The [custom-engine.html#custom-engine-api-reference-rnd_pos rnd_pos()] method behaves in a similar fashion to the rnd_next() method but takes an additional parameter:

int ha_foo::rnd_pos(byte * buf, byte *pos)

The *pos parameter contains positioning information previously recorded using the position() method.

A storage engine must locate the row specified by the position and return it through *buf in the internal MySQL row format.