JPro JPro - 4 years ago 125
C Question

extern and global in c

Can anyone please tell me is there any special requirement to use either

variables in a C program?
I do not see any difference in a program like below, if I change from gloabl to extern.

#include <stdio.h>
#include <stdlib.h>
int myGlobalvar = 10;

int main(int argc, char *argv[])
int myFunc(int);
int i;
i = 12;
myGlobalvar = 100;
printf("Value of myGlobalvar is %d , i = %d\n", myGlobalvar, i);
i = myFunc(10);
printf("Value of passed value : %d\n",i);
printf("again Value of myGlobalvar is %d , i = %d\n", myGlobalvar, i);
return 0;
int myFunc(int i)
i = 20 + 1000;
//extern int myGlobalvar;
myGlobalvar = 20000;
// printf("Value of passed value : %d",i);
return i;

If uncomment
extern int myGlobalvar
, the value does not change.

Is there any
difference between both?

Can anyone please correct me?

Answer Source

The keyword extern means "the storage for this variable is allocated elsewhere". It tells the compiler "I'm referencing myGlobalvar here, and you haven't seen it before, but that's OK; the linker will know what you are talking about." In your specific example it's not particularly useful, because the compiler does know about myGlobalvar -- it's defined earlier in the same translation unit (.c or .cc file.) You normally use extern when you want to refer to something that is not in the current translation unit, such as a variable that's defined in a library you will be linking to.

(Of course, normally that library would declare the extern variables for you, in a header file that you should include.)

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