C Function

Summary: in this tutorial, you will learn about the C functions and how to define your functions.

Introduce to C functions. #

In programming, you often organize a large program in smaller pieces. Each piece is more manageable than the original program. To divide the program into smaller pieces, you use functions.

A function is a named block of code that performs a specific task.

In the previous tutorials, you have learned various C functions such as printf() and scanf() from the stdio.h library.

To output a piece of text to the standard output, you call the printf() function. When you call the printf() function, C will execute the code of the printf() function.

A function may accept inputs. The inputs of a function are called arguments. For example, the printf() function accepts multiple arguments:

printf("Counter %d", counter);Code language: JavaScript (javascript)

In this example, the "Counter %d" and counter variable are the inputs of the printf() function. They are also called the arguments of the printf() function.

A function may return a value. For example, the sqrt() function from the math.h library returns the square root of a number:

float number = 81;
float result = sqrt(number); // 9Code language: JavaScript (javascript)

Besides providing standard functions, C allows you to define your functions. These functions are known as custom functions.

You must specify a function prototype and function definition to define a custom function.

Function prototype #

The following shows how to define a function prototype:

return_type function_name(parameter_list);

In this syntax:

• First, specify the type of return value. If a function doesn’t return any value, you use the void keyword.
• Second, provide the name of the function. The function name should describe exactly what it does.
• Third, specify the parameters within the parentheses that follow the function name. If a function has multiple parameters, you use a comma (,) to separate two parameters. If the function has no parameters, you can leave it empty or use the void keyword.

Function definition #

The syntax of the function definition is as follows:

return_type function_name(parameter_list)
{
    // statements
}Code language: JavaScript (javascript)

The function definition looks like the function prototype, except it has a function body.

The function body is wrapped between curly braces {} that follow the parentheses. Inside the function body, you can have one or more statements.

Return value

To return a value from a function, you use the return statement followed by a return value:

return expression;Code language: JavaScript (javascript)

When the program encounters the return statement, it immediately stops the function and returns the value to the caller.

If you want to stop the function without returning a value, you can omit the expression in the return statement like this:

return;Code language: JavaScript (javascript)

Typically, you place the function prototype at the beginning of the file before the main() function. You define the function after the main() function as follows:

#include <stdio.h>

// function prototypes
return_type function_name(parmeter_list);

int main()
{
    // statements
    return 0;
}

// function definition
return_type function_name(parmeter_list)
{
    // function body
}
Code language: PHP (php)

When you call a function, the C compiler checks the function call against the function prototype. If they don’t match, the compiler issues a compilation error.

For example, if the function prototype has three parameters, but the function call only has two, the compiler will issue a compilation error.

Also, if the function prototype and function definition disagree, the compiler will issue a compilation error.

C function example #

The following example illustrates how to create a custom function. It prompts for two numbers and displays the larger number.

#include <stdio.h>

int max(int a, int b);

int main()
{
    int x, y, m;
    // prompt for two integers
    puts("Enter two numbers to find the max:");
    scanf("%d", &x);
    scanf("%d", &y);

    // find the max
    m = max(x, y);
    printf("The max of %d and %d is %d", x, y, m);

    return 0;
}

// return the max of two numbers
int max(int a, int b)
{
    if (a > b)
        return a;
    else
        return b;
}Code language: C++ (cpp)

How it works.

First, declare a function prototype that finds the maximum number of two numbers:

int max(int a, int b);

The max() function has two parameters of type integer (int). It also returns an integer (int).

Second, define the max() function definition after the main() function:

// return the max of two numbers
int max(int a, int b)
{
    if (a > b)
        return a;
    else
        return b;
}Code language: JavaScript (javascript)

The max() function returns a if a is greater b. Otherwise, it returns b.

Third, prompt for two integers in the main() function:

puts("Enter two numbers to find the max:");
scanf("%d", &x);
scanf("%d", &y);Code language: JavaScript (javascript)

Fourth, call the max() function to get the max of the two input numbers.

// find the max
m = max(x, y);
Code language: JavaScript (javascript)

Finally, show the max of the two numbers:

printf("The max of %d and %d is %d", x, y, m);Code language: JavaScript (javascript)

Summary #

• A function is a named block of code that performs a specific task.
• To define a custom function, specify the function prototype and function definition.
• Use the return statement to return a value from a function.