Keywords in C

C keywords are reserved words that have predefined meanings recognized by the compiler. They form the basic building blocks of the C language and cannot be used as identifiers such as variable names, function names, or structure names.

Have predefined meanings and form part of the C language syntax.
Cannot be used as identifiers such as variable names, function names, or structure names.

#include <stdio.h>

int main(){
    
    int x = 10;

    if (x == 10)
    {
        printf("Successful demonstration of keywords.");
    }
    else
    {
        printf("Failed");
    }

    return 0;
}

Output

Successful demonstration of keywords.

What Happens if We Use a Variable Name Same as a Keyword?

Using a keyword as a variable name results in a compilation error because keywords are reserved by the language.

#include <stdio.h>

int main()
{
    int return = 10;

    printf("%d", return);

    return 0;
}

Output:

error: expected identifier before 'return'

Important Points

Keywords are reserved words and cannot be used as identifiers.
C is a case-sensitive language; if and IF are treated as different tokens.
The number of keywords depends on the C standard version.
ANSI C (C89/C90) contains 32 keywords.
Modern C standards such as C99, C11, and C23 introduce additional keywords.

C Keywords List

The following are the most commonly used keywords in C.

Data Type Keywords

Used to declare variables and define the type of data they can store.

Keyword	Usage
char	Declares a character data type.
int	Declares an integer data type.
float	Declares a single-precision floating-point type.
double	Declares a double-precision floating-point type.
void	Specifies no value or return type.
short	Declares a short integer.
long	Declares a long integer.
signed	Declares a signed data type.
unsigned	Declares an unsigned data type.

Control Flow Keywords

Used to control the execution flow of a program.

Keyword	Usage
if	Executes code when a condition is true.
else	Executes code when an if condition is false.
switch	Selects one block from multiple choices.
case	Defines a branch inside a switch statement.
default	Executes when no case matches.
for	Creates a for loop.
while	Creates a while loop.
do	Creates a do-while loop.
break	Terminates a loop or switch statement.
continue	Skips the current loop iteration.
goto	Transfers control to a labeled statement.
return	Exits a function and optionally returns a value.

Storage Class Keywords

Used to define the scope, lifetime, and visibility of variables.

Keyword	Usage
auto	Declares an automatic local variable.
register	Requests storage in a CPU register.
static	Preserves variable value between function calls.
extern	Refers to a variable defined elsewhere.

User-Defined Type Keywords

Used to create custom data types.

Keyword	Usage
struct	Defines a structure.
union	Defines a union.
enum	Defines an enumeration.
typedef	Creates an alias for an existing data type.

Qualifier Keywords

Used to modify the behavior of variables.

Keyword	Usage
const	Makes a variable read-only.
volatile	Indicates that a variable may change unexpectedly.
restrict	Specifies exclusive access to an object through a pointer (C99).

Memory Management and Utility Keywords

Used for low-level programming and compiler-specific operations.

Keyword	Usage
sizeof	Returns the size of a data type or object in bytes.

Keyword Vs Identifier