kill_fasync usage

Dear list, 

quick advice please,

I wnat to use kill_fasync(struct fasync_struct *PTR,...) to notify userland of 
events.  Can I just call kill_fasync regardless of the state of PTR or does 
PTR actually have to point to something valid.  

In my code PTR=NULL initially and may or may not be set or unset during use. I 
would like to know if I can call kill_fasync without testing what is in PTR.  
If I have to test what would PTR be if there isn't anything in the queue?
If I can't test PTR how can I find out from the arguments to my fasync method 
if I am adding (or removing) processes from the list?

Thanks SA