The following options control the dialect of C (or languages derived from C, such as 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=
c89
iso9899:1990
-ansi).
iso9899:199409
c99
c9x
iso9899:1999
iso9899:199x
c9x and iso9899:199x are deprecated.
gnu89
gnu99
gnu9x
gnu9x is deprecated.
c++98
gnu++98
-std=c++98 plus GNU extensions. This is the
default for C++ code.
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-function__builtin_ as prefix. See Other built-in functions provided by GCC, for details of the functions affected,
including those which are not built-in functions when -ansi or
-std options for strict ISO C conformance are used because they
do not have an ISO standard meaning.
GCC 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.
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
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.
See Language Standards Supported by GCC, for details of
freestanding and hosted environments.
-fms-extensions
-trigraphs
-ansi option (and -std
options for strict ISO C conformance) implies -trigraphs.
-no-integrated-cpp
-B option. The user supplied compilation step can then add in
an additional preprocessing step after normal preprocessing but before
compiling. The default is to use the integrated cpp (internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and
"cc1obj" are merged.
-traditional
-traditional-cpp
-E switch.
The preprocessor continues to support a pre-standard mode. See the GNU
CPP manual for details.
-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.
-fwritable-strings
Writing into string constants is a very bad idea; "constants" should be constant.
This option is deprecated.