Pthread_t invalid value

Hi all,

I need to check a pthread_t variable against an invalid value.
I understood that errno has been set if an error occurs while thread creation.
But this check can’t be made applicable in my situation.

On referring, I understood that pthread_t is int32 type. While searching in internet, it is seen that the invalid value is implementation specific. Can I use zero or -1 as an invalid pthread_t value?
Or can there be threads with zero or -1 as thread id in QNX?