The information in this topic is an extract from the full
documentation of the GNU C compiler, and is limited to the meaning
of the options; that is, it describes only the command options
(switches) and does not describe anything beyond that. For complete
and current documentation, please see Using and Porting the GNU
CC (version 2.0) by Richard M. Stallman.
The C and C++ compilers are integrated. Both process input files
through one or more of four stages: preprocessing, compilation,
assembly, and linking. Source file-name suffixes identify the
source language, but the name you use for the compiler governs
default assumptions:
gcc
Assumes preprocessed (.i) files are C, and assumes
C-style linking.
g++
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 file names (including named .o object files and
.a archives) are passed to the linker in command-line
order.
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
cobjective-cc-headerc++cpp-outputassemblerassembler-with-cpp
-xnone
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 file name suffixes) to
tell gcc(1) where to start, and one of the options
-c, -S, or -E to say where gcc(1) is to
stop. Note that some combinations (for example, -x cpp-output
-E) instruct gcc(1) to do nothing at all.
-c
Compile or assemble the source files, but do not link. The
compiler output is an object file corresponding to each source
file. This option can also be given as --compile.
By default, gcc makes the object file name for a source
file by replacing a suffix such as .c, .i, or .s with .o. Use
-o to select another name.
Also, gcc ignores any unrecognized input files (those
that do not require compilation or assembly) with the -c
option.
-S
Stop after the stage of compilation proper; do not assemble.
The output is an assembler code file for each nonassembler input
file specified. This option can also be given as --assemble.
By default, gcc makes the assembler file name for a
source file by replacing a suffix such as .c or .i with .s. Use
-o to select another name.
The gcc utility ignores any input files that do not
require compilation.
-E
Stop after the preprocessing stage; do not run the compiler
proper. The output is preprocessed source code, which is sent to
the standard output.
The gcc utility ignores input files which do not require
preprocessing.
-ofile
Place output in file file This applies regardless to
whatever sort of output the gcc utility 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.
-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 cannot read from a pipe; but the GNU
assembler has no trouble with this.
The following options control the dialect of C that the compiler
accepts:
-ansi
Support all American National Standards Institute (ANSI)
standard C programs.
This turns off certain features of GNU C that are incompatible
with ANSI C, such as the asminline and typeof
keywords, and predefined macros such as 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,
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 does not call for. This is to
avoid interfering with any programs that might use these names for
other things.
-fno-asm
Do not recognize asminline 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.
-fno-builtin
Do not recognize built-in functions that do not begin with two
leading underscores. Currently, the functions affected include the
following: _exit(3),abort(3), abs(3),
alloc(3), cos(3), exit(3), fabs(3),
labs(3), memcmp(3), memcpy(3), sin(3), sqrt(3), strcmp(3), strcpy(3), and strlen(3).
The -ansi option prevents alloc(3) and _exit(3) from
being built-in functions.
-fhosted
Compile for a hosted environment; this implies the
-fbuiltin option, and implies that suspicious declarations
of main() should be warned about.
-ffreestanding
Compile for a free-standing environment; this implies the
-fno-builtin option, and also implies that main has
no special requirements.
-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 (C++ only). Normally, such a declaration
in C++ means that the function foo takes no arguments.
-trigraphs
Support ANSI C trigraphs. The -ansi option implies
-trigraphs.
-traditional
Attempt to support some aspects of traditional C compilers. For
details, see the GNU C Manual.
It should be noted that in 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.
-traditional-cpp
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.
-fdollars-in-identifiers
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.)
-fenum-int-equiv
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.
-fexternal-templates
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.
-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.
-fcond-mismatch
Allow conditional expressions with mismatched types in the
second and third arguments. The value of such an expression is
void.
-fthis-is-variable
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.
-funsigned-char
Let the type char be unsigned, like unsigned
char.
Each kind of computer 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 the program depends on whether an
object is signed or unsigned. 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 computers 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.
-fsigned-char
Let the type char be signed, like signed char.
Note that this is equivalent to -fno-un-signed-char,
which is the negative form of -funsigned-char. Likewise,
-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 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.
-fwritable-strings
Store string constants in the writable data segment and do not
make them unique. This is for compatibility with old programs that
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.
-aux-infofile
Write function prototypes to file.
-includefile
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 -includefile 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.
