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### 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.