In the C programming language, keywords are reserved words that have a special, predefined meaning to the compiler. These words are the fundamental building blocks of the C language and are used to construct program statements.

You can think of keywords as the verbs and nouns of the C language; they define the structure and actions of a program.

Key Properties

·         Reserved: Keywords are reserved by the language and cannot be used for any other purpose.

·         Predefined Meaning: The compiler knows exactly what each keyword is supposed to do.

·         Lowercase: All keywords in C must be written in lowercase.

Most importantly, you cannot use a keyword as the name for a variable, function, or any other identifier.

List of ANSI C Keywords

The ANSI C standard (C89/C90) defines 32 keywords, which are often grouped by their function:

Note: Newer standards of C (like C99 and C11) have added a few more keywords (e.g., _Bool, inline), but the 32 listed above are the most fundamental.

Example of Keywords in a Program

This example demonstrates how several keywords are used together to create a simple program.

C

#include <stdio.h>

int main() {

    // 'const' and 'int' are keywords

    const int MAX_VALUE = 10;

    // 'for' is a keyword for creating a loop

    for (int i = 1; i <= MAX_VALUE; i++) {

        // 'if' and 'else' are keywords for decision making

        if (i % 2 == 0) {

            printf("%d is an even number.\n", i);

        } else {

            printf("%d is an odd number.\n", i);

        }

    }

    // 'return' is a keyword to exit the function

    return 0;

}

Code Breakdown:

·         int: A data type keyword used to declare integer variables (MAX_VALUE and i).

·         const: A type qualifier keyword that makes the variable MAX_VALUE a read-only constant.

·         for: A control flow keyword used to create a loop that iterates from 1 to 10.

·         if...else: Control flow keywords used to check a condition. The program prints one message if a number is even and another if it's odd.

·         return: A jump keyword that ends the main function and returns the value 0 to the operating system.

Trying to use a keyword as a variable name will result in a compilation error:

C

// THIS IS WRONG AND WILL CAUSE AN ERROR

int if = 5; // Error: 'if' is a reserved keyword.