In C language, functions, labels, variables, etc. are known as identifiers. There is a particular way to name these identifiers:
1) An identifier should not be a keyword of C.
2) Its name should not match to a standard library function.
3) An identifier is case sensitive.
4) It should contain only letters, digits and underscores.
5) The first character must be a letter or an underscore.
6) There is no character limit for the length of an identifier.
7) For external linking no matter how many characters are there in the identifier only first 6 characters are important.
8) For internal linking no matter how many characters are there in the identifier only first 31 characters are important.