Extern Storage Class in C (Keyword)

January 14, 2023

Extern Storage Class in C

Extern is mostly used to indicate that a variable has been defined with external linking in another part of the program.

When we have global variables or methods that are shared by two or more files, we use an external storage class.

No storage is allocated to a variable when the extern specifier is used with a variable declaration since it is presumed that the variable has already been created elsewhere in the program.

Use of Extern Storage Class

We can construct the self-customized header file “external.h” for using extern variables, and there we have stored int a and int b.
The syntax for importing “external.h” into our main “prepinsta.c” code is as follows: #include “external.h”.
The variables declared in the external.h file can then be used after being initialised once more in the prepinsta.c file.
Here sum = a + b will show the following output

Tokens in C

Bitwise Operators in C

Conditional statement Program in C

Switch case program in C

function pointer in C

Example 1

File name: external.h

int a = 100;
int b = 200;

File name: prepinsta.c

Run

#include<stdio.h> 
#include<external.h>
int main()
{
  extern int a,b;
  int sum = a + b;
  printf("%d + %d = %d ", a, b, sum);
  return 0;
}

Output

100 + 200 = 300

Use of Extern Storage Class in C file

We can use the extern keyword variable in our main program that is named as “placement.c”.
We can declare the extern variables before starting main function.
But we also need to give them some value at this stage also.
We can change the values of extern variables in the main function as shown in this program.

File name: placement.c

Run

#include<stdio.h> 
extern int a=10;
extern int b=20;
int main()
{
  a = 100;
  b = 200;
  int sum = a + b;
  printf("%d + %d = %d ", a, b, sum);
  return 0;
}

Output

100 + 200 = 300

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Extern Storage Class in C (Keyword)