determine what column(s) form the primary key, in C extention

Hey. I'm creating an extention in C. An extention will be fired as, or
inside, a trigger.

What I need: dynamically, inside a trigger, identify which column, or
columns if the primary key is composed of multiple columns, form the
primary key -- their indexes or names.

I need a fast way, therefore I don't consider using SPI_exec(..) and the
like.

I was suggested to use `get_primary_key_attnos` from
`src/include/catalog/pg_constraint.h`

extern Bitmapset *get_primary_key_attnos(Oid relid, bool deferrableOk)

It returns *Bitstamp. And it's got "nwords" and "words[]". But those
return just big numbers, not something that look similar to an index of
the primary key column.

And therefore I haven't had any luck thus far.

How to do it?