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ImageSurajit Nandy
Published on : 4th Oct 2021
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Storage classes in C programming
A variable in C programming belongs to a particlular storage class.
Storage classes tell us :
  1. Where the variable will be stroed.
  2. What is the initial value of the variable.
  3. What is the scope of the variable.
  4. What is the life of the variable.

There are four storage classes in C programming.
  1. auto
  2. register
  3. static
  4. extern

Let's discuss them one by one.
1. auto:
Storage : Memory [stack section]
Initial value : Garbage value (an unpredictable value)
Scope : Local to the block where the variable is declared.
Life : Within the block.
Source code :
#include <stdio.h>

int main(int argc, char** argv)
{
	auto int num = 100;
	{
		printf("Number = %d", num);
	}

	return 0;
}
Output :
Number = 100
Note : If we don't specify any storage class within the function, it is auto by default.

2. register:
Storage : Register
Initial value : Garbage value (an unpredictable value)
Scope : Local to the block where the variable is declared.
Life : Within the block.
Source code : 
#include <stdio.h>

int main(int argc, char** argv)
{
	register int i;
	printf("Numbers = ");
	for(i=0; i < 5; i++)
	{
		printf(" %d", i);
		if(i != 4)
		{
			printf(",");
		}
	}

	return 0;
}
Output :
Numbers =  0, 1, 2, 3, 4
Note : Register access is faster than memory access. We cannot access the address of a register variable. So, we cannot use pointer on a register variable.

3. static:
Storage : Memory [static/global section]
Initial value : Zero
Scope : Local to the block where the variable is declared.
Life : Till the end of a program.
Source code :
#include <stdio.h>

void func()
{
	static int i;
	printf("Value = %d\n", i);
	i++;
}

int main(int argc, char** argv)
{	
	register int i;
	for(i=0; i<5; i++)
		func();
		
	return 0;
}
Output :
Value = 0
Value = 1
Value = 2
Value = 3
Value = 4
Note : Here, i static variable persists between different function calls.

4. extern:
Storage : Memory [static/global section]
Initial value : Zero
Scope : Global.
Life : Till the end of a program.
Source code : 
#include <stdio.h>

int i;

void func()
{
	extern int i;
	printf("Value = %d\n", i);
	i++;
}

int main(int argc, char** argv)
{	
	register int i;
	for(i=0; i<5; i++)
		func();
		
	return 0;
}
Output :
Value = 0
Value = 1
Value = 2
Value = 3
Value = 4
Note : Here, i extern variable persists until the program ends.