-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). 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, 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.