C Function Declaration vs. Definition

A function declaration, also known as a prototype, tells the compiler about the function’s name, return type, and parameters without providing the actual body. In contrast, a function definition provides the complete implementation of the function.

Function Declaration (Prototype)

A function declaration informs the compiler about a function’s existence. It specifies the function’s return type, name, and parameters, allowing the compiler to ensure that function calls match the declared signature. Declarations are typically placed at the beginning of a file or in header files.

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// Function declaration (prototype)
int add(int a, int b);

Explanation:

int add(int a, int b); tells the compiler that there is a function named add that takes two integer parameters and returns an integer.
This line does not include the function body, meaning the actual code that performs the addition is defined elsewhere.
Function declarations help in checking the correctness of function calls before the function is defined.

Function Definition

A function definition provides the full implementation of a function. It includes the function body where the actual code logic is written.

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

Explanation:

int add(int a, int b) matches the declaration: it specifies that the function takes two integers and returns an integer.
The curly braces { ... } enclose the function body where the actual addition is performed.
return a + b; calculates the sum of the parameters and returns the result.

Putting It All Together: Example Program

This example demonstrates how to declare and define a function in a single C program. We declare the function prototype at the top and then provide its definition later in the code. Finally, we call the function in the main() function.

main.c

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#include <stdio.h>

// Function declaration (prototype)
int add(int a, int b);

int main() {
    int result = add(5, 3);
    printf("The sum is: %d\n", result);
    return 0;
}

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

Explanation:

The #include <stdio.h> directive includes the standard input/output library for using printf().
The function prototype int add(int a, int b); informs the compiler about the existence and signature of the add function.
Inside the main() function, we call add(5, 3) and store the returned value in the variable result.
printf() is used to print the result to the console.
The function definition int add(int a, int b) { return a + b; } provides the implementation of the function, performing the addition.

Conclusion

In this tutorial, we learned the difference between function declaration and function definition in C:

Function Declaration: Tells the compiler about the function’s signature without providing its body.
Function Definition: Provides the complete implementation of the function.
Example Program: Demonstrated how to declare, define, and call a function within a single C program.

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C Function Declaration vs. Definition