Differentiate between an internal static and external static variable?

Ans:
An internal static variable is declared inside a block with static storage class whereas an external static variable is declared outside all the blocks in a file. An internal static variable has persistent storage, block scope and no linkage. An external static variable has permanent storage, file scope and internal linkage.

Ex - 1: What is the output?
  #include<stdio.h>
       main()
     {
      extern int i;
     printf(“%d”,i);
       }
     int i=200;

Ans:
200.
Explanation:

In this example the variable i is declared as extern. Extern storage class allows the values from outside program .It are also global scope.

Ex - 2: What is the output?
#include<stdio.h>
       main()
     {
      extern int i=20;
     printf(“%d”,i);
       }
     int i=200;

Ans:
Compiler error.
Explanation:
Extern variables cannot be initialized. Extern allows only variable declarations.

Ex - 3: What is the output?

#include<stdio.h>
       main()
     {
      extern int i;
       i=20;
     printf(“%d”,i);
       }
Ans::
Runtime error.
Explanation:

Because extern have the global scope. In the above code the variable i is undefined symbol. Here i do not declare outside the main.