The C and C++ compilers are integrated. Both process input files through one or more of four stages: preprocessing, compilation, assembly, and linking. Source filename suffixes identify the source language, but which name you use for the compiler governs default assumptions:
assumes preprocessed (.i) files are C and assumes C style linking.
assumes preprocessed (.i) files are C++ and assumes C++ style linking.
Suffixes of source file names indicate the language and kind of processing to be done:
.c C source; preprocess, compile, assemble
.C C++ source; preprocess, compile, assemble
.cc C++ source; preprocess, compile, assemble
.cxx C++ source; preprocess, compile, assemble
.m Objective-C source; preprocess, compile, assemble
.i preprocessed C; compile, assemble
.ii preprocessed C++; compile, assemble
.s Assembler source; assemble
.S Assembler source; preprocess, assemble
.h Preprocessor file; not usually named on command line
Files with other suffixes are passed to the linker. Common cases include:
.o Object file
.a Archive file
Linking is always the last stage unless you use one of the -c, -S, or -E options to avoid it (or unless compilation errors stop the whole process). For the link stage, all .o files corresponding to source files, -l libraries, unrecognized filenames (including named .o object files and .a archives) are passed to the linker in command-line order.
Options must be separate: `-dr' is quite different from `-d -r '.
Most `-f' and `-W' options have two contrary forms: -fname and -fno-name (or -Wname and -Wno-name). Only the non-default forms are shown here.
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
-c -S -E -o file -pipe -v -x language
-ansi -fall-virtual -fcond-mismatch -fdollars-in-identifiers -fenum-int-equiv -fexternal-templates -fno-asm -fno-builtin -fhosted -fno-hosted -ffreestanding -fno-freestanding -fno-strict-prototype -fsigned-bitfields -fsigned-char -fthis-is-variable -funsigned-bitfields -funsigned-char -fwritable-strings -traditional -traditional-cpp -trigraphs
-fsyntax-only -pedantic -pedantic-errors -w -W -Wall -Waggregate-return -Wcast-align -Wcast-qual -Wchar-subscript -Wcomment -Wconversion -Wenum-clash -Werror -Wformat -Wid-clash-len -Wimplicit -Wimplicit-int -Wimplicit-function-declaration -Winline -Wlong-long -Wmain -Wmissing-prototypes -Wmissing-declarations -Wnested-externs -Wno-import -Wparentheses -Wpointer-arith -Wredundant-decls -Wreturn-type -Wshadow -Wstrict-prototypes -Wswitch -Wtemplate-debugging -Wtraditional -Wtrigraphs -Wuninitialized -Wunused -Wwrite-strings
-a -dletters -fpretend-float -g -glevel -gcoff -gxcoff -gxcoff+ -gdwarf -gdwarf+ -gstabs -gstabs+ -ggdb -p -pg -save-temps -print-file-name=library -print-libgcc-file-name -print-prog-name=program
-fcaller-saves -fcse-follow-jumps -fcse-skip-blocks -fdelayed-branch -felide-constructors -fexpensive-optimizations -ffast-math -ffloat-store -fforce-addr -fforce-mem -finline-functions -fkeep-inline-functions -fmemoize-lookups -fno-default-inline -fno-defer-pop -fno-function-cse -fno-inline -fno-peephole -fomit-frame-pointer -frerun-cse-after-loop -fschedule-insns -fschedule-insns2 -fstrength-reduce -fthread-jumps -funroll-all-loops -funroll-loops -O -O2 -O3
-Aassertion -C -dD -dM -dN -Dmacro[=defn] -E -H -idirafter dir -include file -imacros file -iprefix file -iwithprefix dir -M -MD -MM -MMD -nostdinc -P -Umacro -undef
-llibrary -nostartfiles -nostdlib -static -shared -symbolic -Xlinker option -Wl,option -u symbol
-Bprefix -Idir -I- -Ldir
-b machine -V version
Configuration Dependent Options
-m68000 -m68020 -m68020-40 -m68030 -m68040 -m68881 -mbitfield -mc68000 -mc68020 -mfpa -mnobitfield -mrtd -mshort -msoft-float
-mg -mgnu -munix
-mepilogue -mfpu -mhard-float -mno-fpu -mno-epilogue -msoft-float -msparclite -mv8 -msupersparc -mcypress
-margcount -mc1 -mc2 -mnoargcount
-m29000 -m29050 -mbw -mdw -mkernel-registers -mlarge -mnbw -mnodw -msmall -mstack-check -muser-registers
-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
-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
-mcpu=cpu type -mips2 -mips3 -mint64 -mlong64 -mlonglong128 -mmips-as -mgas -mrnames -mno-rnames -mgpopt -mno-gpopt -mstats -mno-stats -mmemcpy -mno-memcpy -mno-mips-tfile -mmips-tfile -msoft-float -mhard-float -mabicalls -mno-abicalls -mhalf-pic -mno-half-pic -G num -nocpp
-m486 -mno-486 -msoft-float -mno-fp-ret-in-387
-mpa-risc-1-0 -mpa-risc-1-1 -mkernel -mshared-libs -mno-shared-libs -mlong-calls -mdisable-fpregs -mdisable-indexing -mtrailing-colon
-mcpu-type -mnumerics -msoft-float -mleaf-procedures -mno-leaf-procedures -mtail-call -mno-tail-call -mcomplex-addr -mno-complex-addr -mcode-align -mno-code-align -mic-compat -mic2.0-compat -mic3.0-compat -masm-compat -mintel-asm -mstrict-align -mno-strict-align -mold-align -mno-old-align
DEC Alpha Options
-mfp-regs -mno-fp-regs -mno-soft-float -msoft-float
System V Options
-G -Qy -Qn -YP,paths -Ym,dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -finhibit-size-directive -fnonnull-objects -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
Specify explicitly the language for the following input files (rather than choosing a default based on the file name suffix) . This option applies to all following input files until the next `-x' option. Possible values of language are `c', `objective-c', `c-header', `c++', `cpp-output', `assembler', and `assembler-with-cpp'.
