Keywords are those predefined words that have special meaning in the compiler and they cannot be used for any other purpose. As per the C99 standard, C language has 32 keywords. Keywords cannot be used as identifiers.
The following table has the list of all keywords (reserved words) available in the 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 unsignedAll the keywords in C have lowercase alphabets, although the keywords that have been newly added in C, do have uppercase alphabets in them. C is a case-sensitive language. Hence, int is a keyword but INT, or Int are not recognized as a keyword. The new keywords introduced from C99 onwards start with an underscore character. The compiler checks the source code for the correctness of the syntax of all the keywords and then translates it into the machine code.
Example of C KeywordsIn the following program, we are using a keyword as an identifier i.e., as the name of the user-defined function, that will cause a compilation error.
#include <stdio.h>
void register(int, int);
int main () {
/* variable definition: */
int a=5, b=7;
register(a,b);
return 0;
}
void register(int a, int b)
{
printf("%d", a+b);
}
Errors
main.c:3:15: error: expected identifier or '(' before 'int'
3 | void register(int, int);
| ^~~
main.c: In function 'main':
main.c:8:14: error: expected ')' before ',' token
8 | register(a,b);
| ^
| )
main.c: At top level:
main.c:12:15: error: expected identifier or '(' before 'int'
12 | void register(int a, int b)
| ^
The reason for the errors is that we are using a keyword register as the name of a user-defined function, which is not allowed.
The ANSI C version has 32 keywords. These keywords are the basic element of the program logic. These keywords can be broadly classified in following types −
Let us discuss the keywords in each category.
Primary Types C KeywordsThese keywords are used for variable declaration. C is a statically type language, the variable to be used must be declared. Variables in C are declared with the following keywords:
int Declares an integer variable long Declares a long integer variable short Declares a short integer variable signed Declares a signed variable double Declares a double-precision variable char Declares a character variable float Declares a floating-point variable unsigned Declares an unsigned variable void Specifies a void return type User-defined Types C KeywordsC language allows you to define new data types as per requirement. The user defined type has one or more elements of primary type.
The following keywords are provided for user defined data types −
struct Declares a structure type typedef Creates a new data type union Declares a union type enum Declares an enumeration type Storage Types C KeywordsThe following set of keywords are called storage specifiers. They indicate the location where in the memory the variables stored. Default storage type of a variable is auto, although you can ask the compiler to form a variable with specific storage properties.
auto Specifies automatic storage class extern Declares a variable or function static Specifies static storage class register Specifies register storage class Conditionals C KeywordsThe following set of keywords help you to put conditional logic in the program. The conditional logic expressed with if and else keywords provides two alternative actions for a condition. For multi-way branching, use switch case construct. In C, the jump operation in an assembler is implemented by the goto keyword.
goto Jumps to a labeled statement if Starts an if statement else Executes when the if condition is false case Labels a statement within a switch switch Starts a switch statement default Specifies default statement in switch Loops and Loop Control C KeywordsRepetition or iteration is an essential aspect of the algorithm. C provides different alternatives for forming a loop, and keywords for controlling the behaviour of the loop. Each of the keywords let you form a loop of different characteristics and usage.
Other C KeywordsThe following miscellaneous keywords are also extremely important:
const Specifies a constant value Sizeof Determines the size of a data type Volatile compiler that the value of the variable may change at any timeIn C99 version, five more keywords were added −
In C11, seven more keywords have been added
When the C23 standard will be released it will introduce 14 more keywords −
Most of the recently reserved words begin with an underscore followed by a capital letter, Since existing program source code should not have been using these identifiers.
Following points must be kept in mind when using the keywords:
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