GCC(1) GNU Tools(27dec1991) GCC(1)
NAME
gcc, g++ - GNU project C and C++ Compiler (v2 preliminary)
SYNOPSIS
gcc [ option | filename ]...
g++ [ option | filename ]...
WARNING
The information in this man page is an extract from the full
documentation of the GNU C compiler, and is limited to the meaning of the
options.
For complete, current documentation, refer to the Info file `gcc' or the
manual Using and Porting GNU CC (for version 2.0). Both are made from
the Texinfo source file gcc.texinfo.
DESCRIPTION
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:
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 or C++; compile, assemble
.s Assembler source; assemble
.S Assembler source; preprocess, assemble
.h Preprocessor file; not usually named on command line
?? Other (unrecognized) files passed to linker.
Common cases:
.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.
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OPTIONS
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.
Overall Options
-c -S -E -o file -pipe -v -vpath -vspec -x language
Language Options
-ansi -fall-virtual -fcond-mismatch -fdollars-in-identifiers
-fenum-int-equiv -fno-asm -fno-builtin -fno-strict-prototype
-fsigned-bitfields -fsigned-char -fthis-is-variable
-funsigned-bitfields -funsigned-char -fwritable-strings
-traditional -traditional-cpp -trigraphs
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -W -Wall
-Waggregate-return -Wcast-align -Wcast-qual -Wcomment -Wconversion
-Wenum-clash -Werror -Wformat -Wid-clash-len -Wimplicit -Winline
-Wmissing-prototypes -Wparentheses -Wpointer-arith -Wreturn-type
-Wshadow -Wstrict-prototypes -Wswitch -Wtraditional -Wtrigraphs
-Wuninitialized -Wunused -Wwrite-strings
Debugging Options
-a -dletters -fpretend-float -g -gdbx -gdwarf -ggdb -gsdb -p -pg
-save-temps
Optimization Options
-fcaller-saves -fcse-follow-jumps -fdelayed-branch
-felide-constructors -fexpensive-optimizations -ffloat-store
-fforce-addr -fforce-mem -finline -finline-functions
-fkeep-inline-functions -fmemoize-lookups -fno-default-inline
-fno-defer-pop -fno-function-cse -fomit-frame-pointer
-frerun-cse-after-loop -fschedule-insns -fschedule-insns2
-fstrength-reduce -fthread-jumps -funroll-all-loops -funroll-loops
-O -O2
Preprocessor Options
-C -dD -dM -dN -Dmacro[=defn] -E -H -i file -M -MD -MM -MMD
-nostdinc -P -Umacro -undef
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Linker Options
-llibrary -nostdlib -static
Directory Options
-Bprefix -Idir -I- -Ldir
Target Options
-b machine -V version
Machine Dependent Options
M680x0 Options
-m68000 -m68020 -m68881 -mbitfield -mc68000 -mc68020 -mfpa
-mnobitfield -mrtd -mshort -msoft-float
VAX Options
-mg -mgnu -munix
SPARC Options
-mfpu -mno-epilogue
Convex Options
-margcount -mc1 -mc2 -mnoargcount
AMD29K Options
-m29000 -m29050 -mbw -mdw -mkernel-registers -mlarge -mnbw -mnodw
-msmall -mstack-check -muser-registers
M88K Options
-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-underscores
-mocs-debug-info -mocs-frame-position -moptimize-arg-area
-mshort-data-num -msvr3 -msvr4 -mtrap-large-shift
-muse-div-instruction -mversion-03.00 -mwarn-passed-structs
RS6000 Options
-mfp-in-toc -mno-fop-in-toc
RT Options
-mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs
-mfull-fp-blocks -mhc-struct-return -min-line-mul
-mminimum-fp-blocks -mnohc-struct-return
MIPS Options
-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
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i386 Options
-m486 -mno486 -msoft-float
Code Generation Options
+eN -fcall-saved-reg -fcall-used-reg -ffixed-reg -fno-common
-fno-gnu-binutils -fnonnull-objects -fpcc-struct-return -fpic -fPIC
-fshared-data -fshort-enums -fshort-double -fvolatile
OVERALL OPTIONS
-x language
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'.
-x none
Turn off any specification of a language, so that subsequent files
are handled according to their file name suffixes (as they are if
`-x' has not been used at all).
