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gcc
File: gcc.info, Node: Top, Next: G++ and GCC, Up: (DIR)
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 3.4.6. 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.
*Note Introduction: (gccint)Top.
* Menu:
* 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.
File: gcc.info, Node: G++ and GCC, Next: Standards, Prev: Top, Up: Top
1 Programming Languages Supported by GCC
****************************************
GCC stands for "GNU Compiler Collection". GCC is an integrated
distribution of compilers for several major programming languages.
These languages currently include C, C++, Objective-C, Java, Fortran,
and Ada.
The abbreviation "GCC" has multiple meanings in common use. The
current official meaning is "GNU Compiler Collection", which refers
generically to the complete suite of tools. The name historically stood
for "GNU C Compiler", and this usage is still common when the emphasis
is on compiling C programs. Finally, the name is also used when
speaking of the "language-independent" component of GCC: code shared
among the compilers for all supported languages.
The language-independent component of GCC includes the majority of the
optimizers, as well as the "back ends" that generate machine code for
various processors.
The part of a compiler that is specific to a particular language is
called the "front end". In addition to the front ends that are
integrated components of GCC, there are several other front ends that
are maintained separately. These support languages such as Pascal,
Mercury, and COBOL. To use these, they must be built together with GCC
proper.
Most of the compilers for languages other than C have their own names.
The C++ compiler is G++, the Ada compiler is GNAT, and so on. When we
talk about compiling one of those languages, we might refer to that
compiler by its own name, or as GCC. Either is correct.
Historically, compilers for many languages, including C++ and Fortran,
have been implemented as "preprocessors" which emit another high level
language such as C. None of the compilers included in GCC are
implemented this way; they all generate machine code directly. This
sort of preprocessor should not be confused with the "C preprocessor",
which is an integral feature of the C, C++, and Objective-C languages.
File: gcc.info, Node: Standards, Next: Invoking GCC, Prev: G++ and GCC, Up: Top
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). *Note Options
Controlling C Dialect: C Dialect Options.
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-3.4/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. *Note Extensions to the C
Language Family: C Extensions. 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.
*Note Options Controlling C Dialect: C Dialect Options.
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). *Note Standard Libraries: 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.
*Note GNAT Reference Manual: (gnat_rm)Top, for information on standard
conformance and compatibility of the Ada compiler.
*Note The GNU Fortran Language: (g77)Language, for details of the
Fortran language supported by GCC.
*Note Compatibility with the Java Platform: (gcj)Compatibility, for
details of compatibility between `gcj' and the Java Platform.
File: gcc.info, Node: Invoking GCC, Next: C Implementation, Prev: Standards, Up: Top
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.
*Note Compiling C++ Programs: Invoking G++, 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.
*Note Option Index::, for an index to GCC's options.
* Menu:
* 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.
* Precompiled Headers:: Compiling a header once, and using it many times.
* Running Protoize:: Automatically adding or removing function prototypes.
File: gcc.info, Node: Option Summary, Next: Overall Options, Up: Invoking GCC
3.1 Option Summary
==================
Here is a summary of all the options, grouped by type. Explanations are
in the following sections.
_Overall Options_
*Note Options Controlling the Kind of Output: Overall Options.
-c -S -E -o FILE -pipe -pass-exit-codes
-x LANGUAGE -v -### --help --target-help --version
_C Language Options_
*Note Options Controlling C Dialect: C Dialect Options.
-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_
*Note Options Controlling C++ Dialect: C++ Dialect Options.
-fabi-version=N -fno-access-control -fcheck-new
-fconserve-space -fno-const-strings
-fno-elide-constructors
-fno-enforce-eh-specs
-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 -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
_Objective-C Language Options_
*Note Options Controlling Objective-C Dialect: Objective-C Dialect
Options.
-fconstant-string-class=CLASS-NAME
-fgnu-runtime -fnext-runtime
-fno-nil-receivers
-fobjc-exceptions
-freplace-objc-classes
-fzero-link
-gen-decls
-Wno-protocol -Wselector -Wundeclared-selector
_Language Independent Options_
*Note Options to Control Diagnostic Messages Formatting: Language
Independent Options.
-fmessage-length=N
-fdiagnostics-show-location=[once|every-line]
_Warning Options_
*Note Options to Request or Suppress Warnings: Warning Options.
