Chapter 4 · Lesson 1

Defining Functions

Defining Functions in C

Functions break programs into manageable, reusable pieces.

Function declaration (prototype) Tells the compiler about a function before its definition. Placed at the top of the file or in a header:

int add(int a, int b);  // declaration

Function definition The actual implementation:

int add(int a, int b) { return a + b; }

Return values

return expression; exits the function and returns the value.
void functions use return; with no value (or omit it).

Parameters by value C passes all arguments by value — the function receives copies. Modifying a parameter does not affect the caller's variable.

void functions Perform side effects (printing, modifying via pointers) rather than computing a value.

Key points:

Prototypes allow calling a function before its definition in the file.
void func(void) explicitly declares no parameters in C; void func() accepts any arguments in C89.
Function names and parameter names must follow identifier rules (letters, digits, underscore; not starting with a digit).

Code Examples

Function with return value and prototypec

#include <stdio.h>

// Prototypes
int factorial(int n);
double power(double base, int exp);

int main(void) {
    printf("5! = %d\n", factorial(5));
    printf("2^10 = %.0f\n", power(2.0, 10));
    return 0;
}

int factorial(int n) {
    if (n <= 1) return 1;
    return n * factorial(n - 1);
}

double power(double base, int exp) {
    double result = 1.0;
    for (int i = 0; i < exp; i++) {
        result *= base;
    }
    return result;
}

Prototypes let main() call factorial() and power() before their definitions. The compiler knows signatures from prototypes.

void functions and parameters by valuec

#include <stdio.h>

void printLine(char ch, int count) {
    for (int i = 0; i < count; i++) {
        putchar(ch);
    }
    putchar('\n');
}

void swap(int a, int b) {
    int tmp = a;
    a = b;
    b = tmp;
    // Does NOT affect the caller
}

int main(void) {
    printLine('=', 20);
    printLine('-', 10);

    int x = 5, y = 10;
    swap(x, y);
    printf("After swap: x=%d y=%d\n", x, y); // unchanged!
    return 0;
}

C passes by value — swap() gets copies of x and y. To swap the originals, you would pass pointers (covered in the next section).

Quick Quiz

1. What is a function prototype in C?

2. If you modify a parameter inside a C function, what happens to the caller's variable?

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