Expressions
C language

C identifiers start with a letter (or an underlined blank) and can continue with letters, numbers or underlined blanks. Only the first thirty-one letters are discriminating for the compiler, which differentiates between upper and lower case. There are thirty-two reserved keywords that cannot be used as identifiers (see Box 1).

We also advise against using identifiers for functions or macros predefined in standard libraries, as well as names beginning with underlined blanks, not forgetting the traditional asm...