@c Copyright (C) 1988, 1989, 1992 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node Invoking GCC, Installation, Boycott, Top
@chapter GNU CC Command Options
@cindex GNU CC command options
@cindex command options
@cindex options, GNU CC command

When you invoke GNU CC, it normally does preprocessing, compilation,
assembly and linking.  The ``overall options'' allow you to stop this
process at an intermediate stage.  For example, the @samp{-c} option
says not to run the linker.  Then the output consists of object files
output by the assembler.

Other options are passed on to one stage of processing.  Some options
control the preprocessor and others the compiler itself.  Yet other
options control the assembler and linker; most of these are not
documented here, since you rarely need to use any of them.

@cindex grouping options
@cindex options, grouping
The GNU C compiler uses a command syntax much like the Unix C compiler.
The @code{gcc} program accepts options and file names as operands.
Multiple single-letter options may @emph{not} be grouped: @samp{-dr} is
very different from @w{@samp{-d -r}}.

@cindex order of options
@cindex options, order
You can mix options and other arguments.  For the most part, the order
you use doesn't matter; @code{gcc} reorders the command-line options so
that the choices specified by option flags are applied to all input
files.  Order does matter when you use several options of the same kind;
for example, if you specify @samp{-L} more than once, the directories
are searched in the order specified.

Many options have long names starting with @samp{-f} or with
@samp{-W}---for example, @samp{-fforce-mem},
@samp{-fstrength-reduce}, @samp{-Wformat} and so on.  Most of
these have both positive and negative forms; the negative form of
@samp{-ffoo} would be @samp{-fno-foo}.  This manual documents
only one of these two forms, whichever one is not the default.

Here is a summary of all the options, grouped by type.  Explanations are
in the following sections.

@table @emph
@item Overall Options
@xref{Overall Options,,Options Controlling the Kind of Output}.
@example
-c  -S  -E  -o @var{file}  -pipe  -v  -x @var{language}
@end example

@item Language Options
@xref{Dialect Options,,Options Controlling Dialect}.
@example
-ansi  -fbuiltin  -fcond-mismatch  -fno-asm 
-fsigned-bitfields  -fsigned-char 
-funsigned-bitfields  -funsigned-char  -fwritable-strings
-traditional  -traditional-cpp  -trigraphs
@end example

@item Warning Options
@xref{Warning Options,,Options to Request or Suppress Warnings}.
@example
-fsyntax-only  -pedantic  -pedantic-errors
-w  -W  -Wall  -Waggregate-return 
-Wcast-align  -Wcast-qual  -Wcomment  -Wconversion  -Werror
-Wformat  -Wid-clash-@var{len}  -Wimplicit  -Wmissing-prototypes
-Wno-parentheses  -Wpointer-arith  -Wreturn-type  -Wshadow
-Wstrict-prototypes  -Wswitch  -Wtraditional  -Wtrigraphs
-Wuninitialized  -Wunused  -Wwrite-strings  -Wchar-subscripts
@end example

@item Debugging Options
@xref{Debugging Options,,Options for Debugging Your Program or GCC}.
@example
-a  -d@var{letters}  -fpretend-float 
-g  -ggdb  -gdwarf  -gstabs  -gstabs+  -gcoff
-p  -pg  -save-temps
@end example

@item Optimization Options
@xref{Optimize Options,,Options that Control Optimization}.
@example
-fcaller-saves  -fcse-follow-jumps  -fdelayed-branch
-fexpensive-optimizations  -ffloat-store  -fforce-addr  -fforce-mem
-finline  -finline-functions  -fkeep-inline-functions
-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
@end example

@item Preprocessor Options
@xref{Preprocessor Options,,Options Controlling the Preprocessor}.
@example
-C  -dD  -dM  -dN
-D@var{macro}@r{[}=@var{defn}@r{]}  -E  -H
-include @var{file}  -imacros @var{file}
-M  -MD  -MM  -MMD  -nostdinc  -P  -trigraphs  -U@var{macro}
@end example

@item Linker Options
@xref{Link Options,,Options for Linking}.
@example
@var{object-file-name}
-l@var{library}  -nostdlib  -static
@end example

@item Directory Options
@xref{Directory Options,,Options for Directory Search}.
@example
-B@var{prefix}  -I@var{dir}  -I-  -L@var{dir}
@end example

@item Target Options
@xref{Target Options,,Target Machine and Compiler Version}.
@example
-b @var{machine}  -V @var{version}
@end example

@item Machine Dependent Options
@xref{Submodel Options,,Hardware Models and Configurations}.
@example
@emph{M680x0 Options}
-m68000 -m68020 -m68881 -mbitfield -mc68000 -mc68020 -mfpa
-mnobitfield -mrtd -mshort -msoft-float

@emph{VAX Options}
-mg -mgnu -munix

@emph{SPARC Options}
-mfpu -mno-epilogue

@emph{Convex Options}
-margcount -mc1 -mc2 -mnoargcount

@emph{AMD29K Options}
-m29000 -m29050 -mbw -mdw -mkernel-registers -mlarge 
-mnbw -mnodw -msmall -mstack-check -muser-registers

@emph{M88K Options}
-m88000 -m88100 -m88110 -mbig-pic -mcheck-zero-division
-mhandle-large-shift -midentify-revision
-mno-check-zero-division -mno-ocs-debug-info
-mno-ocs-frame-position -mno-optimize-arg-area -mno-underscores
-mocs-debug-info -mocs-frame-position -moptimize-arg-area
-mshort-data-@var{num} -msvr3 -msvr4 -mtrap-large-shift
-muse-div-instruction -mversion-03.00 -mwarn-passed-structs

@emph{RS/6000 Options}
-mfp-in-toc -mno-fop-in-toc

@emph{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

@emph{MIPS Options}
-mcpu=@var{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 @var{num}
@end example

@item Code Generation Options
@xref{Code Gen Options,,Options for Code Generation Conventions}.
@example
-fcall-saved-@var{reg}  -fcall-used-@var{reg}  -ffixed-@var{reg}
-fno-common  -fpcc-struct-return  -fpic  -fPIC  -fshared-data
-fshort-enums  -fshort-double  -fvolatile
@end example

@end table

@menu
* Overall Options::     Controlling the kind of output:
                        an executable, object files, assembler files,
                        or preprocessed source.
* Dialect Options::     Controlling the variant of C language compiled.
* Warning Options::     How picky should the compiler be?
* Debugging Options::   Symbol tables, measurements, and debugging dumps.
* Optimize Options::    How much optimization?
* Preprocessor Options:: Controlling header files and macro definitions.
                         Also, getting dependency information for Make.
* Link Options::        Specifying libraries and so on.
* Directory Options::   Where to find header files and libraries.
                        Where to find the compiler executable files.
* Target Options::      Running a cross-compiler, or an old version of GNU CC.
* Submodel Options::    Specifying minor hardware or convention variations,
                        such as 68010 vs 68020.
* Code Gen Options::    Specifying conventions for function calls, data layout
                        and register usage.
* Environment Variables:: Env vars that affect GNU CC.
@end menu

@node Overall Options, Dialect Options, Invoking GCC, Invoking GCC
@section Options Controlling the Kind of Output

Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order.  The first three
stages apply to an individual source file, and end by producing an
object file; linking combines all the object files (those newly
compiled, and those specified as input) into an executable file.

@cindex file name suffix
For any given input file, the file name suffix determines what kind of
compilation is done:

@table @code
@item @var{file}.c
C source code which must be preprocessed.

@item @var{file}.i
C source code which should not be preprocessed.

@item @var{file}.m
Objective-C source code

@item @var{file}.h
C header file (not to be compiled or linked).

@item @var{file}.cc
@itemx @var{file}.cxx
@itemx @var{file}.C
C++ source code which must be preprocessed.

@item @var{file}.s 
Assembler code.

@item @var{file}.S
Assembler code which must be preprocessed.

@item @var{other}
An object file to be fed straight into linking.
Any file name with no recognized suffix is treated this way.
@end table

You can specify the input language explicitly with the @samp{-x} option:

@table @code
@item -x @var{language}
Specify explicitly the @var{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 @samp{-x} option.  Possible values of @var{language} are
@samp{c}, @samp{objective-c}, @samp{c-header}, @samp{c++},
@samp{cpp-output}, @samp{assembler}, and @samp{assembler-with-cpp}.

