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 Alternate keywords  

The option, ‘-traditional’, disables certain keywords; ‘-ansi’ disables certain others. This causes trouble when you want to use GNU C extensions, or ANSI C features, in a general-purpose header file that should be usable by all programs, including ANSI C programs and traditional ones. The keywords, asm, typeof and inline , cannot be used since they won’t work in a program compiled with ‘-ansi’, while the keywords, const, volatile, signed, typeof and inline , won’t work in a program compiled with ‘-traditional’.

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, __const__ instead of const, 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 the following example’s input.

‘-pedantic’ causes 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 problem.