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 want only some of the four stages (preprocess, compile, assemble, link), 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.
Compile or assemble the source files, but do not link. The compiler output is an object file corresponding to each source file.
By default, GCC makes the object file name for a source file by replacing the suffix `.c', `.i', `.s', etc., with `.o'. Use -o to select another name.
GCC ignores any unrecognized input files (those that do not require compilation or assembly) with the -c option.
Stop after the stage of compilation proper; do not assemble. The output is an assembler code file for each non-assembler input file specified.
By default, GCC makes the assembler file name for a source file by replacing the suffix `.c', `.i', etc., with `.s'. Use -o to select another name.
GCC ignores any input files that don't require compilation.
Stop after the preprocessing stage; do not run the compiler proper. The output is preprocessed source code, which is sent to the standard output.
GCC ignores input files which don't require preprocessing.
Place output in file file. This applies regardless to whatever sort of output GCC is producing, 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 you do not specify `-o', 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.
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.
Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler cannot read from a pipe; but the GNU assembler has no trouble.
The following options control the dialect of C that the compiler accepts:
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'.
The preprocessor predefines a macro __STRICT_ANSI__ when you use the `-ansi' option. 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.
Do not recognize asm, inline or typeof as a keyword. These words may then be used as identifiers. You can use __asm__, __inline__ and __typeof__ instead. `-ansi' implies `-fno-asm'.
Don't recognize built-in functions that do not begin with two leading underscores. Currently, the functions affected include _exit, abort, abs, alloca, cos, exit, fabs, labs, memcmp, memcpy, sin, sqrt, strcmp, strcpy, and strlen.
The `-ansi' option prevents alloca and _exit from being builtin functions.
Compile for a hosted environment; this implies the `-fbuiltin' option, and implies that suspicious declarations of main should be warned about.
Compile for a freestanding environment; this implies the `-fno-builtin' option, and implies that main has no special requirements.
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 (C++ only). Normally, such a declaration in C++ means that the function foo takes no arguments.
Support ANSI C trigraphs. The `-ansi' option implies `-trigraphs'.
Attempt to support some aspects of traditional C compilers. For details, see the GNU C Manual; the duplicate list here has been deleted so that we won't get complaints when it is out of date.
But one note about C++ programs only (not C). `-traditional' has one additional effect for C++: assignment to this is permitted. This is the same as the effect of `-fthis-is-variable'.
Attempt to support some aspects of traditional C preprocessors. This includes the items that specifically mention the preprocessor above, but none of the other effects of `-traditional'.
Permit the use of `$' in identifiers (C++ only). You can also use `-fno-dollars-in-identifiers' to explicitly prohibit use of `$'. (GNU C++ allows `$' by default on some target systems but not others.)
Permit implicit conversion of int to enumeration types (C++ only). Normally GNU C++ allows conversion of enum to int, but not the other way around.
Produce smaller code for template declarations, by generating only a single copy of each template function where it is defined (C++ only). To use this option successfully, you must also mark all files that use templates with either `#pragma implementation' (the definition) or `#pragma interface' (declarations).
When your code is compiled with `-fexternal-templates', all template instantiations are external. You must arrange for all necessary instantiations to appear in the implementation file; you can do this with a typedef that references each instantiation needed. Conversely, when you compile using the default option `-fno-external-templates', all template instantiations are explicitly internal.
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.
Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void.
Permit assignment to this (C++ only). 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. However, for backwards compatibility, you can make it valid with `-fthis-is-variable'.
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 and unsigned char, even though its behavior is always just like one of those two.
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, `-fno-signed-char' is equivalent to `-funsigned-char'.
These options control whether a bitfield is signed or unsigned, when declared with no explicit `signed' or `unsigned' qualifier. By default, such a bitfield is signed, because this is consistent: the basic integer types such as int are signed types.
However, when you specify `-traditional', bitfields are all unsigned no matter what.
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. `-traditional' also has this effect.
Writing into string constants is a very bad idea; ``constants'' should be constant.
These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the `-E' option, GCC does nothing except preprocessing. Some of these options make sense only together with `-E' because they cause the preprocessor output to be unsuitable for actual compilation.
Process file as input before processing the regular input file. In effect, the contents of file are compiled first. Any `-D' and `-U' options on the command line are always processed before `-include file', regardless of the order in which they are written. All the `-include' and `-imacros' options are processed in the order in which they are written.
Process file as input, discarding the resulting output, before processing the regular input file. Because the output generated from file is discarded, the only effect of `-imacros file' is to make the macros defined in file available for use in the main input. The preprocessor evaluates any `-D' and `-U' options on the command line before processing `-imacrosfile', regardless of the order in which they are written. All the `-include' and `-imacros' options are processed in the order in which they are written.
Add the directory dir to the second include path. The directories on the second include path are searched when a header file is not found in any of the directories in the main include path (the one that `-I' adds to).
Specify prefix as the prefix for subsequent `-iwithprefix' options.
Add a directory to the second include path. The directory's name is made by concatenating prefix and dir, where prefix was specified previously with `-iprefix'.
Do not search the standard system directories for header files. Only the directories you have specified with `-I' options (and the current directory, if appropriate) are searched.
By using both `-nostdinc' and `-I-', you can limit the include-file search file to only those directories you specify explicitly.
Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building `libg++'.)
Do not predefine any nonstandard macros. (Including architecture flags).
Run only the C preprocessor. Preprocess all the C source files specified and output the results to standard output or to the specified output file.
Tell the preprocessor not to discard comments. Used with the `-E' option.
Tell the preprocessor not to generate `#line' commands. Used with the `-E' option.
-M [ -MG ]
Tell the preprocessor to output a rule suitable for make describing the dependencies of each object file. For each source file, the preprocessor outputs one make-rule whose target is the object file name for that source file and whose dependencies are all the files `#include'd in it. This rule may be a single line or may be continued with `\'-newline if it is long. The list of rules is printed on standard output instead of the preprocessed C program.
`-M' implies `-E'.
