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 -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.
-fbounds-check
-ftrapv
-fwrapv
-fexceptions
-fnon-call-exceptions
SIGALRM
.
-funwind-tables
-fexceptions
, except that it will just generate any needed
static data, but will not affect the generated code in any other way.
You will normally not enable this option; instead, a language processor
that needs this handling would enable it on your behalf.
-fasynchronous-unwind-tables
-fpcc-struct-return
struct
and union
values in memory like
longer ones, rather than in registers. This convention is less
efficient, but it has the advantage of allowing intercallability between
GCC-compiled files and files compiled with other compilers, particularly
the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return
switch is not binary compatible with code compiled with the
-freg-struct-return
switch.
Use it to conform to a non-default application binary interface.
-freg-struct-return
struct
and union
values in registers when possible.
This is more efficient for small structures than
-fpcc-struct-return
.
If you specify neither -fpcc-struct-return
nor
-freg-struct-return
, GCC defaults to whichever convention is
standard for the target. If there is no standard convention, GCC
defaults to -fpcc-struct-return
, except on targets where GCC is
the principal compiler. In those cases, we can choose the standard, and
we chose the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return
switch is not binary compatible with code compiled with the
-fpcc-struct-return
switch.
Use it to conform to a non-default application binary interface.
-fshort-enums
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.
Warning: the -fshort-enums
switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fshort-double
double
as for float
.
Warning: the -fshort-double
switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fshort-wchar
wchar_t
to be short
unsigned int
instead of the default for the target. This option is
useful for building programs to run under WINE.
Warning: the -fshort-wchar
switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fshared-data
const
variables of this
compilation be shared data rather than private data. The distinction
makes sense only on certain operating systems, where shared data is
shared between processes running the same program, while private data
exists in one copy per process.
-fno-common
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.
-fno-ident
#ident
directive.
-finhibit-size-directive
.size
assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the
two halves are placed at locations far apart in memory. This option is
used when compiling crtstuff.c
; you should not need to use it
for anything else.
-fverbose-asm
-fno-verbose-asm
, the default, causes the
extra information to be omitted and is useful when comparing two assembler
files.
-fpic
-fpic
does not work; in that case, recompile with -fPIC
instead. (These maximums are 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. For the 386, GCC supports PIC for System V
but not for the Sun 386i. Code generated for the IBM RS/6000 is always
position-independent.
-fPIC
Position-independent code requires special support, and therefore works
only on certain machines.
-fpie
-fPIE
-fpic
and -fPIC
, but
generated position independent code can be only linked into executables.
Usually these options are used when -pie
GCC option will be
used during linking.
-ffixed-
reg
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
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-saved-
reg
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model 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.
-fpack-struct
Warning: the -fpack-struct
switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Additionally, it makes the code suboptimal.
Use it to conform to a non-default application binary interface.
-finstrument-functions
__builtin_return_address
does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn, void *call_site); void __cyg_profile_func_exit (void *this_fn, void *call_site);
The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.
This currently disables function inlining. This restriction is expected to be removed in future releases.
A function may be given the attribute no_instrument_function
, in
which case this instrumentation will not be done. This can be used, for
example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling functions
cannot safely be called (perhaps signal handlers, if the profiling
routines generate output or allocate memory).
-fstack-check
Note that this switch does not actually cause checking to be done; the
operating system must do that. The switch causes generation of code
to ensure that the operating system sees the stack being extended.
-fstack-limit-register=
reg
-fstack-limit-symbol=
sym
-fno-stack-limit
For instance, if the stack starts at absolute address 0x80000000
and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit
and
-Wl,--defsym,__stack_limit=0x7ffe0000
to enforce a stack limit
of 128KB. Note that this may only work with the GNU linker.
-fargument-alias
-fargument-noalias
-fargument-noalias-global
-fargument-alias
specifies that arguments (parameters) may
alias each other and may alias global storage.
-fargument-noalias
specifies that arguments do not alias
each other, but may alias global storage.
-fargument-noalias-global
specifies that arguments do not
alias each other and do not alias global storage.
Each language will automatically use whatever option is required by
the language standard. You should not need to use these options yourself.
-fleading-underscore
-fno-leading-underscore
, forcibly
change the way C symbols are represented in the object file. One use
is to help link with legacy assembly code.
Warning: the -fleading-underscore
switch causes GCC to
generate code that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary interface.
Not all targets provide complete support for this switch.
-ftls-model=
model
global-dynamic
,
local-dynamic
, initial-exec
or local-exec
.
The default without -fpic
is initial-exec
; with
-fpic
the default is global-dynamic
.