- C programming language - Basic C program structure

Let us learn C program structure in this tutorial.

C Program Structure in C Programming

Every C program contains a number of building blocks which include functions, blocks, variable declarations, including header files etc. Each function in C program can perform specific task. A program written in C will have the following structure:

    Include Header File Section

    Global Declaration Section

    /* Comments */

    main() /*Function name * /
    {
        /* comments */

        Local Declaration Part

        Executable Part
     }

    User-defined Function
     {
        Statements
     }

Let us discuss about the various part of C program

Step 1: Include Header File Section in C Programming

Most C program depend upon 1 or more header files. This header file contain function definition that is used in our program.
The header files have a default extension with ".h ".
The header file is included in the program by using " #include " directive.

For example:

#include <stdio.h> or #include "stdio.h"
#include <conio.h>

Some of the header files are:

stdio.h - standard i/o functions
stdlib.h - defines library function
math.h - mathematical functions
string.h - string functions
time.h - time holding functions
conio.h - console in/out, that means, the ways of getting data to and from the console devices (keyboard & screen). This is required if our program has clrscr() and getch( )

Step 2: Global Declaration Section in C Programming

Here declare some variables which are used in more than one function. These variables are called global variables.
This must be declared outside of all the functions.

Step 3: main() function in C Programming

Every C program must contain main() function because this is an entry point of every program.
Program execution is always begin with main() function and then it starts from open brace { and ends with close brace } .
Within ‘{ } ’, we will write an entire program.

Step 3.1: Declaration Part in C Programming

Here declare the entire variables which are used in executable part.
Variable initialization is done in this section, that means, providing initial value to the variables.
These variables are called local variables.

Step 3.2: Executable Part in C Programming

This contains statement which may be single statement or set of statements.

Step 4: User-defined Function in C - Not compulsory but required in all large or complex programs

User or Programer can define their own function in C.
In general, this function can be defined after the main() function.
Sometimes, this function may be defined before the main().
It is not compulsory to write functions in order to execute. However, when the number of lines in main() becomes more, programmers move the logic into multiple meaningful functions

Step 5: Comments in C

Comments are used to understand the flow of the program.

while execution, the compiler does not execute the comments.
The programmers wish to include the comments anywhere in their programs just for better understanding of the program.

Thus, Comments serve two purposes in C programming:

It is primarily used to maintain an explanation of why a particular statement or block of statements is written.
It is also very useful for documentation of software or set of programs written in C.

There are 2 types of comments in C

C Single line comment //

// is used at the start of the line

Example: // This line is to the calculate the area of a square

C Multi line comment /* and */

Usually, the comments are placed between the delimiters /* and */

C Program Writing - Rules and Conventions

Rules in C programming

All keywords should be written in lower case letters.
C program is case sensitive. For example, variable name COUNT is different from the variable name count.
Opening and closing brace should be balanced.
Each statement should be terminated with semicolon. For example:
```
d = a + b;
```
Keywords like for cannot be used for variable or function names

Conventions in C programming

Variable names should be written in lower case letters as a convention though not a rule
Upper case letters used for symbolic constants.
Extra Blank space may be inserted between the words which improve the readability of the statement.
It is not necessary to fix the position of the statements in the program.
More than one statements can be written in one line that are separated by using semicolon. For example:
```
printf(" the main program ");   d = a + b;
```

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