-imacrosfile
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
-imacrosfile 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.
-idirafterdir
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).
-ifoutput
Output cpp(1) conditionals with #failed.
-iprefixprefix
Specify prefix as the prefix for subsequent
-iwithprefix options.
-isystemdir
Add dir as a system include directory.
-iwithprefixdir
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.
-nostdinc
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.
-nostdinc++
Do not search for header files in the C++-specific standard
directories, but do still search the other standard directories.
(This option is used when building libg++.)
-undef
Do not predefine any nonstandard macros (including architecture
flags).
-E
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.
-C
Tell the preprocessor not to discard comments. Used with the
-E option. This option can also be given as
--comments.
-P
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(1) describing the dependencies of each object file. For
each source file, the preprocessor outputs one make(1)-rule
whose target is the object file name for that source file, and
whose dependencies are all the files #include in it. This
rule may be a single line or may be continued with \ 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 that 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.
-MD
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(1) command.
-MMD
Like -MD except mention only user header files, not
system header files.
-H
Print the name of each header file used, in addition to other
normal activities.
-Aquestion(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 computer.
This option can also given as --assert.
-Dmacro
Define macro macro with the string 1 as its
definition. Note that on Interix, the macro INTERIX is
automatically defined.
-Dmacro=defn
Define macro macro as defn (This option can also
be given as --define-macro.) All instances of -D on
the command line are processed before any -U options.
-Umacro
Undefine macro macro. -U options are evaluated
after all -D options, but before any -include and
-imacros options.
-dM
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.
-dD
Tell the preprocessor to pass all macro definitions into the
output, in their proper sequence in the rest of the output.
-dN
Like -dD except that the macro arguments and contents
are omitted. Only #definename is included in the
output.
These options come into play when the compiler links object files
into an executable output file. They are meaningless if the
compiler is not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is
considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If gcc does a link step, these object files are
used as input to the linker.
-llibrary
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.
Usually, 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.
-lobjc
You need this special case of the -l option to link an
Objective C program.
-nostartfiles
Do not use the standard system start-up files when linking. The
standard libraries are used normally.
-nostdlib
Do not use the standard system libraries and start-up files
when linking. Only the files you specify will be passed to the
linker.
-static
On systems that support dynamic linking, this prevents linking
with the shared libraries. On other systems, this option has no
effect.
-shared
Produce a shared object that can then be linked with other
objects to form an executable. Only a few systems support this
option.
-symbolic
Bind references to global symbols when building a shared
object. Warn about any unresolved references (unless overridden by
the link editor option Note that Interix does not support dynamic
linking. -Xlinker -z -Xlinker defs Only a few systems
support this option.
Tbssaddress
Set address of .bss section.
Tdataaddress
Set address of .data section.
Ttextaddress
Set address of .text section.
-Xlinkeroption
Pass option as an option to the linker. You can use this
to supply system-specific linker options that 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.
-Wl,option
Pass option as an option to the linker. If option
contains commas, it is split into multiple options at the
commas.
-usymbol
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 the directories gcc searches to find
header files, libraries, and parts of the compiler.
-Idir
Append directory dir to the list of directories searched
for include files.
-I-
Any directories you specify with -I options before the
-I- option are searched only for the case of #include
"file"; they are not searched for #include
<file>.
If additional directories are specified with -I options
after the -I-, these directories are searched for all
#include directives. (Ordinarily all-I
directories are used this way.)
In addition, the -I- option inhibits the use of the
current directory (where the current input file came from) as the
first search directory for #include "file". There is no way
to override this effect of -I-. With -I., you can
specify searching the directory that was current when the compiler
was invoked. That is not exactly the same as what the preprocessor
does by default, but it is often satisfactory.
-I- does not inhibit the use of the standard system
directories for header files. Thus, -I- and -nostdinc
are independent.
-Ldir
Add directory dir to the list of directories to be
searched for -l.
-Bprefix
This option specifies where to find the executables, libraries
and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms
cpp(1)', cc1(1)' (or, for C++, cc1plus(1)'),
as(1) and ld(1). 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: /usr/lib/gcc/ and /usr/local/lib/gcc-lib/.
If neither of these 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.
Usually, on most computers, libgcc.a is not 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 is used
second.
