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GNU Info File | 1994-11-23 | 50.1 KB | 1,246 lines

This is Info file gcc.info, produced by Makeinfo-1.55 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Published by the Free Software Foundation 675 Massachusetts Avenue Cambridge, MA 02139 USA Copyright (C) 1988, 1989, 1992, 1993, 1994 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English. File: gcc.info, Node: Option Summary, Next: Overall Options, Up: Invoking GCC 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 -v -x LANGUAGE *C Language Options* *Note Options Controlling C Dialect: C Dialect Options. -ansi -fallow-single-precision -fcond-mismatch -fno-asm -fno-builtin -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char -fwritable-strings -traditional -traditional-cpp -trigraphs *C++ Language Options* *Note Options Controlling C++ Dialect: C++ Dialect Options. -fall-virtual -fdollars-in-identifiers -felide-constructors -fenum-int-equiv -fexternal-templates -fhandle-signatures -fmemoize-lookups -fno-default-inline -fno-strict-prototype -fnonnull-objects -fthis-is-variable -nostdinc++ -traditional +eN *Warning Options* *Note Options to Request or Suppress Warnings: Warning Options. -fsyntax-only -pedantic -pedantic-errors -w -W -Wall -Waggregate-return -Wbad-function-cast -Wcast-align -Wcast-qual -Wchar-subscript -Wcomment -Wconversion -Wenum-clash -Werror -Wformat -Wid-clash-LEN -Wimplicit -Wimport -Winline -Wlarger-than-LEN -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wno-import -Woverloaded-virtual -Wparentheses -Wpointer-arith -Wredundant-decls -Wreorder -Wreturn-type -Wshadow -Wstrict-prototypes -Wswitch -Wsynth -Wtemplate-debugging -Wtraditional -Wtrigraphs -Wuninitialized -Wunused -Wwrite-strings *Debugging Options* *Note Options for Debugging Your Program or GCC: Debugging Options. -a -dLETTERS -fpretend-float -g -gLEVEL -gcoff -gdwarf -gdwarf+ -ggdb -gstabs -gstabs+ -gxcoff -gxcoff+ -p -pg -print-file-name=LIBRARY -print-libgcc-file-name -print-prog-name=PROGRAM -save-temps *Optimization Options* *Note Options that Control Optimization: Optimize Options. -fcaller-saves -fcse-follow-jumps -fcse-skip-blocks -fdelayed-branch -fexpensive-optimizations -ffast-math -ffloat-store -fforce-addr -fforce-mem -finline-functions -fkeep-inline-functions -fno-default-inline -fno-defer-pop -fno-function-cse -fno-inline -fno-peephole -fomit-frame-pointer -frerun-cse-after-loop -fschedule-insns -fschedule-insns2 -fstrength-reduce -fthread-jumps -funroll-all-loops -funroll-loops -O -O0 -O1 -O2 -O3 *Preprocessor Options* *Note Options Controlling the Preprocessor: Preprocessor Options. -AQUESTION(ANSWER) -C -dD -dM -dN -DMACRO[=DEFN] -E -H -idirafter DIR -include FILE -imacros FILE -iprefix FILE -iwithprefix DIR -iwithprefixbefore DIR -isystem DIR -M -MD -MM -MMD -MG -nostdinc -P -trigraphs -undef -UMACRO -Wp,OPTION *Assembler Option* *Note Passing Options to the Assembler: Assembler Options. -Wa,OPTION *Linker Options* *Note Options for Linking: Link Options. OBJECT-FILE-NAME -lLIBRARY -nostartfiles -nostdlib -s -static -shared -symbolic -Wl,OPTION -Xlinker OPTION -u SYMBOL *Directory Options* *Note Options for Directory Search: Directory Options. -BPREFIX -IDIR -I- -LDIR *Target Options* *Note Target Options::. -b MACHINE -V VERSION *Machine Dependent Options* *Note Hardware Models and Configurations: Submodel Options. *M680x0 Options* -m68000 -m68020 -m68020-40 -m68030 -m68040 -m68881 -mbitfield -mc68000 -mc68020 -mfpa -mnobitfield -mrtd -mshort -msoft-float *VAX Options* -mg -mgnu -munix *SPARC Options* -mapp-regs -mcypress -mepilogue -mflat -mfpu -mhard-float -mhard-quad-float -mno-app-regs -mno-flat -mno-fpu -mno-epilogue -mno-unaligned-doubles -msoft-float -msoft-quad-float -msparclite -msupersparc -munaligned-doubles -mv8 SPARC V9 compilers support the following options in addition to the above: -mmedlow -mmedany -mint32 -mint64 -mlong32 -mlong64 -mno-stack-bias -mstack-bias *Convex Options* -mc1 -mc2 -mc32 -mc34 -mc38 -margcount -mnoargcount -mlong32 -mlong64 -mvolatile-cache -mvolatile-nocache *AMD29K Options* -m29000 -m29050 -mbw -mnbw -mdw -mndw -mlarge -mnormal -msmall -mkernel-registers -mno-reuse-arg-regs -mno-stack-check -mno-storem-bug -mreuse-arg-regs -msoft-float -mstack-check -mstorem-bug -muser-registers *ARM Options* -mapcs -m2 -m3 -m6 -mbsd -mxopen -mno-symrename *M88K Options* -m88000 -m88100 -m88110 -mbig-pic -mcheck-zero-division -mhandle-large-shift -midentify-revision -mno-check-zero-division -mno-ocs-debug-info -mno-ocs-frame-position -mno-optimize-arg-area -mno-serialize-volatile -mno-underscores -mocs-debug-info -mocs-frame-position -moptimize-arg-area -mserialize-volatile -mshort-data-NUM -msvr3 -msvr4 -mtrap-large-shift -muse-div-instruction -mversion-03.00 -mwarn-passed-structs *RS/6000 Options and PowerPC* -mcpu=CPU TYPE -mpower -mno-power -mpower2 -pno-power2 -mpowerpc -mno-powerpc -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mnew-mnemonics -mno-new-mnemonics -mfull-toc -mminimal-toc -mno-fop-in-toc -mno-sum-in-toc *RT Options* -mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs -mfull-fp-blocks -mhc-struct-return -min-line-mul -mminimum-fp-blocks -mnohc-struct-return *MIPS Options* -mabicalls -mcpu=CPU TYPE -membedded-data -membedded-pic -mfp32 -mfp64 -mgas -mgp32 -mgp64 -mgpopt -mhalf-pic -mhard-float -mint64 -mips1 -mips2 -mips3 -mlong64 -mlong-calls -mmemcpy -mmips-as -mmips-tfile -mno-abicalls -mno-embedded-data -mno-embedded-pic -mno-gpopt -mno-long-calls -mno-memcpy -mno-mips-tfile -mno-rnames -mno-stats -mrnames -msoft-float -mstats -G NUM -nocpp *i386 Options* -m486 -mieee-fp -mno-486 -mno-fancy-math-387 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib -mno-wide-multiply -mreg-alloc=LIST *HPPA Options* -mdisable-fpregs -mdisable-indexing -mjump-in-delay -mgas -mlong-calls -mno-disable-fpregs -mno-disable-indexing -mno-gas -mno-jump-in-delay -mno-long-calls -mno-portable-runtime -mpa-risc-1-0 -mpa-risc-1-1 -mportable-runtime *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* -mfp-regs -mno-fp-regs -mno-soft-float -msoft-float *Clipper Options* -mc300 -mc400 *H8/300 Options* -mrelax -mh *System V Options* -Qy -Qn -YP,PATHS -Ym,DIR *Code Generation Options* *Note Options for Code Generation Conventions: Code Gen Options. -fcall-saved-REG -fcall-used-REG -ffixed-REG -finhibit-size-directive -fno-common -fno-ident -fno-gnu-linker -fpcc-struct-return -fpic -fPIC -freg-struct-return -fshared-data -fshort-enums -fshort-double -fvolatile -fvolatile-global -fverbose-asm +e0 +e1 * 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++. * 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. * Target Options:: Running a cross-compiler, or an old version of GNU CC. File: gcc.info, Node: Overall Options, Next: Invoking G++, Prev: Option Summary, Up: Invoking GCC Options Controlling the Kind of Output ====================================== Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. The first three stages apply to an individual source file, and end by producing an object file; linking combines all the object files (those newly compiled, and those specified as input) into an executable file. For any given input file, the file name suffix determines what kind of compilation is done: `FILE.c' C source code which must be preprocessed. `FILE.i' C source code which should not be preprocessed. `FILE.ii' C++ source code which should not be preprocessed. `FILE.m' Objective-C source code. Note that you must link with the library `libobjc.a' to make an Objective-C program work. `FILE.h' C header file (not to be compiled or linked). `FILE.cc' `FILE.cxx' `FILE.cpp' `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.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 objective-c c++ c-header cpp-output c++-cpp-output assembler assembler-with-cpp `-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). If you only want some of the stages of compilation, you can use `-x' (or filename suffixes) to tell `gcc' where to start, and one of the options `-c', `-S', or `-E' to say where `gcc' is to stop. Note that some combinations (for example, `-x cpp-output -E' instruct `gcc' to do nothing at all. `-c' Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file. By default, the object file name for a source file is made by replacing the suffix `.c', `.i', `.s', etc., with `.o'. Unrecognized input files, not requiring compilation or assembly, are ignored. `-S' Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified. By default, the assembler file name for a source file is made by replacing the suffix `.c', `.i', etc., with `.s'. Input files that don't require compilation are ignored. `-E' Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output. Input files which don't require preprocessing are ignored. `-o FILE' Place output in file FILE. This applies regardless to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. Since only one output file can be specified, it does not make sense to use `-o' when compiling more than one input file, unless you are producing an executable file as output. If `-o' is not specified, the default is to put an executable file in `a.out', the object file for `SOURCE.SUFFIX' in `SOURCE.o', its assembler file in `SOURCE.s', and all preprocessed C source on standard output. `-v' Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper. `-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. File: gcc.info, Node: Invoking G++, Next: