Storage class in c

In c, Storage class specifies variable scope, lifetime, default value and the memory space allocated during execution of the program.

c programming language supports 4 storage class specifiers:

1. auto
2. extern
3. static
4. register

auto

auto is default storage class of variables declared or defined inside a function.

These variables are also called as automatic or local variables. They are only accessible inside a function where they are declared.

Default value for variables with auto storage class is Garbage.

Example

void main()
{
// A variable inside a function, by default becomes automatic variable
	int a = 20;
	// A variable with auto specifier
	auto int b = 30;
}

extern

To access a global variable of one c program/source file in other c program/source files, extern storage class is used.

It is used to declare & use a variable in a program which is already defined in some other c program/source file.

Example

// myfile1.c 
int count;
void num()
{
 	count=10;
}

// myfile2.c

// Including myfile1.c
#include"myfile1.c"
void main()
{
	// Accessing a global variable defined in myfile1.c
	extern int count;
	num();
}

static

variable with static specifier created & initialized once in their lifetime & holds the value between function call until the program execution ends.

Default value for variable with static specifier is 0. static specifier can also be used with automatic & extern variables.

Example

#include<stdio.h>
void mycounter();
int main()
{
	mycounter();
	mycounter();
	mycounter();
	mycounter();
	mycounter();
	mycounter();
return 0;
}
void mycounter()
{
	static int a = 0 ;
	printf( "%d ", a) ;
	a = a + 1;
}

register

Variables with register storage class specifier are stored in CPU register for quicker access.

Following are the facts related to the register storage class:

1. Variables with register storage class are local to the function where they are declared.
2. Compiler may store the variables in memory(RAM), but compiler does not
   guarantee it to store in CPU register.
3. Default value for variables with register storage class is garbage.

Example

int main()
{
  register int a;
return 0;
}