DB2's row_number()

I had a brief look at DB2's row_number function which seems to be pretty
useful.
What it does is:

test=# SELECT row_number(), relname FROM pg_class LIMIT 3;
row_number | relname
------------+----------------
1 | pg_description
2 | pg_group
3 | pg_proc
(3 rows)

This makes sense to me and I need this feature from time to time. My
question is: How do I find out when a query starts? Inside a table
function I can call SRF_IS_FIRSTCALL() to see when it is called first.
Is there an easy way to check that inside an "ordinary" C function
returning just one value?
Currently my function counts the number of times it has been called per
connection. I could write a second function for resetting the counter
but this is not too smart ...

Regards,

Hans

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