-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
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
__inline__ instead of
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.