-fsyntax-only -pedantic -pedantic-errors
-w -Wextra -Wall -Waggregate-return
-Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment
-Wconversion -Wno-deprecated-declarations
-Wdisabled-optimization -Wno-div-by-zero -Wendif-labels
-Werror -Werror-implicit-function-declaration
-Wfloat-equal -Wformat -Wformat=2
-Wno-format-extra-args -Wformat-nonliteral
-Wformat-security -Wformat-y2k
-Wimplicit -Wimplicit-function-declaration -Wimplicit-int
-Wimport -Wno-import -Winit-self -Winline
-Wno-invalid-offsetof -Winvalid-pch
-Wlarger-than-LEN -Wlong-long
-Wmain -Wmissing-braces
-Wmissing-format-attribute -Wmissing-noreturn
-Wno-multichar -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 -Wold-style-definition
-Wstrict-prototypes -Wtraditional
-Wdeclaration-after-statement
_Debugging Options_
*Note Options for Debugging Your Program or GCC: Debugging Options.
-dLETTERS -dumpspecs -dumpmachine -dumpversion
-fdump-unnumbered -fdump-translation-unit[-N]
-fdump-class-hierarchy[-N]
-fdump-tree-original[-N]
-fdump-tree-optimized[-N]
-fdump-tree-inlined[-N]
-feliminate-dwarf2-dups -feliminate-unused-debug-types
-feliminate-unused-debug-symbols -fmem-report -fprofile-arcs
-frandom-seed=STRING -fsched-verbose=N
-ftest-coverage -ftime-report
-g -gLEVEL -gcoff -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_
*Note Options that Control Optimization: Optimize Options.
-falign-functions=N -falign-jumps=N
-falign-labels=N -falign-loops=N
-fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize
-fbranch-target-load-optimize2 -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 -fgcse-las -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
-fprofile-generate -fprofile-use
-freduce-all-givs -fregmove -frename-registers
-freorder-blocks -freorder-functions
-frerun-cse-after-loop -frerun-loop-opt
-frounding-math -fschedule-insns -fschedule-insns2
-fno-sched-interblock -fno-sched-spec -fsched-spec-load
-fsched-spec-load-dangerous
-fsched-stalled-insns=N -sched-stalled-insns-dep=N
-fsched2-use-superblocks
-fsched2-use-traces -fsignaling-nans
-fsingle-precision-constant
-fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps
-funroll-all-loops -funroll-loops -fpeel-loops
-funswitch-loops -fold-unroll-loops -fold-unroll-all-loops
--param NAME=VALUE
-O -O0 -O1 -O2 -O3 -Os
_Preprocessor Options_
*Note Options Controlling the Preprocessor: Preprocessor Options.
-AQUESTION=ANSWER
-A-QUESTION[=ANSWER]
-C -dD -dI -dM -dN
-DMACRO[=DEFN] -E -H
-idirafter DIR
-include FILE -imacros FILE
-iprefix FILE -iwithprefix DIR
-iwithprefixbefore DIR -isystem DIR
-M -MM -MF -MG -MP -MQ -MT -nostdinc
-P -fworking-directory -remap
-trigraphs -undef -UMACRO -Wp,OPTION
-Xpreprocessor OPTION
_Assembler Option_
*Note Passing Options to the Assembler: Assembler Options.
-Wa,OPTION -Xassembler OPTION
_Linker Options_
*Note Options for Linking: Link Options.
OBJECT-FILE-NAME -lLIBRARY
-nostartfiles -nodefaultlibs -nostdlib -pie
-s -static -static-libgcc -shared -shared-libgcc -symbolic
-Wl,OPTION -Xlinker OPTION
-u SYMBOL
_Directory Options_
*Note Options for Directory Search: Directory Options.
-BPREFIX -IDIR -I- -LDIR -specs=FILE
_Target Options_
*Note Target Options::.
-V VERSION -b MACHINE
_Machine Dependent Options_
*Note Hardware Models and Configurations: Submodel Options.
