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'
     Support all ANSI standard C programs.

     This turns off certain features of GNU C that are incompatible
     with ANSI C, such as the `asm', `inline' 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, disallows `$' as part of
     identifiers, and disables recognition of C++ style `//' comments.

     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 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.

`-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 two
     leading underscores.  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.

`-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.

        * In C++ programs, assignment to `this' is permitted with
          `-traditional'.  (The option `-fthis-is-variable' also has
          this effect.)

     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.

`'
     In the preprocessor, comments convert to nothing at all, rather
     than to a space.  This allows traditional token concatenation.

`'
     In preprocessing directive, the `#' symbol must appear as the first
     character of a line.

`'
     In the preprocessor, 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.)

`-traditional-cpp'
     Attempt to support some aspects of traditional C preprocessors.
     This includes the last five items in the table immediately above,
     but none of the other effects of `-traditional'.

`-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'.

`-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).