@item -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 @samp{-x}
has not been used at all).
@end table

If you only want some of the stages of compilation, you can use
@samp{-x} (or filename suffixes) to tell @code{gcc} where to start, and
one of the options @samp{-c}, @samp{-S}, or @samp{-E} to say where
@code{gcc} is to stop.  Note that some combinations (for example,
@samp{-x cpp-output -E} instruct @code{gcc} to do nothing at all.

@table @code
@item -c
Compile or assemble the source files, but do not link.  The linking
stage simply is not done.  The ultimate output is in the form of an
object file for each source file.

By default, the object file name for a source file is made by replacing
the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.

Unrecognized input files, not requiring compilation or assembly, are
ignored.

@item -S
Stop after the stage of compilation proper; do not assemble.  The output
is in the form of an assembler code file for each non-assembler input
file specified.

By default, the assembler file name for a source file is made by
replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.

Input files that don't require compilation are ignored.

@item -E
Stop after the preprocessing stage; do not run the compiler proper.  The
output is in the form of preprocessed source code, which is sent to the
standard output.

Input files which don't require preprocessing are ignored.

@cindex output file option
@item -o @var{file}
Place output in file @var{file}.  This applies regardless to whatever
sort of output is being produced, whether it be an executable file,
an object file, an assembler file or preprocessed C code.

Since only one output file can be specified, it does not make sense to
use @samp{-o} when compiling more than one input file, unless you are
producing an executable file as output.

If @samp{-o} is not specified, the default is to put an executable file
in @file{a.out}, the object file for @file{@var{source}.@var{suffix}} in
@file{@var{source}.o}, its assembler file in @file{@var{source}.s}, and
all preprocessed C source on standard output.@refill

@item -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.

@item -pipe
Use pipes rather than temporary files for communication between the
various stages of compilation.  This fails to work on some systems where
the assembler is unable to read from a pipe; but the GNU assembler has
no trouble.
@end table

@node Dialect Options, Warning Options, Overall Options, Invoking GCC
@section Options Controlling Dialect
@cindex dialect options
@cindex language dialect options
@cindex options, dialect

The following options control the dialect of C that the compiler
accepts:

@table @code
@cindex ANSI support
@item -ansi
Support all ANSI standard C programs.

This turns off certain features of GNU C that are incompatible with ANSI
C, such as the @code{asm}, @code{inline} and @code{typeof} keywords, and
predefined macros such as @code{unix} and @code{vax} that identify the
type of system you are using.  It also enables the undesirable and
rarely used ANSI trigraph feature, and disallows @samp{$} as part of
identifiers.

The alternate keywords @code{__asm__}, @code{__extension__},
@code{__inline__} and @code{__typeof__} continue to work despite
@samp{-ansi}.  You would not want to use them in an ANSI C program, of
course, but it useful to put them in header files that might be included
in compilations done with @samp{-ansi}.  Alternate predefined macros
such as @code{__unix__} and @code{__vax__} are also available, with or
without @samp{-ansi}.

The @samp{-ansi} option does not cause non-ANSI programs to be
rejected gratuitously.  For that, @samp{-pedantic} is required in
addition to @samp{-ansi}.  @xref{Warning Options}.

The macro @code{__STRICT_ANSI__} is predefined when the @samp{-ansi}
option is used.  Some header files may notice this macro and refrain
from declaring certain functions or defining certain macros that the
ANSI standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.

@item -fno-asm
Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
keyword.  These words may then be used as identifiers.  You can
use @code{__asm__}, @code{__inline__} and @code{__typeof__} instead.
@samp{-ansi} implies @samp{-fno-asm}.

@item -fno-builtin
Don't recognize non-ANSI built-in functions.  @samp{-ansi} also has this
effect. Currently, the only function affected is @code{alloca}.

@item -trigraphs
Support ANSI C trigraphs.  You don't want to know about this
brain-damage.  The @samp{-ansi} option implies @samp{-trigraphs}.

@cindex traditional C language
@cindex C language, traditional
@item -traditional
Attempt to support some aspects of traditional C compilers.
Specifically:

@itemize @bullet
@item
All @code{extern} declarations take effect globally even if they
are written inside of a function definition.  This includes implicit
declarations of functions.

@item
The keywords @code{typeof}, @code{inline}, @code{signed}, @code{const}
and @code{volatile} are not recognized.  (You can still use the
alternative keywords such as @code{__typeof__}, @code{__inline__}, and
so on.)

@item
Comparisons between pointers and integers are always allowed.

@item
Integer types @code{unsigned short} and @code{unsigned char} promote
to @code{unsigned int}.

@item
Out-of-range floating point literals are not an error.

@item
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
@samp{-fwritable-strings}.)

@cindex @code{longjmp} and automatic variables
@item
All automatic variables not declared @code{register} are preserved by
@code{longjmp}.  Ordinarily, GNU C follows ANSI C: automatic variables
not declared @code{volatile} may be clobbered.

@item
In the preprocessor, comments convert to nothing at all, rather than
to a space.  This allows traditional token concatenation.

@item
In the preprocessor, macro arguments are recognized within string
constants in a macro definition (and their values are stringified,
though without additional quote marks, when they appear in such a
context).  The preprocessor always considers a string constant to end
at a newline.

@item
The predefined macro @code{__STDC__} is not defined when you use
@samp{-traditional}, but @code{__GNUC__} is (since the GNU extensions
which @code{__GNUC__} indicates are not affected by
@samp{-traditional}).  If you need to write header files that work
differently depending on whether @samp{-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.
@end itemize

@item -traditional-cpp
Attempt to support some aspects of traditional C preprocessors.
This includes the last three items in the table immediately above,
but none of the other effects of @samp{-traditional}.

@item -fcond-mismatch
Allow conditional expressions with mismatched types in the second and
third arguments.  The value of such an expression is void.

@item -funsigned-char
Let the type @code{char} be unsigned, like @code{unsigned char}.

Each kind of machine has a default for what @code{char} should
be.  It is either like @code{unsigned char} by default or like
@code{signed char} by default.

Ideally, a portable program should always use @code{signed char} or
@code{unsigned char} when it depends on the signedness of an object.
But many programs have been written to use plain @code{char} and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for.  This option, and its inverse, let you
make such a program work with the opposite default.

The type @code{char} is always a distinct type from each of
@code{signed char} or @code{unsigned char}, even though its behavior
is always just like one of those two.

@item -fsigned-char
Let the type @code{char} be signed, like @code{signed char}.

Note that this is equivalent to @samp{-fno-unsigned-char}, which is
the negative form of @samp{-funsigned-char}.  Likewise,
@samp{-fno-signed-char} is equivalent to @samp{-funsigned-char}.

@item -fsigned-bitfields
@itemx -funsigned-bitfields
@itemx -fno-signed-bitfields
@itemx -fno-unsigned-bitfields
These options control whether a bitfield is signed or unsigned, when the
declaration does not use either @code{signed} or @code{unsigned}.  By
default, such a bitfield is signed, because this is consistent: the
basic integer types such as @code{int} are signed types.

However, when @samp{-traditional} is used, bitfields are all unsigned
no matter what.

@item -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.  @samp{-traditional} also has this
effect.

Writing into string constants is a very bad idea; ``constants'' should
be constant.
@end table

@node Warning Options, Debugging Options, Dialect Options, Invoking GCC
@section Options to Request or Suppress Warnings
@cindex options to control warnings
@cindex warning messages
@cindex messages, warning
@cindex suppressing warnings

Warnings are diagnostic messages that report constructions which
are not inherently erroneous but which are risky or suggest there
may have been an error.

You can request many specific warnings with options beginning @samp{-W},
for example @samp{-Wimplicit} to request warnings on implicit
declarations.  Each of these specific warning options also has a
negative form beginning @samp{-Wno-} to turn off warnings;
for example, @samp{-Wno-implicit}.  This manual lists only one of the
two forms, whichever is not the default.

