Using and Porting GNU CC
The following options control the dialect of C (or languages derived from C, such as C++ and Objective C) that the compiler accepts:
This turns off certain features of GNU C that are incompatible with ANSI
C, such as the
typeof keywords, and
predefined macros such as
vax that identify the
type of system you are using. It also enables the undesirable and
rarely used ANSI trigraph feature, and disallows `
$' as part of
The alternate keywords
__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
__vax__ are also available, with or
-ansi' option does not cause non-ANSI programs to be
rejected gratuitously. For that, `
-pedantic' is required in
addition to `
-ansi'. See Warning Options.
__STRICT_ANSI__ is predefined when the `
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.
_exit are not builtin functions when `
-ansi' is used.
typeofas a keyword. These words may then be used as identifiers. You can use the keywords
-ansi' implies `
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
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.
-ansi' option prevents
ffs from being
builtin functions, since these functions do not have an ANSI standard
-ansi' option implies `
externdeclarations take effect globally even if they are written inside of a function definition. This includes implicit declarations of functions.
volatileare not recognized. (You can still use the alternative keywords such as
__inline__, and so on.)
unsigned charpromote to
0xe-0xd', are treated as expressions instead.
registerare preserved by
longjmp. Ordinarily, GNU C follows ANSI C: automatic variables not declared
volatilemay be clobbered.
\x' and `
\a' evaluate as the literal characters `
x' and `
a' respectively. Without `
\x' is a prefix for the hexadecimal representation of a character, and `
\a' produces a bell.
thisis permitted with `
-traditional'. (The option `
-fthis-is-variable' also has this effect.)
You may wish to use `
-fno-builtin' as well as `
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.
#' symbol must appear as the first character of a line.
__STDC__is not defined when you use `
__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. See Standard Predefined, for more discussion of these and other predefined macros.
\'). (Without `
-traditional', string constants can contain the newline character as typed.)
charbe unsigned, like
Each kind of machine has a default for what
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
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.
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.
charbe signed, like
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 `
unsigned. By default, such a bitfield is signed, because this is consistent: the basic integer types such as
intare signed types.
However, when `
-traditional' is used, bitfields are all unsigned
no matter what.
-traditional' also has this effect.
Writing into string constants is a very bad idea; ``constants'' should be constant.
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).