If you 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.
-c 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.
-S 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.
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-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.
GCC ignores input files which don't require preprocessing.
-o file
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.
-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.
-vpath
Print (on standard error output) all attempts at finding files,
tracing how the `-B', `-b', and `-V' options interact. Also, print
the commands executed to run the stages of compilation and version
numbers, like the `-v' option.
-vspec
Print (on standard error output) all spec's processed by the
dospec1 function in `gcc.c'. Also, print the commands executed
to run the stages of compilation and version numbers, like the `-v'
option.
-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.
LANGUAGE OPTIONS
The following options control the dialect of C that the compiler accepts:
-ansi Support all ANSI standard C programs.
This turns off certain features of GNU C that are incompatible with
ANSI C, such as the asm, inline and typeof keywords, and predefined
macros such as unix and vax that identify the type of system you
are using. It also enables the undesirable and rarely used ANSI
trigraph feature, and makes the preprocessor accept `$' as part of
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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 STRICTANSI 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.
-fno-asm
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'.
-fno-builtin
(Ignored for C++.) Don't recognize non-ANSI built-in functions.
`-ansi' also has this effect. Currently, the only function affected
is alloca.
-fno-strict-prototype
(C++ only.) Consider the declaration int foo ();. In C++, this
means that the function foo takes no arguments. In ANSI C, this is
declared int foo(void);. With the flag `-fno-strict-prototype',
declaring functions with no arguments is equivalent to declaring
its argument list to be untyped, i.e., int foo (); is equivalent to
saying int foo (...);.
-trigraphs
Support ANSI C trigraphs. The `-ansi' option implies `-trigraphs'.
-traditional
Attempt to support some aspects of traditional C compilers.
Specifically, for both C and C++ programs:
⊕ In the preprocessor, comments convert to nothing at all, rather
than to a space. This allows traditional token concatenation.
⊕ In the preprocessor, macro arguments are recognized within string
constants in a macro definition (and their values are stringified,
though without additional quote marks, when they appear in such a
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context). The preprocessor always considers a string constant to
end at a newline.
⊕ The preprocessor does not predefine the macro STDC when you use
`-traditional', but still predefinesGNUC (since the GNU
extensions indicated by GNUC are not affected by
`-traditional'). If you need to write header files that work
differently depending on whether `-traditional' is in use, by
testing both of these predefined macros you can distinguish four
situations: GNU C, traditional GNU C, other ANSI C compilers, and
other old C compilers.
⊕ String ``constants'' are not necessarily constant; they are stored
in writable space, and identical looking constants are allocated
separately. (This is the same as the effect of
`-fwritable-strings'.)
In addition, `-traditional' has these effects for C programs (not
C++):
⊕ All extern declarations take effect globally even if they are
written inside of a function definition. This includes implicit
declarations of functions.
⊕ The keywords typeof, inline, signed, const and volatile are not
recognized. (You can still use the alternative keywords such as
typeof, inline, and so on.)
⊕ Comparisons between pointers and integers are always allowed.
⊕ Integer types unsigned short and unsigned char promote to unsigned
int.
⊕ Out-of-range floating point literals are not an error.
⊕ All automatic variables not declared register are preserved by
longjmp. Ordinarily, GNU C follows ANSI C: automatic variables not
declared volatile may be clobbered.
Finally, for C++ programs only (not C), `-traditional' has one
additional effect: 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
(C++ only.) Permit the use of `$' in identifiers. (For GNU C,
this is the default, and you can forbid it with `-ansi'.)
Traditional C allowed the character `$' to form part of
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identifiers; by default, GNU C also allows this. However, ANSI C
forbids `$' in identifiers, and GNU C++ also forbids it by default
on most platforms (though on some platforms it's enabled by default
for GNU C++ as well).
-fenum-int-equiv
(C++ only.) Normally GNU C++ allows conversion of enum to int, but
not the other way around. Use this option if you want GNU C++ to
allow conversion of int to enum as well.
-fall-virtual
(C++ only.) When you use the `-fall-virtual', all member functions
(except for constructor functions and new/delete member operators)
declared in the same class with a ``method-call'' operator method
are treated as virtual functions of the given class. In effect,
all of these methods become ``implicitly virtual.''
