GNU Info
(gcc-295.info)C Dialect Options
Options Controlling C Dialect
=============================
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'. Note: 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
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.
`-std='
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'. *Note Standard Predefined Macros:
(cpp.info)Standard Predefined, 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).
automatically generated by info2www version 1.2.2.9