C Function Declaration and Definition

Function Declaration and Definition

You just learned from the previous chapters that you can create and call a function in the following way:

Example

// Create a function
void myFunction() {
printf("I just got executed!");
}

int main() {
myFunction(); // call the function
return 0;
}

Try it Yourself »

A function consist of two parts:

Declaration: the function's name, return type, and parameters (if any)
Definition: the body of the function (code to be executed)

void myFunction() { // declaration
// the body of the function (definition)
}

For code optimization, it is recommended to separate the declaration and the definition of the function.

You will often see C programs that have function declaration above main(), and function definition below main(). This will make the code better organized and easier to read:

Example

// Function declaration
void myFunction();

// The main method
int main() {
myFunction(); // call the function
return 0;
}

// Function definition
void myFunction() {
printf("I just got executed!");
}

Try it Yourself »

Another Example

If we use the example from the previous chapter regarding function parameters and return values:

Example

int myFunction(int x, int y) {
return x + y;
}

int main() {
int result = myFunction(5, 3);
printf("Result is = %d", result);
return 0;
}
// Outputs 8 (5 + 3)

Try it Yourself »

It is considered good practice to write it like this instead:

Example

// Function declaration
int myFunction(int, int);

// The main method
int main() {
int result = myFunction(5, 3); // call the function
printf("Result is = %d", result);
return 0;
}

// Function definition
int myFunction(int x, int y) {
return x + y;
}

Try it Yourself »

❮ Previous Next ❯