CC(1) GNU Tools CC(1)NAME
cc, g++ - GNU project C and C++ Compiler (v2 preliminary)
SYNOPSIS
cc [option | filename ]...
g++ [option | filename ]...
WARNING
The information in this man page is an extract from the full documenta‐
tion of the GNU C compiler, and is limited to the meaning of the
options. This man page is not kept up to date except when volunteers
want to maintain it.
For complete and 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, assem‐
bly, and linking. Source filename suffixes identify the source lan‐
guage, but which name you use for the compiler governs default assump‐
tions:
gcc assumes preprocessed (.i) files are C and assumes C style link‐
ing.
g++ assumes preprocessed (.i) files are C++ and assumes C++ style
linking.
Suffixes of source file names indicate the language and kind of pro‐
cessing 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
?? 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.
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 -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 -gstabs -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
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
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', `objec‐
tive-c', `c-header', `c++', `cpp-output', `assembler', and `as‐
sembler-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, assem‐
ble, 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 com‐
piler 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 re‐
quire 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 in‐
put 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.
-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 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, un‐
less 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', an object file for `source.suffix' in
`source.o', its assembler file in `source.s', and all prepro‐
cessed C source on the standard output.
-v Print (on the 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 com‐
piler 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.
LANGUAGE OPTIONS
The following options control the dialect of C that the compiler ac‐
cepts:
-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 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 re‐
jected gratuitously. For that, `-pedantic' is required in addi‐
tion to `-ansi'.
The preprocessor predefines a macro __STRICT_ANSI__ when you use
the `-ansi' option. Some header files may notice this macro and
refrain from declaring certain functions or defining certain
macros that the ANSI standard doesn't call for; this is to avoid
interfering with any programs that might use these names for
other things.
-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 af‐
fected 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-proto‐
type', 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 `-tri‐
graphs'.
-traditional
Attempt to support some aspects of traditional C compilers. For
details, see the GNU C Manual; the duplicate list here has been
deleted so that we won't get complaints when it is out of date.
But one note about C++ programs only (not C). `-traditional'
has one additional effect for C++: assignment to this is permit‐
ted. 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 preproces‐
sor 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'.) Tra‐
ditional C allowed the character `$' to form part of identi‐
fiers; by default, GNU C also allows this. However, ANSI C for‐
bids `$' in identifiers, and GNU C++ also forbids it by default
on most platforms (though on some platforms it's enabled by de‐
fault 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 func‐
tions (except for constructor functions and new/delete member
operators) declared in the same class with a ``method-call'' op‐
erator method are treated as virtual functions of the given
class. In effect, all of these methods become ``implicitly vir‐
tual.''
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 `op‐
erator->()()' 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 sec‐
ond and third arguments. The value of such an expression is
void.
-fthis-is-variable
(C++ only.) The incorporation of user-defined free store man‐
agement into C++ has made assignment to this an anachronism.
Therefore, by default GNU C++ treats the type of this in a mem‐
ber function of class X to be X *const. In other words, it is
illegal to assign to this within a class member function. How‐
ever, for backwards compatibility, you can invoke the old behav‐
ior 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 un‐
signed 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 ma‐
chines 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 consis‐
tent: the basic integer types such as int are signed types.
However, when you specify `-traditional', bitfields are all un‐
signed 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. `-tradition‐
al' 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 preprocessor evaluates any `-D' and `-U' options on the com‐
mand 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 explicit‐
ly.
-undef Do not predefine any nonstandard macros. (Including architec‐
ture flags).
-E Run only the C preprocessor. Preprocess all the C source files
specified and output the results to the 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 de‐
scribing 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 the standard output instead of the prepro‐
cessed C program.
`-M' implies `-E'.
-MM Like `-M' but the output mentions only the user header files in‐
cluded 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' com‐
mand.
-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.
-Dmacro
Define macro macro with the string `1' as its definition.
-Dmacro=defn
Define macro macro as defn. All instances of `-D' on the com‐
mand 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 defini‐
tions 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 li‐
brary, which is actually a file named `liblibrary.a'. The link‐
er then uses this file as if it had been specified precisely by
name.
The directories searched include several standard system direc‐
tories 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 ar‐
chive file by scanning through it for members which define sym‐
bols that have so far been referenced but not defined. However,
if the linker finds an ordinary object file rather than a li‐
brary, 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.
-nostdlib
Don't use the standard system libraries and startup files when
linking. Only the files you specify will be passed to the link‐
er.
-static
On systems that support dynamic linking, this prevents linking
with the shared libraries. On other systems, this option has no
effect.
DIRECTORY OPTIONS
These options specify directories to search for header files, for li‐
braries 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' di‐
rectives. (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 directo‐
ries for header files. Thus, `-I-' and `-nostdinc' are indepen‐
dent.
