Taking the "varattno" in "args" (where part of a query)

I'm working on a project and one of my task is trying to take the "where" part of the queries.

I'm programming inside the function "exec_simple_query" in "postgres.c".

I searched inside the source code and how I've understood I need to take the "varattno" in "args" in "qpqual", is it right?

If yes, how do I have to save it in order to have it in that function?

If no, what is the easiest way to take it?

Example: 

For a query: select firstname from friend where age=33

The attributes of the table friends are: firstname, lastname, city, state, age

I need to have the number 5 because "age" is the fifth attribute of "friend".

Thanks