These options control the amount and kinds of warnings produced by GNU
CC:

@table @code
@cindex syntax checking
@item -fsyntax-only
Check the code for syntax errors, but don't emit any output.

@item -w
Inhibit all warning messages.

@item -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 @samp{-ansi}).  However,
without this option, certain GNU extensions and traditional C features
are supported as well.  With this option, they are rejected.

@samp{-pedantic} does not cause warning messages for use of the
alternate keywords whose names begin and end with @samp{__}.  Pedantic
warnings are also disabled in the expression that follows
@code{__extension__}.  However, only system header files should use
these escape routes; application programs should avoid them.
@xref{Alternate Keywords}.

This option is not intended to be @i{useful}; it exists only to satisfy
pedants who would otherwise claim that GNU CC fails to support the ANSI
standard.

Some users try to use @samp{-pedantic} to check programs for strict ANSI
C conformance.  They soon find that it does not do quite what they want:
it finds some non-ANSI practices, but not all---only those for which
ANSI C @emph{requires} a diagnostic.

A feature to report any failure to conform to ANSI C might be useful in
some instances, but would require considerable additional work and would
be quite different from @samp{-pedantic}.  We recommend, rather, that
users take advantage of the extensions of GNU C and disregard the
limitations of other compilers.  Aside from certain supercomputers and
obsolete small machines, there is less and less reason ever to use any
other C compiler other than for bootstrapping GNU CC.

@item -pedantic-errors
Like @samp{-pedantic}, except that errors are produced rather than
warnings.

@item -W
Print extra warning messages for these events:

@itemize @bullet
@cindex @code{longjmp} warnings
@item
A nonvolatile automatic variable might be changed by a call to
@code{longjmp}.  These warnings as well are possible only in
optimizing compilation.

The compiler sees only the calls to @code{setjmp}.  It cannot know
where @code{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 @code{longjmp} cannot
in fact be called at the place which would cause a problem.

@item
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:

@example
foo (a)
@{
  if (a > 0)
    return a;
@}
@end example

@item
An expression-statement contains no side effects.

@item
An unsigned value is compared against zero with @samp{>} or @samp{<=}.
@end itemize

@item -Wimplicit
Warn whenever a function or parameter is implicitly declared.

@item -Wreturn-type
Warn whenever a function is defined with a return-type that defaults
to @code{int}.  Also warn about any @code{return} statement with no
return-value in a function whose return-type is not @code{void}.

@item -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.

@item -Wswitch
Warn whenever a @code{switch} statement has an index of enumeral type
and lacks a @code{case} for one or more of the named codes of that
enumeration.  (The presence of a @code{default} label prevents this
warning.)  @code{case} labels outside the enumeration range also
provoke warnings when this option is used.

@item -Wcomment
Warn whenever a comment-start sequence @samp{/*} appears in a comment.

@item -Wtrigraphs
Warn if any trigraphs are encountered (assuming they are enabled).

@item -Wformat
Check calls to @code{printf} and @code{scanf}, etc., to make sure that
the arguments supplied have types appropriate to the format string
specified.

@item -Wchar-subscripts
Warn if an array subscript has type @code{char}.  This is a common cause
of error, as programmers often forget that this type is signed on some
machines.

@item -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 @samp{-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 @code{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:

@example
@{
  int x;
  switch (y)
    @{
    case 1: x = 1;
      break;
    case 2: x = 4;
      break;
    case 3: x = 5;
    @}
  foo (x);
@}
@end example

@noindent
If the value of @code{y} is always 1, 2 or 3, then @code{x} is
always initialized, but GNU CC doesn't know this.  Here is
another common case:

@example
@{
  int save_y;
  if (change_y) save_y = y, y = new_y;
  @dots{}
  if (change_y) y = save_y;
@}
@end example

@noindent
This has no bug because @code{save_y} is used only if it is set.

Some spurious warnings can be avoided if you declare as
@code{volatile} all the functions you use that never return.
@xref{Function Attributes}.

@item -Wall
All of the above @samp{-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.
@end table

The remaining @samp{-W@dots{}} options are not implied by @samp{-Wall}
because they warn about constructions that we consider reasonable to
use, on occasion, in clean programs.

@table @code
@item -Wtraditional
Warn about certain constructs that behave differently in traditional and
ANSI C.

@itemize @bullet
@item
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.

@item
A function declared external in one block and then used after the end of
the block.

@item
A @code{switch} statement has an operand of type @code{long}.
@end itemize

@item -Wshadow
Warn whenever a local variable shadows another local variable.

@item -Wid-clash-@var{len}
Warn whenever two distinct identifiers match in the first @var{len}
characters.  This may help you prepare a program that will compile
with certain obsolete, brain-damaged compilers.

@item -Wpointer-arith
Warn about anything that depends on the ``size of'' a function type or
of @code{void}.  GNU C assigns these types a size of 1, for
convenience in calculations with @code{void *} pointers and pointers
to functions.

@item -Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from
the target type.  For example, warn if a @code{const char *} is cast
to an ordinary @code{char *}.

@item -Wcast-align
Warn whenever a pointer is cast such that the required alignment of the
target is increased.  For example, warn if a @code{char *} is cast to
an @code{int *} on machines where integers can only be accessed at
two- or four-byte boundaries.

@item -Wwrite-strings
Give string constants the type @code{const char[@var{length}]} so that
copying the address of one into a non-@code{const} @code{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 @code{const} in
declarations and prototypes.  Otherwise, it will just be a nuisance;
this is why we did not make @samp{-Wall} request these warnings.

@item -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.

@item -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.)

@item -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.)

@item -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.

@item -Wredundant-decls
Warn if anything is declared more than once in the same scope, even in
cases where multiple declaration is valid and changes nothing.

@item -Wnested-externs
Warn if an @code{extern} declaration is encountered within an function.

@item -Wno-parentheses
Disable warnings that parentheses are suggested around an expression.

@item -Werror
Make all warnings into errors.
@end table

@node Debugging Options, Optimize Options, Warning Options, Invoking GCC
@section Options for Debugging Your Program or GNU CC
@cindex options, debugging
@cindex debugging information options

GNU CC has various special options that are used for debugging
either your program or GCC:

@table @code
@item -g
Produce debugging information in the operating system's native format
(stabs or COFF or DWARF).  GDB can work with this debugging information.

On most systems that use stabs format, @samp{-g} enables use of extra
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but will probably make DBX crash or
refuse to read the program.  If you want to control for certain whether
to generate the extra information, use @samp{-gstabs+} or @samp{-gstabs}
(see below).

Unlike most other C compilers, GNU CC allows you to use @samp{-g} with
@samp{-O}.  The shortcuts taken by optimized code may occasionally
produce surprising results: some variables you declared may not exist
at all; flow of control may briefly move where you did not expect it;
some statements may not be executed because they compute constant
results or their values were already at hand; some statements may
execute in different places because they were moved out of loops.

Nevertheless it proves possible to debug optimized output.  This makes
it reasonable to use the optimizer for programs that might have bugs.

The following options are useful when GNU CC is generated with the
capability for more than one debugging format.

@item -ggdb
Produce debugging information in the native format (if that is supported),
including GDB extensions if at all possible.

@item -gstabs
Produce debugging information in stabs format (if that is supported),
without GDB extensions.  This is the format used by DBX on most BSD
systems.

@item -gstabs+
Produce debugging information in stabs format (if that is supported),
using GDB extensions.  The use of these extensions is likely to make DBX
crash or refuse to read the program.

@item -gcoff
Produce debugging information in COFF format (if that is supported).
This is the format used by SDB on COFF systems.

@item -gdwarf
Produce debugging information in DWARF format (if that is supported).
This is the format used by SDB on systems that use DWARF.

@item -g@var{level}
@itemx -ggdb@var{level}
@itemx -gstabs@var{level}
@itemx -gcoff@var{level}
@itemx -gdwarf@var{level}
Request debugging information and also use @var{level} to specify how
much information.  The default level is 2.

Level 1 produces minimal information, enough for making backtraces in
parts of the program that you don't plan to debug.  This includes
descriptions of functions and external variables, but no information
about local variables and no line numbers.

Level 3 includes extra information, such as all the macro definitions
present in the program.  Some debuggers support macro expansion when
you use @samp{-g3}.