-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 `-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 di‐
rectories 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 neces‐
sary.
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_PRE‐
FIX 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; re‐
ject 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 tradi‐
tional 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 al‐
ternate keywords whose names begin and end with `__'. Pedantic
warnings are also disabled in the expression that follows __ex‐
tension__. 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. These warnings are possible only in optimizing compi‐
lation.
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 prob‐
lem.
· 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 warn‐
ing:
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 re‐
turn.
· 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 de‐
faults to int. Also warn about any return statement with no re‐
turn-value in a function whose return-type is not void.
-Wunused
Warn whenever a local variable is unused aside from its declara‐
tion, 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 enu‐
meration. (The presence of a default label prevents this warn‐
ing.) case labels outside the enumeration range also provoke
warnings when this option is used.
-Wcomment
Warn whenever a comment-start sequence `/*' appears in a com‐
ment.
-Wtrigraphs
Warn if any trigraphs are encountered (assuming they are en‐
abled).
-Wformat
Check calls to printf and scanf, etc., to make sure that the ar‐
guments 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, be‐
cause 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 vari‐
able 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 de‐
spite 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 initial‐
ized, but GNU CC doesn't know this. Here is another common
case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because save_y is used only if it is set.
Some spurious warnings can be avoided if you declare 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 op‐
tions which pertain to usage that we recommend avoiding and that
we believe are 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 occa‐
sion, in clean programs.
-Wtraditional
Warn about certain constructs that behave differently in tradi‐
tional 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 com‐
pile with certain obsolete, brain-damaged 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 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 copy‐
ing 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 de‐
fined 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 proto‐
type declaration. This warning is issued even if the definition
itself provides a prototype. The aim is to detect global func‐
tions that fail to be declared in header files.
-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 erro‐
neous 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 de‐
clared as inline, or else the -finline-functions option was giv‐
en.
-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 occasion‐
ally 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 support‐
ed), including GDB extensions.
-gdbx Produce debugging information in DBX format (if that is support‐
ed), without GDB extensions.
-gsdb Produce debugging information in SDB format (if that is support‐
ed).
-gdwarf
Produce debugging information in DWARF format (if that is sup‐
ported).
-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 back‐
traces 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 speci‐
fied 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 ad‐
dition to normal output.
-dy Dump debugging information during parsing, to the standard er‐
ror.
-dr Dump after RTL generation, to `file.rtl'.
-dx Just generate RTL for a function instead of compiling it. Usu‐
ally 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 the
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'.
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 com‐
pilation 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 `-fun‐
roll-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 pre‐
vents 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 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 func‐
tion (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. Unfortunate‐
ly, using the cache introduces another layer of mechanisms which
must be implemented, and so incurs its own overhead. `-fmemo‐
ize-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 help‐
ful 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 privi‐
leges as every other, and the cache need not be flushed.
-fno-default-inline
(C++ only.) If `-fdefault-inline' is enabled then member func‐
tions 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 de‐
fault. 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
memory references potential common subexpressions. When they
are not common subexpressions, instruction combination should
eliminate the separate register-load. I am interested in hear‐
ing about the difference this makes.
-fforce-addr
Force memory address constants to be copied into registers be‐
fore 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, be‐
cause the standard calling sequence automatically handles the
frame pointer and nothing is saved by pretending it doesn't ex‐
ist. The machine-description macro FRAME_POINTER_REQUIRED con‐
trols 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 de‐
sirable 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 func‐
tion 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 clob‐
bered 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 oth‐
erwise 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-
time callable version of the function.
-fno-function-cse
Do not put function addresses in registers; make each instruc‐
tion that calls a constant function contain the function's ad‐
dress explicitly.
This option results in less efficient code, but some strange
hacks that alter the assembler output may be confused by the op‐
timizations 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 `-fun‐
roll-all-loops'.
The `-O' option usually turns on the `-fthread-jumps' and `-fde‐
layed-branch' options, but specific machines may change the default op‐
timizations.
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 elimi‐
nation 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 des‐
tination 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 com‐
pile 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 instruc‐
tions 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.
-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 tempo‐
rary.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively ex‐
pensive.
-fdelayed-branch
If supported for the target machine, attempt to reorder instruc‐
tions to exploit instruction slots available after delayed
branch instructions.
-fschedule-insns
If supported for the target machine, attempt to reorder instruc‐
tions 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 instruc‐
tion 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 in‐
stalled 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 compila‐
tion. This is useful when you have installed GNU CC as a cross-
compiler.
The value to use for machine is the same as was specified as the
machine type when configuring GNU CC as a cross-compiler. For
example, if a cross-compiler was configured with `configure
i386v', meaning to compile for an 80386 running System V, then
you would specify `-b i386v' to run that cross compiler.
When you do not specify `-b', it normally means to compile for
the same type of machine that you are using.
