Function Declaration and Definition in C Programming

Introduction to Function Declaration and Definition

Function declaration and definition in C programming are core concepts used to create reusable and organized code. These concepts help the compiler understand how a function works and how it should be used.

In C programming, every function must be declared and defined properly for correct execution.

What is Function Declaration in C Programming

Function declaration in C programming specifies the function name, return type, and parameters before it is used in the program. It is also known as a function prototype.

Syntax of Function Declaration

return_type function_name(parameter_list);

Example of Function Declaration

int add(int a, int b);

What is Function Definition in C Programming

Function definition contains the actual implementation or logic of the function.

Syntax of Function Definition

return_type function_name(parameter_list) {

// function body

}

Example of Function Definition

int add(int a, int b) {

return a + b;

}

Complete Example: Declaration, Definition and Function Call

#include <stdio.h>

// Function Declaration
int add(int a, int b);

int main() {
int result;

// Function Call
result = add(10, 5);

printf(“Sum = %d”, result);
return 0;
}

// Function Definition
int add(int a, int b) {
return a + b;
}

Example Without Declaration (Function Before main)

If the function is defined before main(), declaration is optional.

#include <stdio.h>

int add(int a, int b) {
return a + b;
}

int main() {
int result = add(3, 7);
printf(“Result = %d”, result);
return 0;
}

Example with Multiple Functions

#include <stdio.h>

int square(int n);
int cube(int n);

int main() {
int num = 3;

printf(“Square = %d\n”, square(num));
printf(“Cube = %d”, cube(num));

return 0;
}

int square(int n) {
return n * n;
}

int cube(int n) {
return n * n * n;
}

Key Differences Between Declaration and Definition