C Dialect Options, Prev: Overall Options, Up: Invoking GCC Compiling C++ Programs ====================== C++ source files conventionally use one of the suffixes `.C', `.cc', `cpp', or `.cxx'; preprocessed C++ files use the suffix `.ii'. GNU CC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name `gcc'). However, C++ programs often require class libraries as well as a compiler that understands the C++ language--and under some circumstances, you might want to compile programs from standard input, or otherwise without a suffix that flags them as C++ programs. `g++' is a program that calls GNU CC with the default language set to C++, and automatically specifies linking against the GNU class library libg++. (1) On many systems, the script `g++' is also installed with the name `c++'. When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. *Note Options Controlling C Dialect: C Dialect Options, for explanations of options for languages related to C. *Note Options Controlling C++ Dialect: C++ Dialect Options, for explanations of options that are meaningful only for C++ programs. ---------- Footnotes ---------- (1) Prior to release 2 of the compiler, there was a separate `g++' compiler. That version was based on GNU CC, but not integrated with it. Versions of `g++' with a `1.XX' version number--for example, `g++' version 1.37 or 1.42--are much less reliable than the versions integrated with GCC 2. Moreover, combining G++ `1.XX' with a version 2 GCC will simply not work. File: gcc.info, Node: C Dialect Options, Next: C++ Dialect Options, Prev: Invoking G++, Up: Invoking GCC 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' Support all ANSI standard C programs. This turns off certain features of GNU C that are incompatible with ANSI C, such as the `asm', `inline' and `typeof' keywords, and predefined macros such as `unix' and `vax' that identify the type of system you are using. It also enables the undesirable and rarely used ANSI trigraph feature, and disallows `$' as part of identifiers. The alternate keywords `__asm__', `__extension__', `__inline__' and `__typeof__' continue to work despite `-ansi'. You would not want to use them in an ANSI C program, of course, but it 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-ANSI 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 ANSI standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things. The functions `alloca', `abort', `exit', and `_exit' are not builtin functions when `-ansi' is used. `-fno-asm' Do not recognize `asm', `inline' or `typeof' as a keyword. These words may then be used as identifiers. You can use the keywords `__asm__', `__inline__' and `__typeof__' instead. `-ansi' implies `-fno-asm'. `-fno-builtin' Don't recognize builtin functions that do not begin with two leading underscores. Currently, the functions affected include `abort', `abs', `alloca', `cos', `exit', `fabs', `ffs', `labs', `memcmp', `memcpy', `sin', `sqrt', `strcmp', `strcpy', and `strlen'. GCC normally generates special code to handle certain builtin functions more efficiently; for instance, calls to `alloca' may become single instructions that adjust the stack directly, and calls to `memcpy' may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. The `-ansi' option prevents `alloca' and `ffs' from being builtin functions, since these functions do not have an ANSI standard meaning. `-trigraphs' Support ANSI C trigraphs. You don't want to know about this brain-damage. The `-ansi' option implies `-trigraphs'. `-traditional' Attempt to support some aspects of traditional C compilers. Specifically: * All `extern' declarations take effect globally even if they are written inside of a function definition. This includes implicit declarations of functions. * The newer keywords `typeof', `inline', `signed', `const' and `volatile' are not recognized. (You can still use the alternative keywords such as `__typeof__', `__inline__', and so on.) * Comparisons between pointers and integers are always allowed. * Integer types `unsigned short' and `unsigned char' promote to `unsigned int'. * Out-of-range floating point literals are not an error. * Certain constructs which ANSI regards as a single invalid preprocessing number, such as `0xe-0xd', are treated as expressions instead. * String "constants" are not necessarily constant; they are stored in writable space, and identical looking constants are allocated separately. (This is the same as the effect of `-fwritable-strings'.) * All automatic variables not declared `register' are preserved by `longjmp'. Ordinarily, GNU C follows ANSI C: automatic variables not declared `volatile' may be clobbered. * The character escape sequences `\x' and `\a' evaluate as the literal characters `x' and `a' respectively. Without `-traditional', `\x' is a prefix for the hexadecimal representation of a character, and `\a' produces a bell. * In C++ programs, assignment to `this' is permitted with `-traditional'. (The option `-fthis-is-variable' also has this effect.) You may wish to use `-fno-builtin' as well as `-traditional' if your program uses names that are normally GNU C builtin functions for other purposes of its own. You cannot use `-traditional' if you include any header files that rely on ANSI C features. Some vendors are starting to ship systems with ANSI C header files and you cannot use `-traditional' on such systems to compile files that include any system headers. `' In the preprocessor, comments convert to nothing at all, rather than to a space. This allows traditional token concatenation. `' In preprocessor directive, the `#' symbol must appear as the first character of a line. `' In the preprocessor, macro arguments are recognized within string constants in a macro definition (and their values are stringified, though without additional quote marks, when they appear in such a context). The preprocessor always considers a string constant to end at a newline. `' The predefined macro `__STDC__' is not defined when you use `-traditional', but `__GNUC__' is (since the GNU extensions which `__GNUC__' indicates are not affected by `-traditional'). If you need to write header files that work differently depending on whether `-traditional' is in use, by testing both of these predefined macros you can distinguish four situations: GNU C, traditional GNU C, other ANSI C compilers, and other old C compilers. *Note Standard Predefined Macros: (cpp.info)Standard Predefined, for more discussion of these and other predefined macros. `' The preprocessor considers a string constant to end at a newline (unless the newline is escaped with `\'). (Without `-traditional', string constants can contain the newline character as typed.) `-traditional-cpp' Attempt to support some aspects of traditional C preprocessors. This includes the last five items in the table immediately above, but none of the other effects of `-traditional'. `-fcond-mismatch' Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. `-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 bitfield is signed or unsigned, when the declaration does not use either `signed' or `unsigned'. By default, such a bitfield is signed, because this is consistent: the basic integer types such as `int' are signed types. However, when `-traditional' is used, bitfields are all unsigned no matter what. `-fwritable-strings' Store string constants in the writable data segment and don't uniquize them. This is for compatibility with old programs which assume they can write into string constants. The option `-traditional' also has this effect. Writing into string constants is a very bad idea; "constants" should be constant. `-fallow-single-precision' Do not promote single precision math operations to double precision, even when compiling with `-traditional'. Traditional K&R C promotes all floating point operations to double precision, regardless of the sizes of the operands. On the architecture for which you are compiling, single precision may be faster than double precision. If you must use `-traditional', but want to use single precision operations when the operands are single precision, use this option. This option has no effect when compiling with ANSI or GNU C conventions (the default). File: gcc.info, Node: C++ Dialect Options, Next: Warning Options, Prev: C Dialect Options, Up: Invoking GCC 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 -felide-constructors -O -c firstClass.C In this example, only `-felide-constructors' is an option meant only for C++ programs; you can use the other options with any language supported by GNU CC. Here is a list of options that are *only* for compiling C++ programs: `-fno-access-control' Turn off all access checking. This switch is mainly useful for working around bugs in the access control code. `-fall-virtual' Treat all possible member functions as virtual, implicitly. All member functions (except for constructor functions and `new' or `delete' member operators) are treated as virtual functions of the class where they appear. This does not mean that all calls to these member functions will be made through the internal table of virtual functions. Under some circumstances, the compiler can determine that a call to a given virtual function can be made directly; in these cases the calls are direct in any case. `-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 your program mysteriously crashes after `main()' has completed, you may have an object that is being destroyed twice because two