c program structure | 5 basic components of a c program

If you want to know the c program structure then read the complete article.

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Components of c program structure

C progam contains 5 main components are listed below.

• Pre-processor.
• variables.
• Functions.
• C program statements and expressions.
• Comment lines.

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Pre-processor in c program structure

The preprocessor is a simple instruction to compiler. It instructs that pre-processors have to be processed before compiling the program.

Compiler substiutes pre-processor text with its value.

Pre-processors begins with # symbols.

Various pre-processors are as follows:

Name	Description
#define	It defines a pre-processor-macro.
#undef	It undefines a pre-processor macro.
#include	It uincludes a header file.
#pragma	It is to give special commands to the compiler.
#ifdef	It returns true if defined.
#ifndef	It returns true if not defined.
#if	pre-processor if block.
#else	pre-processor else block.
#elif	#else and #if in on estatement.
#endif	It ends the if pre-processor.
#error	It prints error message to stderr.

No lets look at some pre-processor examples.

define MY_MACRO 100
#ifndef MY_MACRO
 printf("My macro is not defined");
#endif
#ifdef MY_MACRO
 printf("My macro is defined");
#endif

Try other macoros yourself.

Predefined macros in c

Macro-name	Description
__FILE__	It is the file name of the c program.
__DATE__	It returns the date in MM DD YYYY format.
__LINE__	It returns current line number as decimal constant.
__TIME__	It returns Time in HH:MM:SS format.
__STDC__	It is defined as 1 if the compiler compiles in ANSI Standard

preprocessor operators

Name	Operator	Description	Example
Macro continuation operator	\	It is used to continue a macro if it doesnot fit in one line.	#define my_error(g) \ printf(#g " Error occurred\n")
String size operator	#	It converts parameter of macro to string constant.	#define my_error(a, b) \ printf(#a " and " #b ":Are the errors\n")
Token pasting operator	##	It combines two arguments	#define token_x(n) printf ("token" #n " = %d", token##n)
Defined operator	defined	It is used to determine a macro is previously defined or not.	#if !defined (MY_MESSAGE) #define MY_MESSAGE "Hello world!" #endif

Parameterized macros: macros which allows to pass parameter like we pass to functions.

Example:

#define square_of_a_number(x) ((x) * (x))

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Variable in c program structure

Variables are nothing but name given to storing area of different data.

There are two types of variables: 1) Global variable 2) Local variable.

Below is the example to declare a variable:

datatype variable name;

Example: int example;

You can directly initialize variable like below

int example=100;

Refer to explanation to variables in hello world program.

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Functions in c program structure

Function is a set of code which is intended to perform a specific task.

Main function in c where the program begins to exeute.

Function prototype in c is as below:

return_type function name (  parameters )
{
 .
 .
 .
}

Please refer to function example in hello world c program.

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C program statements and expressions

Statements are the commands given to the computer to perform specific task.

Types of statements

• Expression Statements.
• Labeled Statements.
• Compound Statements.
• Jump Statements.
• Iteration Statements.
• Selection Statements.

Expression consists one more operand and operator which finally results in a value.

Expressions are used in control statements.

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Comment lines in c program structure

Comment lines are for programmers to understand whats going on in the code.

These comment lines are not compiled and has no effect on the output of the c program.

Comment lines in your code is a good programming practice.

In c programming there are two types of comment lines:

• Single line comments. It begins with //
• Multiple line comments. It begins with /* and ends with */

Example of comment line in c

//This is simgle line comment in c programming.
/*This is multiple line comment in c programming.

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Example program for c program structure

#include<stdio.h>//This is #include pre-processor macro
int global_variable=1;//This is a global variable.
void my_function()
{
	printf("My function is called.\n");
}
int main(int argc, char const *argv[])
{
	//this is a single line comment.
	/*
	This is a multiple line comments.
	Line1
	Line2
	.
	.
	.
	.
	.
	*/
	int local_variable=100;//This is a local variable.
	my_function();
	if( local_variable==100 ) //if is a statement and localvariable==100 is an expression
	{
		printf("Local variable value is 100");
	}
	else
	{
		printf("Local variable value is not 100");
	}
	return 0;
}

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c programming tutorial

• Datatypes in c
• String in c
• C programming history WiKi

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