C keywords

Which are the C keywords and what is their meaning? The answer is below. But let's first clear the term itself.

Keywords (or reserved words) in programming are language-specific words that have special meaning. You cannot use them for names of variables or functions. Note that the term "reserved words" also include words that are reserved for future extensions of the language.

C keywords

The meaning of these words is defined by the language. ANSI C had a small set of keywords. They describe the data types, operations and others. In ANSI C there are 32 keywords, the C99 standard adds 5 and C11 adds 7. Additionally, different compilers may define their own (for instance Microsoft’s Visual Studio defines 19 more).

Keywords defined by ANSI and ISO C90.

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

Words added by ISO C99

Originally, the keywords in C were in lowercase letters only. This was the case with ANSI(which became ISO C90). Later some of the new once started with an underscore and a capital letter. The purpose of this is to keep compatibility with already written code. Since this was an uncommon way for naming in C it is likely that most of the code written through the years does not contain these words.

_Bool	_Imaginary	restrict
_Complex	inline

Words added by ISO C11

Note that not all compilers support the new keywords. You should check with your compiler before using the new once. It is certain that it will support the 32 words from the ANSI standard and most of the time this is enough.

_Alignas	_Generic	_Thread_local
_Alignof	_Noreturn
_Atomic	_Static_assert

C Keywords and macros from the standard library

As we said, starting with an underscore and a capital letter is not common for reserved words in C. That’s why usually the new words are used through their macros, which keeps the code style clean.

Keyword	Macro	Macro defined in
_Alignas	alignas	stdalign.h
_Alignof	alignof	stdalign.h
_Atomic	Depends on the type (atomic_bool, atomic_char, atomic_int….)	stdatomic.h
_Bool	bool	stdbool.h
_Complex	complex	complex.h
_Imaginary	imaginary	complex.h
_Noreturn	noreturn	stdnoreturn.h
_Static_assert	static_assert	assert.h

The rest of the words don’t have macros in the standard library.

Related reading:   
-    Preprocessor directives, used in C language.
-    The standard library
-    C operators

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