Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 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 section entitled “GNU Free Documentation License”.

(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.

Introduction

This manual documents how to use the GNU compilers, as well as their features and incompatibilities, and how to report bugs. It corresponds to GCC version 3.3.5. The internals of the GNU compilers, including how to port them to new targets and some information about how to write front ends for new languages, are documented in a separate manual. See Introduction (GNU Compiler Collection (GCC) Internals).

• G++ and GCC: You can compile C or C++ programs.
• Standards: Language standards supported by GCC.
• Invoking GCC: Command options supported by gcc.
• C Implementation: How GCC implements the ISO C specification.
• C Extensions: GNU extensions to the C language family.
• C++ Extensions: GNU extensions to the C++ language.
• Objective-C: GNU Objective-C runtime features.
• Compatibility: Binary Compatibility
• Gcov: gcov---a test coverage program.
• Trouble: If you have trouble using GCC.
• Bugs: How, why and where to report bugs.
• Service: How to find suppliers of support for GCC.
• Contributing: How to contribute to testing and developing GCC.
• Funding: How to help assure funding for free software.
• GNU Project: The GNU Project and GNU/Linux.
• Copying: GNU General Public License says how you can copy and share GCC.
• GNU Free Documentation License: How you can copy and share this manual.
• Contributors: People who have contributed to GCC.
• Option Index: Index to command line options.
• Keyword Index: Index of concepts and symbol names.

1 Compile C, C++, Objective-C, Ada, Fortran, Java, or treelang

Several versions of the compiler (C, C++, Objective-C, Ada, Fortran, Java and treelang) are integrated; this is why we use the name “GNU Compiler Collection”. GCC can compile programs written in any of these languages. The Ada, Fortran, Java and treelang compilers are described in separate manuals.

“GCC” is a common shorthand term for the GNU Compiler Collection. This is both the most general name for the compiler, and the name used when the emphasis is on compiling C programs (as the abbreviation formerly stood for “GNU C Compiler”).

When referring to C++ compilation, it is usual to call the compiler “G++”. Since there is only one compiler, it is also accurate to call it “GCC” no matter what the language context; however, the term “G++” is more useful when the emphasis is on compiling C++ programs.

Similarly, when we talk about Ada compilation, we usually call the compiler “GNAT”, for the same reasons.

We use the name “GCC” to refer to the compilation system as a whole, and more specifically to the language-independent part of the compiler. For example, we refer to the optimization options as affecting the behavior of “GCC” or sometimes just “the compiler”.

Front ends for other languages, such as Mercury and Pascal exist but have not yet been integrated into GCC. These front ends, like that for C++, are built in subdirectories of GCC and link to it. The result is an integrated compiler that can compile programs written in C, C++, Objective-C, or any of the languages for which you have installed front ends.

In this manual, we only discuss the options for the C, Objective-C, and C++ compilers and those of the GCC core. Consult the documentation of the other front ends for the options to use when compiling programs written in other languages.

G++ is a compiler, not merely a preprocessor. G++ builds object code directly from your C++ program source. There is no intermediate C version of the program. (By contrast, for example, some other implementations use a program that generates a C program from your C++ source.) Avoiding an intermediate C representation of the program means that you get better object code, and better debugging information. The GNU debugger, GDB, works with this information in the object code to give you comprehensive C++ source-level editing capabilities (see C and C++ (Debugging with GDB)).

2 Language Standards Supported by GCC

For each language compiled by GCC for which there is a standard, GCC attempts to follow one or more versions of that standard, possibly with some exceptions, and possibly with some extensions.

GCC supports three versions of the C standard, although support for the most recent version is not yet complete.

The original ANSI C standard (X3.159-1989) was ratified in 1989 and published in 1990. This standard was ratified as an ISO standard (ISO/IEC 9899:1990) later in 1990. There were no technical differences between these publications, although the sections of the ANSI standard were renumbered and became clauses in the ISO standard. This standard, in both its forms, is commonly known as C89, or occasionally as C90, from the dates of ratification. The ANSI standard, but not the ISO standard, also came with a Rationale document. To select this standard in GCC, use one of the options -ansi, -std=c89 or -std=iso9899:1990; to obtain all the diagnostics required by the standard, you should also specify -pedantic (or -pedantic-errors if you want them to be errors rather than warnings). See Options Controlling C Dialect.

Errors in the 1990 ISO C standard were corrected in two Technical Corrigenda published in 1994 and 1996. GCC does not support the uncorrected version.

An amendment to the 1990 standard was published in 1995. This amendment added digraphs and __STDC_VERSION__ to the language, but otherwise concerned the library. This amendment is commonly known as AMD1; the amended standard is sometimes known as C94 or C95. To select this standard in GCC, use the option -std=iso9899:199409 (with, as for other standard versions, -pedantic to receive all required diagnostics).

A new edition of the ISO C standard was published in 1999 as ISO/IEC 9899:1999, and is commonly known as C99. GCC has incomplete support for this standard version; see http://gcc.gnu.org/gcc-3.3/c99status.html for details. To select this standard, use -std=c99 or -std=iso9899:1999. (While in development, drafts of this standard version were referred to as C9X.)

Errors in the 1999 ISO C standard were corrected in a Technical Corrigendum published in 2001. GCC does not support the uncorrected version.

By default, GCC provides some extensions to the C language that on rare occasions conflict with the C standard. See Extensions to the C Language Family. Use of the -std options listed above will disable these extensions where they conflict with the C standard version selected. You may also select an extended version of the C language explicitly with -std=gnu89 (for C89 with GNU extensions) or -std=gnu99 (for C99 with GNU extensions). The default, if no C language dialect options are given, is -std=gnu89; this will change to -std=gnu99 in some future release when the C99 support is complete. Some features that are part of the C99 standard are accepted as extensions in C89 mode.

The ISO C standard defines (in clause 4) two classes of conforming implementation. A conforming hosted implementation supports the whole standard including all the library facilities; a conforming freestanding implementation is only required to provide certain library facilities: those in <float.h>, <limits.h>, <stdarg.h>, and <stddef.h>; since AMD1, also those in <iso646.h>; and in C99, also those in <stdbool.h> and <stdint.h>. In addition, complex types, added in C99, are not required for freestanding implementations. The standard also defines two environments for programs, a freestanding environment, required of all implementations and which may not have library facilities beyond those required of freestanding implementations, where the handling of program startup and termination are implementation-defined, and a hosted environment, which is not required, in which all the library facilities are provided and startup is through a function int main (void) or int main (int, char *[]). An OS kernel would be a freestanding environment; a program using the facilities of an operating system would normally be in a hosted implementation.

GCC aims towards being usable as a conforming freestanding implementation, or as the compiler for a conforming hosted implementation. By default, it will act as the compiler for a hosted implementation, defining __STDC_HOSTED__ as 1 and presuming that when the names of ISO C functions are used, they have the semantics defined in the standard. To make it act as a conforming freestanding implementation for a freestanding environment, use the option -ffreestanding; it will then define __STDC_HOSTED__ to 0 and not make assumptions about the meanings of function names from the standard library, with exceptions noted below. To build an OS kernel, you may well still need to make your own arrangements for linking and startup. See Options Controlling C Dialect.

GCC does not provide the library facilities required only of hosted implementations, nor yet all the facilities required by C99 of freestanding implementations; to use the facilities of a hosted environment, you will need to find them elsewhere (for example, in the GNU C library). See Standard Libraries.

Most of the compiler support routines used by GCC are present in libgcc, but there are a few exceptions. GCC requires the freestanding environment provide memcpy, memmove, memset and memcmp. Some older ports of GCC are configured to use the BSD bcopy, bzero and bcmp functions instead, but this is deprecated for new ports. Finally, if __builtin_trap is used, and the target does not implement the trap pattern, then GCC will emit a call to abort.

For references to Technical Corrigenda, Rationale documents and information concerning the history of C that is available online, see http://gcc.gnu.org/readings.html

There is no formal written standard for Objective-C. The most authoritative manual is “Object-Oriented Programming and the Objective-C Language”, available at a number of web sites

• http://developer.apple.com/techpubs/macosx/Cocoa/ObjectiveC/ is a recent version
• http://www.toodarkpark.org/computers/objc/ is an older example
• http://www.gnustep.org has additional useful information

There is no standard for treelang, which is a sample language front end for GCC. Its only purpose is as a sample for people wishing to write a new language for GCC. The language is documented in gcc/treelang/treelang.texi which can be turned into info or HTML format.

See GNAT Reference Manual (GNAT Reference Manual), for information on standard conformance and compatibility of the Ada compiler.

See The GNU Fortran Language (Using and Porting GNU Fortran), for details of the Fortran language supported by GCC.

See Compatibility with the Java Platform (GNU gcj), for details of compatibility between gcj and the Java Platform.

3 GCC Command Options

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 documented here, since you rarely need to use any of them.

Most of the command line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.

See Compiling C++ Programs, for a summary of special options for compiling C++ programs.

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.

See Option Index, for an index to GCC's options.

• Option Summary: Brief list of all options, without explanations.
• Overall Options: Controlling the kind of output: an executable, object files, assembler files, or preprocessed source.
• Invoking G++: Compiling C++ programs.
• C Dialect Options: Controlling the variant of C language compiled.
• C++ Dialect Options: Variations on C++.
• Objective-C Dialect Options: Variations on Objective-C.
• Language Independent Options: Controlling how diagnostics should be formatted.
• Warning Options: How picky should the compiler be?
• Debugging Options: Symbol tables, measurements, and debugging dumps.
• Optimize Options: How much optimization?
• Preprocessor Options: Controlling header files and macro definitions. Also, getting dependency information for Make.
• Assembler Options: Passing options to the assembler.
• Link Options: Specifying libraries and so on.
• Directory Options: Where to find header files and libraries. Where to find the compiler executable files.
• Spec Files: How to pass switches to sub-processes.
• Target Options: Running a cross-compiler, or an old version of GCC.
• Submodel Options: Specifying minor hardware or convention variations, such as 68010 vs 68020.
• Code Gen Options: Specifying conventions for function calls, data layout and register usage.
• Environment Variables: Env vars that affect GCC.
• Running Protoize: Automatically adding or removing function prototypes.

3.1 Option Summary

Here is a summary of all the options, grouped by type. Explanations are in the following sections.

Overall Options

See Options Controlling the Kind of Output.

          -c  -S  -E  -o file  -pipe  -pass-exit-codes  
          -x language  -v  -###  --help  --target-help  --version
     

C Language Options

See Options Controlling C Dialect.

          -ansi  -std=standard  -aux-info filename 
          -fno-asm  -fno-builtin  -fno-builtin-function 
          -fhosted  -ffreestanding  -fms-extensions 
          -trigraphs  -no-integrated-cpp  -traditional  -traditional-cpp 
          -fallow-single-precision  -fcond-mismatch 
          -fsigned-bitfields  -fsigned-char 
          -funsigned-bitfields  -funsigned-char 
          -fwritable-strings
     

C++ Language Options

See Options Controlling C++ Dialect.

          -fabi-version=n  -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-templates 
          -fno-implicit-inline-templates 
          -fno-implement-inlines  -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  -Wabi  -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

See Options Controlling Objective-C Dialect.

          -fconstant-string-class=class-name 
          -fgnu-runtime  -fnext-runtime  -gen-decls 
          -Wno-protocol  -Wselector  -Wundeclared-selector
     

Language Independent Options

See Options to Control Diagnostic Messages Formatting.

          -fmessage-length=n 
          -fdiagnostics-show-location=[once|every-line]
     

Warning Options

See Options to Request or Suppress Warnings.

          -fsyntax-only  -pedantic  -pedantic-errors 
          -w  -W  -Wall  -Waggregate-return 
          -Wcast-align  -Wcast-qual  -Wchar-subscripts  -Wcomment 
          -Wconversion  -Wno-deprecated-declarations 
          -Wdisabled-optimization  -Wno-div-by-zero  -Werror 
          -Wfloat-equal  -Wformat  -Wformat=2 
          -Wformat-nonliteral  -Wformat-security 
          -Wimplicit  -Wimplicit-int  
          -Wimplicit-function-declaration 
          -Werror-implicit-function-declaration 
          -Wimport  -Winline  -Wno-endif-labels 
          -Wlarger-than-len  -Wlong-long 
          -Wmain  -Wmissing-braces 
          -Wmissing-format-attribute  -Wmissing-noreturn 
          -Wno-multichar  -Wno-format-extra-args  -Wno-format-y2k 
          -Wno-import  -Wnonnull  -Wpacked  -Wpadded 
          -Wparentheses  -Wpointer-arith  -Wredundant-decls 
          -Wreturn-type  -Wsequence-point  -Wshadow 
          -Wsign-compare  -Wstrict-aliasing 
          -Wswitch  -Wswitch-default  -Wswitch-enum 
          -Wsystem-headers  -Wtrigraphs  -Wundef  -Wuninitialized 
          -Wunknown-pragmas  -Wunreachable-code 
          -Wunused  -Wunused-function  -Wunused-label  -Wunused-parameter 
          -Wunused-value  -Wunused-variable  -Wwrite-strings
     

C-only Warning Options

          -Wbad-function-cast  -Wmissing-declarations 
          -Wmissing-prototypes  -Wnested-externs 
          -Wstrict-prototypes  -Wtraditional
     

Debugging Options

See Options for Debugging Your Program or GCC.

          -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] 
          -feliminate-dwarf2-dups  -fmem-report 
          -fprofile-arcs  -frandom-seed=n 
          -fsched-verbose=n -ftest-coverage  -ftime-report 
          -g  -glevel  -gcoff  -gdwarf  -gdwarf-1  -gdwarf-1+  -gdwarf-2 
          -ggdb  -gstabs  -gstabs+  -gvms  -gxcoff  -gxcoff+ 
          -p  -pg  -print-file-name=library  -print-libgcc-file-name 
          -print-multi-directory  -print-multi-lib 
          -print-prog-name=program  -print-search-dirs  -Q 
          -save-temps  -time
     

Optimization Options

See Options that Control Optimization.

