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(gcc-300.info)Alternate Keywords

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Alternate Keywords
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   The option `-traditional' disables certain keywords; `-ansi' and the
various `-std' options disable certain others.  This causes trouble
when you want to use GNU C extensions, or ISO C features, in a
general-purpose header file that should be usable by all programs,
including ISO C programs and traditional ones.  The keywords `asm',
`typeof' and `inline' cannot be used since they won't work in a program
compiled with `-ansi' (although `inline' can be used in a program
compiled with `-std=c99'), while the keywords `const', `volatile',
`signed', `typeof' and `inline' won't work in a program compiled with
`-traditional'.  The ISO C99 keyword `restrict' is only available when
`-std=gnu99' (which will eventually be the default) or `-std=c99' (or
the equivalent `-std=iso9899:1999') is used.

   The way to solve these problems is to put `__' at the beginning and
end of each problematical keyword.  For example, use `__asm__' instead
of `asm', `__const__' instead of `const', and `__inline__' instead of
`inline'.

   Other C compilers won't accept these alternative keywords; if you
want to compile with another compiler, you can define the alternate
keywords as macros to replace them with the customary keywords.  It
looks like this:

     #ifndef __GNUC__
     #define __asm__ asm
     #endif

   `-pedantic' and other options cause warnings for many GNU C
extensions.  You can prevent such warnings within one expression by
writing `__extension__' before the expression.  `__extension__' has no
effect aside from this.