6.49 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 a later standard). 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 wont 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 suppress such warnings using the keyword __extension__. Specifically:

• Writing __extension__ before an expression prevents warnings about extensions within that expression.
• In C, writing:

  [[__extension__ ]]
  

  suppresses warnings about using [[]] attributes in C versions that predate C23.

__extension__ has no effect aside from this.