          -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  -floop-optimize  -fcrossjumping 
          -fif-conversion  -fif-conversion2 
          -finline-functions  -finline-limit=n  -fkeep-inline-functions 
          -fkeep-static-consts  -fmerge-constants  -fmerge-all-constants 
          -fmove-all-movables  -fnew-ra  -fno-branch-count-reg 
          -fno-default-inline  -fno-defer-pop 
          -fno-function-cse  -fno-guess-branch-probability 
          -fno-inline  -fno-math-errno  -fno-peephole  -fno-peephole2 
          -funsafe-math-optimizations  -ffinite-math-only 
          -fno-trapping-math  -fno-zero-initialized-in-bss 
          -fomit-frame-pointer  -foptimize-register-move 
          -foptimize-sibling-calls  -fprefetch-loop-arrays 
          -freduce-all-givs  -fregmove  -frename-registers 
          -freorder-blocks  -freorder-functions 
          -frerun-cse-after-loop  -frerun-loop-opt 
          -fschedule-insns  -fschedule-insns2 
          -fno-sched-interblock  -fno-sched-spec  -fsched-spec-load 
          -fsched-spec-load-dangerous  -fsignaling-nans 
          -fsingle-precision-constant  -fssa  -fssa-ccp  -fssa-dce 
          -fstrength-reduce  -fstrict-aliasing  -ftracer  -fthread-jumps 
          -funroll-all-loops  -funroll-loops  
          --param name=value 
          -O  -O0  -O1  -O2  -O3  -Os
     

Preprocessor Options

See Options Controlling the Preprocessor.

          -$  -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 
          -trigraphs  -undef  -Umacro  -Wp,option
     

Assembler Option

See Passing Options to the Assembler.

          -Wa,option
     

Linker Options

See Options for Linking.

          object-file-name  -llibrary 
          -nostartfiles  -nodefaultlibs  -nostdlib 
          -s  -static  -static-libgcc  -shared  -shared-libgcc  -symbolic 
          -Wl,option  -Xlinker option 
          -u symbol
     

Directory Options

See Options for Directory Search.

          -Bprefix  -Idir  -I-  -Ldir  -specs=file
     

Target Options

See Target Options.

          -V version  -b machine
     

Machine Dependent Options

See Hardware Models and Configurations.

M680x0 Options

          -m68000  -m68020  -m68020-40  -m68020-60  -m68030  -m68040 
          -m68060  -mcpu32  -m5200  -m68881  -mbitfield  -mc68000  -mc68020   
          -mfpa  -mnobitfield  -mrtd  -mshort  -msoft-float  -mpcrel 
          -malign-int  -mstrict-align
     

M68hc1x Options

          -m6811  -m6812  -m68hc11  -m68hc12  -m68hcs12 
          -mauto-incdec  -minmax  -mlong-calls  -mshort 
          -msoft-reg-count=count
     

VAX Options

          -mg  -mgnu  -munix
     

SPARC Options

          -mcpu=cpu-type 
          -mtune=cpu-type 
          -mcmodel=code-model 
          -m32  -m64 
          -mapp-regs  -mbroken-saverestore  -mcypress 
          -mfaster-structs  -mflat 
          -mfpu  -mhard-float  -mhard-quad-float 
          -mimpure-text  -mlittle-endian  -mlive-g0  -mno-app-regs 
          -mno-faster-structs  -mno-flat  -mno-fpu 
          -mno-impure-text  -mno-stack-bias  -mno-unaligned-doubles 
          -msoft-float  -msoft-quad-float  -msparclite  -mstack-bias 
          -msupersparc  -munaligned-doubles  -mv8
          -threads -pthreads
     

ARM Options

          -mapcs-frame  -mno-apcs-frame 
          -mapcs-26  -mapcs-32 
          -mapcs-stack-check  -mno-apcs-stack-check 
          -mapcs-float  -mno-apcs-float 
          -mapcs-reentrant  -mno-apcs-reentrant 
          -msched-prolog  -mno-sched-prolog 
          -mlittle-endian  -mbig-endian  -mwords-little-endian 
          -malignment-traps  -mno-alignment-traps 
          -msoft-float  -mhard-float  -mfpe 
          -mthumb-interwork  -mno-thumb-interwork 
          -mcpu=name  -march=name  -mfpe=name  
          -mstructure-size-boundary=n 
          -mabort-on-noreturn 
          -mlong-calls  -mno-long-calls 
          -msingle-pic-base  -mno-single-pic-base 
          -mpic-register=reg 
          -mnop-fun-dllimport 
          -mpoke-function-name 
          -mthumb  -marm 
          -mtpcs-frame  -mtpcs-leaf-frame 
          -mcaller-super-interworking  -mcallee-super-interworking
     

MN10200 Options

          -mrelax
     

MN10300 Options

          -mmult-bug  -mno-mult-bug 
          -mam33  -mno-am33 
          -mno-crt0  -mrelax
     

M32R/D Options

          -m32rx  -m32r  -mcode-model=model-type 
          -msdata=sdata-type  -G num
     

M88K Options

          -m88000  -m88100  -m88110  -mbig-pic 
          -mcheck-zero-division  -mhandle-large-shift 
          -midentify-revision  -mno-check-zero-division 
          -mno-ocs-debug-info  -mno-ocs-frame-position 
          -mno-optimize-arg-area  -mno-serialize-volatile 
          -mno-underscores  -mocs-debug-info 
          -mocs-frame-position  -moptimize-arg-area 
          -mserialize-volatile  -mshort-data-num  -msvr3 
          -msvr4  -mtrap-large-shift  -muse-div-instruction 
          -mversion-03.00  -mwarn-passed-structs
     

RS/6000 and PowerPC Options

          -mcpu=cpu-type 
          -mtune=cpu-type 
          -mpower  -mno-power  -mpower2  -mno-power2 
          -mpowerpc  -mpowerpc64  -mno-powerpc 
          -maltivec  -mno-altivec 
          -mpowerpc-gpopt  -mno-powerpc-gpopt 
          -mpowerpc-gfxopt  -mno-powerpc-gfxopt 
          -mnew-mnemonics  -mold-mnemonics 
          -mfull-toc   -mminimal-toc  -mno-fp-in-toc  -mno-sum-in-toc 
          -m64  -m32  -mxl-call  -mno-xl-call  -mpe 
          -msoft-float  -mhard-float  -mmultiple  -mno-multiple 
          -mstring  -mno-string  -mupdate  -mno-update 
          -mfused-madd  -mno-fused-madd  -mbit-align  -mno-bit-align 
          -mstrict-align  -mno-strict-align  -mrelocatable 
          -mno-relocatable  -mrelocatable-lib  -mno-relocatable-lib 
          -mtoc  -mno-toc  -mlittle  -mlittle-endian  -mbig  -mbig-endian 
          -mcall-aix  -mcall-sysv  -mcall-netbsd 
          -maix-struct-return  -msvr4-struct-return 
          -mabi=altivec  -mabi=no-altivec 
          -mabi=spe  -mabi=no-spe 
          -misel=yes  -misel=no 
          -mprototype  -mno-prototype 
          -msim  -mmvme  -mads  -myellowknife  -memb  -msdata 
          -msdata=opt  -mvxworks  -mwindiss  -G num  -pthread
     

Darwin Options

          -all_load -allowable_client -arch -arch_errors_fatal 
          -arch_only -bind_at_load -bundle -bundle_loader 
          -client_name -compatibility_version -current_version 
          -dependency-file -dylib_file -dylinker_install_name 
          -dynamic -dynamiclib -exported_symbols_list 
          -filelist -flat_namespace -force_cpusubtype_ALL 
          -force_flat_namespace -headerpad_max_install_names 
          -image_base -init -install_name -keep_private_externs 
          -multi_module -multiply_defined -multiply_defined_unused 
          -noall_load -nomultidefs -noprebind -noseglinkedit 
          -pagezero_size -prebind -prebind_all_twolevel_modules 
          -private_bundle -read_only_relocs -sectalign 
          -sectobjectsymbols -whyload -seg1addr 
          -sectcreate -sectobjectsymbols -sectorder 
          -seg_addr_table -seg_addr_table_filename -seglinkedit 
          -segprot -segs_read_only_addr -segs_read_write_addr 
          -single_module -static -sub_library -sub_umbrella 
          -twolevel_namespace -umbrella -undefined 
          -unexported_symbols_list -weak_reference_mismatches -whatsloaded
     

RT Options

          -mcall-lib-mul  -mfp-arg-in-fpregs  -mfp-arg-in-gregs 
          -mfull-fp-blocks  -mhc-struct-return  -min-line-mul 
          -mminimum-fp-blocks  -mnohc-struct-return
     

MIPS Options

          -mabicalls  -march=cpu-type  -mtune=cpu=type 
          -mcpu=cpu-type  -membedded-data  -muninit-const-in-rodata 
          -membedded-pic  -mfp32  -mfp64  -mfused-madd  -mno-fused-madd 
          -mgas  -mgp32  -mgp64 
          -mgpopt  -mhalf-pic  -mhard-float  -mint64  -mips1 
          -mips2  -mips3  -mips4  -mlong64  -mlong32  -mlong-calls  -mmemcpy 
          -mmips-as  -mmips-tfile  -mno-abicalls 
          -mno-embedded-data  -mno-uninit-const-in-rodata 
          -mno-embedded-pic  -mno-gpopt  -mno-long-calls 
          -mno-memcpy  -mno-mips-tfile  -mno-rnames  -mno-stats 
          -mrnames  -msoft-float 
          -m4650  -msingle-float  -mmad 
          -mstats  -EL  -EB  -G num  -nocpp 
          -mabi=32  -mabi=n32  -mabi=64  -mabi=eabi 
          -mfix7000  -mno-crt0  -mflush-func=func  -mno-flush-func 
          -mbranch-likely  -mno-branch-likely
     

i386 and x86-64 Options

          -mcpu=cpu-type  -march=cpu-type  
          -mfpmath=unit  -masm=dialect  -mno-fancy-math-387 
          -mno-fp-ret-in-387  -msoft-float  -msvr3-shlib 
          -mno-wide-multiply  -mrtd  -malign-double 
          -mpreferred-stack-boundary=num 
          -mmmx  -msse  -msse2 -msse3 -m3dnow 
          -mthreads  -mno-align-stringops  -minline-all-stringops 
          -mpush-args  -maccumulate-outgoing-args  -m128bit-long-double 
          -m96bit-long-double  -mregparm=num  -momit-leaf-frame-pointer 
          -mno-red-zone
          -mcmodel=code-model 
          -m32  -m64
     

HPPA Options

          -march=architecture-type 
          -mbig-switch  -mdisable-fpregs  -mdisable-indexing 
          -mfast-indirect-calls  -mgas  -mgnu-ld  -mhp-ld 
          -mjump-in-delay  -mlinker-opt  -mlong-calls 
          -mlong-load-store  -mno-big-switch  -mno-disable-fpregs 
          -mno-disable-indexing  -mno-fast-indirect-calls  -mno-gas 
          -mno-jump-in-delay  -mno-long-load-store 
          -mno-portable-runtime  -mno-soft-float 
          -mno-space-regs  -msoft-float  -mpa-risc-1-0 
          -mpa-risc-1-1  -mpa-risc-2-0  -mportable-runtime 
          -mschedule=cpu-type  -mspace-regs  -msio  -mwsio 
          -nolibdld  -static  -threads
     

Intel 960 Options

          -mcpu-type  -masm-compat  -mclean-linkage 
          -mcode-align  -mcomplex-addr  -mleaf-procedures 
          -mic-compat  -mic2.0-compat  -mic3.0-compat 
          -mintel-asm  -mno-clean-linkage  -mno-code-align 
          -mno-complex-addr  -mno-leaf-procedures 
          -mno-old-align  -mno-strict-align  -mno-tail-call 
          -mnumerics  -mold-align  -msoft-float  -mstrict-align 
          -mtail-call
     

DEC Alpha Options

          -mno-fp-regs  -msoft-float  -malpha-as  -mgas 
          -mieee  -mieee-with-inexact  -mieee-conformant 
          -mfp-trap-mode=mode  -mfp-rounding-mode=mode 
          -mtrap-precision=mode  -mbuild-constants 
          -mcpu=cpu-type  -mtune=cpu-type 
          -mbwx  -mmax  -mfix  -mcix 
          -mfloat-vax  -mfloat-ieee 
          -mexplicit-relocs  -msmall-data  -mlarge-data 
          -mmemory-latency=time
     

DEC Alpha/VMS Options

          -mvms-return-codes
     

H8/300 Options

          -mrelax  -mh  -ms  -mn  -mint32  -malign-300
     

SH Options

          -m1  -m2  -m3  -m3e 
          -m4-nofpu  -m4-single-only  -m4-single  -m4 
          -m5-64media  -m5-64media-nofpu 
          -m5-32media  -m5-32media-nofpu 
          -m5-compact  -m5-compact-nofpu 
          -mb  -ml  -mdalign  -mrelax 
          -mbigtable  -mfmovd  -mhitachi  -mnomacsave 
          -mieee  -misize  -mpadstruct  -mspace 
          -mprefergot  -musermode
     

System V Options

          -Qy  -Qn  -YP,paths  -Ym,dir
     

ARC Options

          -EB  -EL 
          -mmangle-cpu  -mcpu=cpu  -mtext=text-section 
          -mdata=data-section  -mrodata=readonly-data-section
     

TMS320C3x/C4x Options

          -mcpu=cpu  -mbig  -msmall  -mregparm  -mmemparm 
          -mfast-fix  -mmpyi  -mbk  -mti  -mdp-isr-reload 
          -mrpts=count  -mrptb  -mdb  -mloop-unsigned 
          -mparallel-insns  -mparallel-mpy  -mpreserve-float
     