Incidentally, the Win32 version of gcc sets the
GCC_EXEC_PREFIX environment variable, and that value is
inherited in the Interix space. If you have both Interix
gcc(1) and Win32 gcc(1) installed, the inherited
GCC_EXEC_PRE-FIX value will cause problems. Unset it before
running gcc(1) in the Interix environment.
Warnings are diagnostic messages that report constructions that are
not inherently erroneous, but which are risky or suggest that there
may have been an error.
These options control the amount and kinds of warnings produced
by GNU CC:
-fsyntax-only
Check the code for syntax errors, but do not emit any
output.
-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. 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.
-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(3). These warnings are possible only in
optimizing compilation.
The compiler sees only the calls to setjmp(3). It cannot
know where longjmp(3) will be called. A signal handler could
call it at any point in the code. As a result, you may get a
warning even when there is no problem because longjmp(3) cannot
be called at the place that 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;
}
Spurious warnings can occur because GNU CC does not realize that
certain functions (including abort(3) and longjmp(3)) 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
<=
-Wimplicit
Warn whenever a function or parameter is implicitly
declared.
-Wmain
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.
-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 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.
-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(3), scanf(3), and so on, to
ensure 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 systems.
-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 do not specify -O, you will not 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.
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, x is
always initialized, but GNU CC does not 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 as
volatile all the functions you use that never return.
-Wparentheses
Warn if parentheses are omitted in certain contexts.
-Wtemplate-debugging
When using templates in a C++ program, warn if debugging is not
yet fully available (C++ only).
-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. This
option can also be given as --all-warnings.
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 compilers.
-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.
-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 computers 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 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
-Wall was not designed to request these warnings.
-Wconversion
Warn if a prototype causes a type conversion that differs 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 that change the width of a fixed-point
argument, or whether it is signed, except when the conversion is
the same as the default promotion.
-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 that 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 a
function.
-Wenum-clash
Warn about conversion between different enumeration types (C++
only).
-Woverloaded-virtual
(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 does not match any virtual functions
from the base class.
-Winline
Warn if a function can not be inlined, and either it was
declared as inline, or the -finline-functions option was
given.
-Werror
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:
-g
Produce debugging information in the operating system's native
format (stabs, COFF, XCOFF, or DWARF). GNU debugger (GDB) can work
with this debugging information. This option can also be given as
--debug.
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 stop responding 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; and some statements may execute in different places
because they were moved out of loops.
Nevertheless, it is 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.
-ggdb
Produce debugging information in the native format (if that is
supported), including GDB extensions if at all possible.
-gstabs
Produce debugging information in stabs format (if that is
supported), without GDB extensions. This is the format used by DBX
on most Berkeley Software Distribution (BSD) systems.
-gstabs+
Produce debugging information in stabs format (if that is
supported), using GNU extensions understood only by GDB. The use of
these extensions is likely to make other debuggers stop responding
or refuse to read the program.
-gcoff
Produce debugging information in COFF format (if that is
supported). This is the format used by symbolic debugger (SDB) on
most System V systems prior to System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that is
supported). This is the format used by the DBX debugger on IBM
RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make other
debuggers stop responding or refuse to read the program.
-gdwarf
Produce debugging information in DWARF format (if that is
supported). This is the format used by SDB on most System V Release
4 systems.
-gdwarf+
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 stop responding or refuse to read the program.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gdwarflevel
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 do not 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.
-p
Generate extra code to write profile information suitable for
the analysis program prof(1).
-pg
Generate extra code to write profile information suitable for
the analysis program gprof(1).
-a
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(1). Note, however, that the format of the data is not
what tcov(1) expects. Eventually, GNU gprof(1) should
be extended to process this data.
-ax
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 file names. 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.
-dletters
Says to make debugging dumps during compilation at times
specified by letters (This option can also be given as
--dump.) 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 (for instance, foo.c.rtl or
foo.c.jump.)
-dM
Dump all macro definitions at the end of preprocessing, and
write no output.
-dN
Dump all macro names at the end of preprocessing.
-dD
Dump all macro definitions at the end of preprocessing, in
addition to normal output.
-dy
Dump debugging information during parsing to standard
error.
-dr
Dump after RTL generation, to file.rtl.
-dx
Just generate RTL for a function instead of compiling it.
