Structure of a C program

hello, world!

This is a hello world program in C.

#include <stdio.h>

int main(){
    printf("hello, world!\n");
    return 0;
}

• The main function is the entrypoint of our code. It's the part of code that runs first whenever the program is compiled and executed.
• The printf function displays the provided format string to the console.
• The main function must return either an integer or void (nothing).
• stdio.h header file must be included in order to perform input and output operations with the terminal.

Parts of a c program

Warning

Contains functions and topics not covered yet for illustration purpose.

/*
    This is a program illustrating various parts of a C program.
*/
#include<stdio.h>

#define PI 3.14

int square(int);
float circle(int);

int main() {
  int a = 10;
  printf("area of a square with side %d is %d\n", a, square(a));
  printf("area of a circle with side %d is %.2f\n", a, circle(a));
  return 0;
}

int square(int s) {
  return s * s;
}
float circle(int r) {
  return 3.14 * (float) r * (float) r;
}

This code can be divided into parts:

Documentation

Comments explaining parts of the code comes under documentation

/*
    This is a program illustrating various parts of a C program.
*/

Pre-processor

Header files and definitions in the top of the file are pre-processors

#include<stdio.h>

#define PI 3.14

main function

This is the entrypoint of the program. The main function should always return an int or void. If the program is able to run successfully 0 is returned.

int main(){
    ...
    return 0;
}

User defined functions

User defined functions like square and circle are used as sub-programs for organising code and providing reusability of code.

int square(int);
float circle(int);

int square(int s) {
  return s * s;
}
float circle(int r) {
  return 3.14 * (float) r * (float) r;
}