Keywords and Identifiers of C Language

Identifiers: 
Identifiers are names that are given to various program elements, such as variables, functions and arrays. These are user-defined. Identifiers consist of letters, digits and some characters but the first letter must be a letter. Generally lowercase letter is used but uppercase letter can also be used. Uppercase and lowercase letters are not interchangeable.

Identifiers can be of any length but typically it accepts 31 characters.

The following names are valid identifiers.

a   x   xy   firstname  lastName  area_of_circle   _length  test18

The followings are not valid identifiers.
         18test                   The first character must be a letter
         "xx"                      Illegal character "
         first name             Blank space
         last-name              Illegal character -

Keywords:

In C programming language, there are some certain reserve words. These reserve words are called Keyword. Keywords can be used only in particular purpose that is defined in language. So, keywords cannot be used as identifiers. Keywords have standard predefined meanings.

C keyword list is below:

      

Keywords in C Language

auto	double	int	struct
break	else	long	switch
case	enum	register	typedef
char	extern	return	union
continue	for	signed	void
do	if	static	while
default	goto	sizeof	volatile
const	float	short	unsigned

Note that the keywords are all lowercase. Since uppercase and lowercase characters are not equivalent, it is possible to utilize an uppercase keyword as an identifier. Normally, however, this is not done, as it is considered a poor programming practice.