C - Program Structure

Explore the foundational structure of a C program, starting with preprocessor directives and the essential main() function. This guide covers global declarations, macros, and user-defined functions, providing insight into how to effectively organize and structure your C code for optimal performance.

C - Program Structure

A C program typically starts with preprocessor directives and must have a main() function. It may also include global declarations, macros, and user-defined functions.

The Preprocessor Section

Header files (with ".h" extension) contain predefined functions. Use #include to include them in your program.

For example, #include <stdio.h> is needed for printf() and scanf(). Other common headers are string.h, math.h, and stdlib.h.

Use #define for constants and macros:

#define PI 3.14159

Example with constant:

#include 
#define PI 3.14159
int main(){
   int radius = 5;
   float area = PI*radius*radius;
   printf("Area: %f", area);
   return 0;
}

Area: 78.539749

Example with macro:

#include 
#define PI 3.14159
#define AREA(r) (PI*r*r)
int main(){
   int radius = 5;
   float area = AREA(radius);
   printf("Area: %f", area);
   return 0;
}

Area: 78.539749

The main() Function

Every C program must have a main() function. It is the entry point of the program.

Example of main() function:

#include 
int main() {
   printf("Hello, World! \n");
   return 0;
}

Hello, World!

The Global Declaration Section

This section contains variable and function declarations that are accessible throughout the program.

Example of global variable declaration:

int total = 0;
float average = 0.0;

Example of forward declaration of a function:

float area(float height, float width);

Subroutines in a C Program

C programs can have multiple user-defined functions for better organization and reusability.

Comments in a C Program

Comments help document and debug code. They can be multi-line (/* ... */) or single-line (//).

Example of multi-line comment:

/*
Program to display Hello World
Author: tutorialsarena
Built with codeBlocks
*/

Example of single-line comment:

int age = 20; // variable to store age

Structure of the C Program

Here is an example showing different sections of a C program:

/*Headers*/
#include 
#include 

/*forward declaration*/
float area_of_square(float);

/*main function*/
int main() {
   float side = 5.50;
   float area = area_of_square(side);
   printf ("Side=%5.2f Area=%5.2f", side, area);
   return 0;
}

/*subroutine*/
float area_of_square(float side){
   float area = pow(side,2);
   return area;
}

Side= 5.50 Area=30.25