C Functions

C functions are named blocks of code that perform a specific task. You write a function once and call it as many times as you need, which keeps programs short, organized and easy to test.

Defining and Calling a Function

A function has a return type, a name, a parameter list and a body. void means it returns nothing.

#include <stdio.h>

void greet(const char *name) {
    printf("Hello, %s!\n", name);
}

int main(void) {
    greet("Sara");
    greet("Omar");
    return 0;
}

Hello, Sara!
Hello, Omar!

Return Values and Prototypes

A function must be declared before it is called. If you define it below main, add a prototype (the first line followed by a semicolon) above main.

#include <stdio.h>

double average(int a, int b);   // prototype

int main(void) {
    printf("Average: %.1f\n", average(7, 10));
    return 0;
}

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

Average: 8.5

Pass by Value

C copies arguments into parameters. Changing a parameter inside the function does not affect the caller's variable. To change the original you pass a pointer, covered in the pointers lesson.

#include <stdio.h>

void tryToChange(int n) {
    n = 99;
}

int main(void) {
    int value = 1;
    tryToChange(value);
    printf("value is still %d\n", value);
    return 0;
}

value is still 1

Recursion in C

A recursive function calls itself. It needs a base case that stops the recursion.

#include <stdio.h>

unsigned long long factorial(int n) {
    if (n <= 1) return 1;        // base case
    return n * factorial(n - 1); // recursive case
}

int main(void) {
    printf("10! = %llu\n", factorial(10));
    return 0;
}

10! = 3628800

Variable Scope

• Variables declared inside a function are local and exist only while it runs.
• Variables declared outside all functions are global and visible everywhere after their declaration.
• A static local variable keeps its value between calls.
• Prefer local variables and parameters; globals make code harder to follow.

Frequently asked questions

What is a function prototype in C?

A prototype is a declaration that tells the compiler a function's name, return type and parameter types before its full definition appears. It lets you call functions defined later or in other files.

Can a C function return multiple values?

Not directly. You can return a struct that holds several values, or pass pointers to variables that the function fills in.

Is C pass by value or pass by reference?

C is always pass by value. Passing a pointer copies the address, which lets the function modify the original data, so it simulates pass by reference.