`-MG' says to treat missing header files as generated files and assume they live in the same directory as the source file. It must be specified in addition to `-M'.
-MM [ -MG ]
Like `-M' but the output mentions only the user header files included with `#include file"'. System header files included with `#include <file>' are omitted.
Like `-M' but the dependency information is written to files with names made by replacing `.o' with `.d' at the end of the output file names. This is in addition to compiling the file as specified---`-MD' does not inhibit ordinary compilation the way `-M' does.
The Mach utility `md' can be used to merge the `.d' files into a single dependency file suitable for using with the `make' command.
Like `-MD' except mention only user header files, not system header files.
Print the name of each header file used, in addition to other normal activities.
Assert the answer answer for question, in case it is tested with a preprocessor conditional such as `#if #question(answer)'. `-A-' disables the standard assertions that normally describe the target machine.
(answer) Assert the answer answer for question, in case it is tested with a preprocessor conditional such as `#if #question(answer)'. `-A-' disables the standard assertions that normally describe the target machine.
Define macro macro with the string `1' as its definition.
Define macro macro as defn. All instances of `-D' on the command line are processed before any `-U' options.
Undefine macro macro. `-U' options are evaluated after all `-D' options, but before any `-include' and `-imacros' options.
Tell the preprocessor to output only a list of the macro definitions that are in effect at the end of preprocessing. Used with the `-E' option.
Tell the preprocessor to pass all macro definitions into the output, in their proper sequence in the rest of the output.
Like `-dD' except that the macro arguments and contents are omitted. Only `#define name' is included in the output.
Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.
These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.
A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If GCC does a link step, these object files are used as input to the linker.
Use the library named library when linking.
The linker searches a standard list of directories for the library, which is actually a file named `liblibrary.a'. The linker then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories plus any that you specify with `-L'.
Normally the files found this way are library files---archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. However, if the linker finds an ordinary object file rather than a library, the object file is linked in the usual fashion. The only difference between using an `-l' option and specifying a file name is that `-l' surrounds library with `lib' and `.a' and searches several directories.
You need this special case of the -l option in order to link an Objective C program.
Do not use the standard system startup files when linking. The standard libraries are used normally.
Don't use the standard system libraries and startup files when linking. Only the files you specify will be passed to the linker.
On systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option has no effect.
Produce a shared object which can then be linked with other objects to form an executable. Only a few systems support this option.
Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option `-Xlinker -z -Xlinker defs'). Only a few systems support this option.
Pass option as an option to the linker. You can use this to supply system-specific linker options which GNU CC does not know how to recognize.
If you want to pass an option that takes an argument, you must use `-Xlinker' twice, once for the option and once for the argument. For example, to pass `-assert definitions', you must write `-Xlinker -assert -Xlinker definitions'. It does not work to write `-Xlinker "-assert definitions"', because this passes the entire string as a single argument, which is not what the linker expects.
Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas.
Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use `-u' multiple times with different symbols to force loading of additional library modules.
These options specify directories to search for header files, for libraries and for parts of the compiler:
Append directory dir to the list of directories searched for include files.
Any directories you specify with `-I' options before the `-I-' option are searched only for the case of `#include "file"'; they are not searched for `#include <file>'.
If additional directories are specified with `-I' options after the `-I-', these directories are searched for all `#include' directives. (Ordinarily all `-I' directories are used this way.)
In addition, the `-I-' option inhibits the use of the current directory (where the current input file came from) as the first search directory for `#include "file"'. There is no way to override this effect of `-I-'. With `-I.' you can specify searching the directory which was current when the compiler was invoked. That is not exactly the same as what the preprocessor does by default, but it is often satisfactory.
`-I-' does not inhibit the use of the standard system directories for header files. Thus, `-I-' and `-nostdinc' are independent.
Add directory dir to the list of directories to be searched for `-l'.
This option specifies where to find the executables, libraries and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms `cpp', `cc1' (or, for C++, `cc1plus'), `as' and `ld'. It tries prefix as a prefix for each program it tries to run, both with and without `machine/version/'.
For each subprogram to be run, the compiler driver first tries the `-B' prefix, if any. If that name is not found, or if `-B' was not specified, the driver tries two standard prefixes, which are `/usr/lib/gcc/' and `/usr/local/lib/gcc-lib/'. If neither of those results in a file name that is found, the compiler driver searches for the unmodified program name, using the directories specified in your `PATH' environment variable.
The run-time support file `libgcc.a' is also searched for using the `-B' prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means. Most of the time, on most machines, `libgcc.a' is not actually necessary.
You can get a similar result from the environment variable GCC_EXEC_PREFIX; if it is defined, its value is used as a prefix in the same way. If both the `-B' option and the GCC_EXEC_PREFIX variable are present, the `-B' option is used first and the environment variable value second.
Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error.
These options control the amount and kinds of warnings produced by GNU CC:
Check the code for syntax errors, but don't emit any output.
Inhibit all warning messages.
Inhibit warning messages about the use of #import.
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. There is no reason to use this option; it exists only to satisfy pedants.
`-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.
Like `-pedantic', except that errors are produced rather than warnings.
Print extra warning messages for these events:
A nonvolatile automatic variable might be changed by a call to longjmp. These warnings 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:
if (a > 0)
Spurious warnings can occur because GNU CC does not realize that certain functions (including abort and longjmp) will never return.
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 `<='.
Warn whenever a declaration does not specify a type.
Warn whenever a function is used before being declared.
Same as -Wimplicit-int and -Wimplicit-function-declaration.
Warn if the main function is declared or defined with a suspicious type. Typically, it is a function with external linkage, returning int, and taking zero or two arguments.
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.
Warn whenever a local variable is unused aside from its declaration, whenever a function is declared static but never defined, and whenever a statement computes a result that is explicitly not used.
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.
Warn whenever a comment-start sequence `/*' appears in a comment.
Warn if any trigraphs are encountered (assuming they are enabled).
Check calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified.
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.
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:
case 1: x = 1;
case 2: x = 4;
case 3: x = 5;
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:
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 as volatile all the functions you use that never return.