This does not mean that all calls to these methods will be made
through the internal table of virtual functions. There are some
circumstances under which it is obvious that a call to a given
virtual function can be made directly, and in these cases the calls
still go direct.
The effect of making all methods of a class with a declared
`operator->()()' implicitly virtual using `-fall-virtual' extends
also to all non-constructor methods of any class derived from such
a class.
-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
(C++ only.) The incorporation of user-defined free store
management into C++ has made assignment to this an anachronism.
Therefore, by default GNU C++ treats the type of this in a member
function of class X to be X *const. In other words, it is illegal
to assign to this within a class member function. However, for
backwards compatibility, you can invoke the old behavior by using
`-fthis-is-variable'.
-funsigned-char
Let the type char be unsigned, like unsigned char.
Each kind of machine has a default for what char should be. It is
either like unsigned char by default or like signed char by
default.
Ideally, a portable program should always use signed char or
unsigned char when it depends on the signedness of an object. But
many programs have been written to use plain char and expect it to
be signed, or expect it to be unsigned, depending on the machines
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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-unsigned-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 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.
PREPROCESSOR OPTIONS
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.
-i file
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 `-i file' is to make the
macros defined in file available for use in the main input. The
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preprocessor evaluates any `-D' and `-U' options on the command
line before processing `-i' file.
-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.
-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.
-P Tell the preprocessor not to generate `#line' commands. Used with
the `-E' option.
-M 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'.
-MM 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 `.c' with `.d' at the end of the input 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.
-MMD Like `-MD' except mention only user header files, not system header
files.
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-H Print the name of each header file used, in addition to other
normal activities.
-Dmacro
Define macro macro with the string `1' as its definition.
-Dmacro=defn
Define macro macro as defn. All instances of `-D' on the command
line are processed before any `-U' or `-i' options.
-Umacro
Undefine macro macro. `-U' options are evaluated after all `-D'
options, but before any `-i' 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 preprocessing 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 `#define name' is included in the output.
LINKER OPTIONS
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'.
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
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using an `-l' option and specifying a file name is that `-l'
surrounds library with `lib' and `.a' and searches several
directories.
-nostdlib
Don't use the standard system libraries and startup 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. (`-g' also has this effect.) On other
systems, this option has no effect.
DIRECTORY OPTIONS
These options specify directories to search for header files, for
libraries and for 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 which was current when the compiler was
invoked. That is not exactly the same as what the preprocessor
does by default, but it is often satisfactory.
`-I-' does not inhibit the use of the standard system directories
for header files. Thus, `-I-' and `-nostdinc' are independent.
-Ldir Add directory dir to the list of directories to be searched for
`-l'.
-Bprefix
This option specifies where to find the executables, libraries 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
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`-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
GCCEXECPREFIX; if it is defined, its value is used as a prefix in
the same way. If both the `-B' option and the GCCEXECPREFIX
variable are present, the `-B' option is used first and the
environment variable value second.
WARNING OPTIONS
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:
-fsyntax-only
Check the code for syntax errors, but don't emit any output.
-w Inhibit all warning messages.
-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.
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-W 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:
foo (a)
{
if (a > 0)
return a;
}
Spurious warnings can occur because GNU CC does not realize that
certain functions (including abort and longjmp) will never return.
⊕ An expression-statement contains no side effects.
⊕ An unsigned value is compared against zero with `>' or `<='.
-Wimplicit
Warn whenever a function or parameter is implicitly declared.
-Wreturn-type
Warn whenever a function is defined with a return-type that
defaults to int. Also warn about any return statement with no
return-value in a function whose return-type is not void.
-Wunused
Warn whenever a 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.
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-Wcomment
Warn whenever a comment-start sequence `/*' appears in a comment.
-Wtrigraphs
Warn if any trigraphs are encountered (assuming they are enabled).
-Wformat
Check calls to printf and scanf, etc., to make sure that the
arguments supplied have types appropriate to the format string
specified.
-Wuninitialized
An automatic variable is used without first being initialized.
These warnings are possible only in optimizing compilation, because
they require data flow information that is computed only when
optimizing. If you don't specify `-O', you simply won't get these
warnings.
