c > functions > functions


These are reusable sections of code. They can be built-in or user-defined. Functions allow us to write code that can be modified in one place and change the effect in multiple sections. Functions are also sometimes referred to as routines because of how we use them to group a set of related steps under a single name. A function in C follows the format:

return_type function_name(argument_1,argument_2,...){
    return return_value;

Here is an example of a function being used to draw a line:

#include <stdio.h>

void print_line(){

int main(){
    printf("Hello there!!\n");
    return 0;

↗ Open 📋 Copy c/functions/print_line.c

We call the function print_line() from the main() function. We pass the control from the main() function to the print_line() function and when it is finished executing it passes control back to the main() function.


Functions with arguments but no return values

A parameter is a variable which stores a value being passed into a function. When we give a parameter a value, we refer to this value as an argument. Arguments are the inputs of a function. A function may have any number of arguments even no arguments.

#include <stdio.h>

void count_to(int number){
    printf("Let's count from 1 to %d\n",number);
    for (int i=1;i<=number;i++){

int main(){
    return 0;

↗ Open 📋 Copy c/functions/count_to.c

The function count_to() accepts one integer argument and runs a for loop to count from 1 to that number. Notice that we did not have to create a variable within the function count_to() in order for the program to work. By putting int number in the parentheses of the function definition we are declaring a local automatic variable called number.

A local automatic variable can only be accessed within the function's body and is created without us having to put it in the function's body (automatic).

Declarative programming

C is a procedural language which means that we specify the steps (the procedure) that the computer must take in order to achieve the desired outcome. When we engage in declarative programming, we tell the computer what to do rather than how to do it.

Functions are one way in which we can achieve this declarative style of coding. It is much easier to run count_to(1500) than it is to type out the steps required to do the counting. Thus by using the interface count_to(number), we tell the computer what to do without telling it how to do it.

Of course, someone would have had to write the implementation of the function in the first place and that someone might even be you. The interesting idea is that this implementation can be done using a for loop or a while loop or a goto directive. Regardless of the way we do the implementation, the code count_to(1500) will always do exactly that - count to 1500.

Our coding style should move towards being declarative (telling the computer what to do) instead of always being imperative (telling the computer how to do it). The current n lines of code should make the next n lines of code easier to write.


Unless you've read the implementation of printf() from the source code of your C compiler, you won't know what the printf() function does in order to output text to the terminal. That's the amazing thing about declarative programming. We focus on what should be achieved rather than telling the machine how to achieve it.

Read more in Functions part 2