Share state ( allocated memory ) across two C functions...

Hi,
I have interesting question that stops me now.
Suppose I have 2 functions

1. preparestate
2. doajob

first allocates some state using
MemoryContextAlloc(TopTransactionContext)
or something, another function using that memory.

question is how I lookup that memory in second function doajob?

Of cause I can return a handle from first function, pointer and accept
that pointer in second function, but in this case I have to check
that pointer on validity and so on...

Is there any good practice ( some way to do so ) for that?

Thank you.