@cindex @code{prof}
@item -p
Generate extra code to write profile information suitable for the
analysis program @code{prof}.
@c ??? looks like -p and -pg are now equivalent.  Are they? 11dec91

@cindex @code{gprof}
@item -pg
Generate extra code to write profile information suitable for the
analysis program @code{gprof}.

@cindex @code{tcov}
@item -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 @code{tcov}.  Note,
however, that the format of the data is not what @code{tcov} expects.
Eventually GNU @code{gprof} should be extended to process this data.

@item -d@var{letters}
Says to make debugging dumps during compilation at times specified by
@var{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.  @file{foo.c.rtl} or @file{foo.c.jump}).  Here are the
possible letters for use in @var{letters}, and their meanings:

@table @samp
@item M
Dump all macro definitions, at the end of preprocessing, and write no
output.
@item N
Dump all macro names, at the end of preprocessing.
@item D
Dump all macro definitions, at the end of preprocessing, in addition to
normal output.
@item y
Dump debugging information during parsing, to standard error.
@item r
Dump after RTL generation, to @file{@var{file}.rtl}.
@item x
Just generate RTL for a function instead of compiling it.  Usually used
with @samp{r}.
@item j
Dump after first jump optimization, to @file{@var{file}.jump}.
@item s
Dump after CSE (including the jump optimization that sometimes
follows CSE), to @file{@var{file}.cse}.
@item L
Dump after loop optimization, to @file{@var{file}.loop}.
@item t
Dump after the second CSE pass (including the jump optimization that
sometimes follows CSE), to @file{@var{file}.cse2}.
@item f
Dump after flow analysis, to @file{@var{file}.flow}.
@item c
Dump after instruction combination, to @file{@var{file}.combine}.
@item S
Dump after the first instruction scheduling pass, to
@file{@var{file}.sched}.
@item l
Dump after local register allocation, to @file{@var{file}.lreg}.
@item g
Dump after global register allocation, to @file{@var{file}.greg}.
@item R
Dump after the second instruction scheduling pass, to
@file{@var{file}.sched2}.
@item J
Dump after last jump optimization, to @file{@var{file}.jump2}.
@item d
Dump after delayed branch scheduling, to @file{@var{file}.dbr}.
@item k
Dump after conversion from registers to stack, to @file{@var{file}.stack}.
@item a
Produce all the dumps listed above.
@item m
Print statistics on memory usage, at the end of the run, to
standard error.
@item p
Annotate the assembler output with a comment indicating which
pattern and alternative was used.
@end table

@item -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.

@item -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 @file{foo.c} with @samp{-c -save-temps} would produce files
@file{foo.cpp} and @file{foo.s}, as well as @file{foo.o}.
@end table

@node Optimize Options, Preprocessor Options, Debugging Options, Invoking GCC
@section Options That Control Optimization
@cindex optimize options
@cindex options, optimization

These options control various sorts of optimizations:

@table @code
@item -O
Optimize.  Optimizing compilation takes somewhat more time, and a lot
more memory for a large function.

Without @samp{-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 @samp{-O}, only variables declared @code{register} are
allocated in registers.  The resulting compiled code is a little worse
than produced by PCC without @samp{-O}.

With @samp{-O}, the compiler tries to reduce code size and execution
time.

When @samp{-O} is specified, @samp{-fthread-jumps} and
@samp{-fdelayed-branch} are turned on.  On some machines other
flags may also be turned on.

@item -O2
Highly optimize.  All supported optimizations that do not involve a
space-speed tradeoff are performed.  As compared to @samp{-O}, this
option will increase both compilation time and the performance of the
generated code.

All @samp{-f@var{flag}} options that control optimization are turned on
when @samp{-O2} is specified, except for @samp{-funroll-loops} and
@samp{-funroll-all-loops}.
@end table

Options of the form @samp{-f@var{flag}} specify machine-independent
flags.  Most flags have both positive and negative forms; the negative
form of @samp{-ffoo} would be @samp{-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 @samp{no-} or
adding it.

@table @code
@item -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 @code{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 @samp{-ffloat-store} for such programs.

@item -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.

@item -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 hearing
about the difference this makes.

@item -fforce-addr
Force memory address constants to be copied into registers before
doing arithmetic on them.  This may produce better code just as
@samp{-fforce-mem} may.  I am interested in hearing about the
difference this makes.

@item -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.  @strong{It also makes debugging impossible on
some machines.}

@ifset INTERNALS
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 @code{FRAME_POINTER_REQUIRED} controls
whether a target machine supports this flag.  @xref{Registers}.@refill
@end ifset
@ifclear INTERNALS
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 @code{FRAME_POINTER_REQUIRED} controls
whether a target machine supports this flag.  @xref{Registers,,Register
Usage, gcc.info, Using and Porting GCC}.@refill
@end ifclear

@item -finline
Pay attention to the @code{inline} keyword.  Normally the negation of this
option @samp{-fno-inline} is used to keep the compiler from expanding
any functions inline.  However, the opposite effect may be desirable
when compiling without optimization, since inline expansion is turned
off in that case.

@item -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 @code{static}, then the function is normally not output as
assembler code in its own right.

@item -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.

@item -fkeep-inline-functions
Even if all calls to a given function are integrated, and the function
is declared @code{static}, nevertheless output a separate run-time
callable version of the function.

@item -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.
@end table

The following options control specific optimizations.  The @samp{-O2}
option turns on all of these optimizations except @samp{-funroll-loops}
and @samp{-funroll-all-loops}.  The @samp{-O} option usually turns on
the @samp{-fthread-jumps} and @samp{-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.

@table @code
@item -fstrength-reduce
Perform the optimizations of loop strength reduction and
elimination of iteration variables.

@item -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.

@item -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.

@item -frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations has been
performed.  

@item -fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.

@item -fdelayed-branch
If supported for the target machine, attempt to reorder instructions
to exploit instruction slots available after delayed branch
instructions.

@item -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.

@item -fschedule-insns2
Similar to @samp{-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.

@item -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.
@samp{-funroll-loop} implies @samp{-fstrength-reduce} and
@samp{-frerun-cse-after-loop}.

@item -funroll-all-loops
Perform the optimization of loop unrolling.  This is done for all loops
and usually makes programs run more slowly.  @samp{-funroll-all-loops}
implies @samp{-fstrength-reduce} and @samp{-frerun-cse-after-loop}.

@item -fno-peephole
Disable any machine-specific peephole optimizations.
@end table

@node Preprocessor Options, Link Options, Optimize Options, Invoking GCC
@section Options Controlling the Preprocessor
@cindex preprocessor options
@cindex options, preprocessor

These options control the C preprocessor, which is run on each C source
file before actual compilation.

If you use the @samp{-E} option, nothing is done except preprocessing.
Some of these options make sense only together with @samp{-E} because
they cause the preprocessor output to be unsuitable for actual
compilation.

@table @code
@item -include @var{file}
Process @var{file} as input before processing the regular input file.
In effect, the contents of @var{file} are compiled first.  Any @samp{-D}
and @samp{-U} options on the command line are always processed before
@samp{-include @var{file}}, regardless of the order in which they are
written.  All the @samp{-include} and @samp{-imacros} options are
processed in the order in which they are written.

@item -imacros @var{file}
Process @var{file} as input, discarding the resulting output, before
processing the regular input file.  Because the output generated from
@var{file} is discarded, the only effect of @samp{-imacros @var{file}}
is to make the macros defined in @var{file} available for use in the
main input.

Any @samp{-D} and @samp{-U} options on the command line are always
processed before @samp{-imacros @var{file}}, regardless of the order in
which they are written.  All the @samp{-include} and @samp{-imacros}
options are processed in the order in which they are written.

@item -nostdinc
Do not search the standard system directories for header files.  Only
the directories you have specified with @samp{-I} options (and the
current directory, if appropriate) are searched.  @xref{Directory
Options}, for information on @samp{-I}.

By using both @samp{-nostdinc} and @samp{-I-}, you can limit the include-file
search path to only those directories you specify explicitly.

@item -undef
Do not predefine any nonstandard macros.  (Including architecture flags).

@item -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.