These warnings occur only for variables that are candidates for
register allocation. Therefore, they do not occur for a variable
that is declared volatile, or whose address is taken, or whose size
is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
structures, unions or arrays, even when they are in registers.
Note that there may be no warning about a variable that is used
only to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the
warnings are printed.
These warnings are made optional because GNU CC is not smart enough
to see all the reasons why the code might be correct despite
appearing to have an error. Here is one example of how this can
happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of y is always 1, 2 or 3, then x is always
10/89 Page 15
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initialized, but GNU CC doesn't know this. Here is another common
case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because savey 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.
-Wall All of the above `-W' options combined. These are all the options
which pertain to usage that we recommend avoiding and that we
believe is easy to avoid, even in conjunction with macros.
The remaining `-W...' options are not implied by `-Wall' because they
warn about constructions that we consider reasonable to use, on occasion,
in clean programs.
-Wtraditional
Warn about certain constructs that behave differently in
traditional and ANSI C.
⊕ Macro arguments occurring within string constants in the macro
body. These would substitute the argument in traditional C, but
are part of the constant in ANSI C.
⊕ A function declared external in one block and then used after the
end of the block.
⊕ A switch statement has an operand of type long.
-Wshadow
Warn whenever a local variable shadows another local variable.
-Wid-clash-len
Warn whenever two distinct identifiers match in the first len
characters. This may help you prepare a program that will compile
with certain obsolete, brain-damaged compilers.
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-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 machines where integers can only be accessed at two- or
four-byte boundaries.
-Wwrite-strings
Give string constants the type const char[length] so that copying
the address of one into a non-const char * pointer will get a
warning. These warnings will help you find at compile time code
that can try to write into a string constant, but only if you have
been very careful about using const in declarations and prototypes.
Otherwise, it will just be a nuisance; this is why we did not make
`-Wall' request these warnings.
-Wconversion
Warn if a prototype causes a type conversion that is different from
what would happen to the same argument in the absence of a
prototype. This includes conversions of fixed point to floating
and vice versa, and conversions changing the width or signedness of
a fixed point argument except when the same as the default
promotion.
-Waggregate-return
Warn if any functions that return structures or unions are defined
or called. (In languages where you can return an array, this also
elicits a warning.)
-Wstrict-prototypes
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration which specifies the
argument types.)
-Wmissing-prototypes
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. The aim is to detect global functions that
fail to be declared in header files.
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-Wenum-clash
(C++ only.) Warn when converting between different enumeration
types.
-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 doesn't 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 else the -finline-functions option was given.
-Werror
Treat warnings as errors; abort compilation after any warning.
DEBUGGING OPTIONS
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 (for DBX or SDB or DWARF). GDB also can work with this
debugging information. On most systems that use DBX format, `-g'
enables use of extra debugging information that only GDB can use;
if you want to control for certain whether to generate this
information, use `-ggdb' or `-gdbx'.
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 configured and
compiled with the capability for more than one debugging format.
-ggdb Produce debugging information in DBX format (if that is supported),
including GDB extensions.
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-gdbx Produce debugging information in DBX format (if that is supported),
without GDB extensions.
-gsdb Produce debugging information in SDB format (if that is supported).
-gdwarf
Produce debugging information in DWARF format (if that is
supported).
-glevel
-ggdblevel
-gdbxlevel
-gsdblevel
-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 don't plan to debug. This
includes descriptions of functions and external variables, but no
information about local variables and no line numbers.
-p Generate extra code to write profile information suitable for the
analysis program prof.
-pg Generate extra code to write profile information suitable for the
analysis program gprof.
-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. Note, however,
that the format of the data is not what tcov expects. Eventually
GNU gprof should be extended to process this data.
-dletters
Says to make debugging dumps during compilation at times specified
by letters. This is used for debugging the compiler. The file
names for most of the dumps are made by appending a word to the
source file name (e.g. `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.
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-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 `file.jump'.
-ds Dump after 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'.
-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.
-fpretend-float
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.
-save-temps
Store the usual ``temporary'' intermediate files permanently; place
them in the current directory and name them based on the source
file. Thus, compiling `foo.c' with `-c -save-temps' would produce
files `foo.cpp' and `foo.s', as well as `foo.o'.