Warn if parentheses are omitted in certain contexts.
When using templates in a C++ program, warn if debugging is not yet fully available (C++ only).
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.
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.
Warn whenever a local variable shadows another local variable.
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.
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.
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 *.
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.
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.
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.
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.)
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.)
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.
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.
Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing.
Warn if an extern declaration is encountered within an function.
Warn about conversion between different enumeration types (C++ only).
Warn if long long type is used. This is default. To inhibit the warning messages, use flag `-Wno-long-long'. Flags `-W-long-long' and `-Wno-long-long' are taken into account only when flag `-pedantic' is used.
(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. Use this option to request warnings when a derived class declares a function that may be an erroneous attempt to define a virtual function: that is, warn when a function with the same name as a virtual function in the base class, but with a type signature that doesn't match any virtual functions from the base class.
Warn if a function can not be inlined, and either it was declared as inline, or else the -finline-functions option was given.
Treat warnings as errors; abort compilation after any warning.
GNU CC has various special options that are used for debugging either your program or GCC:
Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information.
On most systems that use stabs format, `-g' enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use `-gstabs+', `-gstabs', `-gxcoff+', `-gxcoff', `-gdwarf+', or `-gdwarf' (see below).
Unlike most other C compilers, GNU CC allows you to use `-g' with `-O'. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.
The following options are useful when GNU CC is generated with the capability for more than one debugging format.
Produce debugging information in the native format (if that is supported), including GDB extensions if at all possible.
Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems.
Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.
Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.
Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.
Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.
Produce debugging information in DWARF format (if that is supported). This is the format used by SDB on most System V Release 4 systems.
Produce debugging information in DWARF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.
Request debugging information and also use level to specify how much information. The default level is 2.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use `-g3'.
Generate extra code to write profile information suitable for the analysis program prof.
Generate extra code to write profile information suitable for the analysis program gprof.
Generate extra code to write profile information for basic blocks, which will record the number of times each basic block is executed. This data could be analyzed by a program like tcov. Note, however, that the format of the data is not what tcov expects. Eventually GNU gprof should be extended to process this data.
Generate extra code to read basic block profiling parameters from file `bb.in' and write profiling results to file `bb.out'. `bb.in' contains a list of functions. Whenever a function on the list is entered, profiling is turned on. When the outmost function is left, profiling is turned off. If a function name is prefixed with `-' the function is excluded from profiling. If a function name is not unique it can be disambiguated by writing `/path/filename.d:functionname'. `bb.out' will list some available filenames. Four function names have a special meaning: `__bb_jumps__' will cause jump frequencies to be written to `bb.out'. `__bb_trace__' will cause the sequence of basic blocks to be piped into `gzip' and written to file `bbtrace.gz'. `__bb_hidecall__' will cause call instructions to be excluded from the trace. `__bb_showret__' will cause return instructions to be included in the trace.
Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the compiler. The file names for most of the dumps are made by appending a word to the source file name (e.g. `foo.c.rtl' or `foo.c.jump').
Dump all macro definitions, at the end of preprocessing, and write no output.
Dump all macro names, at the end of preprocessing.
Dump all macro definitions, at the end of preprocessing, in addition to normal output.
Dump debugging information during parsing, to standard error.
Dump after RTL generation, to `file.rtl'.
Just generate RTL for a function instead of compiling it. Usually used with `r'.
Dump after first jump optimization, to `file.jump'.
Dump after CSE (including the jump optimization that sometimes follows CSE), to `file.cse'.
Dump after loop optimization, to `file.loop'.
Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to `file.cse2'.
Dump after flow analysis, to `file.flow'.
Dump after instruction combination, to `file.combine'.
Dump after the first instruction scheduling pass, to `file.sched'.
Dump after local register allocation, to `file.lreg'.
Dump after global register allocation, to `file.greg'.
Dump after the second instruction scheduling pass, to `file.sched2'.
Dump after last jump optimization, to `file.jump2'.
Dump after delayed branch scheduling, to `file.dbr'.
Dump after conversion from registers to stack, to `file.stack'.
Produce all the dumps listed above.
Print statistics on memory usage, at the end of the run, to standard error.
Annotate the assembler output with a comment indicating which pattern and alternative was used.
When running a cross-compiler, pretend that the target machine uses the same floating point format as the host machine. This causes incorrect output of the actual floating constants, but the actual instruction sequence will probably be the same as GNU CC would make when running on the target machine.
Store the usual ``temporary'' intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling `foo.c' with `-c -save-temps' would produce files `foo.cpp' and `foo.s', as well as `foo.o'.
Print the full absolute name of the library file library that would be used when linking---and do not do anything else. With this option, GNU CC does not compile or link anything; it just prints the file name.
Same as `-print-file-name=libgcc.a'.
Like `-print-file-name', but searches for a program such as `cpp'.
These options control various sorts of optimizations:
Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
Without `-O', the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code.
Without `-O', only variables declared register are allocated in registers. The resulting compiled code is a little worse than produced by PCC without `-O'.
With `-O', the compiler tries to reduce code size and execution time.
When you specify `-O', the two options `-fthread-jumps' and `-fdefer-pop' are turned on. On machines that have delay slots, the `-fdelayed-branch' option is turned on. For those machines that can support debugging even without a frame pointer, the `-fomit-frame-pointer' option is turned on. On some machines other flags may also be turned on.
Optimize even more. Nearly all supported optimizations that do not involve a space-speed tradeoff are performed. Loop unrolling and function inlining are not done, for example. As compared to -O, this option increases both compilation time and the performance of the generated code.
Optimize yet more. This turns on everything -O2 does, along with also turning on -finline-functions.
Do not optimize.
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.
Options of the form `-fflag' specify machine-independent flags. Most flags have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. The following list shows only one form---the one which is not the default. You can figure out the other form by either removing `no-' or adding it.
Do not store floating point variables in registers. This prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have.
For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use `-ffloat-store' for such programs.
Use heuristics to compile faster (C++ only). 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.
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.
