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 this:
#ifndef __GNUC__ #define __asm__ asm #endif
-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.