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OPTIMIZATION OPTIONS
These options control various sorts of optimizations:
-O 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', `-fthread-jumps' and `-fdelayed-branch' are
turned on. On some machines other flags may also be turned on.
-O2 Highly optimize. As compared to `-O', this option will increase
both compilation time and the performance of the generated code.
All `-fflag' options that control optimization are turned on when
you specify `-O2', except `-funroll-loops' and
`-funroll-all-loops'.
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.
-ffloat-store
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.
-fmemoize-lookups
-fsave-memoized
(C++ only.) These flags are used to get the compiler to compile
programs faster using heuristics. They are not on by default since
10/89 Page 21
GCC(1) GNU Tools(27dec1991) GCC(1)
they are only effective about half the time. The other half of the
time programs compile more slowly (and take more memory).
The first time the compiler must build a call to a member function
(or reference to a data member), it must (1) determine whether the
class implements member functions of that name; (2) resolve which
member function to call (which involves figuring out what sorts of
type conversions need to be made); and (3) check the visibility of
the member function to the caller. All of this adds up to slower
compilation. Normally, the second time a call is made to that
member function (or reference to that data member), it must go
through the same lengthy process again. This means that code like
this
cout << "This " << p << " has " << n << " legs.\n";
makes six passes through all three steps. By using a software
cache, a ``hit'' significantly reduces this cost. Unfortunately,
using the cache introduces another layer of mechanisms which must
be implemented, and so incurs its own overhead.
`-fmemoize-lookups' enables the software cache.
Because access privileges (visibility) to members and member
functions may differ from one function context to the next, g++ may
need to flush the cache. With the `-fmemoize-lookups' flag, the
cache is flushed after every function that is compiled. The
`-fsave-memoized' flag enables the same software cache, but when
the compiler determines that the context of the last function
compiled would yield the same access privileges of the next
function to compile, it preserves the cache. This is most helpful
when defining many member functions for the same class: with the
exception of member functions which are friends of other classes,
each member function has exactly the same access privileges as
every other, and the cache need not be flushed.
-fno-default-inline
(C++ only.) If `-fdefault-inline' is enabled then member functions
defined inside class scope are compiled inline by default; i.e.,
you don't need to add `inline' in front of the member function
name. By popular demand, this option is now the default. To keep
GNU C++ from inlining these member functions, specify
`-fno-default-inline'.
-fno-defer-pop
Always pop the arguments to each function call as soon as that
function returns. For machines which must pop arguments after a
function call, the compiler normally lets arguments accumulate on
the stack for several function calls and pops them all at once.
-fforce-mem
Force memory operands to be copied into registers before doing
arithmetic on them. This may produce better code by making all
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GCC(1) GNU Tools(27dec1991) GCC(1)
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.
-fforce-addr
Force memory address constants to be copied into registers before
doing arithmetic on them. This may produce better code just as
`-fforce-mem' may. I am interested in hearing about the difference
this makes.
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that don't
need one. This avoids the instructions to save, set up and restore
frame pointers; it also makes an extra register available in many
functions. It also makes debugging impossible on 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 FRAMEPOINTERREQUIRED controls whether a
target machine supports this flag.
-finline
Pay attention the inline keyword. Normally the negation of this
option `-fno-inline' is used to keep the compiler from expanding
any functions inline. However, the opposite effect may be
desirable when compiling with `-g', since `-g' normally turns off
all inline function expansion.
-finline-functions
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be worth
integrating in this way.
If all calls to a given function are integrated, and the function
is declared static, then 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 machines, usually
those which 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, nevertheless output a separate run-
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GCC(1) GNU Tools(27dec1991) GCC(1)
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 strange hacks
that alter the assembler output may be confused by the
optimizations performed when this option is not used.
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.
-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
in certain cases. This is not as powerful as completely global
CSE, but not as slow either.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations
has been performed.
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-felide-constructors
(C++ only.) Use this option to instruct the compiler to be smarter
about when it can elide constructors. Without this flag, GNU C++
and cfront both generate effectively the same code for:
A foo ();
A x (foo ()); // x initialized by `foo ()', no ctor called
A y = foo (); // call to `foo ()' heads to temporary,
// y is initialized from the temporary.
