Scope and Lifetime of a variable

You will learn about scope and lifetime of local and global variables in this tutorial. Also, you will learn about static and register variables.
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Every variable in C programming has two properties: type and storage class.

Type refers to the data type of a variable. And, storage class determines the scope and lifetime of a variable.

There are 4 types of storage class:

  1. automatic
  2. external
  3. static
  4. register

Local Variable

The variables declared inside the function are automatic or local variables.

The local variables exist only inside the function in which it is declared. When the function exits, the local variables are destroyed.

int main() {
    int n; // n is a local varible to main() function
    ... .. ...
}

void func() {
   int n1; // n1 is local to func() fucntion
}

Global Variable

Variables that are declared outside of all functions are known as external variables. External or global variables are accessible to any function.


Example #1: External Variable

#include <stdio.h>
void display();

int n = 5;  // global variable

int main()
{
    ++n;     // variable n is not declared in the main() function
    display();
    return 0;
}

void display()
{
    ++n;     // variable n is not declared in the display() function
    printf("n = %d", n);
}

Output

n = 7

Suppose, a global variable is declared in file1. If you try to use that variable in a different file file2, the compiler will complain. To solve this problem, keyword extern is used in file2 to indicate that the external variable is declared in another file.

Register Variable

The register keyword is used to declare register variables. Register variables were supposed to be faster than local variables.

However, modern compilers are very good at code optimization and there is a rare chance that using register variables will make your program faster. 

Unless you are working on embedded system where you know how to optimize code for the given application, there is no use of register variables.

Static Variable

A static variable is declared by using keyword static. For example;

static int i;

The value of a static variable persists until the end of the program.


Example #2: Static Variable

#include <stdio.h>
void display();

int main()
{
    display();
    display();
}
void display()
{
    static int c = 0;
    printf("%d  ",c);
    c += 5;
}

Output

0  5

During the first function call, the value of c is equal to 0. Then, it's value is increased by 5.

During the second function call, variable c is not initialized to 0 again. It's because c is a static variable. So, 5 is displayed on the screen.