How to index global variable definition?

[Boson Hubble]

Hi,
It seems gtags (5.9.3) can't index global variable definition as object in C source file. An example:
try.h:
/* === try.h begin === */
extern int var_dec;
int var_def;
void func_dec ();
void func_def () {}
/* === try.h end === */

I ran gtags in the directory try.h reside in:
$ gtags

then printed all object names:
$ global -c

The output:
func_def

Could you give me some hints on how to index global variable definition/declaration and function declaration as object (not symbol) with gtags?

Thanks,

Boson