A C keyword cannot be used as a function name or a variable in a program. C keywords are all in lower cases. C is a case dependent language. That means int and INT are different. Here are the 32 main keywords of C language: auto, double, int, struct, break, else, long, switch, case, enum, register, typedef, char, extern, return, union, const, float, short, unsigned, continue, for, signed, void, default, goto, sizeof, volatile, do, if, static, while. Out of these keywords: enum, const, signed, void, and volatile were defined by ANSI.
Later, some extra keywords were also defined: asm, _cs, _ds, _es, _ss, cdecl, far, huge, interrupt, near, pascal