V850 Options

          -mlong-calls  -mno-long-calls  -mep  -mno-ep 
          -mprolog-function  -mno-prolog-function  -mspace 
          -mtda=n  -msda=n  -mzda=n 
          -mapp-regs  -mno-app-regs 
          -mdisable-callt  -mno-disable-callt 
          -mv850e 
          -mv850  -mbig-switch
     

NS32K Options

          -m32032  -m32332  -m32532  -m32081  -m32381 
          -mmult-add  -mnomult-add  -msoft-float  -mrtd  -mnortd 
          -mregparam  -mnoregparam  -msb  -mnosb 
          -mbitfield  -mnobitfield  -mhimem  -mnohimem
     

AVR Options

          -mmcu=mcu  -msize  -minit-stack=n  -mno-interrupts 
          -mcall-prologues  -mno-tablejump  -mtiny-stack
     

MCore Options

          -mhardlit  -mno-hardlit  -mdiv  -mno-div  -mrelax-immediates 
          -mno-relax-immediates  -mwide-bitfields  -mno-wide-bitfields 
          -m4byte-functions  -mno-4byte-functions  -mcallgraph-data 
          -mno-callgraph-data  -mslow-bytes  -mno-slow-bytes  -mno-lsim 
          -mlittle-endian  -mbig-endian  -m210  -m340  -mstack-increment
     

MMIX Options

          -mlibfuncs  -mno-libfuncs  -mepsilon  -mno-epsilon  -mabi=gnu 
          -mabi=mmixware  -mzero-extend  -mknuthdiv  -mtoplevel-symbols 
          -melf  -mbranch-predict  -mno-branch-predict  -mbase-addresses 
          -mno-base-addresses  -msingle-exit  -mno-single-exit
     

IA-64 Options

          -mbig-endian  -mlittle-endian  -mgnu-as  -mgnu-ld  -mno-pic 
          -mvolatile-asm-stop  -mb-step  -mregister-names  -mno-sdata 
          -mconstant-gp  -mauto-pic  -minline-float-divide-min-latency 
          -minline-float-divide-max-throughput 
          -minline-int-divide-min-latency 
          -minline-int-divide-max-throughput  -mno-dwarf2-asm 
          -mfixed-range=register-range
     

D30V Options

          -mextmem  -mextmemory  -monchip  -mno-asm-optimize 
          -masm-optimize  -mbranch-cost=n  -mcond-exec=n
     

S/390 and zSeries Options

          -mhard-float  -msoft-float  -mbackchain  -mno-backchain 
          -msmall-exec  -mno-small-exec  -mmvcle  -mno-mvcle 
          -m64  -m31  -mdebug  -mno-debug
     

CRIS Options

          -mcpu=cpu  -march=cpu  -mtune=cpu 
          -mmax-stack-frame=n  -melinux-stacksize=n 
          -metrax4  -metrax100  -mpdebug  -mcc-init  -mno-side-effects 
          -mstack-align  -mdata-align  -mconst-align 
          -m32-bit  -m16-bit  -m8-bit  -mno-prologue-epilogue  -mno-gotplt 
          -melf  -maout  -melinux  -mlinux  -sim  -sim2 
          -mmul-bug-workaround  -mno-mul-bug-workaround
     

PDP-11 Options

          -mfpu  -msoft-float  -mac0  -mno-ac0  -m40  -m45  -m10 
          -mbcopy  -mbcopy-builtin  -mint32  -mno-int16 
          -mint16  -mno-int32  -mfloat32  -mno-float64 
          -mfloat64  -mno-float32  -mabshi  -mno-abshi 
          -mbranch-expensive  -mbranch-cheap 
          -msplit  -mno-split  -munix-asm  -mdec-asm
     

Xstormy16 Options

          -msim
     

Xtensa Options

          -mbig-endian  -mlittle-endian 
          -mdensity  -mno-density 
          -mmac16  -mno-mac16 
          -mmul16  -mno-mul16 
          -mmul32  -mno-mul32 
          -mnsa  -mno-nsa 
          -mminmax  -mno-minmax 
          -msext  -mno-sext 
          -mbooleans  -mno-booleans 
          -mhard-float  -msoft-float 
          -mfused-madd  -mno-fused-madd 
          -mserialize-volatile  -mno-serialize-volatile 
          -mtext-section-literals  -mno-text-section-literals 
          -mtarget-align  -mno-target-align 
          -mlongcalls  -mno-longcalls
     

FRV Options

          -mgpr-32  -mgpr-64  -mfpr-32  -mfpr-64 
          -mhard-float  -msoft-float  -malloc-cc  -mfixed-cc 
          -mdword  -mno-dword  -mdouble  -mno-double 
          -mmedia  -mno-media  -mmuladd  -mno-muladd  -mlibrary-pic 
          -macc-4  -macc-8  -mpack  -mno-pack  -mno-eflags 
          -mcond-move  -mno-cond-move -mscc  -mno-scc  
          -mcond-exec  -mno-cond-exec  -mvliw-branch  -mno-vliw-branch 
          -mmulti-cond-exec  -mno-multi-cond-exec  -mnested-cond-exec 
          -mno-nested-cond-exec  -mtomcat-stats 
          -mcpu=cpu
     

Code Generation Options

See Options for Code Generation Conventions.

          -fcall-saved-reg  -fcall-used-reg 
          -ffixed-reg  -fexceptions 
          -fnon-call-exceptions  -funwind-tables 
          -fasynchronous-unwind-tables 
          -finhibit-size-directive  -finstrument-functions 
          -fno-common  -fno-ident  -fno-gnu-linker 
          -fpcc-struct-return  -fpic  -fPIC 
          -freg-struct-return  -fshared-data  -fshort-enums 
          -fshort-double  -fshort-wchar  -fvolatile 
          -fvolatile-global  -fvolatile-static 
          -fverbose-asm  -fpack-struct  -fstack-check 
          -fstack-limit-register=reg  -fstack-limit-symbol=sym 
          -fargument-alias  -fargument-noalias 
          -fargument-noalias-global  -fleading-underscore 
          -ftls-model=model 
          -ftrapv  -fbounds-check
     

• Overall Options: Controlling the kind of output: an executable, object files, assembler files, or preprocessed source.
• C Dialect Options: Controlling the variant of C language compiled.
• C++ Dialect Options: Variations on C++.
• Objective-C Dialect Options: Variations on Objective-C.
• Language Independent Options: Controlling how diagnostics should be formatted.
• Warning Options: How picky should the compiler be?
• Debugging Options: Symbol tables, measurements, and debugging dumps.
• Optimize Options: How much optimization?
• Preprocessor Options: Controlling header files and macro definitions. Also, getting dependency information for Make.
• Assembler Options: Passing options to the assembler.
• Link Options: Specifying libraries and so on.
• Directory Options: Where to find header files and libraries. Where to find the compiler executable files.
• Spec Files: How to pass switches to sub-processes.
• Target Options: Running a cross-compiler, or an old version of GCC.

3.2 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 compiled, and those specified as input) into an executable file.

For any given input file, the file name suffix determines what kind of compilation is done:

file.c

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.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 traditional preprocessor).

file.r

Fortran source code which must be preprocessed with a RATFOR preprocessor (not included with GCC).

See Options Controlling the Kind of Output (Using and Porting GNU Fortran), for more details of the handling of Fortran input files.

file.ads

Ada source code file which contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.

file.adb

Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.

file.s

Assembler code.

file.S

Assembler code which must be preprocessed.

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
          assembler  assembler-with-cpp
          ada
          f77  f77-cpp-input  ratfor
          java
          treelang
     

-x none

Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if -x has not been used at all).

-pass-exit-codes

Normally the gcc program will exit with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program will instead return with numerically 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-assembler 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 compiler driver program and of the preprocessor and the compiler proper.

-###

Like -v except the commands are not executed and all command arguments are quoted. This is useful for shell scripts to capture the driver-generated command lines.

-pipe

Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble.

--help

Print (on the standard output) a description of the command line options understood by gcc. If the -v option is also specified then --help will also be passed on to the various processes invoked by gcc, so that they can display the command line options they accept. If the -W option is also specified then command line options which have no documentation associated with them will also be displayed.

--target-help

Print (on the standard output) a description of target specific command line options for each tool.

--version

Display the version number and copyrights of the invoked GCC.

3.3 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 compiler that understands the C++ language—and under some circumstances, you might want to compile programs from standard input, or otherwise without a suffix that flags them as C++ programs. g++ is a program that calls GCC with the default language set to C++, and automatically specifies linking against the C++ library. On many systems, g++ is also installed with the name c++.

When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. See Options Controlling C Dialect, for explanations of options for languages related to C. See Options Controlling C++ Dialect, for explanations of options that are meaningful only for C++ programs.

3.4 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 C90 programs. In C++ mode, remove GNU extensions that conflict with ISO C++.

This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style // comments as well as the inline keyword.

The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite -ansi. You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with -ansi. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without -ansi.

The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -pedantic is required in addition to -ansi. See Warning Options.

The macro __STRICT_ANSI__ is predefined when the -ansi option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things.

Functions which would normally be built in but do not have semantics defined by ISO C (such as alloca and ffs) are not built-in functions with -ansi is used. See Other built-in functions provided by GCC, for details of the functions affected.

-std=

Determine the language standard. This option is currently only supported when compiling C or C++. A value for this option must be provided; possible values are

c89

iso9899:1990

ISO C90 (same as -ansi).

iso9899:199409

ISO C90 as modified in amendment 1.

c99

c9x

iso9899:1999

iso9899:199x

ISO C99. Note that this standard is not yet fully supported; see http://gcc.gnu.org/gcc-3.3/c99status.html for more information. The names c9x and iso9899:199x are deprecated.

gnu89

Default, ISO C90 plus GNU extensions (including some C99 features).

gnu99

gnu9x

ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will become the default. The name gnu9x is deprecated.

c++98

The 1998 ISO C++ standard plus amendments.

gnu++98

The same as -std=c++98 plus GNU extensions. This is the default for C++ code.

Even when this option is not specified, you can still use some of the features of newer standards in so far as they do not conflict with previous C standards. For example, you may use __restrict__ even when -std=c99 is not specified.

The -std options specifying some version of ISO C have the same effects as -ansi, except that features that were not in ISO C90 but are in the specified version (for example, // comments and the inline keyword in ISO C99) are not disabled.

See Language Standards Supported by GCC, for details of these standard versions.

-aux-info filename

Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C.

Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.

-fno-asm

Do not recognize asm, inline or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __asm__, __inline__ and __typeof__ instead. -ansi implies -fno-asm.

In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords. You may want to use the -fno-gnu-keywords flag instead, which has the same effect. In C99 mode (-std=c99 or -std=gnu99), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99.

-fno-builtin

-fno-builtin-function

Don't recognize built-in functions that do not begin with __builtin_ as prefix. See Other built-in functions provided by GCC, for details of the functions affected, including those which are not built-in functions when -ansi or -std options for strict ISO C conformance are used because they do not have an ISO standard meaning.

GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to alloca may become single instructions that adjust the stack directly, and calls to memcpy may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library.

With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named this is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:

          #define abs(n)          __builtin_abs ((n))
          #define strcpy(d, s)    __builtin_strcpy ((d), (s))
     

-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.

See Language Standards Supported by GCC, for details of freestanding and hosted environments.

-fms-extensions

Accept some non-standard constructs used in Microsoft header files.

-trigraphs

Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance) implies -trigraphs.

-no-integrated-cpp

Performs a compilation in two passes: preprocessing and compiling. This option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the -B option. The user supplied compilation step can then add in an additional preprocessing step after normal preprocessing but before compiling. The default is to use the integrated cpp (internal cpp)

The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.

-traditional

-traditional-cpp

Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are now only supported with the -E switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP manual for details.

-fcond-mismatch

Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++.

-funsigned-char

Let the type char be unsigned, like unsigned char.

Each kind of machine has a default for what char should be. It is either like unsigned char by default or like signed char by default.

Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default.

The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two.

-fsigned-char

Let the type char be signed, like signed char.

Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char.

-fsigned-bitfields

-funsigned-bitfields

-fno-signed-bitfields

-fno-unsigned-bitfields

These options control whether a bit-field is signed or unsigned, when the declaration does not use either signed or unsigned. By default, such a bit-field is signed, because this is consistent: the basic integer types such as int are signed types.

-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.

Writing into string constants is a very bad idea; “constants” should be constant.

3.5 Options Controlling C++ Dialect

This section describes the command-line options that are only meaningful for C++ programs; but you can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file firstClass.C like this:

     g++ -g -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:

-fabi-version=n

Use version n of the C++ ABI. Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always be the version that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed.

The default is version 1.

-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. This check is normally unnecessary because the C++ standard specifies that operator new will only return 0 if it is declared throw(), in which case the compiler will always check the return value even without this option. In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc. 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 standard. Even if you use -fno-const-strings, you cannot actually modify the value of a string constant, unless you also use -fwritable-strings.

This option might be removed in a future release of G++. For maximum portability, you should structure your code so that it works with string constants that have type const char *.

-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 temporary 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 reducing 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. See Template Instantiation, for more information.

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. See Template Instantiation, for more information.

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. See Template Instantiation, for more information.

-fno-implicit-inline-templates

Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles 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.

-fms-extensions

Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard 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 some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive will allow some nonconforming code to compile.

-frepo

Enable automatic template instantiation at link time. This option also implies -fno-implicit-templates. See Template Instantiation, for more information.

-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 compilation. 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 endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17.

-fuse-cxa-atexit

Register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but will only work if your C library supports __cxa_atexit.

-fvtable-gc

Emit special relocations for vtables and virtual function references 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 support 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 available. 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. See Options That Control Optimization. Note that these functions will have linkage like inline functions; they just won't be inlined by default.

-Wabi (C++ only)

Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated will be compatible.

