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The following options control the dialect of C (or languages derived from C, such as C++ and Objective C) that the compiler accepts:
-ansi
In C mode, support all ANSI standard C programs. In C++ mode, remove GNU extensions that conflict with ANSI C++.
This turns off certain features of GCC that are incompatible with ANSI
C (when compiling C code), or of ANSI standard C++ (when compiling C++ code),
such as the asm
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. For the C compiler,
it disables recognition of C++ style `//' comments as well as
the inline
keyword. For the C++ compiler,
`-foperator-names' is enabled as well.
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.
-fstd=
Determine the language standard. A value for this option must be provided; possible values are
Even when this option is not specified, you can still use some of the
features of newer standards in so far as they do not conflict with
previous C standards. For example, you may use __restrict__
even
when -fstd=c9x is not specified.
-fno-asm
Do not recognize 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
Don't recognize builtin functions that do not begin with `__builtin_'
as prefix. Currently, the functions affected include 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.
-fhosted
Assert that compilation takes place in a hosted environment. This implies
`-fbuiltin'. A hosted environment is one in which the
entire standard library is available, and in which main
has a return
type of int
. Examples are nearly everything except a kernel.
This is equivalent to `-fno-freestanding'.
-ffreestanding
Assert that compilation takes place in a freestanding environment. This
implies `-fno-builtin'. A freestanding environment
is one in which the standard library may not exist, and program startup may
not necessarily be at main
. The most obvious example is an OS kernel.
This is equivalent to `-fno-hosted'.
-trigraphs
Support ANSI C trigraphs. You don't want to know about this brain-damage. The `-ansi' option implies `-trigraphs'.
-traditional
Attempt to support some aspects of traditional C compilers. Specifically:
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.
You may wish to use `-fno-builtin' as well as `-traditional' if your program uses names that are normally GNU C builtin functions for other purposes of its own.
You cannot use `-traditional' if you include any header files that rely on ANSI C features. Some vendors are starting to ship systems with ANSI C header files and you cannot use `-traditional' on such systems to compile files that include any system headers.
The `-traditional' option also enables `-traditional-cpp', which is described next.
-traditional-cpp
Attempt to support some aspects of traditional C preprocessors. Specifically:
__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 (cpp.info)Standard Predefined section `Standard Predefined Macros' in The C Preprocessor,
for more discussion of these and other predefined macros.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void.
-funsigned-char
Let the type char
be unsigned, like unsigned char
.
Each kind of machine has a default for what char
should
be. It is either like unsigned char
by default or like
signed char
by default.
Ideally, a portable program should always use signed char
or
unsigned char
when it depends on the signedness of an object.
But many programs have been written to use plain char
and
expect it to be signed, or expect it to be unsigned, depending on the
machines 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
Let the type char
be signed, like signed char
.
Note that this is equivalent to `-fno-unsigned-char', which is the negative form of `-funsigned-char'. Likewise, the option `-fno-signed-char' is equivalent to `-funsigned-char'.
You may wish to use `-fno-builtin' as well as `-traditional' if your program uses names that are normally GNU C builtin functions for other purposes of its own.
You cannot use `-traditional' if you include any header files that rely on ANSI C features. Some vendors are starting to ship systems with ANSI C header files and you cannot use `-traditional' on such systems to compile files that include any system headers.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bitfield is signed or unsigned, when the
declaration does not use either 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
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. The option `-traditional' also has this effect.
Writing into string constants is a very bad idea; "constants" should be constant.
-fallow-single-precision
Do not promote single precision math operations to double precision, even when compiling with `-traditional'.
Traditional K&R C promotes all floating point operations to double precision, regardless of the sizes of the operands. On the architecture for which you are compiling, single precision may be faster than double precision. If you must use `-traditional', but want to use single precision operations when the operands are single precision, use this option. This option has no effect when compiling with ANSI or GNU C conventions (the default).
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