Don't make member functions inline by default merely because they are defined inside the class scope (C++ only).
Always pop the arguments to each function call as soon as that function returns. For machines which must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once.
Force memory operands to be copied into registers before doing arithmetic on them. This may produce better code by making all memory references potential common subexpressions. When they are not common subexpressions, instruction combination should eliminate the separate register-load. I am interested in hearing about the difference this makes.
Force memory address constants to be copied into registers before doing arithmetic on them. This may produce better code just as `-fforce-mem' may. I am interested in hearing about the difference this makes.
Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on most machines.
On some machines, such as the Vax, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag.
Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be worth integrating in this way.
If all calls to a given function are integrated, and the function is declared static, then GCC normally does not output the function as assembler code in its own right.
Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced.
This option is enabled by default on certain machines, usually those which have no call-preserved registers to use instead.
Even if all calls to a given function are integrated, and the function is declared static, nevertheless output a separate run-time callable version of the function.
Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.
Disable any machine-specific peephole optimizations.
This option allows GCC to violate some ANSI or IEEE rules/specifications in the interest of optimizing code for speed. For example, it allows the compiler to assume arguments to the sqrt function are non-negative numbers.
This option should never be turned on by any `-O' option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ANSI rules/specifications for math functions.
The following options control specific optimizations. The `-O2' option turns on all of these optimizations except `-funroll-loops' and `-funroll-all-loops'.
The `-O' option usually turns on the `-fthread-jumps' and `-fdelayed-branch' options, but specific machines may change the default optimizations.
You can use the following flags in the rare cases when ``fine-tuning'' of optimizations to be performed is desired.
Perform the optimizations of loop strength reduction and elimination of iteration variables.
Perform optimizations where we check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false.
Perform the optimization of loop unrolling. This is only done for loops whose number of iterations can be determined at compile time or run time.
Perform the optimization of loop unrolling. This is done for all loops. This usually makes programs run more slowly.
In common subexpression elimination, scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an if statement with an else clause, CSE will follow the jump when the condition tested is false.
This is similar to `-fcse-follow-jumps', but causes CSE to follow jumps which conditionally skip over blocks. When CSE encounters a simple if statement with no else clause, `-fcse-skip-blocks' causes CSE to follow the jump around the body of the if.
Re-run common subexpression elimination after loop optimizations has been performed.
Elide constructors when this seems plausible (C++ only). With this flag, GNU C++ initializes y directly from the call to foo without going through a temporary in the following code:
A foo (); A y = foo ();
Without this option, GNU C++ first initializes y by calling the appropriate constructor for type A; then assigns the result of foo to a temporary; and, finally, replaces the initial value of `y' with the temporary.
The default behavior (`-fno-elide-constructors') is specified by the draft ANSI C++ standard. If your program's constructors have side effects, using `-felide-constructors' can make your program act differently, since some constructor calls may be omitted.
Perform a number of minor optimizations that are relatively expensive.
If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.
If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating point instruction is required.
Similar to `-fschedule-insns', but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle.
By default, GNU CC compiles code for the same type of machine that you are using. However, it can also be installed as a cross-compiler, to compile for some other type of machine. In fact, several different configurations of GNU CC, for different target machines, can be installed side by side. Then you specify which one to use with the `-b' option.
In addition, older and newer versions of GNU CC can be installed side by side. One of them (probably the newest) will be the default, but you may sometimes wish to use another.
The argument machine specifies the target machine for compilation. This is useful when you have installed GNU CC as a cross-compiler.
The value to use for machine is the same as was specified as the machine type when configuring GNU CC as a cross-compiler. For example, if a cross-compiler was configured with `configure i386v', meaning to compile for an 80386 running System V, then you would specify `-b i386v' to run that cross compiler.
When you do not specify `-b', it normally means to compile for the same type of machine that you are using.
The argument version specifies which version of GNU CC to run. This is useful when multiple versions are installed. For example, version might be `2.0', meaning to run GNU CC version 2.0.
The default version, when you do not specify `-V', is controlled by the way GNU CC is installed. Normally, it will be a version that is recommended for general use.
MACHINE DEPENDENT OPTIONS
Each of the target machine types can have its own special options, starting with `-m', to choose among various hardware models or configurations---for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified.
Some configurations of the compiler also support additional special options, usually for command-line compatibility with other compilers on the same platform.
These are the `-m' options defined for the 68000 series:
Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems.
Generate output for a 68020 (rather than a 68000). This is the default when the compiler is configured for 68020-based systems.
Generate output containing 68881 instructions for floating point. This is the default for most 68020-based systems unless -nfp was specified when the compiler was configured.
Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems.
Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems.
Generate output for a 68040, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040.
Generate output containing Sun FPA instructions for floating point.
Generate output containing library calls for floating point. WARNING: the requisite libraries are not part of GNU CC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.
Consider type int to be 16 bits wide, like short int.
Do not use the bit-field instructions. `-m68000' implies `-mnobitfield'.
Do use the bit-field instructions. `-m68020' implies `-mbitfield'. This is the default if you use the unmodified sources.
Use a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)
The rtd instruction is supported by the 68010 and 68020 processors, but not by the 68000.
These `-m' options are defined for the Vax:
Do not output certain jump instructions (aobleq and so on) that the Unix assembler for the Vax cannot handle across long ranges.
Do output those jump instructions, on the assumption that you will assemble with the GNU assembler.
Output code for g-format floating point numbers instead of d-format.
These `-m' switches are supported on the SPARC:
Generate output containing floating point instructions. This is the default.
Generate output containing library calls for floating point. Warning: there is no GNU floating-point library for SPARC. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option.
With -mepilogue (the default), the compiler always emits code for function exit at the end of each function. Any function exit in the middle of the function (such as a return statement in C) will generate a jump to the exit code at the end of the function.
With -mno-epilogue, the compiler tries to emit exit code inline at every function exit.
These three options select variations on the SPARC architecture.
By default (unless specifically configured for the Fujitsu SPARClite), GCC generates code for the v7 variant of the SPARC architecture.
