Functions and Calls

A function is reusable code that performs a clear task. You have already used library functions such as printf(), strlen(), and sqrt(). Now we define our own functions.

Defining functions

Use returnType functionName to define a function. A function can receive input and return output.

#include <stdio.h>

int larger(int num1, int num2) {
    if (num1 > num2) {
        return num1;
    } else {
        return num2;
    }
}

int main(void) {
    printf("%d\n", larger(4, 12));
    printf("%d\n", larger(54, 33));
    return 0;
}

Output:

12
54

return gives a result back to the caller and ends the current function call.

Functions without return values

Some functions perform an action without returning a useful result.

#include <stdio.h>

void print_greeting(const char name[]) {
    printf("Hello, %s!\n", name);
}

int main(void) {
    print_greeting("Alice");
    return 0;
}

A function declared with void does not return a result; control returns to the caller when the function finishes.

Function calls

When a function is called, the program temporarily jumps into that function. When it finishes, execution returns to the original position. This can be understood through the call stack.

The following program splits the distance calculation into distance() and the helper function square(). One function can call another so a larger calculation becomes a set of focused steps.

#include <math.h>
#include <stdio.h>

double square(double value) {
    return value * value;
}

double distance(double x1, double y1, double x2, double y2) {
    return sqrt(square(x2 - x1) + square(y2 - y1));
}

int main(void) {
    double x1, y1, x2, y2;

    printf("Enter the first point, for example 0, 0: ");
    scanf("%lf, %lf", &x1, &y1);
    printf("Enter the second point, for example 3, 4: ");
    scanf("%lf, %lf", &x2, &y2);

    printf("Distance: %.1f\n", distance(x1, y1, x2, y2));
    return 0;
}

One possible run:

Enter the first point: 0, 0
Enter the second point: 3, 4
Distance: 5.0

main()

Every C program starts in main(). Other functions must be declared or defined before they are called.

int main(void) {
    printf("Program starts\n");
    return 0;
}