You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

The known incompatibilities at this point include:

• Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same byte as a base class. For example:

                 struct A { virtual void f(); int f1 : 1; };
                 struct B : public A { int f2 : 1; };
            

  In this case, G++ will place B::f2 into the same byte asA::f1; other compilers will not. You can avoid this problem by explicitly padding A so that its size is a multiple of the byte size on your platform; that will cause G++ and other compilers to layout B identically.

• Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out virtual bases. For example:

                 struct A { virtual void f(); char c1; };
                 struct B { B(); char c2; };
                 struct C : public A, public virtual B {};
            

  In this case, G++ will not place B into the tail-padding for A; other compilers will. You can avoid this problem by explicitly padding A so that its size is a multiple of its alignment (ignoring virtual base classes); that will cause G++ and other compilers to layout C identically.

• Incorrect handling of bit-fields with declared widths greater than that of their underlying types, when the bit-fields appear in a union. For example:

                 union U { int i : 4096; };
            

  Assuming that an int does not have 4096 bits, G++ will make the union too small by the number of bits in an int.

• Empty classes can be placed at incorrect offsets. For example:

                 struct A {};
                 
                 struct B {
                   A a;
                   virtual void f ();
                 };
                 
                 struct C : public B, public A {};
            

  G++ will place the A base class of C at a nonzero offset; it should be placed at offset zero. G++ mistakenly believes that the A data member of B is already at offset zero.

• Names of template functions whose types involve typename or template template parameters can be mangled incorrectly.

                 template <typename Q>
                 void f(typename Q::X) {}
                 
                 template <template <typename> class Q>
                 void f(typename Q<int>::X) {}
            

  Instantiations of these templates may be mangled incorrectly.

-Wctor-dtor-privacy (C++ only)

Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. This warning is enabled by default.

-Wnon-virtual-dtor (C++ only)

Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it declares a non-virtual one. This warning is enabled by -Wall.

-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) { }
          };
     

The compiler will rearrange the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall.

The following -W... options are not affected by -Wall.

-Weffc++ (C++ only)

Warn about violations of the following style guidelines from Scott Meyers' Effective C++ book:

• Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated memory.
• Item 12: Prefer initialization to assignment in constructors.
• Item 14: Make destructors virtual in base classes.
• Item 15: Have operator= return a reference to *this.
• Item 23: Don't try to return a reference when you must return an object.

Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:

• Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.
• Item 7: Never overload &&, ||, or ,.

When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings.

-Wno-deprecated (C++ only)

Do not warn about usage of deprecated features. See Deprecated Features.

-Wno-non-template-friend (C++ only)

Disable warnings when non-templatized friend functions are declared within a template. Since the advent of explicit template specification 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 function. (Section 14.5.3). Before G++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization 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, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search 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:

          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 preserve 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 =.

3.6 Options Controlling Objective-C Dialect

This section describes the command-line options that are only meaningful for Objective-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 some_class.m like this:

     gcc -g -fgnu-runtime -O -c some_class.m

In this example, -fgnu-runtime is an option meant only for Objective-C programs; you can use the other options with any language supported by GCC.

Here is a list of options that are only for compiling Objective-C programs:

-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. The macro __NEXT_RUNTIME__ is predefined if (and only if) this option is used.

-gen-decls

Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.

-Wno-protocol

If a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them.

-Wselector

Warn if multiple methods of different types for the same selector are found during compilation. The check is performed on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing in a @selector(...) expression, and a corresponding method for that selector has been found during compilation. Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached, for example because an error is found during compilation, or because the -fsyntax-only option is being used.

-Wundeclared-selector

Warn if a @selector(...) expression referring to an undeclared selector is found. A selector is considered undeclared if no method with that name has been declared before the @selector(...) expression, either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. This option always performs its checks as soon as a @selector(...) expression is found, while -Wselector only performs its checks in the final stage of compilation. This also enforces the coding style convention that methods and selectors must be declared before being used.

3.7 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 formatting 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 correctly.

-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.

3.8 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 C++ Dialect Options and 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 version 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++ features are supported as well. With this option, they are rejected.

-pedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. Pedantic warnings are also disabled in the expression that follows __extension__. However, only system header files should use these escape routes; application programs should avoid them. See Alternate Keywords.

Some users try to use -pedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all—only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.

A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -pedantic. We don't have plans to support such a feature in the near future.

Where the standard specified with -std represents a GNU extended dialect of C, such as gnu89 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -pedantic are given where they are required by the base standard. (It would not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)

-pedantic-errors

Like -pedantic, except that errors are produced rather than warnings.

-w

Inhibit all warning messages.

-Wno-import

Inhibit warning messages about the use of #import.

-Wchar-subscripts

Warn if an array subscript has type char. This is a common cause of error, as programmers often forget that this type is signed on 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 (see Function 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 C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations. However, if -pedantic is used with -Wformat, warnings will be given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard). See Options Controlling C Dialect.

Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.

-Wformat is included in -Wall. For more control over some aspects of format checking, the options -Wno-format-y2k, -Wno-format-extra-args, -Wno-format-zero-length, -Wformat-nonliteral, -Wformat-security, and -Wformat=2 are available, but are not included in -Wall.

-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 Specification says that such unused arguments are allowed.

-Wno-format-zero-length

If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats 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 currently 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. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security.

-Wnonnull

Warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute.

-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.

-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 bracketed, 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 violations of sequence point rules in the C standard.

The C standard defines the order in which expressions in a C program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap.

It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C standard specifies that “Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.”. If a program breaks these rules, the results on any particular implementation are entirely unpredictable.

Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.

The present implementation of this option only works for C programs. A future implementation may also work for C++ programs.

The C standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on our readings page, at http://gcc.gnu.org/readings.html.

-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 diagnostic 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 enumeration. (The presence of a default label prevents this warning.) case labels outside the enumeration range also provoke warnings when this option is used.

-Wswitch-default

Warn whenever a switch statement does not have a default case.

-Wswitch-enum

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 enumeration. 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 (see Variable Attributes).

-Wunused-parameter

Warn whenever a function parameter is unused aside from its declaration.

To suppress this warning use the unused attribute (see Variable Attributes).

-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 (see Variable Attributes).

-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 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 initialized 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 optimizing. If you don't specify -O, you simply won't get these warnings.

These warnings occur only for variables that are candidates for register allocation. Therefore, they do not occur for a variable that is declared volatile, or whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for structures, unions or arrays, even when they are in registers.

Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.

These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this can happen:

          {
            int x;
            switch (y)
              {
              case 1: x = 1;
                break;
              case 2: x = 4;
                break;
              case 3: x = 5;
              }
            foo (x);
          }
     

If the value of y is always 1, 2 or 3, then x is always initialized, but GCC doesn't know this. Here is another common case:

          {
            int save_y;
            if (change_y) save_y = y, y = new_y;
            ...
            if (change_y) y = save_y;
          }
     

This has no bug because save_y is used only if it is set.

This option also warns when a non-volatile automatic variable might be changed by a call to longjmp. These warnings as well are possible only in optimizing compilation.

The compiler sees only the calls to setjmp. It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because longjmp cannot in fact be called at the place which would cause a problem.

Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn. See Function Attributes.

-Wunknown-pragmas

Warn when a #pragma directive is encountered which is not understood 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.

-Wstrict-aliasing

This option is only active when -fstrict-aliasing is active. It warns about code which might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in -Wall.

-Wall

All of the above -W options combined. This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables some language-specific warnings described in C++ Dialect Options and Objective-C Dialect Options.

The following -W... options are not implied by -Wall. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning.

-W

Print extra warning messages for these events:

• A function can return either with or without a value. (Falling off the end of the function body is considered returning without a value.) For example, this function would evoke such a warning:

                 foo (a)
                 {
                   if (a > 0)
                     return a;
                 }
            

• An expression-statement or the left-hand side of a comma expression contains no side effects. To suppress the warning, cast the unused expression to void. For example, an expression such as x[i,j] will cause a warning, but x[(void)i,j] will not.
• An unsigned value is compared against zero with < or >=.
• A comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation.
• Storage-class specifiers like static are not the first things in a declaration. According to the C Standard, this usage is obsolescent.
• The return type of a function has a type qualifier such as const. Such a type qualifier has no effect, since the value returned by a function is not an lvalue. (But don't warn about the GNU extension of volatile void return types. That extension will be warned about if -pedantic is specified.)
• If -Wall or -Wunused is also specified, warn about unused arguments.
• 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 specified.)
• An aggregate has a partly bracketed initializer. For example, the following code would evoke such a warning, because braces are missing around the initializer for x.h:

                 struct s { int f, g; };
                 struct t { struct s h; int i; };
                 struct t x = { 1, 2, 3 };
            

• An aggregate has an initializer which does not initialize all members. For example, the following code would cause such a warning, because x.h would be implicitly initialized to zero:

                 struct s { int f, g, h; };
                 struct s x = { 3, 4 };
            

-Wno-div-by-zero

Do not warn about compile-time integer division by zero. Floating point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs.

-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.

-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 analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you would check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken.

-Wtraditional (C only)

Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs which should be avoided.

• Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but does not in ISO C.
• In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only consider a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but would ignore because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations would not recognize #elif, so it suggests avoiding it altogether.
• A function-like macro that appears without arguments.
• The unary plus operator.
• The U integer constant suffix, or the F or L floating point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Use of these macros in user code might normally lead to spurious warnings, however gcc's integrated preprocessor has enough context to avoid warning in these cases.
• A function declared external in one block and then used after the end of the block.
• A switch statement has an operand of type long.
• A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.
• The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
• Usage of ISO string concatenation is detected.
• Initialization of automatic aggregates.
• Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
• 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.
• 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.
• Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features will appear in your code when using libiberty's traditional C compatibility macros, PARAMS and VPARAMS. This warning is also bypassed for nested functions because that feature is already a gcc extension and thus not relevant to traditional C compatibility.

-Wundef

Warn if an undefined identifier is evaluated in an #if directive.

-Wendif-labels

Warn whenever an #else or an #endif are followed by text.

-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 convenience in calculations with void * pointers and pointers to functions.

-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 careful 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 prototype. 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 assignment 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 enabled by -W, and by -Wall in C++ only.

-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 (C only)

Warn if a global function is defined without a previous declaration. 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 noreturn. 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 subtle 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 function like vprintf or vscanf, but this might not always be the case, and some functions for which format attributes are appropriate may not be detected. This option has no effect unless -Wformat is enabled (possibly by -Wall).

-Wno-multichar

Do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, as they have implementation-defined values, and should not be used in portable code.

-Wno-deprecated-declarations

Do not warn about uses of functions, variables, and types marked as deprecated by using the deprecated attribute. (see Function Attributes, see Variable Attributes, see 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 element 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 procedure that never returns.

It is possible for this option to produce a warning even though there are circumstances under which part of the affected line can be executed, so care should be taken when removing apparently-unreachable code.

For instance, when a function is inlined, a warning may mean that the line is unreachable in only one inlined copy of the function.

This option is not made part of -Wall because in a debugging version of a program there is often substantial code which checks correct functioning of the program and is, hopefully, unreachable because the program does work. Another common use of unreachable code is to provide behavior which is selectable at compile-time.

-Winline

Warn if a function can not be inlined and it was declared as inline. Even with this option, the compiler will not warn about failures to inline functions declared in system headers.

The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear.

-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 handle 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.

3.9 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 format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging 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 control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, -gdwarf-1+, -gdwarf-1, or -gvms (see below).

Unlike most other C compilers, GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops.

Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.

The following options are useful when GCC is generated with the capability for more than one debugging format.

-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 supported), 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 supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.

-gcoff

Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.

-gxcoff

Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.

-gxcoff+

Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.

-gdwarf

Produce debugging information in DWARF version 1 format (if that is supported). This is the format used by SDB on most System V Release 4 systems.

This option is deprecated.

-gdwarf+

Produce debugging information in DWARF version 1 format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.

This option is deprecated.

-gdwarf-2

Produce debugging information in DWARF version 2 format (if that is supported). This is the format used by DBX on IRIX 6.

-gvms

Produce debugging information in VMS debug format (if that is supported). This is the format used by DEBUG on VMS systems.

-glevel

-ggdblevel

-gstabslevel

-gcofflevel

-gxcofflevel

-gvmslevel

Request debugging information and also use level to specify how much information. The default level is 2.

Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers.

Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3.

Note that in order to avoid confusion between DWARF1 debug level 2, and DWARF2, neither -gdwarf nor -gdwarf-2 accept a concatenated debug level. Instead use an additional -glevel option to change the debug level for DWARF1 or DWARF2.

-feliminate-dwarf2-dups

Compress DWARF2 debugging information by eliminating duplicated information about each symbol. This option only makes sense when generating DWARF2 debugging information with -gdwarf-2.

-p

Generate extra code to write profile information suitable for the analysis program prof. You must use this option when compiling the source files you want data about, and you must also use it when linking.

-pg

Generate extra code to write profile information suitable for the analysis program gprof. You must use this option when compiling the source files you want data about, and you must also use it when linking.

-Q

Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.

-ftime-report

Makes the compiler print some statistics about the time consumed by each pass when it finishes.

-fmem-report

Makes the compiler print some statistics about permanent memory allocation when it finishes.

-fprofile-arcs

Instrument arcs during compilation to generate coverage data or for profile-directed block ordering. During execution the program 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 auxname.da for each source file. auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.da for input file dir/foo.c, or dir/foo.da for output file specified as -o dir/foo.o).

For profile-directed block ordering, compile the program with -fprofile-arcs plus optimization and code generation options, generate the arc profile information by running the program on a selected workload, and then compile the program again with the same optimization and code generation options plus -fbranch-probabilities (see Options that Control Optimization).

The other use of -fprofile-arcs is for use with gcov, when it is used with the -ftest-coverage option.

With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.

-ftest-coverage

Create data files for the gcov code-coverage utility (see gcov—a Test Coverage Program). See -fprofile-arcs option above for a description of auxname.

auxname.bb

A mapping from basic blocks to line numbers, which gcov uses to associate basic block execution counts with line numbers.

auxname.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 auxname.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:

auxname.da

Runtime arc execution counts, used in conjunction with the arc information in the file auxname.bbg.