Usually used with r.
-dj
Dump after first jump optimization, to
filejump.
-ds
Dump after common subexpression (CSE), (including the jump
optimization that sometimes follows CSE), to
file.cse.
-dL
Dump after loop optimization, to file.loop.
-dt
Dump after the second CSE pass (including the jump optimization
that sometimes follows CSE), to file.cse2.
-df
Dump after flow analysis, to file.flow.
-dc
Dump after instruction combination, to
file.combine.
-dS
Dump after the first instruction scheduling pass, to
file.sched.
-dl
Dump after local register allocation, to
file.lreg.
-dg
Dump after global register allocation, to
file.greg.
-dR
Dump after the second instruction scheduling pass, to
file.sched2.
-dJ
Dump after last jump optimization, to
file.jump2'.
-dd
Dump after delayed branch scheduling, to file.dbr.
-dk
Dump after conversion from registers to stack, to
file.stack'.
-da
Produce all the dumps listed above.
-dm
Print statistics on memory usage, at the end of the run, to
standard error.
-dp
Annotate the assembler output with a comment indicating which
pattern and alternative was used.
--dumpbasebasename
Use basename as the file base name for -d dump
output .
-fpretend-float
When running a cross-compiler, pretend that the target computer
uses the same floating-point format as the host computer. 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 computer.
-save-temps
Store the usual temporary intermediate files permanently; place
them in the current directory and name them based on the source
file. Thus, compiling foo.c with -c would produce
files foo.cpp and foo.s as well as foo.o
-print-file-name=library
Print the full absolute name of the library file library
that would be used when linking--and do nothing else. With this
option, GNU CC does not compile or link anything; it just prints
the file name.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
-print-prog-name=program
Like -print-file-name, but searches for a program such
as cpp(1).
These options control various types of optimizations:
-O
-O1
Optimize. Optimizing compilation takes somewhat more time, and
a lot more memory for a large function.
Without -O, the compiler's goal is to reduce the cost of
compilation and 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 that which is produced by the Portable C Compiler (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 -fde-fer-pop are turned on. On
computers that have delay slots, the -fdelayed-branch option
is turned on. For those computers that can support debugging even
without a frame pointer, the -fomit-frame-pointer option is
turned on. On some computers other flags may also be turned
on.
-O2
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.
-O3
Optimize yet more. This turns on everything -O2 does,
along with also turning on -finline-functions.
-O0
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
computer-independent flags. Most flags have both positive and
negative forms; the negative form -ffoo would be
-fno-foo. The following list shows only one form--the one
that is not the default. You can figure out the other form by
either removing no- or adding it.
-ffloat-store
Do not store floating point variables in registers. This
prevents undesirable excess precision on computers, 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.
-fmemoize-lookups
-fsave-memoized
Use heuristics to compile faster (C++ only). These heuristics
are not enabled by default, because 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 (this involves figuring out
what sorts of type conversions must be made); and (3) check the
visibility of the member function to the caller. All of this
results in 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 long process again. This means that code
like the following:
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 can differ from one function context to the next,
g++(1) 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-default-inline
Do not make member functions inline by default merely because
they are defined inside the class scope (C++ only).
-fno-defer-pop
Always pop the arguments to each function call as soon as that
function returns. For computers that must pop arguments after a
function call, the compiler normally lets arguments accumulate on
the stack for several function calls and pops them all at
once.
-fforce-mem
Force memory operands to be copied into registers before doing
arithmetic on them. This 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.
-fforce-addr
Force memory address constants to be copied into registers
before doing arithmetic on them. This can produce better code, just
as -fforce-mem can.
-fomit-frame-pointer
Do not keep the frame pointer in a register for functions that
do not 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
computers.
On some computers, 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 does not exist.
The machine-description macro FRAME_POINTER_REQUIRED
controls whether a target computer supports this flag.
-finline-functions
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be worth
integrating in this way.
If all calls to a given function are integrated, and the
function is declared static, gcc normally does not
output the function as assembler code in its own right.
-fcaller-saves
Enable values to be allocated in registers that will be
clobbered by function calls, by emitting extra instructions to save
and restore the registers around such calls. Such allocation is
done only when it seems to result in better code than would
otherwise be produced.
This option is enabled by default on certain computers, usually
those that have no call-preserved registers to use
instead.
