or to align the whole structure. Sometimes 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. -Wno-long-double Inhibit warning if the long double type is used. (APPLE ONLY) -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 program or GCC: -g Produce debugging information in the operating system's native for- mat (stabs, COFF, XCOFF, or DWARF). GDB can work with this debug- ging information. 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 con- trol for certain whether to generate the extra information, use -gstabs+ or -gstabs (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. (Other debug formats, such as -gcoff, are not supported in Darwin or Mac OS X.) -glevel -ggdblevel -gstabslevel 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. -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. -a Generate extra code to write profile information for basic blocks, which will record the number of times each basic block is executed, the basic block start address, and the function name containing the basic block. If -g is used, the line number and filename of the start of the basic block will also be recorded. If not overridden by the machine description, the default action is to append to the text file bb.out. This data could be analyzed by a program like "tcov". Note, how- ever, that the format of the data is not what "tcov" expects. Eventually GNU "gprof" should be extended to process this data. -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 records how many times each branch is executed and how many times it is taken. When the compiled program exits it saves this data to a file called sourcename.da for each source file. For profile-directed block ordering, compile the program with -fprofile-arcs plus optimization and code generation options, gen- erate 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-probabili- ties. The other use of -fprofile-arcs is for use with "gcov", when it is used with the -ftest-coverage option. GCC supports two methods of determining code coverage: the options that support "gcov", and options -a and -ax, which write information to text files. The options that support "gcov" do not need to instrument every arc in the program, so a program compiled with them runs faster than a program compiled with -a, which adds instrumentation code to every basic block in the program. The tradeoff: since "gcov" does not have execution counts for all branches, it must start with the exe- cution counts for the instrumented branches, and then iterate over the program flow graph until the entire graph has been solved. Hence, "gcov" runs a little more slowly than a program which uses information from -a and -ax. 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. This option makes it possible to estimate branch probabilities and to calculate basic block execution counts. In general, basic block execution counts as provided by -a do not give enough information to estimate all branch probabilities. -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. C Dump after the first if conversion, to the file file.17.ce. d Dump after delayed branch scheduling, to file.31.dbr. D Dump all macro definitions, at the end of preprocessing, in addition to normal output. e Dump after SSA optimizations, to file.04.ssa and file.07.ussa. 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. p Annotate the assembler output with a comment indicating which pattern and alternative was used. The length of each instruc- tion is also printed. P Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation. v For each of the other indicated dump files (except for file.00.rtl), dump a representation of the control flow graph 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 Dump after all tree based optimization, to file.optimized. inlined Dump after function inlining, to file.inlined. -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. -print-prog-name=program Like -print-file-name, but searches for a program such as cpp. -print-libgcc-file-name Same as -print-file-name=libgcc.a. This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do gcc -nostdlib ... `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. When you specify -O, the compiler turns on -fthread-jumps and -fde- fer-pop on all machines. The compiler turns on -fdelayed-branch on machines that have delay slots, and -fomit-frame-pointer on machines that can support debugging even without a frame pointer. On some machines the compiler also turns on other flags. In Apple's version of GCC, -fstrict-aliasing, -freorder-blocks, and -fsched-interblock are disabled by default when optimizing. -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. Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos. -O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions and -frename-registers options. -O0 Do not optimize. -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 functions. It also makes debugging impossible on some machines. On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro "FRAME_POINTER_REQUIRED" controls whether a target machine supports this flag. -foptimize-sibling-calls 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 point constants) accross compilation units. This option is default for optimized compilation if assembler and linker support it. Use -fno-merge-constants to inhibit this behav- ior. -fmerge-all-constants Attempt to merge identical constants and identical variables. 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-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 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 -fno-unsafe-math-optimizations. -fno-trapping-math Compile code assuming that floating-point operations cannot gener- ate user-visible traps. Setting this option may allow faster code 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. The following options control specific optimizations. The -O2 option turns on all of these optimizations except -funroll-loops and -funroll- all-loops. On most machines, the -O option turns on the -fthread-jumps and -fdelayed-branch options, but specific machines may handle it dif- ferently. 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". -frerun-cse-after-loop Re-run common subexpression elimination after loop optimizations has been performed. -frerun-loop-opt Run the loop optimizer twice. -fgcse Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation. Note: When compiling a program using computed gotos, a GCC exten- sion, you may get better runtime performance if you disable the global common subexpression elmination pass by adding -fno-gcse to the command line. -fgcse-lm When -fgcse-lm is enabled, global common subexpression elimination will attempt to move loads which are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop. -fgcse-sm When -fgcse-sm is enabled, A store motion pass is run after global common subexpression elimination. This pass will attempt to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop. -fdelete-null-pointer-checks Use global dataflow analysis to identify and eliminate useless checks for null pointers. The compiler assumes that dereferencing a null pointer would have halted the program. If a pointer is checked after it has already been dereferenced, it cannot be null. In some environments, this assumption is not true, and programs can safely dereference null pointers. Use -fno-delete-null-pointer- checks to disable this optimization for programs which depend on that behavior. -fexpensive-optimizations Perform a number of minor optimizations that are relatively expen- sive. -foptimize-register-move -fregmove Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to maximize the amount of register tying. This is especially helpful on machines with two-operand instructions. GCC enables this optimization by default with -O2 or higher. Note -fregmove and -foptimize-register-move are the same optimiza- tion. -fdelayed-branch If supported for the target machine, attempt to reorder instruc- tions to exploit instruction slots available after delayed branch instructions. -fschedule-insns If supported for the target machine, attempt to reorder instruc- tions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating point instruction is required. -fschedule-insns2 Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small num- ber of registers and where memory load instructions take more than one cycle. -ffunction-sections -fdata-sections Place each function or data item into its own section in the output file if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file. Use these options on systems where the linker can perform optimiza- tions to improve locality of reference in the instruction space. HPPA processors running HP-UX and Sparc processors running Solaris 2 have linkers with such optimizations. Other systems using the ELF object format as well as AIX may have these optimizations in the future. Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker will create larger object and executable files and will also be slower. You will not be able to use "gprof" on all systems if you specify this option and you may have problems with debugging if you specify both this option and -g. -fcaller-saves Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced. This option is always enabled by default on certain machines, usu- ally those which have no call-preserved registers to use instead. For all machines, optimization level 2 and higher enables this flag by default. -funroll-loops Unroll loops whose number of iterations can be determined at com- pile time or upon entry to the loop. -funroll-loops implies both -fstrength-reduce and -frerun-cse-after-loop. This option makes code larger, and may or may not make it run faster. -funroll-all-loops Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. -funroll-all-loops implies the same options as -funroll- loops, -fprefetch-loop-arrays If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays. -fmove-all-movables Forces all invariant computations in loops to be moved outside the loop. -freduce-all-givs Forces all general-induction variables in loops to be strength- reduced. Note: When compiling programs written in Fortran, -fmove-all-mov- ables and -freduce-all-givs are enabled by default when you use the optimizer. These options may generate better or worse code; results are highly dependent on the structure of loops within the source code. These two options are intended to be removed someday, once they have helped determine the efficacy of various approaches to improv- ing loop optimizations. Please let us ( and ) know how use of these options affects the performance of your production code. We're very interested in code that runs slower when these options are enabled. -fno-peephole -fno-peephole2 Disable any machine-specific peephole optimizations. The differ- ence between -fno-peephole and -fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use both. -fbranch-probabilities After running a program compiled with -fprofile-arcs, you can com- pile it a second time using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken. When the program compiled with -fprofile-arcs exits it saves arc execution counts to a file called sourcename.da for each source file The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. With -fbranch-probabilities, GCC puts a REG_EXEC_COUNT note on the first instruction of each basic block, and a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve opti- mization. Currently, they are only used in one place: in reorg.c, instead of guessing which path a branch is mostly to take, the REG_BR_PROB values are used to exactly determine which path is taken more often. -fno-guess-branch-probability Do not guess branch probabilities using a randomized model. Sometimes gcc will opt to use a randomized model to guess branch probabilities, when none are available from either profiling feed- back (-fprofile-arcs) or __builtin_expect. This means that differ- ent runs of the compiler on the same program may produce different object code. In a hard real-time system, people don't want different runs of the compiler to produce code that has different behavior; minimizing non-determinism is of paramount import. This switch allows users to reduce non-determinism, possibly at the expense of inferior optimization. -fstrict-aliasing Allows the compiler to assume the strictest aliasing rules applica- ble to the language being compiled. For C (and C++), this acti- vates optimizations based on the type of expressions. In particu- lar, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same. For example, an "unsigned int" can alias an "int", but not a "void*" or a "double". A character type may alias any other type. Pay special attention to code like this: union a_union { int i; double d; }; int f() { a_union t; t.d = 3.0; return t.i; } The practice of reading from a different union member than the one most recently written to (called ``type-punning'') is common. Even with -fstrict-aliasing, type-punning is allowed, provided the mem- ory is accessed through the union type. So, the code above will work as expected. However, this code might not: int f() { a_union t; int* ip; t.d = 3.0; ip = &t.i; return *ip; } Every language that wishes to perform language-specific alias anal- ysis should define a function that computes, given an "tree" node, an alias set for the node. Nodes in different alias sets are not allowed to alias. For an example, see the C front-end function "c_get_alias_set". -falign-functions -falign-functions=n Align the start of functions to the next power-of-two greater than n, skipping up to n bytes. For instance, -falign-functions=32 aligns functions to the next 32-byte boundary, but -falign-func- tions=24 would align to the next 32-byte boundary only if this can be done by skipping 23 bytes or less. -fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be aligned. Some assemblers only support this flag when n is a power of two; in that case, it is rounded up. If n is not specified, use a machine-dependent default. -falign-labels -falign-labels=n Align all branch targets to a power-of-two boundary, skipping up to n bytes like -falign-functions. This option can easily make code slower, because it must insert dummy operations for when the branch target is reached in the usual flow of the code. If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead. If n is not specified, use a machine-dependent default which is very likely to be 1, meaning no alignment. This option does not work on Mac OS X. -falign-loops -falign-loops=n Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions. The hope is that the loop will be executed many times, which will make up for any execution of the dummy opera- tions. If n is not specified, use a machine-dependent default. This option does not work on Mac OS X. -falign-jumps -falign-jumps=n Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping, skipping up to n bytes like -falign-functions. In this case, no dummy operations need be executed. If n is not specified, use a machine-dependent default. This option does not work on Mac OS X. -fssa Perform optimizations in static single assignment form. Each func- tion's flow graph is translated into SSA form, optimizations are performed, and the flow graph is translated back from SSA form. Users should not specify this option, since it is not yet ready for production use. -fssa-ccp Perform Sparse Conditional Constant Propagation in SSA form. Requires -fssa. Like -fssa, this is an experimental feature. -fssa-dce Perform aggressive dead-code elimination in SSA form. Requires -fssa. Like -fssa, this is an experimental feature. -fsingle-precision-constant Treat floating point constant as single precision constant instead of implicitly converting it to double precision constant. -frename-registers Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimiza- tion will most benefit processors with lots of registers. It can, however, make debugging impossible, since variables will no longer stay in a ``home register''. -fno-cprop-registers After register allocation and post-register allocation instruction splitting, we perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. --param name=value In some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC will not inline func- tions that contain more that a certain number of instructions. You can control some of these constants on the command-line using the --param option. In each case, the value is an integer. The allowable choices for name are given in the following table: max-delay-slot-insn-search The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbi- trary number of instructions is searched, the time savings from filling the delay slot will be minimal so stop searching. Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in exe- cutable run time. max-delay-slot-live-search When trying to fill delay slots, the maximum number of instruc- tions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compile time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph. max-gcse-memory The approximate maximum amount of memory that will be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization will not be done. max-gcse-passes The maximum number of passes of GCSE to run. max-pending-list-length The maximum number of pending dependencies scheduling will allow before flushing the current state and starting over. Large functions with few branches or calls can create exces- sively large lists which needlessly consume memory and resources. max-inline-insns If an function contains more than this many instructions, it will not be inlined. This option is precisely equivalent to -finline-limit. Options Controlling the Preprocessor These options control the C preprocessor, which is run on each C source file before actual compilation. If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation. You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver han- dle the options instead. -D name Predefine name as a macro, with definition "1". -D name=definition Predefine name as a macro, with definition definition. There are no restrictions on the contents of definition, but if you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax. If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you will need to quote the option. With sh and csh, -D'name(args...)=definition' works. -D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are pro- cessed after all -D and -U options. -U name Cancel any previous definition of name, either built in or provided with a -D option. -undef Do not predefine any system-specific macros. The common predefined macros remain defined. -I dir Add the directory dir to the list of directories to be searched for header files. Directories named by -I are searched before the standard system include directories. It is dangerous to specify a standard system include directory in an -I option. This defeats the special treatment of system headers . It can also defeat the repairs to buggy system headers which GCC makes when it is installed. -o file Write output to file. This is the same as specifying file as the second non-option argument to cpp. gcc has a different interpreta- tion of a second non-option argument, so you must use -o to specify the output file. -Wall Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment and -Wtrigraphs. Note that many of the preprocessor's warnings are on by default and have no options to control them. -Wcomment -Wcomments Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a // comment. (Both forms have the same effect.) -Wtrigraphs Warn if any trigraphs are encountered. This option used to take effect only if -trigraphs was also specified, but now works inde- pendently. Warnings are not given for trigraphs within comments, as they do not affect the meaning of the program. -Wtraditional 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 problematic constructs which should be avoided. -Wimport Warn the first time #import is used. -Wundef Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of defined. Such identifiers are replaced with zero. -Werror Make all warnings into