Difference between declaration of variable with extern and without it

Question

1.h

extern int a;

1.c

#include <stdio.h>
#include "1.h"

int main(){ 
 printf("%d\n", a);
 return 0;
}

2.c

#include "1.h"

int a = 6;

This compiles and runs just fine (gcc 1.c 2.c) if you remove extern from 1.h and prints 6. I know that removing it might cause a to be defined in every translation unit (object file), but what is the problem? doesn't the linker just git rid of it when linking since it compiles with no errors?

If you remove `extern`, your `a` in `1.c` will be uninitialized and the `printf` will invoke undefined behavior. — Eugene Sh., Jan 21 '20 at 16:03
but isn't linked to the correct value during linking? is there a gcc rule regarding this? — Josh, Jan 21 '20 at 16:06
Actually you should be getting multiple definitions error when linking... unless I am missing something here. — Eugene Sh., Jan 21 '20 at 16:09
Hm. Maybe no error because the `2.c` is completely optimized out. But there is definitely an undefined behavior, because when I try to reproduce it, it prints `0` for me. — Eugene Sh., Jan 21 '20 at 16:17

score 2 · Answer 1 · edited Jun 20 '20 at 09:12

This technique does not conform to the letter of the C standard and the 'one definition rule' — it is officially undefined behaviour:

J.2 Undefined behavior

An identifier with external linkage is used, but in the program there does not exist exactly one external definition for the identifier, or the identifier is not used and there exist multiple external definitions for the identifier (6.9).

§6.9 External definitions ¶5

An external definition is an external declaration that is also a definition of a function (other than an inline definition) or an object. If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof or _Alignof operator whose result is an integer constant), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one.¹⁶¹⁾

¹⁶¹⁾ Thus, if an identifier declared with external linkage is not used in an expression, there need be no external definition for it.

more here: How do I use extern to share variables between source files?

Difference between declaration of variable with extern and without it

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