C language context

Just how transient is the memory context created for a C language 
function call?

The reason I ask is that I was getting a seg fault when I attempted to 
pfree something that should have been palloced. When I commented out the 
calls to pfree it worked fine. Most annoying ;-)

I'm still trying to get to the bottom of it, but I want to know if I'm 
safe in relying on the context cleanup to handle things for me. It's an 
immutable text function returning text, and is intended for use in a 
functional index.

cheers

andrew