@item -C
Tell the preprocessor not to discard comments.  Used with the
@samp{-E} option.

@item -P
Tell the preprocessor not to generate @samp{#line} commands.
Used with the @samp{-E} option.

@cindex make
@cindex dependencies, make
@item -M
Tell the preprocessor to output a rule suitable for @code{make}
describing the dependencies of each object file.  For each source file,
the preprocessor outputs one @code{make}-rule whose target is the object
file name for that source file and whose dependencies are all the files
@samp{#include}d in it.  This rule may be a single line or may be
continued with @samp{\}-newline if it is long.  The list of rules is
printed on standard output instead of the preprocessed C program.

@samp{-M} implies @samp{-E}.

Another way to specify output of a @code{make} rule is by setting
the environment variable @code{DEPENDENCIES_OUTPUT} (@pxref{Environment
Variables}).

@item -MM
Like @samp{-M} but the output mentions only the user header files
included with @samp{#include "@var{file}"}.  System header files
included with @samp{#include <@var{file}>} are omitted.

@item -MD
Like @samp{-M} but the dependency information is written to files with
names made by replacing @samp{.c} with @samp{.d} at the end of the
input file names.  This is in addition to compiling the file as
specified---@samp{-MD} does not inhibit ordinary compilation the way
@samp{-M} does.

The Mach utility @samp{md} can be used to merge the @samp{.d} files
into a single dependency file suitable for using with the @samp{make}
command.

@item -MMD
Like @samp{-MD} except mention only user header files, not system
header files.

@item -H
Print the name of each header file used, in addition to other normal
activities.

@item -D@var{macro}
Define macro @var{macro} with the string @samp{1} as its definition.

@item -D@var{macro}=@var{defn}
Define macro @var{macro} as @var{defn}.  All instances of @samp{-D} on
the command line are processed before any @samp{-U} options.

@item -U@var{macro}
Undefine macro @var{macro}.  @samp{-U} options are evaluated after all
@samp{-D} options, but before any @samp{-include} and @samp{-imacros}
options.

@item -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 @samp{-E}
option.

@item -dD
Tell the preprocessing to pass all macro definitions into the output, in
their proper sequence in the rest of the output.

@item -dN
Like @samp{-dD} except that the macro arguments and contents are omitted.
Only @samp{#define @var{name}} is included in the output.

@item -trigraphs
Support ANSI C trigraphs.  You don't want to know about this
brain-damage.  The @samp{-ansi} option also has this effect.
@end table

@node Link Options, Directory Options, Preprocessor Options, Invoking GCC
@section Options for Linking
@cindex link options
@cindex options, linking

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.

@table @code
@cindex file names
@item @var{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 linking is done, these object files are used as input
to the linker.

@item -c
@itemx -S
@itemx -E
If any of these options is used, then the linker is not run, and
object file names should not be used as arguments.  @xref{Overall
Options}.

@cindex Libraries
@item -l@var{library}
Search the library named @var{library} when linking.

It makes a difference where in the command you write this option; the
linker searches processes libraries and object files in the order they
are specified.  Thus, @samp{foo.o -lz bar.o} seaches library @samp{z}
after file @file{foo.o} but before @file{bar.o}.  If @file{bar.o} refers
to functions in @samp{z}, those functions may not be loaded.

The linker searches a standard list of directories for the library,
which is actually a file named @file{lib@var{library}.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 @samp{-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.  But if the file that is found is an
ordinary object file, it is linked in the usual fashion.  The only
difference between using an @samp{-l} option and specifying a file name
is that @samp{-l} surrounds @var{library} with @samp{lib} and @samp{.a}
and searches several directories.

@item -nostdlib
Don't use the standard system libraries and startup files when linking.
Only the files you specify will be passed to the linker.

@item -static
On systems that support dynamic linking, this prevents linking with the shared
libraries.  On other systems, this
option has no effect.

@item -dynamic
On systems that support dynamic linking, you can use this option to
request it explicitly.

@item -shared
Produce a shared object which can then be linked with other objects to
form an executable.  Only a few systems support this option.

@item -symbolic
Bind references to global symbols when building a shared object.  Warn
about any unresolved references (unless overridden by the link editor
option @samp{-Xlinker -z -Xlinker defs}).  Only a few systems support
this option.

@item -Xlinker @var{option}
Pass @var{option} as an option to the linker.  You can use this to
supply system-specific linker options which GNU CC does not know how to
recognize.

If you want to pass an option that takes an argument, you must use
@samp{-Xlinker} twice, once for the option and once for the argument.
For example, to pass @samp{-assert definitions}, you must write
@samp{-Xlinker -assert -Xlinker definitions}.  It does not work to write
@samp{-Xlinker "-assert definitions"}, because this passes the entire
string as a single argument, which is not what the linker expects.
@end table

@node Directory Options, Target Options, Link Options, Invoking GCC
@section Options for Directory Search
@cindex directory options
@cindex options, directory search
@cindex search path

These options specify directories to search for header files, for
libraries and for parts of the compiler:

@table @code
@item -I@var{dir}
Append directory @var{dir} to the list of directories searched for
include files.

@item -I-
Any directories you specify with @samp{-I} options before the @samp{-I-}
option are searched only for the case of @samp{#include "@var{file}"};
they are not searched for @samp{#include <@var{file}>}.

If additional directories are specified with @samp{-I} options after
the @samp{-I-}, these directories are searched for all @samp{#include}
directives.  (Ordinarily @emph{all} @samp{-I} directories are used
this way.)

In addition, the @samp{-I-} option inhibits the use of the current
directory (where the current input file came from) as the first search
directory for @samp{#include "@var{file}"}.  There is no way to
override this effect of @samp{-I-}.  With @samp{-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.

@samp{-I-} does not inhibit the use of the standard system directories
for header files.  Thus, @samp{-I-} and @samp{-nostdinc} are
independent.

@item -L@var{dir}
Add directory @var{dir} to the list of directories to be searched
for @samp{-l}.

@item -B@var{prefix}
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
@file{cpp}, @file{cc1}, @file{as} and @file{ld}.  It tries
@var{prefix} as a prefix for each program it tries to run, both with and
without @samp{@var{machine}/@var{version}/} (@pxref{Target Options}).

For each subprogram to be run, the compiler driver first tries the
@samp{-B} prefix, if any.  If that name is not found, or if @samp{-B}
was not specified, the driver tries two standard prefixes, which are
@file{/usr/lib/gcc/} and @file{/usr/local/lib/gcc/}.  If neither of
those results in a file name that is found, the unmodified program
name is searched for using the directories specified in your
@samp{PATH} environment variable.

@samp{-B} prefixes that effectively specify directory names also apply
to libraries in the linker, because the compiler translates these
options into @samp{-L} options for the linker.

The run-time support file @file{libgcc.a} can also be searched for using
the @samp{-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.

Another way to specify a prefix much like the @samp{-B} prefix is to use
the environment variable @code{GCC_EXEC_PREFIX}.  @xref{Environment
Variables}.
@end table

@node Target Options, Submodel Options, Directory Options, Invoking GCC
@section Specifying Target Machine and Compiler Version
@cindex target options
@cindex cross compiling
@cindex specifying machine version
@cindex specifying compiler version and target machine
@cindex compiler version, specifying
@cindex target machine, specifying

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
@samp{-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.

@table @code
@item -b @var{machine}
The argument @var{machine} specifies the target machine for compilation.
This is useful when you have installed GNU CC as a cross-compiler.

The value to use for @var{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 @samp{configure
i386v}, meaning to compile for an 80386 running System V, then you
would specify @samp{-b i386v} to run that cross compiler.

When you do not specify @samp{-b}, it normally means to compile for
the same type of machine that you are using.

@item -V @var{version}
The argument @var{version} specifies which version of GNU CC to run.
This is useful when multiple versions are installed.  For example,
@var{version} might be @samp{2.0}, meaning to run GNU CC version 2.0.