_M680x0 Options_
-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040
-m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020
-mnobitfield -mrtd -mshort -msoft-float -mpcrel
-malign-int -mstrict-align -msep-data -mno-sep-data
-mshared-library-id=n -mid-shared-library -mno-id-shared-library
_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 -mno-app-regs
-mfaster-structs -mno-faster-structs
-mflat -mno-flat -mfpu -mno-fpu
-mhard-float -msoft-float
-mhard-quad-float -msoft-quad-float
-mimpure-text -mno-impure-text -mlittle-endian
-mstack-bias -mno-stack-bias
-munaligned-doubles -mno-unaligned-doubles
-mv8plus -mno-v8plus -mvis -mno-vis
-mcypress -mf930 -mf934
-msparclite -msupersparc -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
-mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns
-mpoke-function-name
-mthumb -marm
-mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking
_MN10300 Options_
-mmult-bug -mno-mult-bug
-mam33 -mno-am33
-mam33-2 -mno-am33-2
-mno-crt0 -mrelax
_M32R/D Options_
-m32r2 -m32rx -m32r
-mdebug
-malign-loops -mno-align-loops
-missue-rate=NUMBER
-mbranch-cost=NUMBER
-mmodel=CODE-SIZE-MODEL-TYPE
-msdata=SDATA-TYPE
-mno-flush-func -mflush-func=NAME
-mno-flush-trap -mflush-trap=NUMBER
-G NUM
_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-compat -mno-xl-compat -mpe
-malign-power -malign-natural
-msoft-float -mhard-float -mmultiple -mno-multiple
-mstring -mno-string -mupdate -mno-update
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable
-mno-relocatable -mrelocatable-lib -mno-relocatable-lib
-mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian
-mdynamic-no-pic
-mprioritize-restricted-insns=PRIORITY
-msched-costly-dep=DEPENDENCE_TYPE
-minsert-sched-nops=SCHEME
-mcall-sysv -mcall-netbsd
-maix-struct-return -msvr4-struct-return
-mabi=altivec -mabi=no-altivec
-mabi=spe -mabi=no-spe
-misel=yes -misel=no
-mspe=yes -mspe=no
-mfloat-gprs=yes -mfloat-gprs=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 -nofixprebinding -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
_MIPS Options_
-EL -EB -march=ARCH -mtune=ARCH
-mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64
-mips16 -mno-mips16 -mabi=ABI -mabicalls -mno-abicalls
-mxgot -mno-xgot -membedded-pic -mno-embedded-pic
-mgp32 -mgp64 -mfp32 -mfp64 -mhard-float -msoft-float
-msingle-float -mdouble-float -mint64 -mlong64 -mlong32
-GNUM -membedded-data -mno-embedded-data
-muninit-const-in-rodata -mno-uninit-const-in-rodata
-msplit-addresses -mno-split-addresses
-mexplicit-relocs -mno-explicit-relocs
-mrnames -mno-rnames
-mcheck-zero-division -mno-check-zero-division
-mmemcpy -mno-memcpy -mlong-calls -mno-long-calls
-mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp
-mfix-sb1 -mno-fix-sb1 -mflush-func=FUNC
-mno-flush-func -mbranch-likely -mno-branch-likely
_i386 and x86-64 Options_
-mtune=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 -mno-tls-direct-seg-refs
-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
-msmall-text -mlarge-text
-mmemory-latency=TIME
_DEC Alpha/VMS Options_
-mvms-return-codes
_H8/300 Options_
-mrelax -mh -ms -mn -mint32 -malign-300
_SH Options_
-m1 -m2 -m2e -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
-mv850e1
-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
-minline-sqrt-min-latency -minline-sqrt-max-throughput
-mno-dwarf2-asm -mearly-stop-bits
-mfixed-range=REGISTER-RANGE -mtls-size=TLS-SIZE
-mtune=CPU-TYPE -mt -pthread -milp32 -mlp64
_D30V Options_
-mextmem -mextmemory -monchip -mno-asm-optimize
-masm-optimize -mbranch-cost=N -mcond-exec=N
_S/390 and zSeries Options_
-mtune=CPU-TYPE -march=CPU-TYPE
-mhard-float -msoft-float -mbackchain -mno-backchain
-msmall-exec -mno-small-exec -mmvcle -mno-mvcle
-m64 -m31 -mdebug -mno-debug -mesa -mzarch -mfused-madd -mno-fused-madd
_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_
-mconst16 -mno-const16
-mfused-madd -mno-fused-madd
-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_
*Note Options for Code Generation Conventions: Code Gen Options.
-fcall-saved-REG -fcall-used-REG
-ffixed-REG -fexceptions
-fnon-call-exceptions -funwind-tables
-fasynchronous-unwind-tables
-finhibit-size-directive -finstrument-functions
-fno-common -fno-ident
-fpcc-struct-return -fpic -fPIC -fpie -fPIE
-freg-struct-return -fshared-data -fshort-enums
-fshort-double -fshort-wchar
-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 -fwrapv -fbounds-check
* Menu:
* 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.
File: gcc.info, Node: Overall Options, Next: Invoking G++, Prev: Option Summary, Up: Invoking 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. GCC is capable of
preprocessing and compiling several files either into several assembler
input files, or into one assembler input file; then each assembler
input file produces an object file, and 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 or C++ header file to be turned into a precompiled header.
