Thread: Tentative variable in header file problem

Hello. I decided to find out what really tentative variable is. I thought that I understand that, but when I run this code I realized that I don't. Tentative definition without any initialization is initialized to zero, isn't it? I expect that this code is not possible to compile and output from gcc will be something like "Multiple definition error" because:

If the end of the translation unit is reached and no definition has appeared with an initializer for the identifier, then the tentative definition becomes a full definition, and the object defined has uninitialized (zero-filled) space reserved for it.

Full definition means that this variable is definied and initialized to zero in this case. That is the reason why I expect Multiple definition error. But It works good and the output was:

>0<0040da20
>0<0040da20

Was that tentative variable mysteriously changed to extern storage class or what? Does it mean that I don't need an extern keyword to share variable among .c files?

I'm so sorry for ma bad English. I hope you understand my problem. Thanks!

Code:

test.h

int a;      //tentative variable without extern
void test(void);

test.c

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

void test(void){

    printf(">%d<%p", a,&a);
}


main.c

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

int main(void) {

printf(">%d<%p\n", a,&a);

     test();

return 0;
}

Was that tentative variable mysteriously changed to extern storage class or what?

O_o

No. The default for tentative definitions is external linkage which a linker can combine in the case of multiple tentative definitions.

Does it mean that I don't need an extern keyword to share variable among .c files?

No. You should preferably use `extern` with an explicit definition.

The tentative definition mechanic is more of a bandage than an intended feature you should be using.

Soma