The default version, when you do not specify @samp{-V}, is controlled
by the way GNU CC is installed.  Normally, it will be a version that
is recommended for general use.
@end table

The @samp{-b} and @samp{-V} options actually work by controlling part of
the file name used for the executable files and libraries used for
compilation.  A given version of GNU CC, for a given target machine, is
normally kept in the directory @file{/usr/local/lib/gcc/@var{machine}/@var{version}}.@refill

It follows that sites can customize the effect of @samp{-b} or @samp{-V}
either by changing the names of these directories or adding
alternate names (or symbolic links).  Thus, if
@file{/usr/local/lib/gcc/80386} is a link to
@file{/usr/local/lib/gcc/i386v}, then @samp{-b 80386} will be an alias
for @samp{-b i386v}.@refill

In one respect, the @samp{-b} or @samp{-V} do not completely change
to a different compiler: the top-level driver program @code{gcc}
that you originally invoked continues to run and invoke the other
executables (preprocessor, compiler per se, assembler and linker)
that do the real work.  However, since no real work is done in the
driver program, it usually does not matter that the driver program
in use is not the one for the specified target and version.

The only way that the driver program depends on the target machine is
in the parsing and handling of special machine-specific options.
However, this is controlled by a file which is found, along with the
other executables, in the directory for the specified version and
target machine.  As a result, a single installed driver program adapts
to any specified target machine and compiler version.

The driver program executable does control one significant thing,
however: the default version and target machine.  Therefore, you can
install different instances of the driver program, compiled for
different targets or versions, under different names.

For example, if the driver for version 2.0 is installed as @code{ogcc}
and that for version 2.1 is installed as @code{gcc}, then the command
@code{gcc} will use version 2.1 by default, while @code{ogcc} will use
2.0 by default.  However, you can choose either version with either
command with the @samp{-V} option.

@node Submodel Options, Code Gen Options, Target Options, Invoking GCC
@section Specifying Hardware Models and Configurations
@cindex submodel options
@cindex specifying hardware config
@cindex hardware models and configurations, specifying
@cindex machine dependent options

Earlier we discussed the standard option @samp{-b} which chooses among
different installed compilers for completely different target
machines, such as Vax vs. 68000 vs. 80386.

In addition, each of these target machine types can have its own
special options, starting with @samp{-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.

@ifset INTERNALS
These options are defined by the macro @code{TARGET_SWITCHES} in the
machine description.  The default for the options is also defined by
that macro, which enables you to change the defaults.
@end ifset

@menu
* M680x0 Options::
* VAX Options::
* SPARC Options::
* Convex Options::
* AMD29K Options::
* M88K Options::
* RS/6000 Options::
* RT Options::
* MIPS Options::
@end menu

@node M680x0 Options, Vax Options, Submodel Options, Submodel Options
@subsection M680x0 Options
@cindex M680x0 options

These are the @samp{-m} options defined for the 68000 series.  The default
values for these options depends on which style of 68000 was selected when
the compiler was configured; the defaults for the most common choices are
given below.

@table @code
@item -m68020
@itemx -mc68020
Generate output for a 68020 (rather than a 68000).  This is the
default when the compiler is configured for 68020-based systems.

@item -m68000
@itemx -mc68000
Generate output for a 68000 (rather than a 68020).  This is the default
when the compiler is configured for a 68000-based systems.

@item -m68881
Generate output containing 68881 instructions for floating point.
This is the default for most 68020 systems unless @samp{-nfp} was
specified when the compiler was configured.

@item -mfpa
Generate output containing Sun FPA instructions for floating point.

@item -msoft-float
Generate output containing library calls for floating point.
@strong{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.

@item -mshort
Consider type @code{int} to be 16 bits wide, like @code{short int}.

@item -mnobitfield
Do not use the bit-field instructions.  @samp{-m68000} implies
@samp{-mnobitfield}.

@item -mbitfield
Do use the bit-field instructions.  @samp{-m68020} implies
@samp{-mbitfield}.  This is the default if you use the unmodified
sources configured for a 68020.

@item -mrtd
Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the @code{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 @code{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 @code{rtd} instruction is supported by the 68010 and 68020
processors, but not by the 68000.
@end table

@node VAX Options, Sparc Options, M680x0 Options, Submodel Options
@subsection VAX Options
@cindex VAX options

These @samp{-m} options are defined for the Vax:

@table @code
@item -munix
Do not output certain jump instructions (@code{aobleq} and so on)
that the Unix assembler for the Vax cannot handle across long
ranges.

@item -mgnu
Do output those jump instructions, on the assumption that you
will assemble with the GNU assembler.

@item -mg
Output code for g-format floating point numbers instead of d-format.
@end table

@node Sparc Options, Convex Options, Vax Options, Submodel Options
@subsection SPARC Options
@cindex SPARC options

These @samp{-m} switches are supported on the Sparc:

@table @code
@ignore
@item -mfpu
Generate output containing floating point instructions.  This is the
default if you use the unmodified sources.

@item -msoft-float
Generate output containing library calls for floating point.
@strong{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.

@item -malign
This doesn't completely work yet.
@end ignore
@item -mno-epilogue
Generate separate return instructions for @code{return} statements.
This has both advantages and disadvantages; I don't recall what they
are.
@end table

@node Convex Options, AMD29K Options, SPARC Options, Submodel Options
@subsection Convex Options
@cindex Convex options

These @samp{-m} options are defined for the Convex:

@table @code
@item -mc1
Generate output for a C1.  This is the default when the compiler is
configured for a C1.

@item -mc2
Generate output for a C2.  This is the default when the compiler is
configured for a C2.

@item -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.)

@item -mnoargcount
Omit the argument count word.  This is the default if you use the
unmodified sources.
@end table

@node AMD29K Options, M88K Options, Convex Options, Submodel Options
@subsection AMD29K Options
@cindex AMD29K options

These @samp{-m} options are defined for the AMD Am29000:

@table @code
@item -mdw
Generate code that assumes the @code{DW} bit is set, i.e., that byte and
halfword operations are directly supported by the hardware.  This is the
default.

@item -mnodw
Generate code that assumes the @code{DW} bit is not set.

@item -mbw
Generate code that assumes the system supports byte and halfword write
operations.  This is the default.

@item -mnbw
Generate code that assumes the systems does not support byte and
halfword write operations.  @samp{-mnbw} implies @samp{-mnodw}.

@item -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 @code{call} instruction to be used instead
of a @code{const}, @code{consth}, @code{calli} sequence.

@item -mlarge
Do not assume that the @code{call} instruction can be used; this is the
default.

@item -m29050
Generate code for the Am29050.

@item -m29000
Generate code for the Am29000.  This is the default.

@item -mkernel-registers
Generate references to registers @code{gr64-gr95} instead of
@code{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 @samp{-f} flags
must use the normal, user-mode, names.

@item -muser-registers
Use the normal set of global registers, @code{gr96-gr127}.  This is the
default.

@item -mstack-check
Insert a call to @code{__msp_check} after each stack adjustment.  This
is often used for kernel code.
@end table

@node M88K Options, RS/6000 Options, AMD29K Options, Submodel Options
@subsection M88K Options
@cindex M88k options

These @samp{-m} options are defined for Motorola 88K architectures:

@table @code
@item -m88000
@kindex -m88000
Generate code that works well on both the m88100 and the
m88110.

@item -m88100
@kindex -m88100
Generate code tha
Generate code that works best for the m88100, but that also
runs on the m88110.

@item -m88110
@kindex -m88110
Generate code that works best for the m88110, and may not run
on the m88100.

@item -midentify-revision
@kindex -midentify-revision
@kindex ident
@cindex identifying source, compiler (88k)
Include an @code{ident} directive in the assembler output recording the
source file name, compiler name and version, timestamp, and compilation
flags used.

@item -mno-underscores
@kindex -mno-underscores
@cindex underscores, avoiding (88k)
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.

@item -mocs-debug-info
@itemx -mno-ocs-debug-info
@kindex -mocs-debug-info
@kindex -mno-ocs-debug-info
@cindex OCS (88k)
@cindex debugging, 88k OCS
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 allows debugging of code that
has had the frame pointer eliminated.  The default for DG/UX, SVr4, and
Delta 88 SVr3.2 is to include this information; other 88k configurations
omit this information by default.