-mv8 will give you SPARC v8 code. The only difference from v7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC v8 but not in SPARC v7.
-msparclite will give you SPARClite code. This adds the integer multiply, integer divide step and scan (ffs) instructions which exist in SPARClite but not in SPARC v7.
These two options select the processor for which the code is optimised.
With -mcypress (the default), the compiler optimises code for the Cypress CY7C602 chip, as used in the SparcStation/SparcServer 3xx series. This is also appropriate for the older SparcStation 1, 2, IPX etc.
With -msupersparc the compiler optimises code for the SuperSparc cpu, as used in the SparcStation 10, 1000 and 2000 series. This flag also enables use of the full SPARC v8 instruction set.
These `-m' options are defined for the Convex:
Generate output for a C1. This is the default when the compiler is configured for a C1.
Generate output for a C2. This is the default when the compiler is configured for a C2.
Generate code which puts an argument count in the word preceding each argument list. Some nonportable Convex and Vax programs need this word. (Debuggers don't, except for functions with variable-length argument lists; this info is in the symbol table.)
Omit the argument count word. This is the default if you use the unmodified sources.
These `-m' options are defined for the AMD Am29000:
Generate code that assumes the DW bit is set, i.e., that byte and halfword operations are directly supported by the hardware. This is the default.
Generate code that assumes the DW bit is not set.
Generate code that assumes the system supports byte and halfword write operations. This is the default.
Generate code that assumes the systems does not support byte and halfword write operations. This implies `-mnodw'.
Use a small memory model that assumes that all function addresses are either within a single 256 KB segment or at an absolute address of less than 256K. This allows the call instruction to be used instead of a const, consth, calli sequence.
Do not assume that the call instruction can be used; this is the default.
Generate code for the Am29050.
Generate code for the Am29000. This is the default.
Generate references to registers gr64-gr95 instead of gr96-gr127. This option can be used when compiling kernel code that wants a set of global registers disjoint from that used by user-mode code.
Note that when this option is used, register names in `-f' flags must use the normal, user-mode, names.
Use the normal set of global registers, gr96-gr127. This is the default.
Insert a call to __msp_check after each stack adjustment. This is often used for kernel code.
These `-m' options are defined for Motorola 88K architectures:
Generate code that works well on both the m88100 and the m88110.
Generate code that works best for the m88100, but that also runs on the m88110.
Generate code that works best for the m88110, and may not run on the m88100.
Include an ident directive in the assembler output recording the source file name, compiler name and version, timestamp, and compilation flags used.
In assembler output, emit symbol names without adding an underscore character at the beginning of each name. The default is to use an underscore as prefix on each name.
Early models of the 88K architecture had problems with division by zero; in particular, many of them didn't trap. Use these options to avoid including (or to include explicitly) additional code to detect division by zero and signal an exception. All GCC configurations for the 88K use `-mcheck-zero-division' by default.
Include (or omit) additional debugging information (about registers used in each stack frame) as specified in the 88Open Object Compatibility Standard, ``OCS''. This extra information is not needed by GDB. The default for DG/UX, SVr4, and Delta 88 SVr3.2 is to include this information; other 88k configurations omit this information by default.
Force (or do not require) register values to be stored in a particular place in stack frames, as specified in OCS. The DG/UX, Delta88 SVr3.2, and BCS configurations use `-mocs-frame-position'; other 88k configurations have the default `-mno-ocs-frame-position'.
Control how to store function arguments in stack frames. `-moptimize-arg-area' saves space, but may break some debuggers (not GDB). `-mno-optimize-arg-area' conforms better to standards. By default GCC does not optimize the argument area.
num Generate smaller data references by making them relative to r0, which allows loading a value using a single instruction (rather than the usual two). You control which data references are affected by specifying num with this option. For example, if you specify `-mshort-data-512', then the data references affected are those involving displacements of less than 512 bytes. `-mshort-data-num' is not effective for num greater than 64K.
Do, or do not, generate code to guarantee sequential consistency of volatile memory references.
GNU CC always guarantees consistency by default, for the preferred processor submodel. How this is done depends on the submodel.
The m88100 processor does not reorder memory references and so always provides sequential consistency. If you use `-m88100', GNU CC does not generate any special instructions for sequential consistency.
The order of memory references made by the m88110 processor does not always match the order of the instructions requesting those references. In particular, a load instruction may execute before a preceding store instruction. Such reordering violates sequential consistency of volatile memory references, when there are multiple processors. When you use `-m88000' or `-m88110', GNU CC generates special instructions when appropriate, to force execution in the proper order.
The extra code generated to guarantee consistency may affect the performance of your application. If you know that you can safely forgo this guarantee, you may use the option `-mno-serialize-volatile'.
If you use the `-m88100' option but require sequential consistency when running on the m88110 processor, you should use `-mserialize-volatile'.
Turn on (`-msvr4') or off (`-msvr3') compiler extensions related to System V release 4 (SVr4). This controls the following:
Which variant of the assembler syntax to emit (which you can select independently using `-mversion-03.00').
`-msvr4' makes the C preprocessor recognize `#pragma weak'
`-msvr4' makes GCC issue additional declaration directives used in SVr4.
`-msvr3' is the default for all m88K configurations except the SVr4 configuration.
Include code to detect bit-shifts of more than 31 bits; respectively, trap such shifts or emit code to handle them properly. By default GCC makes no special provision for large bit shifts.
Very early models of the 88K architecture didn't have a divide instruction, so GCC avoids that instruction by default. Use this option to specify that it's safe to use the divide instruction.
In the DG/UX configuration, there are two flavors of SVr4. This option modifies -msvr4 to select whether the hybrid-COFF or real-ELF flavor is used. All other configurations ignore this option.
Warn when a function passes a struct as an argument or result. Structure-passing conventions have changed during the evolution of the C language, and are often the source of portability problems. By default, GCC issues no such warning.
These options are defined for the IBM RS6000:
Control whether or not floating-point constants go in the Table of Contents (TOC), a table of all global variable and function addresses. By default GCC puts floating-point constants there; if the TOC overflows, `-mno-fp-in-toc' will reduce the size of the TOC, which may avoid the overflow.