`FILE.cc'
`FILE.cp'
`FILE.cxx'
`FILE.cpp'
`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.hh'
`FILE.H'
C++ header file to be turned into a precompiled header.
`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).
*Note Options Controlling the Kind of Output: (g77)Overall
Options, 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++-header c++-cpp-output
objective-c objective-c-header 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.
If you specify `-o' when compiling more than one input file, or
you are producing an executable file as output, all the source
files on the command line will be compiled at once.
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 `-Wextra' 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.
File: gcc.info, Node: C Dialect Options, Next: C++ Dialect Options, Prev: Invoking G++, Up: Invoking GCC
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'. *Note 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. *Note Other built-in
functions provided by GCC: Other Builtins, 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-3.4/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.
*Note Language Standards Supported by GCC: Standards, 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. *Note Other built-in functions provided
by GCC: Other Builtins, 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'.
*Note Language Standards Supported by GCC: Standards, 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.
This option is deprecated.
File: gcc.info, Node: C++ Dialect Options, Next: Objective-C Dialect Options, Prev: C Dialect Options, Up: Invoking GCC
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 2 is the version of the C++
ABI that first appeared in G++ 3.4. 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 2.
`-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 *'.
`-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.
`-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. *Note 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'. *Note 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'.
`-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.
*Note Options That Control Optimization: Optimize Options. 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
as`A::f1'; other compilers will not. You can avoid this
problem by explicitly padding `A' so that its size is a
multiple of the byte size on your platform; that will cause
G++ and other compilers to layout `B' identically.
* 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.
`-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. *Note 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
enumerated 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.
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 ='.
File: gcc.info, Node: Objective-C Dialect Options, Next: Language Independent Options, Prev: C++ Dialect Options, Up: Invoking GCC
3.6 Options Controlling Objective-C Dialect
===========================================
(NOTE: This manual does not describe the Objective-C language itself.
See `http://gcc.gnu.org/readings.html' for references.)
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' if the GNU runtime is being used,
and `NSConstantString' if the NeXT runtime is being used (see
below). The `-fconstant-cfstrings' option, if also present, will
override the `-fconstant-string-class' setting and cause `@"..."'
literals to be laid out as constant CoreFoundation strings.
`-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.
`-fno-nil-receivers'
Assume that all Objective-C message dispatches (e.g., `[receiver
message:arg]') in this translation unit ensure that the receiver
is not `nil'. This allows for more efficient entry points in the
runtime to be used. Currently, this option is only available in
conjunction with the NeXT runtime on Mac OS X 10.3 and later.
`-fobjc-exceptions'
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++ and Java.
Currently, this option is only available in conjunction with the
NeXT runtime on Mac OS X 10.3 and later.
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
The `@throw' statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a `@catch' block, the
`@throw' may appear without an argument (as shown above), in which
case the object caught by the `@catch' will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and
caught using this scheme. When an object is thrown, it will be
caught by the nearest `@catch' clause capable of handling objects
of that type, analogously to how `catch' blocks work in C++ and
Java. A `@catch(id ...)' clause (as shown above) may also be
provided to catch any and all Objective-C exceptions not caught by
previous `@catch' clauses (if any).
The `@finally' clause, if present, will be executed upon exit from
the immediately preceding `@try ... @catch' section. This will
happen regardless of whether any exceptions are thrown, caught or
rethrown inside the `@try ... @catch' section, analogously to the
behavior of the `finally' clause in Java.
There are several caveats to using the new exception mechanism:
* Although currently designed to be binary compatible with
`NS_HANDLER'-style idioms provided by the `NSException'
class, the new exceptions can only be used on Mac OS X 10.3
(Panther) and later systems, due to additional functionality
needed in the (NeXT) Objective-C runtime.
* As mentioned above, the new exceptions do not support handling
types other than Objective-C objects. Furthermore, when
used from Objective-C++, the Objective-C exception model does
not interoperate with C++ exceptions at this time. This
means you cannot `@throw' an exception from Objective-C and
`catch' it in C++, or vice versa (i.e., `throw ... @catch').
The `-fobjc-exceptions' switch also enables the use of
synchronization blocks for thread-safe execution:
@synchronized (ObjCClass *guard) {
...