Coverage data will map better to the source files if -ftest-coverage 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 dumpname. dumpname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.00.rtl or foo.01.sibling). 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.32.bbro.

c

Dump after instruction combination, to the file file.19.combine.

C

Dump after the first if conversion, to the file file.15.ce1.

d

Dump after delayed branch scheduling, to file.34.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.29.ce3.

f

Dump after control and data flow analysis, to file.14.cfg. Also dump after life analysis, to file.18.life.

F

Dump after purging ADDRESSOF codes, to file.10.addressof.

g

Dump after global register allocation, to file.24.greg.

G

Dump after GCSE, to file.11.gcse.

h

Dump after finalization of EH handling code, to file.02.eh.

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.31.stack.

l

Dump after local register allocation, to file.23.lreg.

L

Dump after loop optimization, to file.12.loop.

M

Dump after performing the machine dependent reorganization pass, to file.33.mach.

n

Dump after register renumbering, to file.28.rnreg.

N

Dump after the register move pass, to file.21.regmove.

o

Dump after post-reload optimizations, to file.25.postreload.

r

Dump after RTL generation, to file.00.rtl.

R

Dump after the second scheduling pass, to file.30.sched2.

s

Dump after CSE (including the jump optimization that sometimes follows CSE), to file.09.cse.

S

Dump after the first scheduling pass, to file.22.sched.

t

Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to file.17.cse2.

T

Dump after running tracer, to file.16.tracer.

u

Dump after null pointer elimination pass to file.08.null.

w

Dump after the second flow pass, to file.26.flow2.

W

Dump after SSA conditional constant propagation, to file.05.ssaccp.

X

Dump after SSA dead code elimination, to file.06.ssadce.

z

Dump after the peephole pass, to file.27.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 instruction 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. Usually used with r.

y

Dump debugging information during parsing, to standard error.

-fdump-unnumbered

When doing debugging dumps (see -d option above), suppress instruction numbers and line number note output. This makes it more feasible 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 translation 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 function 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 intermediate language tree to a file. The file name is generated by appending 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 applicable 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 environment.

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.

-frandom-seed=string

This option provides a seed that GCC uses when it would otherwise use random numbers. At present, this is used to generate certain symbol names that have to be different in every compiled file.

The string should be different for every file you compile.

-fsched-verbose=n

On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints. This information is written to standard error, unless -dS or -dR is specified, in which case it is output to the usual dump listing file, .sched or .sched2 respectively. However for n greater than nine, the output is always printed to standard error.

For n greater than zero, -fsched-verbose outputs the same information as -dRS. For n greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info.

-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 preprocessed 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 -, without spaces between multiple switches. This is supposed 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 files... `gcc -print-libgcc-file-name`
     

-print-search-dirs

Print the name of the configured installation directory and a list of program and library directories gcc will search—and don't do anything else.

This is useful when gcc prints the error message installation problem, 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 '/'. See Environment Variables.

-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 anything else.

-dumpspecs

Print the compiler's built-in specs—and don't do anything else. (This is used when GCC itself is being built.) See Spec Files.

3.10 Options That Control Optimization

These options control various sorts of optimizations.

Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements 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 function and get exactly the results you would expect from the source code.

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed.

-O

-O1

Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.

-O turns on the following optimization flags:

          -fdefer-pop 
          -fmerge-constants 
          -fthread-jumps 
          -floop-optimize 
          -fcrossjumping 
          -fif-conversion 
          -fif-conversion2 
          -fdelayed-branch 
          -fguess-branch-probability 
          -fcprop-registers
     

-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.

-O2

Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when you specify -O2. As compared to -O, this option increases both compilation time and the performance of the generated code.

-O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags:

          -fforce-mem 
          -foptimize-sibling-calls 
          -fstrength-reduce 
          -fcse-follow-jumps  -fcse-skip-blocks 
          -frerun-cse-after-loop  -frerun-loop-opt 
          -fgcse   -fgcse-lm   -fgcse-sm 
          -fdelete-null-pointer-checks 
          -fexpensive-optimizations 
          -fregmove 
          -fschedule-insns  -fschedule-insns2 
          -fsched-interblock -fsched-spec 
          -fcaller-saves 
          -fpeephole2 
          -freorder-blocks  -freorder-functions 
          -fstrict-aliasing 
          -falign-functions  -falign-jumps 
          -falign-loops  -falign-labels
     

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. This is the default.

-Os

Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size.

-Os disables the following optimization flags:

          -falign-functions  -falign-jumps  -falign-loops 
          -falign-labels  -freorder-blocks  -fprefetch-loop-arrays
     

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 you typically will use. You can figure out the other form by either removing no- or adding it.

The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when “fine-tuning” of optimizations to be performed is desired.

-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 compiled 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.

Disabled at levels -O, -O2, -O3, -Os.

-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 common subexpressions, instruction combination should eliminate the separate register-load.

Enabled at levels -O2, -O3, -Os.

-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. See Register Usage (GNU Compiler Collection (GCC) Internals).

Enabled at levels -O, -O2, -O3, -Os.

-foptimize-sibling-calls

Optimize sibling and tail recursive calls.

Enabled at levels -O2, -O3, -Os.

-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.

Enabled at level -O3.

-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 (i.e., 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++.

Inlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters as follows:

max-inline-insns

is set to n.

max-inline-insns-single

is set to n/2.

max-inline-insns-auto

is set to n/2.

min-inline-insns

is set to 130 or n/4, whichever is smaller.

max-inline-insns-rtl

is set to n.

Using -finline-limit=600 thus results in the default settings for these parameters. See below for a documentation of the individual parameters controlling inlining.

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 function 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 compiler 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) across compilation units.

This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.

Enabled at levels -O, -O2, -O3, -Os.

-fmerge-all-constants

Attempt to merge identical constants and identical variables.

This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized 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-conforming behavior.

-fno-branch-count-reg

Do not use “decrement and branch” instructions on a count register, but instead generate a sequence of instructions that decrement a register, compare it against zero, then branch based upon the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390.

The default is -fbranch-count-reg, enabled when -fstrength-reduce is enabled.

-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 optimizations performed when this option is not used.

The default is -ffunction-cse

-fno-zero-initialized-in-bss

If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This can save space in the resulting code.

This option turns off this behavior because some programs explicitly rely on variables going to the data section. E.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.

The default is -fzero-initialized-in-bss.

-fstrength-reduce

Perform the optimizations of loop strength reduction and elimination of iteration variables.

Enabled at levels -O2, -O3, -Os.

-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.

Enabled at levels -O, -O2, -O3, -Os.

-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.

Enabled at levels -O2, -O3, -Os.

-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.

Enabled at levels -O2, -O3, -Os.

-frerun-cse-after-loop

Re-run common subexpression elimination after loop optimizations has been performed.

Enabled at levels -O2, -O3, -Os.

-frerun-loop-opt

Run the loop optimizer twice.

Enabled at levels -O2, -O3, -Os.

-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 extension, you may get better runtime performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.

Enabled at levels -O2, -O3, -Os.

-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.

Enabled by default when gcse is enabled.

-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.

Enabled by default when gcse is enabled.

-floop-optimize

Perform loop optimizations: move constant expressions out of loops, simplify exit test conditions and optionally do strength-reduction and loop unrolling as well.

Enabled at levels -O, -O2, -O3, -Os.

-fcrossjumping

Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The resulting code may or may not perform better than without cross-jumping.

Enabled at levels -O, -O2, -O3, -Os.

-fif-conversion

Attempt to transform conditional jumps into branch-less equivalents. This include use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on chips where it is available is controlled by if-conversion2.

Enabled at levels -O, -O2, -O3, -Os.

-fif-conversion2

Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.

Enabled at levels -O, -O2, -O3, -Os.

-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.

Enabled at levels -O2, -O3, -Os.

-fexpensive-optimizations

Perform a number of minor optimizations that are relatively expensive.

Enabled at levels -O2, -O3, -Os.

-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.

Note -fregmove and -foptimize-register-move are the same optimization.

Enabled at levels -O2, -O3, -Os.

-fdelayed-branch

If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.

Enabled at levels -O, -O2, -O3, -Os.

-fschedule-insns

If supported for the target machine, attempt to reorder instructions 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.

Enabled at levels -O2, -O3, -Os.

-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 number of registers and where memory load instructions take more than one cycle.

Enabled at levels -O2, -O3, -Os.

-fno-sched-interblock

Don't schedule instructions across basic blocks. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fno-sched-spec

Don't allow speculative motion of non-load instructions. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-spec-load

Allow speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-spec-load-dangerous

Allow speculative motion of more load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-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, usually those which have no call-preserved registers to use instead.

Enabled at levels -O2, -O3, -Os.

-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-movables 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 improving loop optimizations.

Please let us (gcc@gcc.gnu.org and fortran@gnu.org) 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 difference 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.

-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.

-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 feedback (-fprofile-arcs) or __builtin_expect. This means that different 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.

The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.

-freorder-blocks

Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality.

Enabled at levels -O2, -O3.

-freorder-functions

Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality. This is implemented by using special subsections text.hot for most frequently executed functions and text.unlikely for unlikely executed functions. Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way.

Also profile feedback must be available in to make this option effective. See -fprofile-arcs for details.

Enabled at levels -O2, -O3, -Os.

-fstrict-aliasing

Allows the compiler to assume the strictest aliasing rules applicable to the language being compiled. For C (and C++), this activates optimizations based on the type of expressions. In particular, 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 memory 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 analysis 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.

Enabled at levels -O2, -O3, -Os.

-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-functions=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 or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-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.

-fno-align-labels and -falign-labels=1 are equivalent and mean that labels will not be aligned.

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 or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment.

Enabled at levels -O2, -O3.

-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 operations.

-fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-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.

-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-frename-registers

Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization will most benefit processors with lots of registers. It can, however, make debugging impossible, since variables will no longer stay in a “home register”.

Enabled at levels -O3.

-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.

Disabled at levels -O, -O2, -O3, -Os.

The following options control compiler behavior regarding floating point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.

-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.

-ffast-math

Sets -fno-math-errno, -funsafe-math-optimizations,
-fno-trapping-math, -ffinite-math-only and
-fno-signaling-nans.

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 functions.

-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 functions.

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 functions.

The default is -fno-unsafe-math-optimizations.

-ffinite-math-only

Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.

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.

The default is -fno-finite-math-only.

-fno-trapping-math

Compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option implies -fno-signaling-nans. 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 functions.

The default is -ftrapping-math.

-fsignaling-nans

Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies -ftrapping-math.

This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined.

The default is -fno-signaling-nans.

This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.

-fsingle-precision-constant

Treat floating point constant as single precision constant instead of implicitly converting it to double precision constant.

The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code.

-fbranch-probabilities

After running a program compiled with -fprofile-arcs (see Options for Debugging Your Program or gcc), you can compile 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_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. 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.

-fnew-ra

Use a graph coloring register allocator. Currently this option is meant only for testing. Users should not specify this option, since it is not yet ready for production use.

-ftracer

Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do better job.

-funroll-loops

Unroll loops whose number of iterations can be determined at compile 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.

Disabled at level -Os.

-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 optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format and SPARC processors running Solaris 2 have linkers with such optimizations. 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.

-fssa

Perform optimizations in static single assignment form. Each function'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.

--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 functions that contain more that a certain number of instructions. You can control some of these constants on the command-line using the --param option.

The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases.

In each case, the value is an integer. The allowable choices for name are given in the following table:

max-crossjump-edges

The maximum number of incoming edges to consider for crossjumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable size.

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 arbitrary 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 executable run time.

max-delay-slot-live-search

When trying to fill delay slots, the maximum number of instructions 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 excessively large lists which needlessly consume memory and resources.

max-inline-insns-single

Several parameters control the tree inliner used in gcc. This number sets the maximum number of instructions (counted in gcc's internal representation) in a single function that the tree inliner will consider for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). The default value is 300.

max-inline-insns-auto

When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler will be investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied. The default value is 300.

max-inline-insns

The tree inliner does decrease the allowable size for single functions to be inlined after we already inlined the number of instructions given here by repeated inlining. This number should be a factor of two or more larger than the single function limit. Higher numbers result in better runtime performance, but incur higher compile-time resource (CPU time, memory) requirements and result in larger binaries. Very high values are not advisable, as too large binaries may adversely affect runtime performance. The default value is 600.

max-inline-slope

After exceeding the maximum number of inlined instructions by repeated inlining, a linear function is used to decrease the allowable size for single functions. The slope of that function is the negative reciprocal of the number specified here. The default value is 32.

min-inline-insns

The repeated inlining is throttled more and more by the linear function after exceeding the limit. To avoid too much throttling, a minimum for this function is specified here to allow repeated inlining for very small functions even when a lot of repeated inlining already has been done. The default value is 130.

max-inline-insns-rtl

For languages that use the RTL inliner (this happens at a later stage than tree inlining), you can set the maximum allowable size (counted in RTL instructions) for the RTL inliner with this parameter. The default value is 600.

max-unrolled-insns

The maximum number of instructions that a loop should have if that loop is unrolled, and if the loop is unrolled, it determines how many times the loop code is unrolled.

hot-bb-count-fraction

Select fraction of the maximal count of repetitions of basic block in program given basic block needs to have to be considered hot.

hot-bb-frequency-fraction

Select fraction of the maximal frequency of executions of basic block in function given basic block needs to have to be considered hot

tracer-dynamic-coverage

tracer-dynamic-coverage-feedback

This value is used to limit superblock formation once the given percentage of executed instructions is covered. This limits unnecessary code size expansion.

The tracer-dynamic-coverage-feedback is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value.

tracer-max-code-growth

Stop tail duplication once code growth has reached given percentage. This is rather hokey argument, as most of the duplicates will be eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth.

tracer-min-branch-ratio

Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).

tracer-min-branch-ratio

tracer-min-branch-ratio-feedback

Stop forward growth if the best edge do have probability lower than this threshold.

Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile feedback and one for compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.

ggc-min-expand

GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation.

The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If getrlimit is available, the notion of "RAM" is the smallest of actual RAM, RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This is extremely slow, but can be useful for debugging.

ggc-min-heapsize

Minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed, and has no effect on code generation.

The default is RAM/8, with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If getrlimit is available, the notion of "RAM" is the smallest of actual RAM, RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If GCC is not able to calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.

3.11 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 handle 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 processed 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 or GCC-specific macros. The standard 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. If the directory dir is a standard system include directory, the option is ignored to ensure that the default search order for system directories and the special treatment of system headers are not defeated .

-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 interpretation 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 independently. 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 traditional 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.

-Wunused-macros

Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested for existence at least once. The preprocessor will also warn if the macro has not been used at the time it is redefined or undefined.

Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.

Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:

          #if defined the_macro_causing_the_warning
          #endif
     

-Wendif-labels

Warn whenever an #else or an #endif are followed by text. This usually happens in code of the form

          #if FOO
          ...
          #else FOO
          ...
          #endif FOO
     

The second and third FOO should be in comments, but often are not in older programs. This warning is on by default.

-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 diagnostics 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 (see Environment Variables). Debug output will still be sent to the regular output stream as normal.

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.

-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

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.

-MG

In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the #include directive without prepending any path. -MG also suppresses preprocessed output, as a missing header file renders this useless.

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 platform'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 see -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 assembler-with-cpp

Specify the source language: C, 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, 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 knows about C and C++ standards; others may 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.

c++98

The 1998 ISO C++ standard plus amendments.

gnu++98

The same as -std=c++98 plus GNU extensions. This is the default for C++ code.

-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 <file>. 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 directories, 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 directory 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 specified 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. -iwithprefixbefore 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, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with -C to the compiler 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 extensions .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 understand 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.

-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 directives 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 predefined 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 preprocessing.

-P

Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something 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 ordinary source line, since the first token on the line is no longer a #.

-CC

Do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.

In addition to the side-effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.

The -CC option is generally used to support lint comments.

-traditional-cpp

Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.

-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 character 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.

--help

--target-help

Print text describing all the command line options instead of preprocessing 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 normal 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 preprocess as normal. With two dashes, exit immediately.

3.12 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 commas, it is split into multiple options at the commas.

3.13 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.

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. See Overall Options.

-llibrary

-l library

Search the library named library when linking. (The second alternative 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 directories 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 fashion. 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 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.

-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 memcmp, 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.

One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines that GCC uses to overcome shortcomings of particular machines, or special needs for some languages. (See Interfacing to GCC Output (GNU Compiler Collection (GCC) Internals), for more discussion of libgcc.a.) 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++ constructors will be called; see collect2 (GNU Compiler Collection (GCC) Internals).)

-s

Remove all symbol table and relocation information from the executable.

-static

On systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option has no effect.

-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

-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 configured, 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 relocation 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 support 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 recognize.

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.

3.14 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 vendor-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, the -I option will be ignored. The directory will still be searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options.

-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 <file>.

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 directory (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.

-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/ (see Target Options).

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 environment 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 standard 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. See Environment Variables.

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 compiler.

-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.

3.15 Specifying subprocesses and the switches to pass to them

gcc is a driver program. It performs its job by invoking a sequence of other programs to do the work of compiling, assembling and linking. GCC interprets its command-line parameters and uses these to deduce which programs it should invoke, and which command-line options it ought to place on their command lines. This behavior is controlled by spec strings. In most cases there is one spec string for each program that GCC can invoke, but a few programs have multiple spec strings to control their behavior. The spec strings built into GCC can be overridden by using the -specs= command-line switch to specify a spec file.

Spec files are plaintext files that are used to construct spec strings. They consist of a sequence of directives separated by blank lines. The type of directive is determined by the first non-whitespace character on the line and it can be one of the following:

%command

Issues a command to the spec file processor. The commands that can appear here are:

%include <file>

Search for file and insert its text at the current point in the specs file.

%include_noerr <file>

Just like %include, but do not generate an error message if the include file cannot be found.

%rename old_name new_name

Rename the spec string old_name to new_name.

*[spec_name]:

This tells the compiler to create, override or delete the named spec string. All lines after this directive up to the next directive or blank line are considered to be the text for the spec string. If this results in an empty string then the spec will be deleted. (Or, if the spec did not exist, then nothing will happened.) Otherwise, if the spec does not currently exist a new spec will be created. If the spec does exist then its contents will be overridden by the text of this directive, unless the first character of that text is the + character, in which case the text will be appended to the spec.

[suffix]:

Creates a new [suffix] spec pair. All lines after this directive and up to the next directive or blank line are considered to make up the spec string for the indicated suffix. When the compiler encounters an input file with the named suffix, it will processes the spec string in order to work out how to compile that file. For example:

          .ZZ:
          z-compile -input %i
     

This says that any input file whose name ends in .ZZ should be passed to the program z-compile, which should be invoked with the command-line switch -input and with the result of performing the %i substitution. (See below.)

As an alternative to providing a spec string, the text that follows a suffix directive can be one of the following:

@language

This says that the suffix is an alias for a known language. This is similar to using the -x command-line switch to GCC to specify a language explicitly. For example:

               .ZZ:
               @c++
          

Says that .ZZ files are, in fact, C++ source files.

#name

This causes an error messages saying:

               name compiler not installed on this system.
          

GCC already has an extensive list of suffixes built into it. This directive will add an entry to the end of the list of suffixes, but since the list is searched from the end backwards, it is effectively possible to override earlier entries using this technique.

GCC has the following spec strings built into it. Spec files can override these strings or create their own. Note that individual targets can also add their own spec strings to this list.

     asm          Options to pass to the assembler
     asm_final    Options to pass to the assembler post-processor
     cpp          Options to pass to the C preprocessor
     cc1          Options to pass to the C compiler
     cc1plus      Options to pass to the C++ compiler
     endfile      Object files to include at the end of the link
     link         Options to pass to the linker
     lib          Libraries to include on the command line to the linker
     libgcc       Decides which GCC support library to pass to the linker
     linker       Sets the name of the linker
     predefines   Defines to be passed to the C preprocessor
     signed_char  Defines to pass to CPP to say whether char is signed
                  by default
     startfile    Object files to include at the start of the link

Here is a small example of a spec file:

     %rename lib                 old_lib
     
     *lib:
     --start-group -lgcc -lc -leval1 --end-group %(old_lib)

This example renames the spec called lib to old_lib and then overrides the previous definition of lib with a new one. The new definition adds in some extra command-line options before including the text of the old definition.

Spec strings are a list of command-line options to be passed to their corresponding program. In addition, the spec strings can contain %-prefixed sequences to substitute variable text or to conditionally insert text into the command line. Using these constructs it is possible to generate quite complex command lines.

Here is a table of all defined %-sequences for spec strings. Note that spaces are not generated automatically around the results of expanding these sequences. Therefore you can concatenate them together or combine them with constant text in a single argument.

%%

Substitute one % into the program name or argument.

%i

Substitute the name of the input file being processed.

%b

Substitute the basename of the input file being processed. This is the substring up to (and not including) the last period and not including the directory.

%B

This is the same as %b, but include the file suffix (text after the last period).

%d

Marks the argument containing or following the %d as a temporary file name, so that that file will be deleted if GCC exits successfully. Unlike %g, this contributes no text to the argument.

%gsuffix

Substitute a file name that has suffix suffix and is chosen once per compilation, and mark the argument in the same way as %d. To reduce exposure to denial-of-service attacks, the file name is now chosen in a way that is hard to predict even when previously chosen file names are known. For example, %g.s ... %g.o ... %g.s might turn into ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s. suffix matches the regexp [.A-Za-z]* or the special string %O, which is treated exactly as if %O had been preprocessed. Previously, %g was simply substituted with a file name chosen once per compilation, without regard to any appended suffix (which was therefore treated just like ordinary text), making such attacks more likely to succeed.

%usuffix

Like %g, but generates a new temporary file name even if %usuffix was already seen.

%Usuffix

Substitutes the last file name generated with %usuffix, generating a new one if there is no such last file name. In the absence of any %usuffix, this is just like %gsuffix, except they don't share the same suffix space, so %g.s ... %U.s ... %g.s ... %U.s would involve the generation of two distinct file names, one for each %g.s and another for each %U.s. Previously, %U was simply substituted with a file name chosen for the previous %u, without regard to any appended suffix.

%jsuffix

Substitutes the name of the HOST_BIT_BUCKET, if any, and if it is writable, and if save-temps is off; otherwise, substitute the name of a temporary file, just like %u. This temporary file is not meant for communication between processes, but rather as a junk disposal mechanism.

%.SUFFIX

Substitutes .SUFFIX for the suffixes of a matched switch's args when it is subsequently output with %*. SUFFIX is terminated by the next space or %.

%w

Marks the argument containing or following the %w as the designated output file of this compilation. This puts the argument into the sequence of arguments that %o will substitute later.

%o

Substitutes the names of all the output files, with spaces automatically placed around them. You should write spaces around the %o as well or the results are undefined. %o is for use in the specs for running the linker. Input files whose names have no recognized suffix are not compiled at all, but they are included among the output files, so they will be linked.

%O

Substitutes the suffix for object files. Note that this is handled specially when it immediately follows %g, %u, or %U, because of the need for those to form complete file names. The handling is such that %O is treated exactly as if it had already been substituted, except that %g, %u, and %U do not currently support additional suffix characters following %O as they would following, for example, .o.

%p

Substitutes the standard macro predefinitions for the current target machine. Use this when running cpp.

%P

Like %p, but puts __ before and after the name of each predefined macro, except for macros that start with __ or with _L, where L is an uppercase letter. This is for ISO C.

%I

Substitute any of -iprefix (made from GCC_EXEC_PREFIX), -isysroot (made from TARGET_SYSTEM_ROOT), and -isystem (made from COMPILER_PATH and -B options) as necessary.

%s

Current argument is the name of a library or startup file of some sort. Search for that file in a standard list of directories and substitute the full name found.

%estr

Print str as an error message. str is terminated by a newline. Use this when inconsistent options are detected.

%|

Output - if the input for the current command is coming from a pipe.

%(name)

Substitute the contents of spec string name at this point.

%[name]

Like %(...) but put __ around -D arguments.

%x{option}

Accumulate an option for %X.

%X

Output the accumulated linker options specified by -Wl or a %x spec string.

%Y

Output the accumulated assembler options specified by -Wa.

%Z

Output the accumulated preprocessor options specified by -Wp.

%v1

Substitute the major version number of GCC. (For version 2.9.5, this is 2.)

%v2

Substitute the minor version number of GCC. (For version 2.9.5, this is 9.)

%v3

Substitute the patch level number of GCC. (For version 2.9.5, this is 5.)

%a

Process the asm spec. This is used to compute the switches to be passed to the assembler.

%A

Process the asm_final spec. This is a spec string for passing switches to an assembler post-processor, if such a program is needed.

%l

Process the link spec. This is the spec for computing the command line passed to the linker. Typically it will make use of the %L %G %S %D and %E sequences.

%D

Dump out a -L option for each directory that GCC believes might contain startup files. If the target supports multilibs then the current multilib directory will be prepended to each of these paths.

%M

Output the multilib directory with directory separators replaced with _. If multilib directories are not set, or the multilib directory is . then this option emits nothing.

%L

Process the lib spec. This is a spec string for deciding which libraries should be included on the command line to the linker.

%G

Process the libgcc spec. This is a spec string for deciding which GCC support library should be included on the command line to the linker.

%S

Process the startfile spec. This is a spec for deciding which object files should be the first ones passed to the linker. Typically this might be a file named crt0.o.

%E

Process the endfile spec. This is a spec string that specifies the last object files that will be passed to the linker.

%C

Process the cpp spec. This is used to construct the arguments to be passed to the C preprocessor.

%c

Process the signed_char spec. This is intended to be used to tell cpp whether a char is signed. It typically has the definition:

          %{funsigned-char:-D__CHAR_UNSIGNED__}
     

%1

Process the cc1 spec. This is used to construct the options to be passed to the actual C compiler (cc1).

%2

Process the cc1plus spec. This is used to construct the options to be passed to the actual C++ compiler (cc1plus).

%*

Substitute the variable part of a matched option. See below. Note that each comma in the substituted string is replaced by a single space.

%:function(args)

Call the named function function, passing it args. args is first processed as a nested spec string, then split into an argument vector in the usual fashion. The function returns a string which is processed as if it had appeared literally as part of the current spec.

The following built-in spec functions are provided:

if-exists

The if-exists spec function takes one argument, an absolute pathname to a file. If the file exists, if-exists returns the pathname. Here is a small example of its usage:

               *startfile:
               crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
          

if-exists-else

The if-exists-else spec function is similar to the if-exists spec function, except that it takes two arguments. The first argument is an absolute pathname to a file. If the file exists, if-exists-else returns the pathname. If it does not exist, it returns the second argument. This way, if-exists-else can be used to select one file or another, based on the existence of the first. Here is a small example of its usage:

               *startfile:
               crt0%O%s %:if-exists(crti%O%s) \
               %:if-exists-else(crtbeginT%O%s crtbegin%O%s)
          

%{S}

Substitutes the -S switch, if that switch was given to GCC. If that switch was not specified, this substitutes nothing. Note that the leading dash is omitted when specifying this option, and it is automatically inserted if the substitution is performed. Thus the spec string %{foo} would match the command-line option -foo and would output the command line option -foo.

%W{S}

Like %{S} but mark last argument supplied within as a file to be deleted on failure.

%{S*}

Substitutes all the switches specified to GCC whose names start with -S, but which also take an argument. This is used for switches like -o, -D, -I, etc. GCC considers -o foo as being one switch whose names starts with o. %{o*} would substitute this text, including the space. Thus two arguments would be generated.