-fkeep-inline-functions
Even if all calls to a given function are integrated, and the
function is declared static, output a separate run-time
callable version of the function.
-fno-function-cse
Do not put function addresses in registers; make each
instruction that calls a constant function contain the function's
address explicitly.
This option results in less efficient code, but some code that
alters the assembler output can be confused by the optimizations
performed when this option is not used.
-fno-peephole
Disable any computer-specific peephole optimizations.
-ffast-math
This option allows gcc to violate some ANSI or IEEE
rules and specifications in the interest of optimizing code for
speed. or 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
because it can result in incorrect output for programs that depend
on an exact implementation of or 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 computers might
change the default optimizations.
You can use the following flags in the rare cases when
fine-tuning of optimizations to be performed is desired.
-fstrength-reduce
Perform the optimizations of loop strength reduction and
elimination of iteration variables.
-fthread-jumps
Perform optimizations where we check to see if a jump branches
to a location where another comparison subsumed by the first is
found. If so, the first branch is redirected to either the
destination of the second branch or a point immediately following
it, depending on whether the condition is known to be true or
false.
-funroll-loops
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.
-funroll-all-loops
Perform the optimization of loop unrolling. This is done for
all loops. This usually makes programs run more slowly.
-fcse-follow-jumps
In common subexpression elimination, scan through jump
instructions when the target of the jump is not reached by any
other path. or example, when CSE encounters an if statement
with an else clause, CSE will follow the jump when the
condition tested is false.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to
follow jumps that conditionally skip over blocks. When CSE
encounters a simple if statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the
body of the if.
-frerun-cse-after-loop
Rerun common subexpression elimination after loop optimizations
has been performed.
-felide-constructors
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 C++ standard. If your program's constructors
have side effects, using -felide-constructors can make your
program act differently because some constructor calls might be
omitted.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively
expensive.
-fdelayed-branch
If supported for the target computer, attempt to reorder
instructions to exploit instruction slots available after delayed
branch instructions.
-fschedule-insns
If supported for the target computer, attempt to reorder
instructions to eliminate execution stalls due to required data
being unavailable. This helps computers that have slow
floating-point or memory-load instructions by allowing other
instructions to be issued until the result of the load or
floating-point instruction is required.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional
pass of instruction scheduling after register allocation has been
done. This is especially useful on computers 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 computer that
you are using. However, it can also be installed as a
cross-compiler, to compile for some other type of computer. In
fact, several different configurations of GNU CC, for different
target computers, can be installed side by side. You then 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 might sometimes want to use another.
-bmachine
The argument machine specifies the target computer 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 computer 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 usually means to compile
for the same type of computer that you are using.
-Vversion
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.
Each target computer type can have its own special options,
starting with -m, to choose among various hardware models or
configurations; for example, 68010 vs 68020, floating co-processor
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 -m switches are supported on the SPARC:
-mfpu, -mhard-float
Generate output containing floating point instructions. This is
the default.
-mno-fpu, -msoft-float
Generate output containing library calls for floating point.
Note, however, that there is no GNU floating-point library for
SPARC. Normally, the facilities of the computer'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.
-mno-epilogue, -mepilogue
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.
-mno-v8, -mv8, msparclite
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 that 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
that exist in SPARClite but not in SPARC v7.
-mcypress, -msupersparc
These two options select the processor for which the code is
optimized.
With -mcypress (the default), the compiler optimizes code
for the Cypress CY7C602 chip, as used in the
SparcStation/SparcServer 3xx series. This is also appropriate for
the older SparcStation 1, 2, IPX and so on.
With -msupersparc the compiler optimizes code for the
SuperSparc CPU, as used in the SparcStation 10, 1000 and 2000
series. This flag also enables use of the full SPARC v8 instruction
set.
These -m options are defined for the Convex:
-mc1
Generate output for a C1. This is the default when the compiler
is configured for a C1.
-mc2
Generate output for a C2. This is the default when the compiler
is configured for a C2.
-margcount
Generate code which puts an argument count in the word
preceding each argument list. Some nonportable Convex and Vax
programs need this word. (Debuggers do not, except for functions
with variable-length argument lists; this information is in the
symbol table.)
-mnoargcount
Omit the argument count word. This is the default if you use
the unmodified sources.
