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The ‘extern’ Keyword in C Programming

   January 1, 2024
C Programming @ Freshers.in

In C programming, the ‘extern’ keyword is a powerful tool for declaring external variables and functions, enabling modular and efficient code organization. This article offers a comprehensive explanation of the ‘extern’ keyword, its significance in handling external declarations, and includes real-world examples with outputs to illustrate its role in facilitating code modularity and linkage.

Understanding the ‘extern’ Keyword

The ‘extern’ keyword in C is used to declare external variables and functions. It informs the compiler that the variable or function is defined in another source file, allowing for their use in the current file. ‘extern’ helps achieve code modularity, reusability, and efficient linkage.

Syntax for Declaring External Variables:

extern data_type variable_name;

Syntax for Declaring External Functions:

extern return_type function_name(parameters);

Example 1: Using ‘extern’ for External Variables

// File 1: main.c
#include <stdio.h>
extern int globalVariable; // Declaration of the external variable
int main() {
    printf("The value of globalVariable is %d\n", globalVariable);
    return 0;
}

Output:

The value of globalVariable is 42

In this example, ‘extern’ is used to declare an external variable named globalVariable in main.c. The actual definition of this variable is located in another source file, other.c, where it is assigned the value 42. The ‘extern’ declaration allows main.c to use the variable defined externally.

Benefits of Using ‘extern’

  1. Modular Programming: ‘extern’ facilitates modular programming by separating variable/function declarations from their definitions, promoting code organization.
  2. Code Reusability: ‘extern’ enables the reuse of variables and functions across multiple source files, reducing redundancy.

Example 2: Using ‘extern’ for External Functions

// File 1: main.c
#include <stdio.h>
extern void greet(); // Declaration of the external function
int main() {
    greet(); // Calling the external function
    return 0;
}
// File 2: other.c
#include <stdio.h>
void greet() { // Definition of the external function
    printf("Hello, World!\n");
}

Output:

Hello, World!

In this example, ‘extern’ is used to declare an external function named greet() in main.c. The actual definition of this function is located in another source file, other.c, where it is implemented to print “Hello, World!”.

Common Use Cases for ‘extern’

  1. Multi-File Projects: ‘extern’ is essential in multi-file projects to share variables and functions among different source files.
  2. Library Linkage: ‘extern’ is used in libraries to provide external access to functions and variables defined within the library.
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