Note the difference! With this flag, GNU C++ initializes `y'
directly from the call to foo () without going through a temporary.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively
expensive.
-fdelayed-branch
If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after delayed
branch instructions.
-fschedule-insns
If supported for the target machine, attempt to reorder
instructions to eliminate execution stalls due to required data
being unavailable. This helps machines that have slow floating
point or memory load instructions by allowing other instructions to
be issued until the result of the load or floating point
instruction is required.
-fschedule-insns2
Similar to `-fschedule-insns', but requests an additional pass of
instruction scheduling after register allocation has been done.
This is especially useful on machines with a relatively small
number of registers and where memory load instructions take more
than one cycle.
TARGET OPTIONS
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.
-b machine
The argument machine specifies the target machine for compilation.
This is useful when you have installed GNU CC as a cross-compiler.
10/89 Page 25
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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.
-V version
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.
These are the `-m' options defined for the 68000 series:
-m68020
-mc68020
Generate output for a 68020 (rather than a 68000). This is the
default if you use the unmodified sources.
-m68000
-mc68000
Generate output for a 68000 (rather than a 68020).
-m68881
Generate output containing 68881 instructions for floating point.
This is the default if you use the unmodified sources.
-mfpa Generate output containing Sun FPA instructions for floating point.
-msoft-float
Generate output containing library calls for floating point.
WARNING: the requisite libraries are not 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.
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-mshort
Consider type int to be 16 bits wide, like short int.
-mnobitfield
Do not use the bit-field instructions. `-m68000' implies
`-mnobitfield'.
-mbitfield
Do use the bit-field instructions. `-m68020' implies `-mbitfield'.
This is the default if you use the unmodified sources.
-mrtd Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the rtd
instruction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to pop
the arguments there.
This calling convention is incompatible with the one normally used
on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including printf); otherwise
incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
The rtd instruction is supported by the 68010 and 68020 processors,
but not by the 68000.
These `-m' options are defined for the Vax:
-munix
Do not output certain jump instructions (aobleq and so on) that the
Unix assembler for the Vax cannot handle across long ranges.
-mgnu Do output those jump instructions, on the assumption that you will
assemble with the GNU assembler.
-mg Output code for g-format floating point numbers instead of d-
format.
These `-m' switches are supported on the Sparc:
-mfpu Generate output containing floating point instructions. This is
the default if you use the unmodified sources.
-mno-epilogue
Generate separate return instructions for return statements. This
has both advantages and disadvantages; I don't recall what they
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GCC(1) GNU Tools(27dec1991) GCC(1)
are.
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 don't, except for functions with variable-
length argument lists; this info 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, i.e., that byte and
halfword 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 halfword
write operations. This is the default.
-mnbw Generate code that assumes the systems does not support byte and
halfword write operations. This implies `-mnodw'.
-msmall
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.
-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.
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-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 mspcheck after each stack adjustment. This is
often used for kernel code.
These `-m' options are defined for Motorola 88K architectures:
-mbig-pic
Emit position-independent code, suitable for dynamic linking, even
if branches need large displacements. Equivalent to the general-
use option `-fPIC'. The general-use option `-fpic', by contrast,
only emits valid 88k code if all branches involve small
displacements. GCC does not emit position-independent code by
default.
-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 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.
-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 GDB. The default for DG/UX, SVr4, and Delta 88 SVr3.2 is
10/89 Page 29
GCC(1) GNU Tools(27dec1991) GCC(1)
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
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.
-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 `-mversion03.00').
⊕ `-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
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shifts.
-muse-div-instruction
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.
-mversion-03.00
Use alternative assembler syntax for the assembler version
corresponding to SVr4, but without enabling the other features
triggered by `-svr4'. This is implied by `-svr4', is the default
for the SVr4 configuration of GCC, and is permitted by the DG/UX
configuration only if `-svr4' is also specified. The Delta 88
SVr3.2 configuration ignores this option.
-mwarn-passed-structs
Warn when a function passes a struct as an argument or result.
Structure-passing conventions have changed during the evolution of
the C language, and are often the source of portability problems.
By default, GCC issues no such warning.
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 may avoid the overflow.
These `-m' options are defined for the IBM RT PC:
-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, since scratch
space must be allocated dynamically.