-V version
The argument version specifies which version of GNU CC to run.
This is useful when multiple versions are installed. For exam‐
ple, 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 configu‐
rations—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. Nor‐
mally 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.
-mshort
Consider type int to be 16 bits wide, like short int.
-mnobitfield
Do not use the bit-field instructions. `-m68000' implies `-mno‐
bitfield'.
-mbitfield
Do use the bit-field instructions. `-m68020' implies `-mbit‐
field'. 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 in‐
struction, 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); oth‐
erwise incorrect code will be generated for calls to those func‐
tions.
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 proces‐
sors, 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 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 non-portable Convex and Vax programs
need this word. (Debuggers don't, except for functions with
variable-length argument lists; this information is in the sym‐
bol 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 address‐
es 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.
-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:
-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 com‐
pilation flags used.
-mno-underscores
In assembler output, emit symbol names without adding an under‐
score 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 op‐
tions 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 regis‐
ters used in each stack frame) as specified in the 88Open Object
Compatibility Standard, ``OCS''. This extra information is not
needed by GDB. The default for DG/UX, SVr4, and Delta 88 SVr3.2
is to include this information; other 88k configurations omit
this information by default.
-mocs-frame-position
-mno-ocs-frame-position
Force (or do not require) register values to be stored in a par‐
ticular place in stack frames, as specified in OCS. The DG/UX,
Delta88 SVr3.2, and BCS configurations use `-mocs-frame-posi‐
tion'; 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. `-mop‐
timize-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 af‐
fected 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 se‐
lect 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; respec‐
tively, 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 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 instruc‐
tion.
-mversion-03.00
Use alternative assembler syntax for the assembler version cor‐
responding to SVr4, but without enabling the other features
triggered by `-svr4'. This is implied by `-svr4', is the de‐
fault 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 prob‐
lems. 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.
-mfp-arg-in-fpregs
Use a calling sequence incompatible with the IBM calling conven‐
tion 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 compatibili‐
ty 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 schedul‐
ing instructions. The default cpu-type is default, which picks
the longest cycles times for any of the machines, in order that
the code run at reasonable rates on all MIPS cpu's. Other
choices for cpu-type are r2000, r3000, r4000, and r6000. While
picking a specific cpu-type will schedule things appropriately
for that particular chip, the compiler will not generate any
code that does not meet level 1 of the MIPS ISA (instruction set
architecture) without the -mips2 or -mips3 switches being used.
-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 instruc‐
tions). The -mcpu=r4000 switch must be used in conjunction with
-mips2.
-mint64
-mlong64
-mlonglong128
These options don't 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 plat‐
forms 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 encapsu‐
late 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 as‐
sembler 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 caus‐
es 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 as‐
sembler has generated it to add debug support. If mips-tfile is
not run, then no local variables will be available to the debug‐
ger. In addition, stage2 and stage3 objects will have the tem‐
porary 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. Nor‐
mally 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 ref‐
erences 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 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 specifica‐
tion, 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 com‐
piler must check that `a' is not null in code like the follow‐
ing:
obj &a = g ();
a.f (2);
Checking that references of this sort have non-null values re‐
quires extra code, however, and it is unnecessary for many pro‐
grams. 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 con‐
vention is less efficient for small structures, and on many ma‐
chines it fails to be reentrant; but it has the advantage of al‐
lowing intercallability between GCC-compiled code and PCC-com‐
piled 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 compila‐
tion 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 pri‐
vate 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 de‐
clared (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-indepen‐
dent 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 point‐
er, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted
are machine-specific and are defined in the REGISTER_NAMES macro
in the machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-used-reg
Treat the register named reg as an allocatable register that is
clobbered by function calls. It may be allocated for tempo‐
raries 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 vari‐
ables that live across a call. Functions compiled this way will
save and restore the register reg if they use it.
Use of this flag for a register that has a fixed pervasive role
in the machine's execution model, such as the stack pointer or
frame pointer, will produce disastrous results.
A different sort of disaster will result from the use of this
flag for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a
three-way choice.
-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 spec‐
ify 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 ob‐
ject 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 informa‐
tion, and the internal tables that implement virtual functions)
must be kept in each object file that includes class defini‐
tions. 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 imple‐
mentation'). 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 func‐
tions, 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 it‐
self 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 ALSOcpp(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, Novem‐
ber 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 El‐
sner, 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.
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 en‐
tire resulting derived work is distributed under the terms of a permis‐
sion notice identical to this one.
Permission is granted to copy and distribute translations of this manu‐
al into another language, under the above conditions for modified ver‐
sions, except that this permission notice may be included in transla‐
tions approved by the Free Software Foundation instead of in the origi‐
nal English.
AUTHORS
See the GNU CC Manual for the contributors to GNU CC.
GNU Tools 28may1992 CC(1)
