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 2.6 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
iso9899:1990
Same as -ansi
iso9899:199409
ISO C as modified in amend. 1
iso9899:199x
ISO C 9x
c89
same as -std=iso9899:1990
c9x
same as -std=iso9899:199x
gnu89
default, iso9899:1990 + gnu extensions
gnu9x
iso9899:199x + gnu extensions
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:
All extern declarations take effect globally even if they
are written inside of a function definition. This includes implicit
declarations of functions.
The newer keywords typeof, inline, signed, const
and volatile are not recognized. (You can still use the
alternative keywords such as __typeof__, __inline__, and
so on.)
Comparisons between pointers and integers are always allowed.
Integer types unsigned short and unsigned char promote
to unsigned int.
Out-of-range floating point literals are not an error.
Certain constructs which ANSI regards as a single invalid preprocessing
number, such as `0xe-0xd', are treated as expressions instead.
String "constants" are not necessarily constant; they are stored in
writable space, and identical looking constants are allocated
separately. (This is the same as the effect of
`-fwritable-strings'.)
All automatic variables not declared register are preserved by
longjmp. Ordinarily, GNU C follows ANSI C: automatic variables
not declared volatile may be clobbered.
The character escape sequences `\x' and `\a' evaluate as the
literal characters `x' and `a' respectively. Without
`-traditional', `\x' is a prefix for the hexadecimal
representation of a character, and `\a' produces a bell.
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:
Comments convert to nothing at all, rather than to a space. This allows
traditional token concatenation.
In a preprocessing directive, the `#' symbol must appear as the first
character of a line.
Macro arguments are recognized within string constants in a macro
definition (and their values are stringified, though without additional
quote marks, when they appear in such a context). The preprocessor
always considers a string constant to end at a newline.
The predefined macro __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.
The preprocessor considers a string constant to end at a newline (unless
the newline is escaped with `\'). (Without `-traditional',
string constants can contain the newline character as typed.)
-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).