C Functions

A named piece of work

You already use functions: main is one, and printf is another. A function packages a block of statements behind a name. You write the steps once. You call the name whenever you need those steps.

Every function has a return type (what it hands back, or void if it hands back nothing), a name, parentheses, and a body. Execution of a program still starts at main. Other functions run only when something calls them. This course writes int main(void).

Return a value

Put helpers above main so the compiler has already seen the full definition whenmain calls them. The return type on the left must match what return sends back.

#include <stdio.h>

double square(double x) {
  return x * x;
}

int main(void) {
  printf("%.1f\n", square(3.0));
  double area = square(5.0);
  printf("%.1f\n", area);
  return 0;
}

square(3.0) is a call. The argument 3.0 becomes the parameter x inside the function. When return runs, the call is replaced by the value 9.0. You can print that value or store it.

void means no value back

A void function does work — usually printing — and then stops. There is no returnvalue to assign. You can write a bare return; to leave early. You cannot writeint n = greet();.

#include <stdio.h>

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

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

Two calls, one definition. Change the message in greet and every call site updates. That is the whole reason to extract a function: one place to fix, not six pasted copies.

Prototype versus definition

A definition is the function with a body. A prototype (a declaration) is the return type, the name, and the parameter list, ending with a semicolon and no body. The compiler only needs a prototype before the first call. The definition can sit later in the file.

#include <stdio.h>

int max_of(int a, int b);

int main(void) {
  printf("%d\n", max_of(4, 9));
  printf("%d\n", max_of(12, 3));
  return 0;
}

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

The prototype above main promises that max_of exists. The definition belowmain keeps the file readable when helpers grow. The parameter names in the prototype are optional; the types are not. They must match the definition.

Call order and one main

C reads the file top to bottom. If you call a function the compiler has never seen, you get an error. Two legal layouts:

• Define helpers first, then main.
• Declare prototypes first, then main, then define the helpers.

A program you run has exactly one main. Helpers return to whoever called them. mainreturns to the operating system. Keep using return 0; at the end of main.

What belongs in a function

Give a function one job and a name that says that job: square, greet,max_of. If you cannot name it without the word “and”, it is probably two functions. Leave program setup in main and factor the repeated pieces.

#include <stdio.h>

int square(int n) {
  return n * n;
}

int cube(int n) {
  return n * n * n;
}

int main(void) {
  for (int i = 1; i <= 4; i++) {
    printf("%d squared %d, cubed %d\n", i, square(i), cube(i));
  }
  return 0;
}

Next: how arguments get into the function — copies, and pointers that mutate the caller.