Functions

Big idea

A function is a named piece of code. Write it once, then call it by name as often as you like.

The idea

You've used functions from day one — printf and scanf are functions someone else wrote. main is one too: it's where every C program starts. Now you'll write your own.

void greet(void) {
    printf("Hello!\n");
}

• void (first) — this function doesn't give back a value.
• greet — its name.
• (void) — it takes no inputs.
• { … } — the body: what runs each time it's called.

Calling it runs the body: greet();

#include <stdio.h>

void greet(void) {
    printf("Hello!\n");
}

int main(void) {
    greet();
    greet();
    greet();
    return 0;
}

output
Hello!
Hello!
Hello!

Why bother?

• No copy-paste. Fix a bug in one place and every call gets the fix.
• Names explain. print_receipt(); says what's happening; twenty lines of printf don't.
• Small pieces. A long program becomes a set of short functions you can understand one at a time.

Define before you call

C reads your file from top to bottom. If main calls greet before C has seen it, you get a warning or an error. Either put the function above main, or put a prototype — the first line with a ; — at the top:

#include <stdio.h>

void greet(void);        // prototype: "greet exists, the body comes later"

int main(void) {
    greet();
    return 0;
}

void greet(void) {
    printf("Hello!\n");
}

#include <stdio.h>

void middle(void) {
    printf("2\n");
}

int main(void) {
    printf("1\n");
    middle();
    printf("3\n");
    return 0;
}

1
2
3

A call jumps into the function, runs it to the end, then comes back to exactly where it left off.

Watch it

Think of it like

A recipe card named "Pancakes". Instead of rewriting the steps every Sunday, you say "make Pancakes" and follow the card.

🕷️ Spidey-sense: thwip(); — I didn't re-engineer my web-shooters every time I fire them. Built once, called a thousand times a night.

Key terms

• Function — A named, reusable block of code.
• Call — Running a function by writing its name with brackets: greet();
• Definition — The function's header plus its body.
• Prototype — Just the header with a ;, telling C the function exists.