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-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 machine type cpu-type when scheduling
insturctions. 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.
-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 conjuction with -mips2.
-mips3
Issue instructions from level 3 of the MIPS ISA (64 bit
instructions). The -mcpu=r4000 switch must be used in conjuction
with -mips2.
-mint64
-mlong64
-mlonglong128
These options don't work at present.
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-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 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.
-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 (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.
-mgpopt
-mno-gpopt
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.
-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 about the program (number of registers saved, stack
size, etc.).
-mmemcpy
-mno-memcpy
The -mmemcpy switch makes all block moves call the appropriate
string function (memcpy 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
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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.
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.
-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
point 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
The -mabicalls switch says to 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 (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.
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
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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.
+eN (C++ only.) control whether virtual function definitions in
classes are used to generate code, or only to define interfaces for
their callers. These options are provided for compatibility with
cfront 1.x usage; the recommended GNU C++ usage is to use #pragma
interface and #pragma implementation, instead.
With `+e0', virtual function definitions in classes are declared
extern; the declaration is used only as an interface specification,
not to generate code for the virtual functions (in this
compilation).
With `+e1', g++ actually generates the code implementing virtual
functions defined in the code, and makes them publicly visible.
-fnonnull-objects
(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 the default 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 machines it fails
to be reentrant; but it has the advantage of allowing
intercallability between GCC-compiled code and PCC-compiled code.
-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 which 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.
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-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 wish
to verify that the program will work on other systems which always
work this way.
-fvolatile
Consider all memory references through pointers to be volatile.
-fpic If supported for the target machines, generate position-independent
code, suitable for use in a shared library.
-fPIC If supported for the target machine, 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 machine-specific and are defined in the REGISTERNAMES macro in
the machine 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 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.
-fcall-saved-reg
Treat the register named reg as an allocatable 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
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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.
-fgnu-binutils
-fno-gnu-binutils
(C++ only.) `-fgnu-binutils ' (the default for most, but not all,
platforms) makes GNU C++ emit extra information for static
initialization and finalization. This information has to be passed
from the assembler to the GNU linker. Some assemblers won't pass
this information; you must either use GNU as or specify the option
`-fno-gnu-binutils'.
With `-fno-gnu-binutils', you must use the program collect (part of
the GCC distribution) for linking.
PRAGMAS
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 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
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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.
FILES
file.c C source file
file.h C header (preprocessor) file
file.i preprocessed C source file
file.C C++ source file
file.cc C++ source file
file.cxx C++ source file
file.m Objective-C source file
file.s assembly language file
file.o object file
a.out link edited output
TMPDIR/cc* temporary files
LIBDIR/cpp preprocessor
LIBDIR/cc1 compiler for C
LIBDIR/cc1plus compiler for C++
LIBDIR/collect linker front end needed on some machines
LIBDIR/libgcc.a GCC subroutine library
/lib/crt[01n].o start-up routine
LIBDIR/ccrt0 additional start-up routine for C++
/lib/libc.a standard C library, see intro(3)
/usr/include standard directory for #include files
LIBDIR/include standard gcc directory for #include files
LIBDIR/g++-include additional g++ directory for #include
LIBDIR is usually /usr/local/lib/machine/version.
TMPDIR comes from the environment variable TMPDIR (default /usr/tmp if
available, else /tmp).
SEE ALSO
cpp(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1).
`gcc', `cpp', `as',`ld', and `gdb' entries in info.
Using and Porting GNU CC (for version 2.0), Richard M. Stallman, November
1990; The C Preprocessor, Richard M. Stallman, July 1990; Using GDB: A
Guide to the GNU Source-Level Debugger, Richard M. Stallman and Roland H.
Pesch, December 1991; Using as: the GNU Assembler, Dean Elsner, Jay
Fenlason & friends, March 1991; gld: the GNU linker, Steve Chamberlain
and Roland Pesch, April 1991.
BUGS
Report bugs to bug-gcc@prep.ai.mit.edu. Bugs tend actually to be fixed
if they can be isolated, so it is in your interest to report them in such
a way that they can be easily reproduced.
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COPYING
Copyright (c) 1991 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided 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.
AUTHORS
See the GNU CC Manual for the contributors to GNU CC.
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