definitions were merged. `-fdollars-in-identifiers' Accept `$' in identifiers. You can also explicitly prohibit use of `$' with the option `-fno-dollars-in-identifiers'. (GNU C++ allows `$' by default on some target systems but not others.) Traditional C allowed the character `$' to form part of identifiers. However, ANSI C and C++ forbid `$' in identifiers. `-fenum-int-equiv' Permit implicit conversion of `int' to enumeration types. Normally GNU C++ allows conversion of `enum' to `int', but not the other way around. `-fexternal-templates' Cause template instantiations to obey `#pragma interface' and `implementation'; template instances are emitted or not according to the location of the template definition. *Note Template Instantiation::, for more information. `-falt-external-templates' Similar to -fexternal-templates, but template instances are emitted or not according to the place where they are first instantiated. *Note Template Instantiation::, for more information. `-fno-implicit-templates' Never emit code for templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. *Note Template Instantiation::, for more information. `-fhandle-signatures' Recognize the `signature' and `sigof' keywords for specifying abstract types. The default (`-fno-handle-signatures') is not to recognize them. *Note Type Abstraction using Signatures: C++ Signatures. `-fhuge-objects' Support virtual function calls for objects that exceed the size representable by a `short int'. Users should not use this flag by default; if you need to use it, the compiler will tell you so. If you compile any of your code with this flag, you must compile *all* of your code with this flag (including libg++, if you use it). This flag is not useful when compiling with -fvtable-thunks. `-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. `-fmemoize-lookups' `-fsave-memoized' Use heuristics to compile faster. These heuristics are not enabled by default, since they are only effective for certain input files. Other input files compile more slowly. The first time the compiler must build a call to a member function (or reference to a data member), it must (1) determine whether the class implements member functions of that name; (2) resolve which member function to call (which involves figuring out what sorts of type conversions need to be made); and (3) check the visibility of the member function to the caller. All of this adds up to slower compilation. Normally, the second time a call is made to that member function (or reference to that data member), it must go through the same lengthy process again. This means that code like this: cout << "This " << p << " has " << n << " legs.\n"; makes six passes through all three steps. By using a software cache, a "hit" significantly reduces this cost. Unfortunately, using the cache introduces another layer of mechanisms which must be implemented, and so incurs its own overhead. `-fmemoize-lookups' enables the software cache. Because access privileges (visibility) to members and member functions may differ from one function context to the next, G++ may need to flush the cache. With the `-fmemoize-lookups' flag, the cache is flushed after every function that is compiled. The `-fsave-memoized' flag enables the same software cache, but when the compiler determines that the context of the last function compiled would yield the same access privileges of the next function to compile, it preserves the cache. This is most helpful when defining many member functions for the same class: with the exception of member functions which are friends of other classes, each member function has exactly the same access privileges as every other, and the cache need not be flushed. `-fno-strict-prototype' Treat a function declaration with no arguments, such as `int foo ();', as C would treat it--as saying nothing about the number of arguments or their types. Normally, such a declaration in C++ means that the function `foo' takes no arguments. This option does not work with operator overloading, which places constraints on the parameter types. `-fnonnull-objects' Assume that objects reached through references are not null. Normally, GNU C++ makes conservative assumptions about objects reached through references. For example, the compiler must check that `a' is not null in code like the following: obj &a = g (); a.f (2); Checking that references of this sort have non-null values requires extra code, however, and it is unnecessary for many programs. You can use `-fnonnull-objects' to omit the checks for null, if your program doesn't require checking. This checking is currently only done for conversions to virtual base classes. `-fthis-is-variable' Permit assignment to `this'. The incorporation of