6.45 Alternate Keywords

‘-ansi’ and the various ‘-std’ options disable certain keywords. This causes trouble when you want to use GNU C extensions, or a general-purpose header file that should be usable by all programs, including ISO C programs. The keywords asm, typeof and inline are not available in programs compiled with ‘-ansi’ or ‘-std’ (although inline can be used in a program compiled with ‘-std=c99’ or ‘-std=c11’). 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’), or an option for a later standard version, 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, 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.

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