These `-m' options are defined for the IBM RT PC:
Use an in-line code sequence for integer multiplies. This is the default.
Call lmul$$ for integer multiples.
Generate full-size floating point data blocks, including the minimum amount of scratch space recommended by IBM. This is the default.
Do not include extra scratch space in floating point data blocks. This results in smaller code, but slower execution, since scratch space must be allocated dynamically.
Use a calling sequence incompatible with the IBM calling convention in which floating point arguments are passed in floating point registers. Note that varargs.h and stdargs.h will not work with floating point operands if this option is specified.
Use the normal calling convention for floating point arguments. This is the default.
Return structures of more than one word in memory, rather than in a register. This provides compatibility with the MetaWare HighC (hc) compiler. Use `-fpcc-struct-return' for compatibility with the Portable C Compiler (pcc).
Return some structures of more than one word in registers, when convenient. This is the default. For compatibility with the IBM-supplied compilers, use either `-fpcc-struct-return' or `-mhc-struct-return'.
These `-m' options are defined for the MIPS family of computers:
Assume the defaults for the machine type cpu-type when scheduling instructions. The default cpu-type is default, which picks the longest cycles times for any of the machines, in order that the code run at reasonable rates on all MIPS cpu's. Other choices for cpu-type are r2000, r3000, r4000, and r6000. While picking a specific cpu-type will schedule things appropriately for that particular chip, the compiler will not generate any code that does not meet level 1 of the MIPS ISA (instruction set architecture) without the -mips2 or -mips3 switches being used.
Issue instructions from level 2 of the MIPS ISA (branch likely, square root instructions). The -mcpu=r4000 or -mcpu=r6000 switch must be used in conjunction with -mips2.
Issue instructions from level 3 of the MIPS ISA (64 bit instructions). The -mcpu=r4000 switch must be used in conjunction with -mips2.
These options don't work at present.
Generate code for the MIPS assembler, and invoke mips-tfile to add normal debug information. This is the default for all platforms except for the OSF/1 reference platform, using the OSF/rose object format. If any of the -ggdb, -gstabs, or -gstabs+ switches are used, the mips-tfile program will encapsulate the stabs within MIPS ECOFF.
Generate code for the GNU assembler. This is the default on the OSF/1 reference platform, using the OSF/rose object format.
The -mrnames switch says to output code using the MIPS software names for the registers, instead of the hardware names (ie, a0 instead of $4). The GNU assembler does not support the -mrnames switch, and the MIPS assembler will be instructed to run the MIPS C preprocessor over the source file. The -mno-rnames switch is default.
The -mgpopt switch says to write all of the data declarations before the instructions in the text section, to all the MIPS assembler to generate one word memory references instead of using two words for short global or static data items. This is on by default if optimization is selected.
For each non-inline function processed, the -mstats switch causes the compiler to emit one line to the standard error file to print statistics about the program (number of registers saved, stack size, etc.).
The -mmemcpy switch makes all block moves call the appropriate string function (memcpy or bcopy) instead of possibly generating inline code.
The -mno-mips-tfile switch causes the compiler not postprocess the object file with the mips-tfile program, after the MIPS assembler has generated it to add debug support. If mips-tfile is not run, then no local variables will be available to the debugger. In addition, stage2 and stage3 objects will have the temporary file names passed to the assembler embedded in the object file, which means the objects will not compare the same.
Generate output containing library calls for floating point. WARNING: the requisite libraries are not part of GNU CC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.
Generate output containing floating point instructions. This is the default if you use the unmodified sources.
Assume that the FR bit in the status word is on, and that there are 32 64-bit floating point registers, instead of 32 32-bit floating point registers. You must also specify the -mcpu=r4000 and -mips3 switches.
Assume that there are 32 32-bit floating point registers. This is the default.
Emit (or do not emit) the .abicalls, .cpload, and .cprestore pseudo operations that some System V.4 ports use for position independent code.
The -mhalf-pic switch says to put pointers to extern references into the data section and load them up, rather than put the references in the text section. This option does not work at present. -Gnum Put global and static items less than or equal to num bytes into the small data or bss sections instead of the normal data or bss section. This allows the assembler to emit one word memory reference instructions based on the global pointer (gp or $28), instead of the normal two words used. By default, num is 8 when the MIPS assembler is used, and 0 when the GNU assembler is used. The -Gnum switch is also passed to the assembler and linker. All modules should be compiled with the same -Gnum value.
Tell the MIPS assembler to not run its preprocessor over user assembler files (with a `.s' suffix) when assembling them.
These `-m' options are defined for the Intel 80386 family of computers: -m486
Control whether or not code is optimized for a 486 instead of an 386. Code generated for a 486 will run on a 386 and vice versa.
Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GNU CC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.
On machines where a function returns floating point results in the 80387 register stack, some floating point opcodes may be emitted even if `-msoft-float' is used.
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types float and double in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU.
The option `-mno-fp-ret-in-387' causes such values to be returned in ordinary CPU registers instead.
These `-m' options are defined for the HPPA family of computers:
Generate code for a PA 1.0 processor.
Generate code for a PA 1.1 processor.
Generate code which is suitable for use in kernels. Specifically, avoid add instructions in which one of the arguments is the DP register; generate addil instructions instead. This avoids a rather serious bug in the HP-UX linker.
Generate code that can be linked against HP-UX shared libraries. This option is not fully function yet, and is not on by default for any PA target. Using this option can cause incorrect code to be generated by the compiler.
Don't generate code that will be linked against shared libraries. This is the default for all PA targets.
Generate code which allows calls to functions greater than 256K away from the caller when the caller and callee are in the same source file. Do not turn this option on unless code refuses to link with ``branch out of range errors from the linker.
Prevent floating point registers from being used in any manner. This is necessary for compiling kernels which perform lazy context switching of floating point registers. If you use this option and attempt to perform floating point operations, the compiler will abort.
Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.
