GNU Info
(gcc-300.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 ISO C89 programs. In C++ mode, remove GNU
extensions that conflict with ISO C++.
This turns off certain features of GCC that are incompatible with
ISO C89 (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'. 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
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. Note: Other built-in
functions provided by GCC, for details of the
functions affected.
`-std='
Determine the language standard. This option is currently only
supported when compiling C. A value for this option must be
provided; possible values are
`c89'
`iso9899:1990'
ISO C89 (same as `-ansi').
`iso9899:199409'
ISO C89 as modified in amendment 1.
`c99'
`c9x'
`iso9899:1999'
`iso9899:199x'
ISO C99. Note that this standard is not yet fully supported;
see `http://gcc.gnu.org/c99status.html' for more information.
The names `c9x' and `iso9899:199x' are deprecated.
`gnu89'
Default, ISO C89 plus GNU extensions (including some C99
features).
`gnu99'
`gnu9x'
ISO C99 plus GNU extensions. When ISO C99 is fully
implemented in GCC, this will become the default. The name
`gnu9x' is deprecated.
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 C89
but are in the specified version (for example, `//' comments and
the `inline' keyword in ISO C99) are not disabled.
Note: Language Standards Supported by GCC, for details
of these standard versions.
`-aux-info FILENAME'
Output to the given filename prototyped declarations for all
functions declared and/or defined in a translation unit, including
those in header files. This option is silently ignored in any
language other than C.
Besides 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'
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, 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'
Don't recognize built-in functions that do not begin with
`__builtin_' as prefix. Note: 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.
In C++, `-fno-builtin' is always in effect. The `-fbuiltin'
option has no effect. Therefore, in C++, the only way to get the
optimization benefits of built-in functions is to call the function
using the `__builtin_' prefix. The GNU C++ Standard Library uses
built-in functions to implement many functions (like
`std::strchr'), so that you automatically get efficient code.
`-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'.
Note: Language Standards Supported by GCC, for details
of freestanding and hosted environments.
`-trigraphs'
Support ISO C trigraphs. The `-ansi' option (and `-std' options
for strict ISO C conformance) 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 ISO 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 ISO 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 built-in functions
for other purposes of its own.
You cannot use `-traditional' if you include any header files that
rely on ISO C features. Some vendors are starting to ship systems
with ISO 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 ISO 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.
This option is not supported for C++.
`-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'.
`-fsigned-bitfields'
`-funsigned-bitfields'
`-fno-signed-bitfields'
`-fno-unsigned-bitfields'
These options control whether a bit-field is signed or unsigned,
when the declaration does not use either `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.
However, when `-traditional' is used, bit-fields 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 ISO or GNU C conventions (the default).
`-fshort-wchar'
Override the underlying type for `wchar_t' to be `short unsigned
int' instead of the default for the target. This option is useful
for building programs to run under WINE.
automatically generated by info2www version 1.2.2.9