%{^S*}

Like %{S*}, but don't put a blank between a switch and its argument. Thus %{^o*} would only generate one argument, not two.

%{S*&T*}

Like %{S*}, but preserve order of S and T options (the order of S and T in the spec is not significant). There can be any number of ampersand-separated variables; for each the wild card is optional. Useful for CPP as %{D*&U*&A*}.

%{<S}

Remove all occurrences of -S from the command line. Note—this command is position dependent. % commands in the spec string before this option will see -S, % commands in the spec string after this option will not.

%{S*:X}

Substitutes X if one or more switches whose names start with -S are specified to GCC. Note that the tail part of the -S option (i.e. the part matched by the *) will be substituted for each occurrence of %* within X.

%{S:X}

Substitutes X, but only if the -S switch was given to GCC.

%{!S:X}

Substitutes X, but only if the -S switch was not given to GCC.

%{|S:X}

Like %{S:X}, but if no S switch, substitute -.

%{|!S:X}

Like %{!S:X}, but if there is an S switch, substitute -.

%{.S:X}

Substitutes X, but only if processing a file with suffix S.

%{!.S:X}

Substitutes X, but only if not processing a file with suffix S.

%{S|P:X}

Substitutes X if either -S or -P was given to GCC. This may be combined with ! and . sequences as well, although they have a stronger binding than the |. For example a spec string like this:

          %{.c:-foo} %{!.c:-bar} %{.c|d:-baz} %{!.c|d:-boggle}
     

will output the following command-line options from the following input command-line options:

          fred.c        -foo -baz
          jim.d         -bar -boggle
          -d fred.c     -foo -baz -boggle
          -d jim.d      -bar -baz -boggle
     

The conditional text X in a %{S:X} or %{!S:X} construct may contain other nested % constructs or spaces, or even newlines. They are processed as usual, as described above.

The -O, -f, -m, and -W switches are handled specifically in these constructs. If another value of -O or the negated form of a -f, -m, or -W switch is found later in the command line, the earlier switch value is ignored, except with {S*} where S is just one letter, which passes all matching options.

The character | at the beginning of the predicate text is used to indicate that a command should be piped to the following command, but only if -pipe is specified.

It is built into GCC which switches take arguments and which do not. (You might think it would be useful to generalize this to allow each compiler's spec to say which switches take arguments. But this cannot be done in a consistent fashion. GCC cannot even decide which input files have been specified without knowing which switches take arguments, and it must know which input files to compile in order to tell which compilers to run).

GCC also knows implicitly that arguments starting in -l are to be treated as compiler output files, and passed to the linker in their proper position among the other output files.

3.16 Specifying Target Machine and Compiler Version

The usual way to run GCC is to run the executable called gcc, or <machine>-gcc when cross-compiling, or <machine>-gcc-<version> to run a version other than the one that was installed last. Sometimes this is inconvenient, so GCC provides options that will switch to another cross-compiler or version.

-b machine

The argument machine specifies the target machine for compilation.

The value to use for machine is the same as was specified as the machine type when configuring GCC as a cross-compiler. For example, if a cross-compiler was configured with configure i386v, meaning to compile for an 80386 running System V, then you would specify -b i386v to run that cross compiler.

-V version

The argument version specifies which version of GCC to run. This is useful when multiple versions are installed. For example, version might be 2.0, meaning to run GCC version 2.0.

The -V and -b options work by running the <machine>-gcc-<version> executable, so there's no real reason to use them if you can just run that directly.

3.17 Hardware Models and Configurations

Earlier we discussed the standard option -b which chooses among different 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 special 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.

• M680x0 Options
• M68hc1x Options
• VAX Options
• SPARC Options
• ARM Options
• MN10200 Options
• MN10300 Options
• M32R/D Options
• M88K Options
• RS/6000 and PowerPC Options
• Darwin Options
• RT Options
• MIPS Options
• i386 and x86-64 Options
• HPPA Options
• Intel 960 Options
• DEC Alpha Options
• DEC Alpha/VMS Options
• H8/300 Options
• SH Options
• System V Options
• TMS320C3x/C4x Options
• V850 Options
• ARC Options
• NS32K Options
• AVR Options
• MCore Options
• IA-64 Options
• D30V Options
• S/390 and zSeries Options
• CRIS Options
• MMIX Options
• PDP-11 Options
• Xstormy16 Options
• Xtensa Options
• FRV Options

3.17.1 M680x0 Options

These are the -m options defined for the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was configured; the defaults for the most common choices are given below.

-m68000

-mc68000

Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems.

Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.

-m68020

-mc68020

Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems.

-m68881

Generate output containing 68881 instructions for floating point. This is the default for most 68020 systems unless --nfp was specified when the compiler was configured.

-m68030

Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems.

-m68040

Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems.

This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions.

-m68060

Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems.

This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions.

-mcpu32

Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems.

Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

-m5200

Generate output for a 520X “coldfire” family cpu. This is the default when the compiler is configured for 520X-based systems.

Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5202.

-m68020-40

Generate output for a 68040, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.

-m68020-60

Generate output for a 68060, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.

-mfpa

Generate output containing Sun FPA instructions for floating point.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all m68k targets. 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. The embedded targets m68k-*-aout and m68k-*-coff do provide software floating point support.

-mshort

Consider type int to be 16 bits wide, like short int.

-mnobitfield

Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.

-mbitfield

Do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use a configuration designed for a 68020.

-mrtd

Use a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.

This calling convention is incompatible with the one normally 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.)

The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

-malign-int

-mno-align-int

Control whether GCC aligns int, long, long long, float, double, and long double variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.

Warning: if you use the -malign-int switch, GCC will align structures containing the above types differently than most published application binary interface specifications for the m68k.

-mpcrel

Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.

-mno-strict-align

-mstrict-align

Do not (do) assume that unaligned memory references will be handled by the system.

3.17.2 M68hc1x Options

These are the -m options defined for the 68hc11 and 68hc12 microcontrollers. The default values for these options depends on which style of microcontroller was selected when the compiler was configured; the defaults for the most common choices are given below.

-m6811

-m68hc11

Generate output for a 68HC11. This is the default when the compiler is configured for 68HC11-based systems.

-m6812

-m68hc12

Generate output for a 68HC12. This is the default when the compiler is configured for 68HC12-based systems.

-m68S12

-m68hcs12

Generate output for a 68HCS12.

-mauto-incdec

Enable the use of 68HC12 pre and post auto-increment and auto-decrement addressing modes.

-minmax

-nominmax

Enable the use of 68HC12 min and max instructions.

-mlong-calls

-mno-long-calls

Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will use the call instruction to call a function and the rtc instruction for returning.

-mshort

Consider type int to be 16 bits wide, like short int.

-msoft-reg-count=count

Specify the number of pseudo-soft registers which are used for the code generation. The maximum number is 32. Using more pseudo-soft register may or may not result in better code depending on the program. The default is 4 for 68HC11 and 2 for 68HC12.

3.17.3 VAX Options

These -m options are defined for the VAX:

-munix

Do not output certain jump instructions (aobleq and so on) that the Unix assembler for the VAX cannot handle across long ranges.

-mgnu

Do output those jump instructions, on the assumption that you will assemble with the GNU assembler.

-mg

Output code for g-format floating point numbers instead of d-format.

3.17.4 SPARC Options

These -m switches are supported on the SPARC:

-mno-app-regs

-mapp-regs

Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. This is the default.

To be fully SVR4 ABI compliant at the cost of some performance loss, specify -mno-app-regs. You should compile libraries and system software with this option.

-mfpu

-mhard-float

Generate output containing floating point instructions. This is the default.

-mno-fpu

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide software floating point support.

-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

-mhard-quad-float

Generate output containing quad-word (long double) floating point instructions.

-msoft-quad-float

Generate output containing library calls for quad-word (long double) floating point instructions. The functions called are those specified in the SPARC ABI. This is the default.

As of this writing, there are no sparc implementations that have hardware support for the quad-word floating point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default.

-mno-flat

-mflat

With -mflat, the compiler does not generate save/restore instructions and will use a “flat” or single register window calling convention. This model uses %i7 as the frame pointer and is compatible with the normal register window model. Code from either may be intermixed. The local registers and the input registers (0–5) are still treated as “call saved” registers and will be saved on the stack as necessary.

With -mno-flat (the default), the compiler emits save/restore instructions (except for leaf functions) and is the normal mode of operation.

-mno-unaligned-doubles

-munaligned-doubles

Assume that doubles have 8 byte alignment. This is the default.

With -munaligned-doubles, GCC assumes that doubles have 8 byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4 byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating point code.

-mno-faster-structs

-mfaster-structs

With -mfaster-structs, the compiler assumes that structures should have 8 byte alignment. This enables the use of pairs of ldd and std instructions for copies in structure assignment, in place of twice as many ld and st pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus, it's intended only for use on targets where the developer acknowledges that their resulting code will not be directly in line with the rules of the ABI.

-mimpure-text

-mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object.

-mimpure-text suppresses the “relocations remain against allocatable but non-writable sections” linker error message. However, the necessary relocations will trigger copy-on-write, and the shared object is not actually shared across processes. Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC.

This option is only available on SunOS and Solaris.

-mv8

-msparclite

These two options select variations on the SPARC architecture.

By default (unless specifically configured for the Fujitsu SPARClite), GCC generates code for the v7 variant of the SPARC architecture.

-mv8 will give you SPARC v8 code. The only difference from v7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC v8 but not in SPARC v7.

-msparclite will give you SPARClite code. This adds the integer multiply, integer divide step and scan (ffs) instructions which exist in SPARClite but not in SPARC v7.

These options are deprecated and will be deleted in a future GCC release. They have been replaced with -mcpu=xxx.

-mcypress

-msupersparc

These two options select the processor for which the code is optimized.

With -mcypress (the default), the compiler optimizes code for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.

With -msupersparc the compiler optimizes code for the SuperSPARC cpu, as used in the SPARCStation 10, 1000 and 2000 series. This flag also enables use of the full SPARC v8 instruction set.

These options are deprecated and will be deleted in a future GCC release. They have been replaced with -mcpu=xxx.

-mcpu=cpu_type

Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are v7, cypress, v8, supersparc, sparclite, hypersparc, sparclite86x, f930, f934, sparclet, tsc701, v9, ultrasparc, and ultrasparc3.

Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are v7, v8, sparclite, sparclet, v9.

Here is a list of each supported architecture and their supported implementations.

              v7:             cypress
              v8:             supersparc, hypersparc
              sparclite:      f930, f934, sparclite86x
              sparclet:       tsc701
              v9:             ultrasparc, ultrasparc3
     

-mtune=cpu_type

Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type would.

The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular cpu implementation. Those are cypress, supersparc, hypersparc, f930, f934, sparclite86x, tsc701, ultrasparc, and ultrasparc3.

These -m switches are supported in addition to the above on the SPARCLET processor.

-mlittle-endian

Generate code for a processor running in little-endian mode.

-mlive-g0

Treat register %g0 as a normal register. GCC will continue to clobber it as necessary but will not assume it always reads as 0.

-mbroken-saverestore

Generate code that does not use non-trivial forms of the save and restore instructions. Early versions of the SPARCLET processor do not correctly handle save and restore instructions used with arguments. They correctly handle them used without arguments. A save instruction used without arguments increments the current window pointer but does not allocate a new stack frame. It is assumed that the window overflow trap handler will properly handle this case as will interrupt handlers.

These -m switches are supported in addition to the above on SPARC V9 processors in 64-bit environments.

-mlittle-endian

Generate code for a processor running in little-endian mode. It is only available for a few configurations and most notably not on Solaris.

-m32

-m64

Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.

-mcmodel=medlow

Generate code for the Medium/Low code model: the program must be linked in the low 32 bits of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked.

-mcmodel=medmid

Generate code for the Medium/Middle code model: the program must be linked in the low 44 bits of the address space, the text segment must be less than 2G bytes, and data segment must be within 2G of the text segment. Pointers are 64 bits.

-mcmodel=medany

Generate code for the Medium/Anywhere code model: the program may be linked anywhere in the address space, the text segment must be less than 2G bytes, and data segment must be within 2G of the text segment. Pointers are 64 bits.

-mcmodel=embmedany

Generate code for the Medium/Anywhere code model for embedded systems: assume a 32-bit text and a 32-bit data segment, both starting anywhere (determined at link time). Register %g4 points to the base of the data segment. Pointers are still 64 bits. Programs are statically linked, PIC is not supported.

-mstack-bias

-mno-stack-bias

With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by −2047 which must be added back when making stack frame references. Otherwise, assume no such offset is present.

These switches are supported in addition to the above on Solaris:

-threads

Add support for multithreading using the Solaris threads library. This option sets flags for both the preprocessor and linker. This option does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it.

-pthreads

Add support for multithreading using the POSIX threads library. This option sets flags for both the preprocessor and linker. This option does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it.

3.17.5 ARM Options

These -m options are defined for Advanced RISC Machines (ARM) architectures:

-mapcs-frame

Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with this option will cause the stack frames not to be generated for leaf functions. The default is -mno-apcs-frame.

-mapcs

This is a synonym for -mapcs-frame.

-mapcs-26

Generate code for a processor running with a 26-bit program counter, and conforming to the function calling standards for the APCS 26-bit option. This option replaces the -m2 and -m3 options of previous releases of the compiler.

-mapcs-32

Generate code for a processor running with a 32-bit program counter, and conforming to the function calling standards for the APCS 32-bit option. This option replaces the -m6 option of previous releases of the compiler.

-mthumb-interwork

Generate code which supports calling between the ARM and Thumb instruction sets. Without this option the two instruction sets cannot be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified.

-mno-sched-prolog

Prevent the reordering of instructions in the function prolog, or the merging of those instruction with the instructions in the function's body. This means that all functions will start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start if functions inside an executable piece of code. The default is -msched-prolog.

