How to find free npc_id

[vykimo 68]

Hello,

I need to find npc_id to store it in 

 

sd->progressbar.npc_id =?;

 

I think it's like an unique id stored in this field.

What kind of function can give me one unique?

I see that 

	st = script->alloc_state(ev->nd->u.scr.script, ev->pos, sd->bl.id, ev->nd->bl.id);

in npc.c. Then what I understand that's I need to allocate a bl?

 

Please give me some advices.