These -m options are defined for the AMD Am29000:
-mdw
Generate code that assumes the DW bit is set; that is, that
byte and half-word operations are directly supported by the
hardware. This is the default.
-mnodw
Generate code that assumes the DW bit is not set.
-mbw
Generate code that assumes the system supports byte and
half-word write operations. This is the default.
-mnbw
Generate code that assumes the systems does not support byte
and half-word write operations. This implies -mnodw.
-msmall
Use a small memory model that assumes all function addresses
are either within a single 256 KB segment or at an absolute address
of less than 256 KB. This allows the call instruction to be
used instead of a constconsthcalli
sequence.
-mlarge
Do not assume that the call instruction can be used;
this is the default.
-m29050
Generate code for the Am29050.
-m29000
Generate code for the Am29000. This is the default.
-mkernel-registers
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.
-muser-registers
Use the normal set of global registers, gr96-gr127 This
is the default.
-mstack-check
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:
-m88000
Generate code that works well on both the m88100 and the
m88110.
-m88100
Generate code that works best for the m88100, but that also
runs on the m88110.
-m88110
Generate code that works best for the m88110, and may not run
on the m88100.
-midentify-revision
Include an ident directive in the assembler output
recording the source file name, compiler name and version,
timestamp, and compilation flags used.
-mno-underscores
In assembler output, emit symbol names without adding an
underscore character at the beginning of each name. The default is
to use an underscore as prefix on each name.
-mno-check-zero-division
-mcheck-zero-division
Early models of the 88K architecture had problems with division
by zero. In particular, many of them did not 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.
-mocs-debug-info
-mno-ocs-debug-info
Include (or omit) additional debugging information (about
registers used in each stack frame) as specified in the 88Open
Object Compatibility Standard, OCS. This extra information is not
needed by GNU Debugger (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.
-mocs-frame-position
-mno-ocs-frame-position
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.
-moptimize-arg-area
-mno-optimize-arg-area
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.
-mshort-data-num
Generate smaller data references by making them relative to
r0, which allows loading a value using a single instruction
(rather than the usual two). You can control which data references
are affected by specifying num with this option. For
example, if you specify -mshort-data-512, the data
references affected are those involving displacements of less than
512 bytes. -mshort-data-num is not effective for
num greater than 64 KB.
-mserialize-volatile
-mno-serialize-volatile
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
it 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 can 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 might affect
the performance of your application. If you know that you can
safely forgo this guarantee, you can 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.
-msvr4
-msvr3
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.
-mtrap-large-shift
-mhandle-large-shift
Include code to detect bit-shifts of more than 31 bits;
respectively, trap such shifts or emit code to handle them
properly. By default, gcc makes no special provision for
large bit shifts.
-muse-div-instruction
Very early models of the 88K architecture did not have a divide
instruction, so gcc avoids that instruction by default. Use
this option to specify that it is safe to use the divide
instruction.
-mversion-03.00
In the DG/UX configuration, there are two versions of SVr4.
This option modifies -msvr4 to select whether the
hybrid-COFF or real-ELF version is used. All other configurations
ignore this option.
-mwarn-passed-structs
Warn when a function passes a structure 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:
-mfp-in-toc
-mno-fp-in-toc
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 might prevent the overflow.
These -m options are defined for the IBM RT Personal
Computer:
-min-line-mul
Use an in-line code sequence for integer multiplies. This is
the default.
-mcall-lib-mul
Call lmul$$ for integer multiples.
-mfull-fp-blocks
Generate full-size floating-point data blocks, including the
minimum amount of scratch space recommended by IBM. This is the
default.
-mminimum-fp-blocks
Do not include extra scratch space in floating-point data
blocks. This results in smaller code but slower execution because
scratch space must be allocated dynamically.
-mfp-arg-in-fpregs
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.
-mfp-arg-in-gregs
Use the normal calling convention for floating-point arguments.
This is the default.
-mhc-struct-return
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).
-mnohc-struct-return
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:
-mcpu=cpu-type
Assume the defaults for the computer type cpu-type when
scheduling instructions. The default cpu-type is
default, which picks the longest cycles times for any of the
computers so that the code runs at reasonable rates on all MIPS
CPUs.Other choices for cpu-type are r2000,
r3000, r4000, and r6000. Although 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 instruction set architecture
(ISA) without the -mips2 or -mips3 switches being
used.
