Storage classes are used to define scope and life time of a variable. There are four storage classes in C programming.
| Storage Classes | Storage Place | Default Value | Scope | Lifetime |
| auto | RAM | Garbage Value | Local | Within function |
| extern | RAM | Zero | Global | Till the end of main program, May be declared anywhere in the program |
| static | RAM | Zero | Local | Till the end of main program, Retains value between multiple functions call |
| register | Register | Garbage Value | Local | Within function |
1)auto
The auto keyword is applied to all local variables automatically. It is the default storage class that is why it is known as automatic variable.
Example
#include<stdio.h>
int main()
{
int a=10;
auto int b=10;//same like above
printf("%d %d",a,b);
return 0;
}
10 10
2)register
The register variable allocates memory in register than RAM. Its size is same of register size. It has a faster access than other variables.It is recommended to use register variable only for quick access such as in counter.
Example
int counter=0;
The static variable is initialized only once and exists till the end of the program. It retains its value between multiple functions call.The static variable has the default value 0 which is provided by compiler.
#include<stdio.h>
int func()
{
static int i=0;//static variable
int j=0;//local variable
i++;
j++;
printf("i= %d and j= %d
", i, j);
}
int main() {
func();
func();
func();
return 0;
}
i= 1 and j= 1 i= 2 and j= 1 i= 3 and j= 1
The extern variable is visible to all the programs. It is used if two or more files are sharing same variable or function.