Functions

A function is a block of code that can be called to perform a specific task. Functions are used to make code more modular and reusable. Functions can be called from other functions, or from the main program.

They are written as follows:

type function_name(type parameter1_name, type parameter2_name, ...) {
  ...
}

They follow the same naming rules as variables.

Function Example

This function takes two integers and prints their sum:

#include <stdio.h>
 
void add(int a, int b) {
  // We invoke printf() which is a function from the standard library
  // that takes the arguments we pass to it and prints them to the console.
  // The format string "%d\n" tells printf() to print the first argument.
  printf("%d\n", a + b);
}
 
int main() {
  add(1, 2); // Prints 3
  add(3, 4); // Prints 7
  add(5, 6); // Prints 11
 
  // We invoke the exit() function from the standard library to exit the program.
  // This is equivalent to returning 0 from main().
  exit(0);
}

Function Return Values

Functions can return a value to the caller. The return type is specified before the function name. If the function does not return a value, the return type is void. The function above did not return a value, so its return type was void.

Here is an example of a function that returns the sum of two integers:

#include <stdio.h>
 
int add(int a, int b) {
  return a + b;
}
 
int main() {
  int sum = add(1, 2);
  printf("%d\n", sum); // Prints 3
 
  return 0;
}

Function Prototypes

Function prototypes are used to tell the compiler about a function before it is defined. Function declarations at the top of a file are called prototypes:

#include <stdio.h>
 
// Function prototype
int add(int a, int b);
 
int main() {
  int sum = add(1, 2);
  printf("%d\n", sum); // Prints 3
 
  return 0;
}
 
// Function definition
int add(int a, int b) {
  return a + b;
}

You can also just declare and define the function at the same time like above

Why do we need function prototypes? The compiler reads the file from top to bottom. If we call a function before it is defined, the compiler will not know what to do. We can fix this by defining the function before we call it, or by using a function prototype.

In this way, we can declare prototypes in a header file and define the functions in a separate source file so those source files can call each other's functions without having to redefine their functions.

Return Code of Main

The return code of main() is used to tell the operating system whether the program exited successfully or not. A return code of 0 means the program exited successfully. Any other return code means the program exited with an error.

Control Flow Arrays