-mips2
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.
-mips3
Issue instructions from level 3 of the MIPS ISA (64-bit
instructions). The -mcpu=r4000 switch must be used in
conjunction with -mips2.
-mint64
-mlong64
-mlonglong128
These options do not work at present.
-mmips-as
Generate code for the MIPS assembler, and invoke
mips-tfile to add normal debug information. This is the
default for all platforms except 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.
-mgas
Generate code for the GNU assembler. This is the default on the
OSF/1 reference platform, using the OSF/rose object format.
-mrnames
-mno-rnames
The -mrnames switch says to output code using the MIPS
software names for the registers, instead of the hardware names
(that is, it uses 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 the default.
-mgpopt
-mno-gpopt
The -mgpopt switch says to write all of the data
declarations before the instructions in the text section. This
allows 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.
-mstats
-mno-stats
For each non-inline function processed, the -mstats
switch causes the compiler to emit one line to the standard error
file to print statistics, such as number of registers saved, and
stack size, about the program.
-mmemcpy
-mno-memcpy
The -mmemcpy switch makes all block moves call the
appropriate string function or bcopy) instead of possibly
generating inline code.
-mmips-tfile
-mno-mips-tfile
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.
-msoft-float
Generate output containing library calls for floating point.
Note that the requisite libraries are not part of GNU CC. Normally
the facilities of the computer'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.
-mhard-float
Generate output containing floating-point instructions. This is
the default if you use the unmodified sources.
-mfp64
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 -oint registers. You must also specify the
-mcpu=r4000 and -mips3 switches.
-mfp32
Assume that there are 32 32-bit floating-point registers. This
is the default.
-mabicalls
-mno-abicalls
Emit (or do not emit) the .abicalls, .cpload, and
.cprestore pseudo operations that some System V.4 ports use
for position-independent code.
-mhalf-pic
-mno-half-pic
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 or
$28), instead of the normal two words used. By default,
num is eight when the MIPS assembler is used, and zero 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.
-nocpp
Tell the MIPS assembler to not run it's 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
-mno-486
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.
-msoft-float
Generate output containing library calls for floating point.
Note that the requisite libraries are not part of GNU CC. Normally,
the facilities of the computer'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.
On computers where a function returns floating-point results in
the 80387 register stack, some floating-point op-codes may be
emitted even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the floating-point unit (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 Hewlett-Packard
Precision Architecture (HPPA) family of computers:
-mpa-risc-1-0
Generate code for a PA 1.0 processor.
-mpa-risc-1-1
Generate code for a PA 1.1 processor.
-mkernel
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.
-mshared-libs
Generate code that can be linked against HP-UX shared
libraries. This option is not yet fully functional, and is not on
by default for any PA target. Using this option can cause incorrect
code to be generated by the compiler. Note that Interix does not
support dynamic linking.
-mno-shared-libs
Do not generate code that will be linked against shared
libraries. This is the default for all PA targets.
-mlong-calls
Generate code that allows calls to functions greater than 256
KB away from the caller when the caller and call recipient 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.
-mdisable-fpregs
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.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This
prevents some rather obscure problems when compiling MIG generated
code under MACH.
-mtrailing-colon
Add a colon to the end of label definitions (for ELF
assemblers).
These -m options are defined for the Intel 80960 family
of computers:
-mcpu-type
Assume the defaults for the computer 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.
-mnumerics
-msoft-float
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.
-mleaf-procedures
-mno-leaf-procedures
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
through function pointers, or using a linker that does not support
this optimization.
-mtail-call
-mno-tail-call
Do (or do not) make additional attempts (beyond those of the
computer-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
complete. The default is -mno-tail-call.
-mcomplex-addr
-mno-complex-addr
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 might 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.
-mcode-align
-mno-code-align
Align code to 8-byte boundaries for faster fetching (or do not
bother). Currently turned on by default for C-series
implementations only.
-mic-compat
-mic2.0-compat
-mic3.0-compat
Enable compatibility with iC960 v2.0 or v3.0.
-masm-compat
-mintel-asm
Enable compatibility with the iC960 assembler.
-mstrict-align
-mno-strict-align
Do not permit (do permit) unaligned accesses.