hard errors. Source code which triggers warnings will be rejected. -Wsystem-headers Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your own code, therefore suppressed. If you are responsible for the system library, you may want to see them. -w Suppress all warnings, including those which GNU CPP issues by default. -pedantic Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since they trigger frequently on harmless code. -pedantic-errors Issue all the mandatory diagnostics, and make all mandatory diag- nostics into errors. This includes mandatory diagnostics that GCC issues without -pedantic but treats as warnings. -M Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from -include or -imacros command line options. Unless specified explicitly (with -MT or -MQ), the object file name consists of the basename of the source file with any suffix replaced with object file suffix. If there are many included files then the rule is split into several lines using \-newline. The rule has no commands. This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output will still be sent to the regular output stream as normal. Passing -M to the driver implies -E. -MM Like -M but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header. This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header will appear in -MM dependency output. This is a slight change in semantics from GCC versions 3.0 and earlier. -MF file @anchor{-MF} When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the preprocessor sends the rules to the same place it would have sent preprocessed output. When used with the driver options -MD or -MMD, -MF overrides the default dependency output file. -dependency-file Like -MF. (APPLE ONLY) -MG When used with -M or -MM, -MG says to treat missing header files as generated files and assume they live in the same directory as the source file. It suppresses preprocessed output, as a missing header file is ordinarily an error. This feature is used in automatic updating of makefiles. -MP This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors make gives if you remove header files without updating the Makefile to match. This is typical output: test.o: test.c test.h test.h: -MT target Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, including any path, deletes any file suffix such as .c, and appends the plat- form's usual object suffix. The result is the target. An -MT option will set the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options. For example, -MT '$(objpfx)foo.o' might give $(objpfx)foo.o: foo.c -MQ target Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives $$(objpfx)foo.o: foo.c The default target is automatically quoted, as if it were given with -MQ. -MD -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise it take the basename of the input file and applies a .d suffix. If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file (but @pxref{-MF}), but if used without -E, each -o is understood to specify a target object file. Since -E is not implied, -MD can be used to generate a dependency output file as a side-effect of the compilation process. -MMD Like -MD except mention only user header files, not system -header files. -x c -x c++ -x objective-c -x objective-c++ -x assembler-with-cpp Specify the source language: C, C++, Objective-C, Objective-C++, or assembly. This has nothing to do with standards conformance or extensions; it merely selects which base syntax to expect. If you give none of these options, cpp will deduce the language from the extension of the source file: .c, .cc, .m, .mm, or .S. Some other common extensions for C++ and assembly are also recognized. If cpp does not recognize the extension, it will treat the file as C; this is the most generic mode. Note: Previous versions of cpp accepted a -lang option which selected both the language and the standards conformance level. This option has been removed, because it conflicts with the -l option. -std=standard -ansi Specify the standard to which the code should conform. Currently cpp only knows about the standards for C; other language standards will be added in the future. standard may be one of: ""iso9899:1990"" ""c89"" The ISO C standard from 1990. c89 is the customary shorthand for this version of the standard. The -ansi option is equivalent to -std=c89. ""iso9899:199409"" The 1990 C standard, as amended in 1994. ""iso9899:1999"" ""c99"" ""iso9899:199x"" ""c9x"" The revised ISO C standard, published in December 1999. Before publication, this was known as C9X. ""gnu89"" The 1990 C standard plus GNU extensions. This is the default. ""gnu99"" ""gnu9x"" The 1999 C standard plus GNU extensions. -I- Split the include path. Any directories specified with -I options before -I- are searched only for headers requested with "#include "file""; they are not searched for "#include ". If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives. In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for "#include "file"". -nostdinc Do not search the standard system directories for header files. Only the directories you have specified with -I options (and the directory of the current file, if appropriate) are searched. -nostdinc++ Do not search for header files in the C++-specific standard direc- tories, but do still search the other standard directories. (This option is used when building the C++ library.) -include file Process file as if "#include "file"" appeared as the first line of the primary source file. However, the first directory searched for file is the preprocessor's working directory instead of the direc- tory containing the main source file. If not found there, it is searched for in the remainder of the "#include "..."" search chain as normal. If multiple -include options are given, the files are included in the order they appear on the command line. -imacros file Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines remain defined. This allows you to acquire all the macros from a header without also processing its declarations. All files specified by -imacros are processed before all files specified by -include. -idirafter dir Search dir for header files, but do it after all directories speci- fied with -I and the standard system directories have been exhausted. dir is treated as a system include directory. -iprefix prefix Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /. -iwithprefix dir -iwithprefixbefore dir Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithpre- fixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would. Use of these options is discouraged. -isystem dir Search dir for header files, after all directories specified by -I but before the standard system directories. Mark it as a system directory, so that it gets the same special treatment as is applied to the standard system directories. -fpreprocessed Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, tri- graph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes com- ments, so that you can pass a file preprocessed with -C to the com- piler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends. -fpreprocessed is implicit if the input file has one of the exten- sions .i, .ii or .mi. These are the extensions that GCC uses for preprocessed files created by -save-temps. -ftabstop=width Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8. -fno-show-column Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not under- stand the column numbers, such as dejagnu. -A predicate=answer Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters. -A -predicate=answer Cancel an assertion with the predicate predicate and answer answer. -A- Cancel all predefined assertions and all assertions preceding it on the command line. Also, undefine all predefined macros and all macros preceding it on the command line. (This is a historical wart and may change in the future.) -dCHARS CHARS is a sequence of one or more of the following characters, and must not be preceded by a space. Other characters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify characters whose behavior conflicts, the result is undefined. M Instead of the normal output, generate a list of #define direc- tives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command touch foo.h; cpp -dM foo.h will show all the predefined macros. D Like M except in two respects: it does not include the prede- fined macros, and it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file. N Like D, but emit only the macro names, not their expansions. I Output #include directives in addition to the result of prepro- cessing. -P Inhibit generation of linemarkers in the output from the preproces- sor. This might be useful when running the preprocessor on some- thing that is not C code, and will be sent to a program which might be confused by the linemarkers. -C Do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive. You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordi- nary source line, since the first token on the line is no longer a #. -gcc Define the macros __GNUC__, __GNUC_MINOR__ and __GNUC_PATCHLEVEL__. These are defined automatically when you use gcc -E; you can turn them off in that case with -no-gcc. -traditional Try to imitate the behavior of old-fashioned C, as opposed to ISO C. -trigraphs Process trigraph sequences. These are three-character sequences, all starting with ??, that are defined by ISO C to stand for single characters. For example, ??/ stands for \, so '??/n' is a charac- ter constant for a newline. By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options. The nine trigraphs and their replacements are Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??