user-defined free store management into C++ has made assignment to `this' an anachronism. Therefore, by default it is invalid to assign to `this' within a class member function; that is, GNU C++ treats `this' in a member function of class `X' as a non-lvalue of type `X *'. However, for backwards compatibility, you can make it valid with `-fthis-is-variable'. `-fvtable-thunks' Use `thunks' to implement the virtual function dispatch table (`vtable'). The traditional (cfront-style) approach to implementing vtables was to store a pointer to the function and two offsets for adjusting the `this' pointer at the call site. Newer implementations store a single pointer to a `thunk' function which does any necessary adjustment and then calls the target function. This option also enables a heuristic for controlling emission of vtables; if a class has any non-inline virtual functions, the vtable will be emitted in the translation unit containing the first one of those. `-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 libg++.) `-traditional' For C++ programs (in addition to the effects that apply to both C and C++), this has the same effect as `-fthis-is-variable'. *Note Options Controlling C Dialect: C Dialect Options. 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. `-Wenum-clash' `-Woverloaded-virtual' `-Wtemplate-debugging' Warnings that apply only to C++ programs. *Note Options to Request or Suppress Warnings: Warning Options. `+eN' Control how virtual function definitions are used, in a fashion compatible with `cfront' 1.x. *Note Options for Code Generation Conventions: Code Gen Options. File: gcc.info, Node: Warning Options, Next: Debugging Options, Prev: C++ Dialect Options, Up: Invoking GCC 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. These options control the amount and kinds of warnings produced by GNU CC: `-fsyntax-only' Check the code for syntax errors, but don't do anything beyond that. `-w' Inhibit all warning messages. `-Wno-import' Inhibit warning messages about the use of `#import'. `-pedantic' Issue all the warnings demanded by strict ANSI standard C; reject all programs that use forbidden extensions. Valid ANSI standard C programs should compile properly with or without this option (though a rare few will require `-ansi'). However, without this option, certain GNU extensions and traditional C features are supported as well. With this option, they are rejected. `-pedantic' does not cause warning messages for use of the alternate keywords whose names begin and end with `__'. Pedantic 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::. This option is not intended to be useful; it exists only to satisfy pedants who would otherwise claim that GNU CC fails to support the ANSI standard. Some users try to use `-pedantic' to check programs for strict ANSI C conformance. They soon find that it does not do quite what they want: it finds some non-ANSI practices, but not all--only those for which ANSI C *requires* a diagnostic. A feature to report any failure to conform to ANSI C might be useful in some instances, but would require considerable additional work and would be quite different from `-pedantic'. We recommend, rather, that users take advantage of the extensions of GNU C and disregard the limitations of other compilers. Aside from certain supercomputers and obsolete small machines, there is less and less reason ever to use any other C compiler other than for bootstrapping GNU CC. `-pedantic-errors' Like `-pedantic', except that errors are produced rather than warnings. `-W' Print extra warning messages for these events: * A nonvolatile 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. * 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 contains no side effects. * 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. * An aggregate has a partly bracketed initializer. For example, the following code would evoke such a warning, because braces are missing around the initializer for `x.h': struct s { int f, g; }; struct t { struct s h; int i; }; struct t x = { 1, 2, 3 }; `-Wimplicit' Warn whenever a function or parameter is implicitly declared. `-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'. `-Wunused' Warn whenever a variable is unused aside from its declaration, whenever a function is declared static but never defined, whenever a label is declared but not used, and whenever a statement computes a result that is explicitly not used. To suppress this warning for a local variable or expression, simply cast it to void. This will also work for file-scope variables, but if you want to mark them used at the point of definition, you can use this macro: #define USE(var) \ static void *const use_##var = (&use_##var, &var, 0) USE (string); `-Wswitch' Warn whenever a `switch' statement has an index of enumeral type and lacks a `case' for one or more of the named codes of that enumeration. (The presence of a `default' label prevents this warning.) `case' labels outside the enumeration range also provoke warnings when this option is used. `-Wcomment' Warn whenever a comment-start sequence `/*' appears in a comment. `-Wtrigraphs' Warn if any trigraphs are encountered (assuming they are enabled). `-Wformat' Check calls to `printf' and `scanf', etc., to make sure that the arguments supplied have types appropriate to the format string specified. `-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. `-Wuninitialized' An automatic variable is used without first being initialized. These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don't specify `-O', you simply won't get these warnings. These warnings occur only for variables that are candidates for register allocation. Therefore, they do not occur for a variable that is declared `volatile', or whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for structures, unions or arrays, even when they are in registers. Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed. These warnings are made optional because GNU CC 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 GNU CC 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. Some spurious warnings can be avoided if you declare all the functions you use that never return as `noreturn'. *Note Function Attributes::. `-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. `-Wenum-clash' Warn about conversion between different enumeration types. (C++ only). `-Wtemplate-debugging' When using templates in a C++ program, warn if debugging is not yet fully available (C++ only). `-Wreorder (C++ only)' Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance: struct A { int i; int j; A(): j (0), i (1) { } }; Here the compiler will warn that the member initializers for `i' and `j' will be rearranged to match the declaration order of the members. `-Wall' All of the above `-W' options combined. These are all the options which pertain to usage that we recommend avoiding and that we believe is easy to avoid, even in conjunction with macros. The remaining `-W...' options are not implied by `-Wall' because they warn about constructions that we consider reasonable to use, on occasion, in clean programs. `-Wtraditional' Warn about certain constructs that behave differently in traditional and ANSI C. * Macro arguments occurring within string constants in the macro body. These would substitute the argument in traditional C, but are part of the constant in ANSI C. * 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'. `-Wshadow' Warn whenever a local variable shadows another local variable. `-Wid-clash-LEN' Warn whenever two distinct identifiers match in the first LEN characters. This may help you prepare a program that will compile with certain obsolete, brain-damaged compilers. `-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' 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' 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. 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'. `-Waggregate-return' Warn if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.) `-Wstrict-prototypes' Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration which specifies the argument types.) `-Wmissing-prototypes' Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. The aim is to detect global functions that fail to be declared in header files. `-Wmissing-declarations' Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files. `-Wredundant-decls' Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing. `-Wnested-externs' Warn if an `extern' declaration is encountered within an function. `-Winline' Warn if a function can not be inlined, and either it was declared as inline, or else the `-finline-functions' option was given. `-Woverloaded-virtual' Warn when a derived class function declaration may be an error in defining a virtual function (C++ only). In a derived class, the definitions of virtual functions must match the type signature of a virtual function declared in the base class. With this option, the compiler warns when you define a function with the same name as a virtual function, but with a type signature that does not match any declarations from the base class. `-Wsynth (C++ only)' Warn when g++'s synthesis behavior does not match that of cfront. For instance: struct A { operator int (); A& operator = (int); }; main () { A a,b; a = b; } In this example, g++ will synthesize a default `A& operator = (const A&);', while cfront will use the user-defined `operator ='. `-Werror' Make all warnings into errors.