C programs are constructed from a set of reserved words which provide control and from libraries which perform special functions. The basic instructions are built up using a reserved set of words, such as main, for, if, while, default, double, extern, for, and int, etc., C demands that they are used only for giving commands or making statements. You cannot use a default, for example, as the name of a variable. An attempt to do so will result in a compilation error.

Restrictions apply to keywords

Keywords are the words whose meaning has already been explained to the C compiler and their meanings cannot be changed.
Keywords can be used only for their intended purpose.
Keywords cannot be used as user-defined variables.
All keywords must be written in lowercase.

Keywords have standard, predefined meanings in C. These keywords can be used only for their intended purpose; they cannot be used as programmer-defined identifiers. Keywords are an essential part of a language definition. They implement specific features of the language. Every C word is classified as either a keyword or an identifier. A keyword is a sequence of characters that the C compiler readily accepts and recognizes while being used in a program. 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.

32 Keywords in C Programming 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

Keyword Types

Keyword Types	Keywords
Data types	int char float double
Qualifiers	signed unsigned short long
User-defined	typedef Enum
Storage Classes	auto register static extern
Loop	For While Do
Decision	if else switch case default
Jump	Break continue Goto
Derived	struct union
function	void return
Others	const volatile sizeof

Data types

int	Specifies the integer type of value a variable will hold
char	Specifies the character type of value a variable will hold
float	Specifies the single-precision floating-point of value a variable will hold
double	Specifies the double-precision floating-point type of value a variable will

Qualifiers

signed	Specifies a variable can hold positive and negative integer type of data
unsigned	Specifies a variable can hold only the positive integer type of data
short	Specifies a variable can hold fairly small integer type of data
long	Specifies a variable can hold fairly large integer type of data

User-defined

typedef	Used to define a new name for an existing data type
Enum	Gives an opportunity to invent own data type and define what values the variable of this data type can take

Loop

For	Loop is used when the number of passes is known in advance
While	Loop is used when the number of passes is not known in advance
Do	Loop is used to handle menu-driven programs

Decision

Jump

Break	Used to force immediate termination of a loop, bypassing the conditional expression and any remaining code in the body of the loop
continue	Used to take the control to the beginning of the loop bypassing the statements inside the loop
Goto	Used to take the control to required place in the program

Storage Classes

Storage Classes	Storage	Default initial value	Scope	Life
auto	Memory	An unpredictable value	Local	Till the control remains within the block
register	CPU registers	Garbage value	Local	Till the control remains within the block
static	Memory	Zero	Local	Value of the variable persists between different function calls
extern	Memory	Zero	Global	Till the program's execution doesn't come to an end

Remember: Some Important points

The keywords are also called ‘Reserved words’.
Keywords are the words whose meaning has already been explained to the C compiler and their meanings cannot be changed.
Keywords serve as basic building blocks for program statements.
Keywords can be used only for their intended purpose.
Keywords cannot be used as user-defined variables.
All keywords must be written in lowercase.
32 keywords available in C.

Keywords in C Programming Language: A Comprehensive Guide