Add a colon to the end of label definitions (for ELF assemblers).
These `-m' options are defined for the Intel 80960 family of computers:
Assume the defaults for the machine type cpu-type for instruction and addressing-mode availability and alignment. The default cpu-type is kb; other choices are ka, mc, ca, cf, sa, and sb.
The -mnumerics option indicates that the processor does support floating-point instructions. The -msoft-float option indicates that floating-point support should not be assumed.
Do (or do not) attempt to alter leaf procedures to be callable with the bal instruction as well as call. This will result in more efficient code for explicit calls when the bal instruction can be substituted by the assembler or linker, but less efficient code in other cases, such as calls via function pointers, or using a linker that doesn't support this optimization.
Do (or do not) make additional attempts (beyond those of the machine-independent portions of the compiler) to optimize tail-recursive calls into branches. You may not want to do this because the detection of cases where this is not valid is not totally complete. The default is -mno-tail-call.
Assume (or do not assume) that the use of a complex addressing mode is a win on this implementation of the i960. Complex addressing modes may not be worthwhile on the K-series, but they definitely are on the C-series. The default is currently -mcomplex-addr for all processors except the CB and CC.
Align code to 8-byte boundaries for faster fetching (or don't bother). Currently turned on by default for C-series implementations only.
Enable compatibility with iC960 v2.0 or v3.0.
Enable compatibility with the iC960 assembler.
Do not permit (do permit) unaligned accesses.
Enable structure-alignment compatibility with Intel's gcc release version 1.3 (based on gcc 1.37). Currently this is buggy in that #pragma align 1 is always assumed as well, and cannot be turned off.
These `-m' options are defined for the DEC Alpha implementations:
Use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in `libgcc1.c' will be used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines will issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.
Note that Alpha implementations without floating-point operations are required to have floating-point registers.
Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option.
A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.
These additional options are available on System V Release 4 for compatibility with other compilers on those systems:
On SVr4 systems, gcc accepts the option `-G' (and passes it to the system linker), for compatibility with other compilers. However, we suggest you use `-symbolic' or `-shared' as appropriate, instead of supplying linker options on the gcc command line.
Identify the versions of each tool used by the compiler, in a .ident assembler directive in the output.
Refrain from adding .ident directives to the output file (this is the default).
Search the directories dirs, and no others, for libraries specified with `-l'. You can separate directory entries in dirs from one another with colons.
Look in the directory dir to find the M4 preprocessor. The assembler uses this option.
CODE GENERATION OPTIONS
These machine-independent options control the interface conventions used in code generation.
Most of them begin with `-f'. These options have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed---the one which is not the default. You can figure out the other form by either removing `no-' or adding it.
Assume that objects reached through references are not null (C++ only).
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.
Use the same convention for returning struct and union values that is used by the usual C compiler on your system. This convention is less efficient for small structures, and on many machines it fails to be reentrant; but it has the advantage of allowing intercallability between GCC-compiled code and PCC-compiled code.
Use the convention that struct and union values are returned in registers when possible. This is more efficient for small structures than -fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GNU CC defaults to whichever convention is standard for the target. If there is no standard convention, GNU CC defaults to -fpcc-struct-return.
Allocate to an enum type only as many bytes as it needs for the declared range of possible values. Specifically, the enum type will be equivalent to the smallest integer type which has enough room.
Use the same size for double as for float .
Requests that the data and non-const variables of this compilation be shared data rather than private data. The distinction makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private data exists in one copy per process.
Allocate even uninitialized global variables in the bss section of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without extern) in two different compilations, you will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work this way.
Ignore the `#ident' directive.
Do not output global initializations (such as C++ constructors and destructors) in the form used by the GNU linker (on systems where the GNU linker is the standard method of handling them). Use this option when you want to use a non-GNU linker, which also requires using the collect2 program to make sure the system linker includes constructors and destructors. (collect2 is included in the GNU CC distribution.) For systems which must use collect2, the compiler driver gcc is configured to do this automatically.
Don't output a .size assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling `crtstuff.c'; you should not need to use it for anything else.
Put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself).
Consider all memory references through pointers to be volatile.
Consider all memory references to extern and global data items to be volatile.
If supported for the target machines, generate position-independent code, suitable for use in a shared library.
If supported for the target machine, emit position-independent code, suitable for dynamic linking, even if branches need large displacements.
Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
Treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the register reg.
Use of this flag for a register that has a fixed pervasive role in the machine's execution model, such as the stack pointer or frame pointer, will produce disastrous results.
This flag does not have a negative form, because it specifies a three-way choice.
Treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg if they use it.
Use of this flag for a register that has a fixed pervasive role in the machine's execution model, such as the stack pointer or frame pointer, will produce disastrous results.
A different sort of disaster will result from the use of this flag for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a three-way choice.
Two `#pragma' directives are supported for GNU C++, to permit using the same header file for two purposes: as a definition of interfaces to a given object class, and as the full definition of the contents of that object class.
(C++ only.) Use this directive in header files that define object classes, to save space in most of the object files that use those classes. Normally, local copies of certain information (backup copies of inline member functions, debugging information, and the internal tables that implement virtual functions) must be kept in each object file that includes class definitions. You can use this pragma to avoid such duplication. When a header file containing `#pragma interface' is included in a compilation, this auxiliary information will not be generated (unless the main input source file itself uses `#pragma implementation'). Instead, the object files will contain references to be resolved at link time.
#pragma implementation "objects.h"
(C++ only.) Use this pragma in a main input file, when you want full output from included header files to be generated (and made globally visible). The included header file, in turn, should use `#pragma interface'. Backup copies of inline member functions, debugging information, and the internal tables used to implement virtual functions are all generated in implementation files.
If you use `#pragma implementation' with no argument, it applies to an include file with the same basename as your source file; for example, in `allclass.cc', `#pragma implementation' by itself is equivalent to `#pragma implementation "allclass.h"'. Use the string argument if you want a single implementation file to include code from multiple header files.
There is no way to split up the contents of a single header file into multiple implementation files.