}
Upon entering the `@synchronized' block, a thread of execution
shall first check whether a lock has been placed on the
corresponding `guard' object by another thread. If it has, the
current thread shall wait until the other thread relinquishes its
lock. Once `guard' becomes available, the current thread will
place its own lock on it, execute the code contained in the
`@synchronized' block, and finally relinquish the lock (thereby
making `guard' available to other threads).
Unlike Java, Objective-C does not allow for entire methods to be
marked `@synchronized'. Note that throwing exceptions out of
`@synchronized' blocks is allowed, and will cause the guarding
object to be unlocked properly.
`-freplace-objc-classes'
Emit a special marker instructing `ld(1)' not to statically link in
the resulting object file, and allow `dyld(1)' to load it in at
run time instead. This is used in conjunction with the
Fix-and-Continue debugging mode, where the object file in question
may be recompiled and dynamically reloaded in the course of
program execution, without the need to restart the program itself.
Currently, Fix-and-Continue functionality is only available in
conjunction with the NeXT runtime on Mac OS X 10.3 and later.
`-fzero-link'
When compiling for the NeXT runtime, the compiler ordinarily
replaces calls to `objc_getClass("...")' (when the name of the
class is known at compile time) with static class references that
get initialized at load time, which improves run-time performance.
Specifying the `-fzero-link' flag suppresses this behavior and
causes calls to `objc_getClass("...")' to be retained. This is
useful in Zero-Link debugging mode, since it allows for individual
class implementations to be modified during program execution.
`-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.
`-print-objc-runtime-info'
Generate C header describing the largest structure that is passed
by value, if any.
File: gcc.info, Node: Language Independent Options, Next: Warning Options, Prev: Objective-C Dialect Options, Up: Invoking GCC
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.
File: gcc.info, Node: Warning Options, Next: Debugging Options, Prev: Language Independent Options, Up: Invoking GCC
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 *Note C++
Dialect Options:: and *Note 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.
*Note 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 (*note 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). *Note Options Controlling C Dialect: C
Dialect Options.
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 `-Wformat-y2k',
`-Wno-format-extra-args', `-Wno-format-zero-length',
`-Wformat-nonliteral', `-Wformat-security', and `-Wformat=2' are
available, but are not included in `-Wall'.
`-Wformat-y2k'
If `-Wformat' is specified, also 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 -Wformat-y2k'.
`-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.
`-Winit-self (C, C++, and Objective-C only)'
Warn about uninitialized variables which are initialized with
themselves. Note this option can only be used with the
`-Wuninitialized' option, which in turn only works with `-O1' and
above.
For example, GCC will warn about `i' being uninitialized in the
following snippet only when `-Winit-self' has been specified:
int f()
{
int i = i;
return i;
}
`-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 the GCC 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 enumerated 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 enumerated 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 (*note
Variable Attributes::).
`-Wunused-parameter'
Warn whenever a function parameter is unused aside from its
declaration.
To suppress this warning use the `unused' attribute (*note
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 (*note
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 `-Wextra -Wunused' (note that `-Wall' implies
`-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.
If you want to warn about code which uses the uninitialized value
of the variable in its own initializer, use the `-Winit-self'
option.
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'. *Note 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 *Note C++ Dialect
Options:: and *Note 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.
`-Wextra'
(This option used to be called `-W'. The older name is still
supported, but the newer name is more descriptive.) 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 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 };
* A function parameter is declared without a type specifier in
K&R-style functions:
void foo(bar) { }
* An empty body occurs in an `if' or `else' statement.
* A pointer is compared against integer zero with `<', `<=',
`>', or `>='.
* A variable might be changed by `longjmp' or `vfork'.
* Any of several floating-point events that often indicate
errors, such as overflow, underflow, loss of precision, etc.
* (C++ only) An enumerator and a non-enumerator both appear in
a conditional expression.
* (C++ only) A non-static reference or non-static `const'
member appears in a class without constructors.
* (C++ only) Ambiguous virtual bases.
* (C++ only) Subscripting an array which has been declared
`register'.
* (C++ only) Taking the address of a variable which has been
declared `register'.
* (C++ only) A base class is not initialized in a derived
class' copy constructor.
`-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.
`-Wdeclaration-after-statement (C only)'
Warn when a declaration is found after a statement in a block.
This construct, known from C++, was introduced with ISO C99 and is
by default allowed in GCC. It is not supported by ISO C90 and was
not supported by GCC versions before GCC 3.0. *Note Mixed
Declarations::.
`-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 also enabled by `-Wextra'; to get the
other warnings of `-Wextra' without this warning, use `-Wextra
-Wno-sign-compare'.
`-Wa | | |
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