The following options control the dialect of C (or languages derived from C, such as C++ and Objective C) that the compiler accepts:
-ansi
asm
, inline
and typeof
keywords, and
predefined macros such as unix
and vax
that identify the
type of system you are using. It also enables the undesirable and
rarely used ANSI trigraph feature, disallows `$' as part of
identifiers, and disables recognition of C++ style `//' comments.
The alternate keywords __asm__
, __extension__
,
__inline__
and __typeof__
continue to work despite
`-ansi'. You would not want to use them in an ANSI C program, of
course, but it is useful to put them in header files that might be included
in compilations done with `-ansi'. Alternate predefined macros
such as __unix__
and __vax__
are also available, with or
without `-ansi'.
The `-ansi' option does not cause non-ANSI programs to be
rejected gratuitously. For that, `-pedantic' is required in
addition to `-ansi'. See section Options to Request or Suppress Warnings.
The macro __STRICT_ANSI__
is predefined when the `-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.
The functions alloca
, abort
, exit
, and
_exit
are not builtin functions when `-ansi' is used.
-fno-asm
asm
, inline
or typeof
as a
keyword, so that code can use these words as identifiers. You can use
the keywords __asm__
, __inline__
and __typeof__
instead. `-ansi' implies `-fno-asm'.
In C++, this switch only affects the typeof
keyword, since
asm
and inline
are standard keywords. You may want to
use the `-fno-gnu-keywords' flag instead, as it also disables the
other, C++-specific, extension keywords such as headof
.
-fno-builtin
abort
,
abs
, alloca
, cos
, exit
, fabs
,
ffs
, labs
, memcmp
, memcpy
, sin
,
sqrt
, strcmp
, strcpy
, and strlen
.
gcc
normally generates special code to handle certain builtin functions
more efficiently; for instance, calls to alloca
may become single
instructions that adjust the stack directly, and calls to memcpy
may become inline copy loops. The resulting code is often both smaller
and faster, but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library.
The `-ansi' option prevents alloca
and ffs
from being
builtin functions, since these functions do not have an ANSI standard
meaning.
-trigraphs
-traditional
extern
declarations take effect globally even if they
are written inside of a function definition. This includes implicit
declarations of functions.
typeof
, inline
, signed
, const
and volatile
are not recognized. (You can still use the
alternative keywords such as __typeof__
, __inline__
, and
so on.)
unsigned short
and unsigned char
promote
to unsigned int
.
register
are preserved by
longjmp
. Ordinarily, GNU C follows ANSI C: automatic variables
not declared volatile
may be clobbered.
this
is permitted with
`-traditional'. (The option `-fthis-is-variable' also has
this effect.)
__STDC__
is not defined when you use
`-traditional', but __GNUC__
is (since the GNU extensions
which __GNUC__
indicates are not affected by
`-traditional'). If you need to write header files that work
differently depending on whether `-traditional' is in use, by
testing both of these predefined macros you can distinguish four
situations: GNU C, traditional GNU C, other ANSI C compilers, and other
old C compilers. The predefined macro __STDC_VERSION__
is also
not defined when you use `-traditional'. See section `Standard Predefined Macros' in The C Preprocessor,
for more discussion of these and other predefined macros.
-traditional-cpp
-fcond-mismatch
-funsigned-char
char
be unsigned, like unsigned char
.
Each kind of machine has a default for what char
should
be. It is either like unsigned char
by default or like
signed char
by default.
Ideally, a portable program should always use signed char
or
unsigned char
when it depends on the signedness of an object.
But many programs have been written to use plain char
and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let you
make such a program work with the opposite default.
The type char
is always a distinct type from each of
signed char
or unsigned char
, even though its behavior
is always just like one of those two.
-fsigned-char
char
be signed, like signed char
.
Note that this is equivalent to `-fno-unsigned-char', which is
the negative form of `-funsigned-char'. Likewise, the option
`-fno-signed-char' is equivalent to `-funsigned-char'.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
signed
or unsigned
. By
default, such a bitfield is signed, because this is consistent: the
basic integer types such as int
are signed types.
However, when `-traditional' is used, bitfields are all unsigned
no matter what.
-fwritable-strings
-fallow-single-precision