-mhard-float

Generate output containing floating point instructions. This is the default.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all ARM targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

-mlittle-endian

Generate code for a processor running in little-endian mode. This is the default for all standard configurations.

-mbig-endian

Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.

-mwords-little-endian

This option only applies when generating code for big-endian processors. Generate code for a little-endian word order but a big-endian byte order. That is, a byte order of the form 32107654. Note: this option should only be used if you require compatibility with code for big-endian ARM processors generated by versions of the compiler prior to 2.8.

-malignment-traps

Generate code that will not trap if the MMU has alignment traps enabled. On ARM architectures prior to ARMv4, there were no instructions to access half-word objects stored in memory. However, when reading from memory a feature of the ARM architecture allows a word load to be used, even if the address is unaligned, and the processor core will rotate the data as it is being loaded. This option tells the compiler that such misaligned accesses will cause a MMU trap and that it should instead synthesize the access as a series of byte accesses. The compiler can still use word accesses to load half-word data if it knows that the address is aligned to a word boundary.

This option is ignored when compiling for ARM architecture 4 or later, since these processors have instructions to directly access half-word objects in memory.

-mno-alignment-traps

Generate code that assumes that the MMU will not trap unaligned accesses. This produces better code when the target instruction set does not have half-word memory operations (i.e. implementations prior to ARMv4).

Note that you cannot use this option to access unaligned word objects, since the processor will only fetch one 32-bit aligned object from memory.

The default setting for most targets is -mno-alignment-traps, since this produces better code when there are no half-word memory instructions available.

-mshort-load-bytes

-mno-short-load-words

These are deprecated aliases for -malignment-traps.

-mno-short-load-bytes

-mshort-load-words

This are deprecated aliases for -mno-alignment-traps.

-mcpu=name

This specifies the name of the target ARM processor. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: arm2, arm250, arm3, arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100, arm7500, arm7500fe, arm7tdmi, arm8, strongarm, strongarm110, strongarm1100, arm8, arm810, arm9, arm9e, arm920, arm920t, arm940t, arm9tdmi, arm10tdmi, arm1020t, xscale.

-mtune=name

This option is very similar to the -mcpu= option, except that instead of specifying the actual target processor type, and hence restricting which instructions can be used, it specifies that GCC should tune the performance of the code as if the target were of the type specified in this option, but still choosing the instructions that it will generate based on the cpu specified by a -mcpu= option. For some ARM implementations better performance can be obtained by using this option.

-march=name

This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the -mcpu= option. Permissible names are: armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5t, armv5te.

-mfpe=number

-mfp=number

This specifies the version of the floating point emulation available on the target. Permissible values are 2 and 3. -mfp= is a synonym for -mfpe=, for compatibility with older versions of GCC.

-mstructure-size-boundary=n

The size of all structures and unions will be rounded up to a multiple of the number of bits set by this option. Permissible values are 8 and 32. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. Specifying the larger number can produce faster, more efficient code, but can also increase the size of the program. The two values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with the other value, if they exchange information using structures or unions.

-mabort-on-noreturn

Generate a call to the function abort at the end of a noreturn function. It will be executed if the function tries to return.

-mlong-calls

-mno-long-calls

Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function will lie outside of the 64 megabyte addressing range of the offset based version of subroutine call instruction.

Even if this switch is enabled, not all function calls will be turned into long calls. The heuristic is that static functions, functions which have the short-call attribute, functions that are inside the scope of a #pragma no_long_calls directive and functions whose definitions have already been compiled within the current compilation unit, will not be turned into long calls. The exception to this rule is that weak function definitions, functions with the long-call attribute or the section attribute, and functions that are within the scope of a #pragma long_calls directive, will always be turned into long calls.

This feature is not enabled by default. Specifying -mno-long-calls will restore the default behavior, as will placing the function calls within the scope of a #pragma long_calls_off directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.

-mnop-fun-dllimport

Disable support for the dllimport attribute.

-msingle-pic-base

Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The run-time system is responsible for initializing this register with an appropriate value before execution begins.

-mpic-register=reg

Specify the register to be used for PIC addressing. The default is R10 unless stack-checking is enabled, when R9 is used.

-mpoke-function-name

Write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:

               t0
                   .ascii "arm_poke_function_name", 0
                   .align
               t1
                   .word 0xff000000 + (t1 - t0)
               arm_poke_function_name
                   mov     ip, sp
                   stmfd   sp!, {fp, ip, lr, pc}
                   sub     fp, ip, #4
     

When performing a stack backtrace, code can inspect the value of pc stored at fp + 0. If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000).

-mthumb

Generate code for the 16-bit Thumb instruction set. The default is to use the 32-bit ARM instruction set.

-mtpcs-frame

Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame.

-mtpcs-leaf-frame

Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame.

-mcallee-super-interworking

Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code.

-mcaller-super-interworking

Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled.

3.17.6 MN10200 Options

These -m options are defined for Matsushita MN10200 architectures:

-mrelax

Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step.

This option makes symbolic debugging impossible.

3.17.7 MN10300 Options

These -m options are defined for Matsushita MN10300 architectures:

-mmult-bug

Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.

-mno-mult-bug

Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.

-mam33

Generate code which uses features specific to the AM33 processor.

-mno-am33

Do not generate code which uses features specific to the AM33 processor. This is the default.

-mno-crt0

Do not link in the C run-time initialization object file.

-mrelax

Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step.

This option makes symbolic debugging impossible.

3.17.8 M32R/D Options

These -m options are defined for Mitsubishi M32R/D architectures:

-m32rx

Generate code for the M32R/X.

-m32r

Generate code for the M32R. This is the default.

-mcode-model=small

Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction), and assume all subroutines are reachable with the bl instruction. This is the default.

The addressability of a particular object can be set with the model attribute.

-mcode-model=medium

Assume objects may be anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses), and assume all subroutines are reachable with the bl instruction.

-mcode-model=large

Assume objects may be anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses), and assume subroutines may not be reachable with the bl instruction (the compiler will generate the much slower seth/add3/jl instruction sequence).

-msdata=none

Disable use of the small data area. Variables will be put into one of .data, bss, or .rodata (unless the section attribute has been specified). This is the default.

The small data area consists of sections .sdata and .sbss. Objects may be explicitly put in the small data area with the section attribute using one of these sections.

-msdata=sdata

Put small global and static data in the small data area, but do not generate special code to reference them.

-msdata=use

Put small global and static data in the small data area, and generate special instructions to reference them.

-G num

Put global and static objects less than or equal to num bytes into the small data or bss sections instead of the normal data or bss sections. The default value of num is 8. The -msdata option must be set to one of sdata or use for this option to have any effect.

All modules should be compiled with the same -G num value. Compiling with different values of num may or may not work; if it doesn't the linker will give an error message—incorrect code will not be generated.

3.17.9 M88K Options

These -m options are defined for Motorola 88k architectures:

-m88000

Generate code that works well on both the m88100 and the m88110.

-m88100

Generate code that works best for the m88100, but that also runs on the m88110.

-m88110

Generate code that works best for the m88110, and may not run on the m88100.

-mbig-pic

Obsolete option to be removed from the next revision. Use -fPIC.

-midentify-revision

Include an ident directive in the assembler output recording the source file name, compiler name and version, timestamp, and compilation flags used.

-mno-underscores

In assembler output, emit symbol names without adding an underscore character at the beginning of each name. The default is to use an underscore as prefix on each name.

-mocs-debug-info

-mno-ocs-debug-info

Include (or omit) additional debugging information (about registers used in each stack frame) as specified in the 88open Object Compatibility Standard, “OCS”. This extra information allows debugging of code that has had the frame pointer eliminated. The default for SVr4 and Delta 88 SVr3.2 is to include this information; other 88k configurations omit this information by default.

-mocs-frame-position

When emitting COFF debugging information for automatic variables and parameters stored on the stack, use the offset from the canonical frame address, which is the stack pointer (register 31) on entry to the function. The SVr4 and Delta88 SVr3.2, and BCS configurations use -mocs-frame-position; other 88k configurations have the default -mno-ocs-frame-position.

-mno-ocs-frame-position

When emitting COFF debugging information for automatic variables and parameters stored on the stack, use the offset from the frame pointer register (register 30). When this option is in effect, the frame pointer is not eliminated when debugging information is selected by the -g switch.

-moptimize-arg-area

Save space by reorganizing the stack frame. This option generates code that does not agree with the 88open specifications, but uses less memory.

-mno-optimize-arg-area

Do not reorganize the stack frame to save space. This is the default. The generated conforms to the specification, but uses more memory.

-mshort-data-num

Generate smaller data references by making them relative to r0, which allows loading a value using a single instruction (rather than the usual two). You control which data references are affected by specifying num with this option. For example, if you specify -mshort-data-512, then the data references affected are those involving displacements of less than 512 bytes. -mshort-data-num is not effective for num greater than 64k.

-mserialize-volatile

-mno-serialize-volatile

Do, or don't, generate code to guarantee sequential consistency of volatile memory references. By default, consistency is guaranteed.

The order of memory references made by the MC88110 processor does not always match the order of the instructions requesting those references. In particular, a load instruction may execute before a preceding store instruction. Such reordering violates sequential consistency of volatile memory references, when there are multiple processors. When consistency must be guaranteed, GCC generates special instructions, as needed, to force execution in the proper order.

The MC88100 processor does not reorder memory references and so always provides sequential consistency. However, by default, GCC generates the special instructions to guarantee consistency even when you use -m88100, so that the code may be run on an MC88110 processor. If you intend to run your code only on the MC88100 processor, you may use -mno-serialize-volatile.

The extra code generated to guarantee consistency may affect the performance of your application. If you know that you can safely forgo this guarantee, you may use -mno-serialize-volatile.

-msvr4

-msvr3

Turn on (-msvr4) or off (-msvr3) compiler extensions related to System V release 4 (SVr4). This controls the following:

1. Which variant of the assembler syntax to emit.
2. -msvr4 makes the C preprocessor recognize #pragma weak that is used on System V release 4.
3. -msvr4 makes GCC issue additional declaration directives used in SVr4.

-msvr4 is the default for the m88k-motorola-sysv4 configuration. -msvr3 is the default for all other m88k configurations.

-mversion-03.00

This option is obsolete, and is ignored.

-mno-check-zero-division

-mcheck-zero-division

Do, or don't, generate code to guarantee that integer division by zero will be detected. By default, detection is guaranteed.

Some models of the MC88100 processor fail to trap upon integer division by zero under certain conditions. By default, when compiling code that might be run on such a processor, GCC generates code that explicitly checks for zero-valued divisors and traps with exception number 503 when one is detected. Use of -mno-check-zero-division suppresses such checking for code generated to run on an MC88100 processor.

GCC assumes that the MC88110 processor correctly detects all instances of integer division by zero. When -m88110 is specified, no explicit checks for zero-valued divisors are generated, and both -mcheck-zero-division and -mno-check-zero-division are ignored.

-muse-div-instruction

Use the div instruction for signed integer division on the MC88100 processor. By default, the div instruction is not used.

On the MC88100 processor the signed integer division instruction div) traps to the operating system on a negative operand. The operating system transparently completes the operation, but at a large cost in execution time. By default, when compiling code that might be run on an MC88100 processor, GCC emulates signed integer division using the unsigned integer division instruction divu), thereby avoiding the large penalty of a trap to the operating system. Such emulation has its own, smaller, execution cost in both time and space. To the extent that your code's important signed integer division operations are performed on two nonnegative operands, it may be desirable to use the div instruction directly.

On the MC88110 processor the div instruction (also known as the divs instruction) processes negative operands without trapping to the operating system. When -m88110 is specified, -muse-div-instruction is ignored, and the div instruction is used for signed integer division.

Note that the result of dividing INT_MIN by −1 is undefined. In particular, the behavior of such a division with and without -muse-div-instruction may differ.

-mtrap-large-shift

-mhandle-large-shift

Include code to detect bit-shifts of more than 31 bits; respectively, trap such shifts or emit code to handle them properly. By default GCC makes no special provision for large bit shifts.

-mwarn-passed-structs

Warn when a function passes a struct as an argument or result. Structure-passing conventions have changed during the evolution of the C language, and are often the source of portability problems. By default, GCC issues no such warning.

3.17.10 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 instructions 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 register 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. Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the General Purpose 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 register. 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 assembler 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. Supported 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 generated 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 specify 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-powerpc

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.

-mabi=spe

Extend the current ABI with SPE ABI extensions. This does not change the default ABI, instead it adds the SPE ABI extensions to the current ABI.

-mabi=no-spe

Disable Booke SPE ABI extensions for the current ABI.

-misel=yes/no

-misel

This switch enables or disables the generation of ISEL instructions.

-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 overflowed 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 calculate 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 arguments than declared. AIX XL compilers access floating point arguments 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 application 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 appropriate directory location. The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompatible.

-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 instructions that update the base register to the address of the calculated 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 structures 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 runtime. 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 runtime. 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.

-mcall-sysv

On System V.4 and embedded PowerPC systems compile code using calling 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 calling 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.

-mabi=no-altivec

Disable AltiVec ABI extensions for the current ABI.

-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 functions 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. configurations.

-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.

-mwindiss

Specify that you are compiling for the WindISS simulation environment.

-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 initialization 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 powerpc*-*-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, compile 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 register names in the assembly language output using symbolic forms.

-mlongcall

-mno-longcall

Default to making all function calls via pointers, so that functions which reside further than 64 megabytes (67,108,864 bytes) from the current location can be called. This setting can be overridden by the shortcall function attribute, or by #pragma longcall(0).

Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On these systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do this, as can the GNU linker for PowerPC/64. It is planned to add this feature to the GNU linker for 32-bit PowerPC systems as well.

In the future, we may cause GCC to ignore all longcall specifications when the linker is known to generate glue.

-pthread

Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker.

3.17.11 Darwin Options

These options are defined for all architectures running the Darwin operating system. They are useful for compatibility with other Mac OS compilers.