- Replacement: [ ] { } # \ ^ | ~ -remap Enable special code to work around file systems which only permit very short file names, such as MS-DOS. -$ Forbid the use of $ in identifiers. The C standard allows imple- mentations to define extra characters that can appear in identi- fiers. By default GNU CPP permits $, a common extension. -h --help --target-help Print text describing all the command line options instead of pre- processing anything. -v Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final form of the include path. -H Print the name of each header file used, in addition to other nor- mal activities. Each name is indented to show how deep in the #include stack it is. -version --version Print out GNU CPP's version number. With one dash, proceed to pre- process as normal. With two dashes, exit immediately. Passing Options to the Assembler You can pass options to the assembler. -Wa,option Pass option as an option to the assembler. If option contains com- mas, it is split into multiple options at the commas. Options for Linking These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step. In addition to the options listed below, Apple's GCC also accepts and passes nearly all of the options defined by the linker ld and by the library tool libtool. Common options include -framework, -dynamic, -bundle, -flat_namespace, and so forth. See the ld and libtool man pages for further details. object-file-name A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker. -c -S -E If any of these options is used, then the linker is not run, and object file names should not be used as arguments. -llibrary -l library Search the library named library when linking. (The second alter- native with the library as a separate argument is only for POSIX compliance and is not recommended.) It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded. The linker searches a standard list of directories for the library, which is actually a file named liblibrary.a. The linker then uses this file as if it had been specified precisely by name. The directories searched include several standard system directo- ries plus any that you specify with -L. Normally the files found this way are library files---archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fash- ion. The only difference between using an -l option and specifying a file name is that -l surrounds library with lib and .a and searches several directories. -lobjc You need this special case of the -l option in order to link an Objective-C program. -nostartfiles Do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib or -nodefaultlibs is used. -nodefaultlibs Do not use the standard system libraries when linking. Only the libraries you specify will be passed to the linker. The standard startup files are used normally, unless -nostartfiles is used. The compiler may generate calls to memcmp, memset, and memcpy for Sys- tem V (and ISO C) environments or to bcopy and bzero for BSD envi- ronments. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified. -nostdlib Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify will be passed to the linker. The compiler may generate calls to mem- cmp, memset, and memcpy for System V (and ISO C) environments or to bcopy and bzero for BSD environments. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified. -no-c++filt By default all linker diagnostic output is piped through c++filt. This option suppresses that behavior. (APPLE ONLY) One of the standard libraries bypassed by -nostdlib and -nodefault- libs is libgcc.a, a library of internal subroutines that GCC uses to overcome shortcomings of particular machines, or special needs for some languages. In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (For example, __main, used to ensure C++ con- structors will be called.) -s Remove all symbol table and relocation information from the exe- cutable. -static On systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option has no effect. This option will not work on Mac OS X unless all of your libraries (including libgcc.a) have also been compiled with -static. -shared Produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options that were used to generate code (-fpic, -fPIC, or model suboptions) when you specify this option.[1] This option is not supported on Mac OS X. -shared-libgcc -static-libgcc On systems that provide libgcc as a shared library, these options force the use of either the shared or static version respectively. If no shared version of libgcc was built when the compiler was con- figured, these options have no effect. There are several situations in which an application should use the shared libgcc instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc. Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared library or a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to do. If, instead, you use the GCC driver to create shared libraries, you may find that they will not always be linked with the shared libgcc. If GCC finds, at its configuration time, that you have a GNU linker that does not support option --eh-frame-hdr, it will link the shared version of libgcc into shared libraries by default. Otherwise, it will take advantage of the linker and optimize away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring reloca- tion costs at library load time. However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ or GCJ driver, as appropriate for the languages used in the program, or using the option -shared-libgcc, such that it is linked with the shared libgcc. -symbolic Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems sup- port this option. -Xlinker option Pass option as an option to the linker. You can use this to supply system-specific linker options which GCC does not know how to rec- ognize. If you want to pass an option that takes an argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects. -Wl,option Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. -u symbol Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u multiple times with different symbols to force loading of additional library modules. Options for Directory Search These options specify directories to search for header files, for libraries and for parts of the compiler: -Idir Add the directory dir to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories. However, you should not use this option to add directories that contain ven- dor-supplied system header files (use -isystem for that). If you use more than one -I option, the directories are scanned in left- to-right order; the standard system directories come after. If a standard system include directory, or a directory specified with -isystem, is also specified with -I, it will be searched only in the position requested by -I. Also, it will not be considered a system include directory. If that directory really does contain system headers, there is a good chance that they will break. For instance, if GCC's installation procedure edited the headers in /usr/include to fix bugs, -I/usr/include will cause the original, buggy headers to be found instead of the corrected ones. GCC will issue a warning when a system include directory is hidden in this way. -I- Any directories you specify with -I options before the -I- option are searched only for the case of #include "file"; they are not searched for #include . If additional directories are specified with -I options after the -I-, these directories are searched for all #include directives. (Ordinarily all -I directories are used this way.) In addition, the -I- option inhibits the use of the current direc- tory (where the current input file came from) as the first search directory for #include "file". There is no way to override this effect of -I-. With -I. you can specify searching the directory which was current when the compiler was invoked. That is not exactly the same as what the preprocessor does by default, but it is often satisfactory. -I- does not inhibit the use of the standard system directories for header files. Thus, -I- and -nostdinc are independent. -Ldir Add directory dir to the list of directories to be searched for -l. -Fdir In Apple's version of GCC only, add the directory dir to the head of the list of directories to be searched for frameworks. The framework search algorithm is, for an inclusion of , to look for files named path/Fmwk.framework/Head- ers/Header.h or path/Fmwk.framework/PrivateHeaders/Header.h where path includes /System/Library/Frameworks/ /Library/Frameworks/, and /Local/Library/Frameworks/, plus any additional paths specified by -F. All the -F options are also passed to the linker. -Bprefix This option specifies where to find the executables, libraries, include files, and data files of the compiler itself. The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/. For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B was not specified, the driver tries two standard prefixes, which are /usr/lib/gcc/ and /usr/local/lib/gcc-lib/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH envi- ronment variable. The compiler will check to see if the path provided by the -B refers to a directory, and if necessary it will add a directory separator character at the end of the path. -B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to includes files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix. The run-time support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two stan- dard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means. Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX. As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it will be replaced by [dir/]include. This is to help with boot-strapping the com- piler. -specs=file Process file after the compiler reads in the standard specs file, in order to override the defaults that the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they are processed in order, from left to right. Specifying Target Machine and Compiler Version By default, GCC compiles code for the same type of machine that you are using. However, it can also be installed as a cross-compiler, to com- pile for some other type of machine. In fact, several different con- figurations of GCC, for different target machines, can be installed side by side. Then you specify which one to use with the -b option. In addition, older and newer versions of GCC can be installed side by side. One of them (probably the newest) will be the default, but you may sometimes wish to use another. -b machine The argument machine specifies the target machine for compilation. This is useful when you have installed GCC as a cross-compiler. The value to use for machine is the same as was specified as the machine type when configuring GCC as a cross-compiler. For exam- ple, if a cross-compiler was configured with configure i386v, mean- ing to compile for an 80386 running System V, then you would spec- ify -b i386v to run that cross compiler. When you do not specify -b, it normally means to compile for the same type of machine that you are using. -V version The argument version specifies which version of GCC to run. This is useful when multiple versions are installed. For example, ver- sion might be 2.0, meaning to run GCC version 2.0. The default version, when you do not specify -V, is the last ver- sion of GCC that you installed. The -b and -V options actually work by controlling part of the file name used for the executable files and libraries used for compilation. A given version of GCC, for a given target machine, is normally kept in the directory /usr/local/lib/gcc-lib/machine/version. Thus, sites can customize the effect of -b or -V either by changing the names of these directories or adding alternate names (or symbolic links). If in directory /usr/local/lib/gcc-lib/ the file 80386 is a link to the file i386v, then -b 80386 becomes an alias for -b i386v. In one respect, the -b or -V do not completely change to a different compiler: the top-level driver program gcc that you originally invoked continues to run and invoke the other executables (preprocessor, com- piler per se, assembler and linker) that do the real work. However, since no real work is done in the driver program, it usually does not matter that the driver program in use is not the one for the specified target. It is common for the interface to the other executables to change incompatibly between compiler versions, so unless the version specified is very close to that of the driver (for example, -V 3.0 with a driver program from GCC version 3.0.1), use of -V may not work; for example, using -V 2.95.2 will not work with a driver program from GCC 3.0. The only way that the driver program depends on the target machine is in the parsing and handling of special machine-specific options. How- ever, this is controlled by a file which is found, along with the other executables, in the directory for the specified version and target machine. As a result, a single installed driver program adapts to any specified target machine, and sufficiently similar compiler versions. The driver program executable does control one significant thing, how- ever: the default version and target machine. Therefore, you can install different instances of the driver program, compiled for differ- ent targets or versions, under different names. For example, if the driver for version 2.0 is installed as ogcc and that for version 2.1 is installed as gcc, then the command gcc will use version 2.1 by default, while ogcc will use 2.0 by default. However, you can choose either version with either command with the -V option. Hardware Models and Configurations Earlier we discussed the standard option -b which chooses among differ- ent installed compilers for completely different target machines, such as VAX vs. 68000 vs. 80386. In addition, each of these target machine types can have its own spe- cial options, starting with -m, to choose among various hardware models or configurations---for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified. Some configurations of the compiler also support additional special options, usually for compatibility with other compilers on the same platform. These options are defined by the macro "TARGET_SWITCHES" in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults. IBM RS/6000 and PowerPC Options These -m options are defined for the IBM RS/6000 and PowerPC: -mpower -mno-power -mpower2 -mno-power2 -mpowerpc -mno-powerpc -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mpowerpc64 -mno-powerpc64 GCC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER instruction set are those instruc- tions supported by the rios chip set used in the original RS/6000 systems and the PowerPC instruction set is the architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx microprocessors. Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both. An MQ reg- ister is included in processors supporting the POWER architecture. You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options listed above. The -mpower option allows GCC to generate instructions that are found only in the POWER architecture and to use the MQ register. Specifying -mpower2 implies -power and also allows GCC to generate instructions that are present in the POWER2 architecture but not the original POWER architecture. The -mpowerpc option allows GCC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Spec- ifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the General Pur- pose group, including floating-point square root. Specifying -mpowerpc-gfxopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select. The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64. If you specify both -mno-power and -mno-powerpc, GCC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ regis- ter. Specifying both -mpower and -mpowerpc permits GCC to use any instruction from either architecture and to allow use of the MQ register; specify this for the Motorola MPC601. -mnew-mnemonics -mold-mnemonics Select which mnemonics to use in the generated assembler code. With -mnew-mnemonics, GCC uses the assembler mnemonics defined for the PowerPC architecture. With -mold-mnemonics it uses the assem- bler mnemonics defined for the POWER architecture. Instructions defined in only one architecture have only one mnemonic; GCC uses that mnemonic irrespective of which of these options is specified. GCC defaults to the mnemonics appropriate for the architecture in use. Specifying -mcpu=cpu_type sometimes overrides the value of these option. Unless you are building a cross-compiler, you should normally not specify either -mnew-mnemonics or -mold-mnemonics, but should instead accept the default. -mcpu=cpu_type Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for machine type cpu_type. Sup- ported values for cpu_type are rios, rios1, rsc, rios2, rs64a, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, power, power2, powerpc, 403, 505, 801, 821, 823, and 860 and common. -mcpu=common selects a completely generic processor. Code gener- ated under this option will run on any POWER or PowerPC processor. GCC will use only the instructions in the common subset of both architectures, and will not use the MQ register. GCC assumes a generic processor model for scheduling purposes. -mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 spec- ify generic POWER, POWER2, pure 32-bit PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes. The other options specify a specific processor. Code generated under those options will run best on that processor, and may not run at all on others. The -mcpu options automatically enable or disable other -m options as follows: common -mno-power, -mno-powerc power power2 rios1 rios2 rsc -mpower, -mno-powerpc, -mno-new-mnemonics powerpc rs64a 602 603 603e 604 620 630 740 7400 7450 750 505 -mno-power, -mpowerpc, -mnew-mnemonics 601 -mpower, -mpowerpc, -mnew-mnemonics 403 821 860 -mno-power, -mpowerpc, -mnew-mnemonics, -msoft-float -mtune=cpu_type Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type, register usage, or choice of mnemonics, as -mcpu=cpu_type would. The same values for cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated will use the architecture, registers, and mnemonics set by -mcpu, but the scheduling parameters set by -mtune. -maltivec -mno-altivec These switches enable or disable the use of built-in functions that allow access to the AltiVec instruction set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements. This option is not supported on Mac OS X; use -faltivec instead. -mfull-toc -mno-fp-in-toc -mno-sum-in-toc -mminimal-toc Modify generation of the TOC (Table Of Contents), which is created for every executable file. The -mfull-toc option is selected by default. In that case, GCC will allocate at least one TOC entry for each unique non-automatic variable reference in your program. GCC will also place floating-point constants in the TOC. However, only 16,384 entries are available in the TOC. If you receive a linker error message that saying you have over- flowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to cal- culate the sum of an address and a constant at run-time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space. If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC will produce code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently executed code. -maix64 -maix32 Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64 and -mpowerpc, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32. -mxl-call -mno-xl-call On AIX, pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer argu- ments than declared. AIX XL compilers access floating point argu- ments which do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating- point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by AIX XL compilers without optimization. -mpe Support IBM RS/6000 SP Parallel Environment (PE). Link an applica- tion written to use message passing with special startup code to enable the application to run. The system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the appropri- ate directory location. The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompati- ble. -malign-mac68k -malign-power -malign-natural The option -malign-mac68k causes structure fields to be aligned on 2-byte boundaries, in order to be compatible with m68k compiler output. The option -malign-power is the standard alignment mode for the PowerPC. The option -malign-natural is an extension of PowerPC alignment that aligns larger data types such as doubles on their natural boundaries. (APPLE ONLY) -msoft-float -mhard-float Generate code that does not use (uses) the floating-point register set. Software floating point emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking. -mmultiple -mno-multiple Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mmultiple on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode. -mstring -mno-string Generate code that uses (does not use) the load string instructions and the store string word instructions to save multiple registers and do small block moves. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mstring on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode. -mupdate -mno-update Generate code that uses (does not use) the load or store instruc- tions that update the base register to the address of the calcu- lated memory location. These instructions are generated by default. If you use -mno-update, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data. -mfused-madd -mno-fused-madd Generate code that uses (does not use) the floating point multiply and accumulate instructions. These instructions are generated by default if hardware floating is used. -mno-bit-align -mbit-align On System V.4 and embedded PowerPC systems do not (do) force struc- tures and unions that contain bit-fields to be aligned to the base type of the bit-field. For example, by default a structure containing nothing but 8 "unsigned" bit-fields of length 1 would be aligned to a 4 byte boundary and have a size of 4 bytes. By using -mno-bit-align, the structure would be aligned to a 1 byte boundary and be one byte in size. -mno-strict-align -mstrict-align On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be handled by the system. -mrelocatable -mno-relocatable On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at run- time. If you use -mrelocatable on any module, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. -mrelocatable-lib -mno-relocatable-lib On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at run- time. Modules compiled with -mrelocatable-lib can be linked with either modules compiled without -mrelocatable and -mrelocatable-lib or with modules compiled with the -mrelocatable options. -mno-toc -mtoc On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program. -mlittle -mlittle-endian On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode. The -mlittle-endian option is the same as -mlittle. -mbig -mbig-endian On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode. The -mbig-endian option is the same as -mbig. -mdynamic-no-pic On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared libraries. (APPLE ONLY) -mlong-branch On Darwin and Mac OS X systems, compile calls to use a 32-bit des- tination address. This is to support kernel extensions, which may load anywhere within the kernel address space. (APPLE ONLY) -mcall-sysv On System V.4 and embedded PowerPC systems compile code using call- ing conventions that adheres to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using powerpc-*-eabiaix. -mcall-sysv-eabi Specify both -mcall-sysv and -meabi options. -mcall-sysv-noeabi Specify both -mcall-sysv and -mno-eabi options. -mcall-aix On System V.4 and embedded PowerPC systems compile code using call- ing conventions that are similar to those used on AIX. This is the default if you configured GCC using powerpc-*-eabiaix. -mcall-solaris On System V.4 and embedded PowerPC systems compile code for the Solaris operating system. -mcall-linux On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system. -mcall-gnu On System V.4 and embedded PowerPC systems compile code for the Hurd-based GNU system. -mcall-netbsd On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system. -maix-struct-return Return all structures in memory (as specified by the AIX ABI). -msvr4-struct-return Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI). -mabi=altivec Extend the current ABI with AltiVec ABI extensions. This does not change the default ABI, instead it adds the AltiVec ABI extensions to the current ABI. This option is effectively permanently enabled on Mac OS X. -mabi=no-altivec Disable AltiVec ABI extensions for the current ABI. This option will not work on Mac OS X. -mprototype -mno-prototype On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether floating point values were passed in the floating point registers in case the function takes a variable arguments. With -mprototype, only calls to prototyped variable argument func- tions will set or clear the bit. -msim On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the standard C libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim. configura- tions. -mmvme On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a. -mads On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libads.a and libc.a. -myellowknife On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a. -mvxworks On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system. -memb On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that eabi extended relocations are used. -meabi -mno-eabi On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface (eabi) which is a set of modifications to the System V.4 specifications. Selecting -meabi means that the stack is aligned to an 8 byte boundary, a function "__eabi" is called to from "main" to set up the eabi environment, and the -msdata option can use both "r2" and "r13" to point to two separate small data areas. Selecting -mno-eabi means that the stack is aligned to a 16 byte boundary, do not call an initializa- tion function from "main", and the -msdata option will only use "r13" to point to a single small data area. The -meabi option is on by default if you configured GCC using one of the pow- erpc*-*-eabi* options. -msdata=eabi On System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in the .sdata2 section, which is pointed to by register "r2". Put small initialized non-"const" global and static data in the .sdata section, which is pointed to by register "r13". Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section. The -msdata=eabi option is incompatible with the -mrelocatable option. The -msdata=eabi option also sets the -memb option. -msdata=sysv On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, which is pointed to by register "r13". Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section. The -msdata=sysv option is incompatible with the -mrelocatable option. -msdata=default -msdata On System V.4 and embedded PowerPC systems, if -meabi is used, com- pile code the same as -msdata=eabi, otherwise compile code the same as -msdata=sysv. -msdata-data On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section. Put small uninitialized global and static data in the .sbss section. Do not use register "r13" to address small data however. This is the default behavior unless other -msdata options are used. -msdata=none -mno-sdata On embedded PowerPC systems, put all initialized global and static data in the .data section, and all uninitialized data in the .bss section. -G num On embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or bss sections instead of the normal data or bss section. By default, num is 8. The -G num switch is also passed to the linker. All modules should be compiled with the same -G num value. -mregnames -mno-regnames On System V.4 and embedded PowerPC systems do (do not) emit regis- ter names in the assembly language output using symbolic forms. -pthread Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker. Intel 386 and AMD x86-64 Options These -m options are defined for the i386 and x86-64 family of comput- ers: -mcpu=cpu-type Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for cpu-type are i386, i486, i586, i686, pentium, pentium- mmx, pentiumpro, pentium2, pentium3, pentium4, k6, k6-2, k6-3, athlon, athlon-tbird, athlon-4, athlon-xp and athlon-mp. While picking a specific cpu-type will schedule things appropri- ately for that particular chip, the compiler will not generate any code that does not run on the i386 without the -march=cpu-type option being used. i586 is equivalent to pentium and i686 is equivalent to pentiumpro. k6 and athlon are the AMD chips as opposed to the Intel ones. -march=cpu-type Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for -mcpu. Moreover, specifying -march=cpu-type implies -mcpu=cpu-type. -m386 -m486 -mpentium -mpentiumpro These options are synonyms for -mcpu=i386, -mcpu=i486, -mcpu=pen- tium, and -mcpu=pentiumpro respectively. These synonyms are depre- cated. -mfpmath=unit generate floating point arithmetics for selected unit unit. the choices for unit are: 387 Use the standard 387 floating point coprocessor present major- ity of chips and emulated otherwise. Code compiled with this option will run almost everywhere. The temporary results are computed in 80bit precesion instead of precision specified by the type resulting in slightly different results compared to most of other chips. See -ffloat-store for more detailed description. This is the default choice for i386 compiler. sse Use scalar floating point instructions present in the SSE instruction set. This instruction set is supported by Pentium3 and newer chips, in the AMD line by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE instruction set supports only single precision arithmetics, thus the double and extended precision arithmetics is still done using 387. Later version, present only in Pentium4 and the future AMD x86-64 chips supports double precision arithmetics too. For i387 you need to use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effec- tive. For x86-64 compiler, these extensions are enabled by default. The resulting code should be considerably faster in majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80bit. This is the default choice for x86-64 compiler. sse,387 Attempt to utilize both instruction sets at once. This effec- tivly double the amount of available registers and on chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is still experimental, because gcc register allocator does not model separate functional units well resulting in instable perfor- mance. -masm=dialect