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Functions in C

Split a program into small functions with parameters and return types.

A function in C has a return type, a name, and a list of parameters, each with its own type. Here is a function that takes two int values and returns their larger one.

C
Open in playground
#include <stdio.h>

int max_of(int a, int b) {
    if (a > b) {
        return a;
    }
    return b;
}

int main(void) {
    printf("Larger: %d\n", max_of(8, 13));
    printf("Larger: %d\n", max_of(-2, -9));
    return 0;
}
Output
Larger: 13
Larger: -2

void functions

If a function does not return anything, its return type is void.

C
Open in playground
#include <stdio.h>

void print_line(int length) {
    for (int i = 0; i < length; i++) {
        printf("-");
    }
    printf("\n");
}

int main(void) {
    print_line(10);
    printf("  Report\n");
    print_line(10);
    return 0;
}
Output
----------
  Report
----------

Declaring before use

The compiler reads your file from top to bottom. If you want to put main first, write a prototype (just the first line of the function followed by a semicolon) above it.

C
Open in playground
#include <stdio.h>

double celsius_to_fahrenheit(double c);   /* prototype */

int main(void) {
    printf("37 C = %.1f F\n", celsius_to_fahrenheit(37));
    return 0;
}

double celsius_to_fahrenheit(double c) {
    return c * 9 / 5 + 32;
}
Output
37 C = 98.6 F

GATE-style practice questions

GATE-styleMCQ · one correct answer

What does the following program print?

#include <stdio.h>
void swap(int a, int b) {
    int t = a;
    a = b;
    b = t;
}
int main(void) {
    int x = 1, y = 2;
    swap(x, y);
    printf("%d %d\n", x, y);
    return 0;
}
Show answer and explanationAnswer: A

C passes arguments by value. swap receives copies of x and y and exchanges only the copies; the originals in main do not change. Swapping the callers' variables needs pointers: swap(&x, &y).

GATE-styleNAT · numerical answer

What does the following program print?

#include <stdio.h>
int f(int n) {
    return n * 2;
}
int g(int n) {
    return f(n) + f(n + 1);
}
int main(void) {
    printf("%d\n", g(f(3)));
    return 0;
}
Show answer and explanationAnswer: 26

f(3) = 6. g(6) = f(6) + f(7) = 12 + 14 = 26.