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The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:
-ansi
This turns off certain features of GCC that are incompatible with ISO
C90 (when compiling C code), or of 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 ISO trigraph feature. For the C compiler,
it disables recognition of C++ style `//' comments as well as
the inline
keyword.
The alternate keywords __asm__
, __extension__
,
__inline__
and __typeof__
continue to work despite
-ansi. You would not want to use them in an ISO 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-ISO programs to be rejected gratuitously. For that, -pedantic is required in addition to -ansi. See Warning Options.
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
ISO standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions which would normally be built in but do not have semantics
defined by ISO C (such as alloca
and ffs
) are not built-in
functions with -ansi is used. See Other built-in functions provided by GCC, for details of the functions
affected.
-std=
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 -std=c99 is not specified.
The -std options specifying some version of ISO C have the same
effects as -ansi, except that features that were not in ISO C90
but are in the specified version (for example, `//' comments and
the inline
keyword in ISO C99) are not disabled.
See Language Standards Supported by GCC, for details of
these standard versions.
-aux-info
filenameBesides declarations, the file indicates, in comments, the origin of
each declaration (source file and line), whether the declaration was
implicit, prototyped or unprototyped (`I', `N' for new or
`O' for old, respectively, in the first character after the line
number and the colon), and whether it came from a declaration or a
definition (`C' or `F', respectively, in the following
character). In the case of function definitions, a K&R-style list of
arguments followed by their declarations is also provided, inside
comments, after the declaration.
-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, which has the same
effect. In C99 mode (-std=c99 or -std=gnu99), this
switch only affects the asm
and typeof
keywords, since
inline
is a standard keyword in ISO C99.
-fno-builtin
-fno-builtin-
functionGCC normally generates special code to handle certain built-in 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. In addition,
when a function is recognized as a built-in function, GCC may use
information about that function to warn about problems with calls to
that function, or to generate more efficient code, even if the
resulting code still contains calls to that function. For example,
warnings are given with -Wformat for bad calls to
printf
, when printf
is built in, and strlen
is
known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with `__builtin_'. If a function is named this is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n)) #define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
main
has a return
type of int
. Examples are nearly everything except a kernel.
This is equivalent to -fno-freestanding.
-ffreestanding
main
. The most obvious example is an OS kernel.
This is equivalent to -fno-hosted.
See Language Standards Supported by GCC, for details of
freestanding and hosted environments.
-fms-extensions
Some cases of unnamed fields in structures and unions are only
accepted with this option. See Unnamed struct/union fields within structs/unions, for details.
-trigraphs
-no-integrated-cpp
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
-traditional
-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 bit-field is signed, because this is consistent: the
basic integer types such as int
are signed types.