Output asm instructions using selected dialect. Supported choices are intel or att (the default one). -mieee-fp -mno-ieee-fp Control whether or not the compiler uses IEEE floating point com- parisons. These handle correctly the case where the result of a comparison is unordered. -msoft-float Generate output containing library calls for floating point. Warn- ing: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross- compilation. On machines where a function returns floating point results in the 80387 register stack, some floating point opcodes may be emitted even if -msoft-float is used. -mno-fp-ret-in-387 Do not use the FPU registers for return values of functions. The usual calling convention has functions return values of types "float" and "double" in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU. The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead. -mno-fancy-math-387 Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this option to avoid generating those instructions. This option is the default on FreeBSD, OpenBSD and NetBSD. This option is overridden when -march indicates that the target cpu will always have an FPU and so the instruction will not need emulation. As of revision 2.6.1, these instructions are not generated unless you also use the -funsafe-math-optimizations switch. -malign-double -mno-align-double Control whether GCC aligns "double", "long double", and "long long" variables on a two word boundary or a one word boundary. Aligning "double" variables on a two word boundary will produce code that runs somewhat faster on a Pentium at the expense of more memory. -m128bit-long-double Control the size of "long double" type. i386 application binary interface specify the size to be 12 bytes, while modern architec- tures (Pentium and newer) prefer "long double" aligned to 8 or 16 byte boundary. This is impossible to reach with 12 byte long dou- bles in the array accesses. Warning: if you use the -m128bit-long-double switch, the structures and arrays containing "long double" will change their size as well as function calling convention for function taking "long double" will be modified. -m96bit-long-double Set the size of "long double" to 96 bits as required by the i386 application binary interface. This is the default. -msvr3-shlib -mno-svr3-shlib Control whether GCC places uninitialized local variables into the "bss" or "data" segments. -msvr3-shlib places them into "bss". These options are meaningful only on System V Release 3. -mrtd Use a different function-calling convention, in which functions that take a fixed number of arguments return with the "ret" num instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. You can specify that an individual function is called with this calling sequence with the function attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl. Warning: this calling convention is incompatible with the one nor- mally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (including "printf"); otherwise incorrect code will be generated for calls to those functions. In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) -mregparm=num Control how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a spe- cific function by using the function attribute regparm. Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. -mpreferred-stack-boundary=num Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits), except when optimizing for code size (-Os), in which case the default is the minimum correct alignment (4 bytes for x86, and 8 bytes for x86-64). On Pentium and PentiumPro, "double" and "long double" values should be aligned to an 8 byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type "__m128" suffers similar penalties if it is not 16 byte aligned. To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary will most likely misalign the stack. It is recommended that libraries that use callbacks always use the default setting. This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2. -mmmx -mno-mmx -msse -mno-sse -msse2 -mno-sse2 -m3dnow -mno-3dnow These switches enable or disable the use of built-in functions that allow direct access to the MMX, SSE and 3Dnow extensions of the instruction set. -mpush-args -mno-push-args Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependen- cies. -maccumulate-outgoing-args If enabled, the maximum amount of space required for outgoing argu- ments will be computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved schedul- ing and reduced stack usage when preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies -mno-push-args. -mthreads Support thread-safe exception handling on Mingw32. Code that relies on thread-safe exception handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per thread exception handling data. -mno-align-stringops Do not align destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but gcc don't know about it. -minline-all-stringops By default GCC inlines string operations only when destination is known to be aligned at least to 4 byte boundary. This enables more inlining, increase code size, but may improve performance of code that depends on fast memcpy, strlen and memset for short lengths. -momit-leaf-frame-pointer Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. The option -fomit-frame-pointer removes the frame pointer for all functions which might make debugging harder. These -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments. -m32 -m64 Generate code for a 32-bit or 64-bit environment. The 32-bit envi- ronment sets int, long and pointer to 32 bits and generates code that runs on any i386 system. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits and generates code for AMD's x86-64 architecture. -mno-red-zone Do not use a so called red zone for x86-64 code. The red zone is mandated by the x86-64 ABI, it is a 128-byte area beyond the loca- tion of the stack pointer that will not be modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone dis- ables this red zone. Options for Code Generation Conventions These machine-independent options control the interface conventions used in code generation. Most of them 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. -fexceptions Enable exception handling. Generates extra code needed to propa- gate exceptions. For some targets, this implies GCC will generate frame unwind information for all functions, which can produce sig- nificant data size overhead, although it does not affect execution. If you do not specify this option, GCC will enable it by default for languages like C++ which normally require exception handling, and disable it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling. -fnon-call-exceptions Generate code that allows trapping instructions to throw excep- tions. Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or float- ing point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as "SIGALRM". -funwind-tables Similar to -fexceptions, except that it will just generate any needed static data, but will not affect the generated code in any other way. You will normally not enable this option; instead, a language processor that needs this handling would enable it on your behalf. -fasynchronous-unwind-tables Generate unwind table in dwarf2 format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector). -fpcc-struct-return Return ``short'' "struct" and "union" values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC- compiled files and files compiled with other compilers. The precise convention for returning structures in memory depends on the target configuration macros. Short structures and unions are those whose size and alignment match that of some integer type. -freg-struct-return Return "struct" and "union" values in registers when possible. This is more efficient for small structures than -fpcc-struct- return. If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct- return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative. -fshort-enums Allocate to an "enum" type only as many bytes as it needs for the declared range of possible values. Specifically, the "enum" type will be equivalent to the smallest integer type which has enough room. -fshort-double Use the same size for "double" as for "float". -fshared-data Requests that the data and non-"const" variables of this compila- tion be shared data rather than private data. The distinction makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private data exists in one copy per process. -fno-common In C, allocate even uninitialized global variables in the data sec- tion of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without "extern") in two different compilations, you will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work this way. -fno-ident Ignore the #ident directive. -fno-gnu-linker Do not output global initializations (such as C++ constructors and destructors) in the form used by the GNU linker (on systems where the GNU linker is the standard method of handling them). Use this option when you want to use a non-GNU linker, which also requires using the collect2 program to make sure the system linker includes constructors and destructors. (collect2 is included in the GCC distribution.) For systems which must use collect2, the compiler driver gcc is configured to do this automatically. -finhibit-size-directive Don't output a ".size" assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling crtstuff.c; you should not need to use it for anything else. -fverbose-asm Put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (per- haps while debugging the compiler itself). -fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files. -fvolatile Consider all memory references through pointers to be volatile. -fvolatile-global Consider all memory references to extern and global data items to be volatile. GCC does not consider static data items to be volatile because of this switch. -fvolatile-static Consider all memory references to static data to be volatile. -fpic Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the pro- gram starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead. (These maximums are 16k on the m88k, 8k on the Sparc, and 32k on the m68k and RS/6000. The 386 has no such limit.) Position-independent code requires special support, and therefore works only on certain machines. For the 386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent. -fpic is not supported on Mac OS X. -fPIC If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on the m68k, m88k, and the Sparc. Position-independent code requires special support, and therefore works only on certain machines. -fPIC is the default on Darwin and Mac OS X. -ffixed-reg Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). reg must be the name of a register. The register names accepted are machine-specific and are defined in the "REGISTER_NAMES" macro in the machine description macro file. This flag does not have a negative form, because it specifies a three-way choice. -fcall-used-reg Treat the register named reg as an allocable register that is clob- bered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the register reg. It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed per- vasive roles in the machine's execution model will produce disas- trous results. This flag does not have a negative form, because it specifies a three-way choice. -fcall-saved-reg Treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg if they use it. It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed per- vasive roles in the machine's execution model will produce disas- trous results. A different sort of disaster will result from the use of this flag for a register in which function values may be returned. This flag does not have a negative form, because it specifies a three-way choice. -fpack-struct Pack all structure members together without holes. Usually you would not want to use this option, since it makes the code subopti- mal, and the offsets of structure members won't agree with system libraries. -finstrument-functions Generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the fol- lowing profiling functions will be called with the address of the current function and its call site. (On some platforms, "__builtin_return_address" does not work beyond the current func- tion, so the call site information may not be available to the pro- filing functions otherwise.) void __cyg_profile_func_enter (void *this_fn, void *call_site); void __cyg_profile_func_exit (void *this_fn, void *call_site); The first argument is the address of the start of the current func- tion, which may be looked up exactly in the symbol table. This instrumentation is also done for functions expanded inline in other functions. The profiling calls will indicate where, concep- tually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an addi- tional expansion of code size. If you use extern inline in your C code, an addressable version of such functions must be provided. (This is normally the case anyways, but if you get lucky and the optimizer always expands the functions inline, you might have got- ten away without providing static copies.) A function may be given the attribute "no_instrument_function", in which case this instrumentation will not be done. This can be used, for example, for the profiling functions listed above, high- priority interrupt routines, and any functions from which the pro- filing functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory). -fstack-check Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but only rarely need to specify it in a single-threaded environment since stack overflow is auto- matically detected on nearly all systems if there is only one stack. Note that this switch does not actually cause checking to be done; the operating system must do that. The switch causes generation of code to ensure that the operating system sees the stack being extended. -fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit Generate code to ensure that the stack does not grow beyond a cer- tain value, either the value of a register or the address of a sym- bol. If the stack would grow beyond the value, a signal is raised. For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions. For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-sym- bol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker. -fargument-alias -fargument-noalias -fargument-noalias-global Specify the possible relationships among parameters and between parameters and global data. -fargument-alias specifies that arguments (parameters) may alias each other and may alias global storage.-fargument-noalias speci- fies that arguments do not alias each other, but may alias global storage.-fargument-noalias-global specifies that arguments do not alias each other and do not alias global storage. Each language will automatically use whatever option is required by the language standard. You should not need to use these options yourself. -fleading-underscore This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code. Be warned that you should know what you are doing when invoking this option, and that not all targets provide complete support for it. ENVIRONMENT This section describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment. Note that you can also specify places to search using options such as -B, -I and -L. These take precedence over places specified using envi- ronment variables, which in turn take precedence over those specified by the configuration of GCC. LANG LC_CTYPE LC_MESSAGES LC_ALL These environment variables control the way that GCC uses localiza- tion information that allow GCC to work with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. These locale cate- gories can be set to any value supported by your installation. A typical value is en_UK for English in the United Kingdom. The LC_CTYPE environment variable specifies character classifica- tion. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that would otherwise be interpreted as a string end or escape. The LC_MESSAGES environment variable specifies the language to use in diagnostic messages. If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If none of these variables are set, GCC defaults to traditional C English behavior. TMPDIR If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper. GCC_EXEC_PREFIX If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish. If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an appropriate prefix to use based on the pathname it was invoked with. If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram. The default value of GCC_EXEC_PREFIX is prefix/lib/gcc-lib/ where prefix is the value of "prefix" when you ran the configure script. Other prefixes specified with -B take precedence over this prefix. This prefix is also used for finding files such as crt0.o that are used for linking. In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with /usr/local/lib/gcc-lib (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. Thus, with -Bfoo/, GCC will search foo/bar where it would normally search /usr/local/lib/bar. These alternate directories are searched first; the standard directories come next. COMPILER_PATH The value of COMPILER_PATH is a colon-separated list of directo- ries, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it can't find the subprograms using GCC_EXEC_PREFIX. LIBRARY_PATH The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it can't find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first). LANG This variable is used to pass locale information to the compiler. One way in which this information is used is to determine the char- acter set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized: C-JIS Recognize JIS characters. C-SJIS Recognize SJIS characters. C-EUCJP Recognize EUCJP characters. If LANG is not defined, or if it has some other value, then the compiler will use mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters. Some additional environments variables affect the behavior of the pre- processor. CPATH C_INCLUDE_PATH CPLUS_INCLUDE_PATH OBJC_INCLUDE_PATH Each variable's value is a list of directories separated by a spe- cial character, much like PATH, in which to look for header files. The special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time. For Windows-based targets it is a semicolon, and for almost all other targets it is a colon. CPATH specifies a list of directories to be searched as if speci- fied with -I, but after any paths given with -I options on the com- mand line. The environment variable is used regardless of which language is being preprocessed. The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of direc- tories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line. DEPENDENCIES_OUTPUT @anchor{DEPENDENCIES_OUTPUT} If this variable is set, its value specifies how to output dependencies for Make based on the non-sys- tem header files processed by the compiler. System header files are ignored in the dependency output. The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form file target, in which case the rules are written to file file using target as the target name. In other words, this environment variable is equivalent to combin- ing the options -MM and -MF, with an optional -MT switch too. SUNPRO_DEPENDENCIES This variable is the same as the environment variable DEPENDEN- CIES_OUTPUT, except that system header files are not ignored, so it implies -M rather than -MM. BUGS To report bugs to Apple, see . FOOTNOTES 1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous. SEE ALSO gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), g77(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1) and the Info entries for gcc, cpp, g77, as, ld, binutils and gdb. AUTHOR See the Info entry for gcc, or , for contributors to GCC. COPYRIGHT Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being ``GNU General Public License'' and ``Funding Free Software'', the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7) man page. (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development. 3rd Berkeley Distribution gcc-3.1 GCC(1)