C Basics - Extern Storage Class in C

Keyword	extern
Storage	Data Memory
Default Initial Value	Zero
Scope	Global
Life	As long as the program’s execution doesn’t come to an end.

int i ; 
int main( ) 
{ 
   printf ( "\nvalue of i = %d", i ) ; 
   increment( ) ; 
   increment( ) ; 
   decrement( ) ; 
   decrement( ) ; 
   return 0;
} 
int increment( ) 
{ 
   i = i + 1 ; 
   printf ( "\n value on incrementing i = %d", i ) ; 
   return 0;
} 
int decrement( ) 
{ 
   i = i - 1 ; 
   printf ( "\nvalue on decrementing i = %d", i ) ; 
   return 0;
} 

The output of the above programs would be:

value of i = 0 
value on incrementing i = 1 
value on incrementing i = 2 
value on decrementing i = 1 
value on decrementing i = 0

The value of i is available to the functions increment( ) and decrement( ) since i has been declared outside all functions.

The extern modifier is most commonly used when there are two or more files sharing the same global variables or functions as explained below.

/*main.c*/
#include &ly;stdio.h>
 
int count ;
extern void write_extern();
 
main() {

   count = 5;
   write_extern();
}

/*support.c*/
#include &ly;stdio.h>
 
extern int count;
 
void write_extern(void) {
   printf("count is %d\n", count);
}

Here the variable count is defined in main.c and declared in support.c as external variable. Similarly, the function write_extern() is defined in support.c and declared in main.c as extern.

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Related topics:
Overview of Storage Class in C   |   Auto Storage Class in C   |   Register Storage Class in C   |   Static Storage Class in C   |   Summary of Storage Class in C

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List of topics: C Programming

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