5.39 Alternate Keywords

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.