Run positions on Tape

Hi gurus.

I'm working on runs formation [ tuplesort.c & tuplestore.c ]
Is there a way to know and store the address of the first and the last position of a run on a tape?

I would store the location of the first tuple while arranging the current run on the current destination tape.
On the other hand I would store the location of the last tuple of the same run just before writing the first tuple of the possible following run during run formation.

I tried to follow all of the "write" function calls starting from LogicalTapeWrite() [tuplestore.c]. They pass through a lot of buffers before possibly writing to disk.

Should I follow those buffers or those infos are already stored somewhere else or can be retreived in a simples way?

Thanks for your atention.

Regards, Manolo.