-mold-align
Enable structure-alignment compatibility with Intel's
gcc release version 1.3 (based on gcc 1.37). Currently, this
is problematic: #pragma is always assumed as well, and
cannot be turned off.
These -m options are defined for the DEC Alpha
implementations:
-mno-soft-float
-msoft-float
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.
-mfp-reg
-mno-fp-regs
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:
-G
On SVr4 systems, gcc(1) accepts the option -G
(and passes it to the system linker), for compatibility with other
compilers. However, it might be best to use -symbolic or
-shared as appropriate, instead of supplying linker options
on the gcc(1) command line.
-Qy
Identify the versions of each tool used by the compiler, in a
.ident assembler directive in the output.
-Qn
Refrain from adding .ident directives to the output file
(this is the default).
-YP,dirs
Search the directories dirs and no others for libraries
specified with -l. You can separate directory entries in
dirs from one another with colons.
-Ym,dir
Look in the directory dir to find the M4 preprocessor.
The assembler uses this option.
These computer-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.
-fnonnull-objects
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 does not require checking.
-fpcc-struct-return
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 computers it fails to be reentrant, but it has the
advantage of allowing intercallability between gcc-compiled
code and pcc-compiled code.
-freg-struct-return
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. f there is no standard convention, GNU
CC defaults to -fpcc-struct-return.
-fshort-enums
Allocate to an enum type only as many bytes as it needs
for the declared range of possible values. Specifically, the
enum type will be equivalent to the smallest integer type
that has enough room.
-fshort-double
Use the same size for double as for float.
-fshared-data
Requests that the data and non-const variables of this
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.
-fno-common
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
want to verify that the program will work on other systems that
always work this way.
-fno-ident
Ignore the #ident directive.
-fno-gnu-linker
Do not output global initializations (such as C++ constructors
and destructors) in the form used by the GNU linker (on systems
where the GNU linker is the standard method of handling them). Use
this option when you want to use a non-GNU linker, which also
requires using the collect2(1) program to make sure the
system linker includes constructors and destructors. (collect2(1) is included in the GNU CC
distribution.) For systems that must use collect2(1), the
compiler driver gcc(1) is configured to do this
automatically.
-finhibit-size-directive
Do not output a .size assembler directive, or anything
else that would cause trouble if the function is split in the
middle, and the two halves are placed at locations far apart in
memory. This option is used when compiling crtstuff.c; you
should not need to use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code
to make it more readable. This option is generally only of use to
those who actually need to read the generated assembly code
(perhaps while debugging the compiler itself).
-fvolatile
Consider all memory references through pointers to be
volatile.
-fvolatile-global
Consider all memory references to extern and global data items
to be volatile.
-fpic
If supported for the target computers, generate
position-independent code that is suitable for use in a shared
library.
-fPIC
If supported for the target computer, emit position-independent
code, suitable for dynamic linking, even if branches need large
displacements.
-ffixed-reg
Treat the register named reg as a fixed register.
Generated code should never refer to it (except perhaps as a stack
pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names
accepted are computer-specific and are defined in the
REGISTER_NAMES macro in the computer description macro
file.
This flag does not have a negative form because it specifies a
three-way choice.
-fcall-used-reg
Treat the register named reg as an allocatable 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 computer'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.
-fcall-saved-reg
Treat the register named reg as an allocatable register
saved by functions. It can 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 computer'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.
#pragma interface
(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
#pragma implementationobjects.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 method for splitting up the contents of a single
header file into multiple implementation files.
LIBDIR is usually
/usr/local/lib/machine/version For Interix, it
is /usr/contrib/machine/version;
machine is i386-pc-interix. TMPDIR comes from the environment variable TMPDIR
(default /usr/tmp if available; otherwise, it is
/tmp).
Using and Porting GNU CC (for version 2.0) Richard M.
Stallman; The C Preprocessor Richard M. Stallman;
Debugging with GDB: the GNU Source-Level Debugger Richard M.
Stallman and Roland H. Pesch; Using as: the GNU Assembler
Dean Elsner, Jay Fenlason & friends; ld: the GNU linker
Steve Chamberlain and Roland Pesch.
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
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 this permission notice may be
included in translations approved by the Free Software Foundation
instead of in the original English.