@item -mocs-frame-position
@kindex -mocs-frame-position
@cindex register positions in frame (88k)
When emitting COFF debugging information for automatic variables and
parameters stored on the stack, use the offset from the canonical frame
address, which is the stack pointer (register 31) on entry to the
function.  The DG/UX, SVr4, Delta88 SVr3.2, and BCS configurations use
@samp{-mocs-frame-position}; other 88k configurations have the default
@samp{-mno-ocs-frame-position}.

@item -mno-ocs-frame-position
@kindex -mno-ocs-frame-position
@cindex register positions in frame (88k)
When emitting COFF debugging information for automatic variables and
parameters stored on the stack, use the offset from the frame pointer
register (register 30).  When this option is in effect, the frame
pointer is not eliminated when debugging information is selected by the
-g switch.

@item -moptimize-arg-area
@itemx -mno-optimize-arg-area
@kindex -moptimize-arg-area
@kindex -mno-optimize-arg-area
@cindex arguments in frame (88k)
Control how to store function arguments in stack frames.
@samp{-moptimize-arg-area} saves space, but was ruled illegal by 88open.
@samp{-mno-optimize-arg-area} conforms to the 88open standards.  By
default GNU CC does not optimize the argument area.

@item -mshort-data-@var{num}
@kindex -mshort-data-@var{num}
@cindex smaller data references (88k)
@cindex r0-relative references (88k)
Generate smaller data references by making them relative to @code{r0},
which allows loading a value using a single instruction (rather than the
usual two).  You control which data references are affected by
specifying @var{num} with this option.  For example, if you specify
@samp{-mshort-data-512}, then the data references affected are those
involving displacements of less than 512 bytes.
@samp{-mshort-data-@var{num}} is not effective for @var{num} greater
than 64K.

@item -msvr4
@itemx -msvr3
@kindex -msvr4
@kindex -msvr3
@cindex assembler syntax, 88k
@cindex SVr4
Turn on (@samp{-msvr4}) or off (@samp{-msvr3}) compiler extensions
related to System V release 4 (SVr4).  This controls the following:

@enumerate
@item 
Which variant of the assembler syntax to emit (which you can select
independently using @samp{-mversion-03.00}).  
@item
@samp{-msvr4} makes the C preprocessor recognize @samp{#pragma weak}
that is used on System V release 4.
@item
@samp{-msvr4} makes GNU CC issue additional declaration directives used in
SVr4.  
@end enumerate

@samp{-msvr3} is the default for all m88K configurations except
the SVr4 configuration.

@item -mversion-03.00
@kindex -mversion-03.00
In the DG/UX configuration, there are two flavors of SVr4.  This option
modifies @samp{-msvr4} to select whether the hybrid-COFF or real-ELF
flavor is used.  All other configurations ignore this option.
@c ??? which asm syntax better for GAS?  option there too?

@item -mno-check-zero-division
@itemx -mcheck-zero-division
@kindex -mno-check-zero-divixion
@kindex -mcheck-zero-division
@cindex zero division on 88k
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 GNU CC configurations for the 88K use
@samp{-mcheck-zero-division} by default.

@item -muse-div-instruction
@kindex -muse-div-instruction
@cindex divide instruction, 88k
Do not emit code to check both the divisor and dividend when doing
signed integer division to see if either is negative, and adjust the
signs so the divide is done using non-negative numbers.  Instead, rely
on the operating system to calculate the correct value when the
@code{div} instruction traps.  This results in different behavior when
the most negative number is divided by -1, but is useful when most or
all signed integer divisions are done with positive numbers.

@item -mtrap-large-shift
@itemx -mhandle-large-shift
@kindex -mtrap-large-shift
@kindex -mhandle-large-shift
@cindex bit shift overflow (88k)
@cindex large bit shifts (88k)
Include code to detect bit-shifts of more than 31 bits; respectively,
trap such shifts or emit code to handle them properly.  By default GNU CC
makes no special provision for large bit shifts.

@item -mwarn-passed-structs
@kindex -mwarn-passed-structs
@cindex structure passing (88k)
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,
GNU CC issues no such warning.
@end table

@node RS/6000 Options, RT Options, M88K Options, Submodel Options
@subsection IBM RS/6000 Options
@cindex RS/6000 Options
@cindex IBM RS/6000 Options

Only one pair of @samp{-m} options is defined for the IBM RS/6000:

@table @code
@item -mfp-in-toc
@itemx -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 GNU CC puts floating-point constants there; if the TOC overflows,
@samp{-mno-fp-in-toc} will reduce the size of the TOC, which may avoid
the overflow.
@end table

@node RT Options, MIPS Options, RS/6000 Options, Submodel Options
@subsection IBM RT Options
@cindex RT options
@cindex IBM RT options

These @samp{-m} options are defined for the IBM RT PC:

@table @code
@item -min-line-mul
Use an in-line code sequence for integer multiplies.  This is the
default.

@item -mcall-lib-mul
Call @code{lmul$$} for integer multiples.

@item -mfull-fp-blocks
Generate full-size floating point data blocks, including the minimum
amount of scratch space recommended by IBM.  This is the default.

@item -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.

@cindex @file{varargs.h} and RT PC
@cindex @file{stdarg.h} and RT PC
@item -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 @code{varargs.h} and @code{stdargs.h} will not work with
floating point operands if this option is specified.

@item -mfp-arg-in-gregs
Use the normal calling convention for floating point arguments.  This is
the default.

@item -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 @samp{-fpcc-struct-return} for compatibility with the
Portable C Compiler (pcc).

@item -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 @samp{-fpcc-struct-return} or
@samp{-mhc-struct-return}.
@end table

@node MIPS Options, , RT Options, Submodel Options
@subsection MIPS Options
@cindex MIPS options

These @samp{-m} options are defined for the MIPS family of computers:

@table @code
@item -mcpu=@var{cpu type}
Assume the defaults for the machine type @var{cpu type} when
scheduling insturctions.  The default @var{cpu type} is
@samp{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 @var{cpu type} are @samp{r2000},
@samp{r3000}, @samp{r4000}, and @samp{r6000}.  While picking a
specific @var{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 @samp{-mips2} or @samp{-mips3} switches being used.

@item -mips2
Issue instructions from level 2 of the MIPS ISA (branch likely, square
root instructions).  The @samp{-mcpu=r4000} or @samp{-mcpu=r6000}
switch must be used in conjuction with @samp{-mips2}.

@item -mips3
Issue instructions from level 3 of the MIPS ISA (64 bit instructions).
You must use the @samp{-mcpu=r4000} switch along with @samp{-mips3}.

@item -mint64
@item -mlong64
@item -mlonglong128
These options don't work at present.

@item -mmips-as
Generate code for the MIPS assembler, and invoke @file{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 the either of the @samp{-gstabs} or @samp{-gstabs+}
switches are used, the @file{mips-tfile} program will encapsulate the
stabs within MIPS ECOFF.

@item -mgas
Generate code for the GNU assembler.  This is the default on the OSF/1
reference platform, using the OSF/rose object format.

@item -mrnames
@itemx -mno-rnames
The @samp{-mrnames} switch says to output code using the MIPS software
names for the registers, instead of the hardware names (ie, @var{a0}
instead of @var{$4}).  The GNU assembler does not support the
@samp{-mrnames} switch, and the MIPS assembler will be instructed to
run the MIPS C preprocessor over the source file.  The
@samp{-mno-rnames} switch is default.

@item -mgpopt
@itemx -mno-gpopt
The @samp{-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.

@item -mstats
@itemx -mno-stats
For each non-inline function processed, the @samp{-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.).

@item -mmemcpy
@itemx -mno-memcpy
The @samp{-mmemcpy} switch makes all block moves call the appropriate
string function (@samp{memcpy} or @samp{bcopy}) instead of possibly
generating inline code.

@item -mmips-tfile
@itemx -mno-mips-tfile
The @samp{-mno-mips-tfile} switch causes the compiler not
postprocess the object file with the @file{mips-tfile} program,
after the MIPS assembler has generated it to add debug support.  If
@file{mips-tfile} is not run, then no local variables will be
available to the debugger.  In addition, @file{stage2} and
@file{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.

@item -msoft-float
Generate output containing library calls for floating point.
@strong{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.

@item -mhard-float
Generate output containing floating point instructions.  This is the
default if you use the unmodified sources.

