A question regarding C string functions

I cannot find any definitive statement in my copies of the various
C language standards that addresses the behaviour of the C string
functions when given a NULL pointer.

Specifically, what does the C standard dictate about the behaviour of
  strrchr(NULL,'/')
but the question could apply to any of the string functions
(strlen(NULL), etc.)

My gut impression is that
  strrchr(NULL,'/'), etc
invoke undefined behaviour, and should be avoided.

Can anyone comment?

--
Lew Pitcher
"In Skills We Trust"