Return Values

The function print_line from the previous exercise prints, and that's it. But what if you want to keep calculating with a result, for example to add up the areas of all the rooms in a flat? A printed value only shows up in the output, and the program can't work with it any further.

That is why functions can return a value. The statement return sends a value back to the place where the function was called. Before the name of the function, you then write the data type of the value it returns, instead of void:

#include <stdio.h>

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

int main(void) {
    int result = add(2, 3);
    printf("%d\n", result);
    return 0;
}

This program prints:

5

The call add(2, 3) runs the function, and the function returns 5. You can imagine that the call is replaced with the value it returned, so int result = add(2, 3); becomes int result = 5;. The value that a function gives back is called its return value, and its type, here int, is the return type. A function can return a double, a char or a bool too:

#include <stdio.h>

double average(int a, int b) {
    return (a + b) / 2.0;
}

int main(void) {
    printf("%.2f\n", average(7, 8));
    printf("%.2f\n", average(3, 4));
    return 0;
}

This program prints:

7.50
3.50

Notice the 2.0: with 2, it would be whole-number division, and average(3, 4) would return 3.00.

You can use a call anywhere you could write a value: print it right away, calculate with it or put it in a condition:

#include <stdio.h>

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

int main(void) {
    printf("%d\n", add(2, 3));
    int total = add(2, 3) * 2;
    printf("%d\n", total);
    if (add(2, 3) > 4) {
        printf("More than 4\n");
    }
    return 0;
}

This program prints:

5
10
More than 4

In printf("%d\n", add(2, 3)), the program first calls add(2, 3) and then prints the 5 that the function returned. In add(2, 3) * 2, the call becomes 5, and 5 * 2 is 10.

Now you also know what the int before main means: main returns a whole number too. It returns it to whoever started the program, and 0 means that the program ended fine. That is the job of return 0; at the end of main.

A function that isn't void has promised to return a value, so it must return one. Forgetting return is a common mistake. This function prints the sum, but it doesn't return it:

#include <stdio.h>

int add(int a, int b) {
    printf("%d\n", a + b);
}

int main(void) {
    int result = add(2, 3);
    printf("%d\n", result);
    return 0;
}

The program doesn't compile:

main.c:5:1: error: non-void function does not return a value [-Werror,-Wreturn-type]
    5 | }
      | ^
1 error generated.

The error message points to the } at the end of the function and says that a non-void function (a function that isn't void) does not return a value. Elsewhere, compilers often only warn about this; here it's an error, so you can't miss it. The fix is return a + b; in place of the printf.