@item -mfp64
Assume that the @var{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 @samp{-mcpu=r4000} and
@samp{-mips3} switches.

@item -mfp32
Assume that there are 32 32-bit floating point registers.  This is the
default.

@item -mabicalls
@itemx -mno-abicalls
Emit the @samp{.abicalls}, @samp{.cpload}, and @samp{.cprestore}
pseudo operations that some System V.4 ports use for position
independent code.

@item -mhalf-pic
@itemx -mno-half-pic
Put pointers to extern references into the data section and load them
up, rather than put the references in the text section.  These options
do not work at present.

@item -G @var{num}
@cindex smaller data references (MIPS)
@cindex gp-relative references (MIPS)
Put global and static items less than or equal to @var{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 (@var{gp} or @var{$28}),
instead of the normal two words used.  By default, @var{num} is 8 when
the MIPS assembler is used, and 0 when the GNU assembler is used.  The
@samp{-G @var{num}} switch is also passed to the assembler and linker.
All modules should be compiled with the same @samp{-G @var{num}} value.
@end table

@ifset INTERNALS
These options are defined by the macro
@code{TARGET_SWITCHES} in the machine description.  The default for the
options is also defined by that macro, which enables you to change the
defaults.
@end ifset

@node Code Gen Options, Environment Variables, Submodel Options, Invoking GCC
@section Options for Code Generation Conventions
@cindex code generation conventions
@cindex options, code generation 
@cindex run-time options

These machine-independent options control the interface conventions
used in code generation.

Most of them have both positive and negative forms; the negative form
of @samp{-ffoo} would be @samp{-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 @samp{no-} or adding
it.

@table @code
@item -fpcc-struct-return
Use the same convention for returning @code{struct} and @code{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 GNU CC-compiled code and PCC-compiled
code.

@item -fshort-enums
Allocate to an @code{enum} type only as many bytes as it needs for the
declared range of possible values.  Specifically, the @code{enum} type
will be equivalent to the smallest integer type which has enough room.

@item -fshort-double
Use the same size for @code{double} as for @code{float}.

@item -fshared-data
Requests that the data and non-@code{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.

@item -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 @code{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.

@item -fno-ident
Ignore the @samp{#ident} directive.

@item -fno-gnu-linker
Don't output global initializations such as C++ constructors and
destructors in the form used by the GNU linker (on systems where the GNU
linker is the standard method of handling them).  Use this option when
you want to use a ``collect'' program and a non-GNU linker.

@item -finhibit-size-directive
Don't output a @code{.size} assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the 
two halves are placed at locations far apart in memory.  This option is
used when compiling @file{crtstuff.c}; you should not need to use it
for anything else.

@item -fvolatile
Consider all memory references through pointers to be volatile.

@item -fpic
@cindex global offset table
If supported for the target machines, generate position-independent
code, suitable for use in a shared library.  All addresses will be
accessed through a global offset table (GOT).  If the GOT size for the
linked executable exceeds a machine-specific maximum size, you will get
an error message from the linker indicating that @samp{-fpic} does not
work; recompile with @samp{-fPIC} instead.  (These maximums are 16k on
the m88k, 8k on the Sparc, and 32k on the m68k and RS/6000.  The 386 has
no such limit.)

Position-independent code requires special support, and therefore works
only on certain machines.  Code generated for the IBM RS/6000 is always
position-independent.

@item -fPIC
If supported for the target machine, emit position-independent code,
suitable for dynamic linking and avoiding any limit on the size of the
global offset table.  This option makes a difference on the m68k, m88k
and the Sparc.

Position-independent code requires special support, and therefore works
only on certain machines.

@item -ffixed-@var{reg}
Treat the register named @var{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).

@var{reg} must be the name of a register.  The register names accepted
are machine-specific and are defined in the @code{REGISTER_NAMES}
macro in the machine description macro file.

This flag does not have a negative form, because it specifies a
three-way choice.

@item -fcall-used-@var{reg}
Treat the register named @var{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 @var{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.

@item -fcall-saved-@var{reg}
Treat the register named @var{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 @var{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.
@end table

@node Environment Variables,, Code Gen Options, Invoking GCC
@section Environment Variables Affecting GNU CC
@cindex environment variables

This section describes several environment variables that affect how GNU
CC operates.  They work by specifying directories or prefixes to use
when searching for various kinds of files.

@ifclear INTERNALS
Note that you can also specify places to search using options such as
@samp{-B}, @samp{-I} and @samp{-L} (@pxref{Directory Options}).  These
take precedence over places specified using environment variables, which
in turn take precedence over those specified by the configuration of GNU
CC. 
@end ifclear
@ifset INTERNALS
Note that you can also specify places to search using options such as
@samp{-B}, @samp{-I} and @samp{-L} (@pxref{Directory Options}).  These
take precedence over places specified using environment variables, which
in turn take precedence over those specified by the configuration of GNU
CC.  @xref{Driver}.
@end ifset

@table @code
@item TMPDIR
@findex TMPDIR
If @code{TMPDIR} is set, it specifies the directory to use for temporary
files.  GNU CC uses temporary files to hold the output of one stage of
compilation which is to be used as input to the next stage: for example,
the output of the preprocessor, which is the input to the compiler
proper.

@item GCC_EXEC_PREFIX
@findex GCC_EXEC_PREFIX
If @code{GCC_EXEC_PREFIX} is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler.  No slash is added
when this prefix is combined with the name of a subprogram, but you can
specify a prefix that ends with a slash if you wish.

If GNU CC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.

Other prefixes specified with @samp{-B} take precedence over this prefix.

This prefix is also used for finding files such as @file{crt0.o} that are
used for linking.

In addition, the prefix is used in an unusual way in finding the
directories to search for header files.  For each of the standard
directories whose name normally begins with @samp{/usr/local/lib/gcc}
(more precisely, with the value of @code{GCC_INCLUDE_DIR}), GNU CC tries
replacing that beginning with the specified prefix to produce an
alternate directory name.  Thus, with @samp{-Bfoo/}, GNU CC will search
@file{foo/bar} where it would normally search @file{/usr/local/lib/bar}.
These alternate directories are searched first; the standard directories
come next.

@item COMPILER_PATH
@findex COMPILER_PATH
The value of @code{COMPILER_PATH} is a colon-separated list of
directories, much like @code{PATH}.  GNU CC tries the directories thus
specified when searching for subprograms, if it can't find the
subprograms using @code{GCC_EXEC_PREFIX}.

@item LIBRARY_PATH
@findex LIBRARY_PATH
The value of @code{LIBRARY_PATH} is a colon-separated list of
directories, much like @code{PATH}.  GNU CC tries the directories thus
specified when searching for special linker files, if it can't find them
using @code{GCC_EXEC_PREFIX}.  Linking using GNU CC also uses these
directories when searching for ordinary libraries for the @samp{-l}
option (but directories specified with @samp{-L} come first).

@item C_INCLUDE_PATH
@itemx C++_INCLUDE_PATH
@itemx OBJC_INCLUDE_PATH
@findex C_INCLUDE_PATH
@findex C++_INCLUDE_PATH
@findex OBJC_INCLUDE_PATH
@c @itemx OBJC++_INCLUDE_PATH
These environment variables pertain to particular languages.  Each
variable's value is a colon-separated list of directories, much like
@code{PATH}.  When GNU CC searches for header files, it tries the
directories listed in the variable for the language you are using, after
the directories specified with @samp{-I} but before the standard header
file directories.

@item DEPENDENCIES_OUTPUT
@findex DEPENDENCIES_OUTPUT
@cindex dependencies for make as output 
If this variable is set, its value specifies how to output dependencies
for Make based on the header files processed by the compiler.  This
output looks much like the output from the @samp{-M} option
(@pxref{Preprocessor Options}), but it goes to a separate file, and is
in addition to the usual results of compilation.

The value of @code{DEPENDENCIES_OUTPUT} can be just a file name, in
which case the Make rules are written to that file, guessing the target
name from the source file name.  Or the value can have the form
@samp{@var{file} @var{target}}, in which case the rules are written to
file @var{file} using @var{target} as the target name.
@end table
