unix:dup/2

dup(+FromStream, +ToStream) is det: Interface to Unix dup2(), copying the underlying filedescriptor and thus making both streams point to the same underlying object. This is normally used together with fork/1 and pipe/2 to talk to an external program that is designed to communicate using standard I/O.
Both FromStream and ToStream either refer to a Prolog stream or an integer descriptor number to refer directly to OS descriptors. See also demo/pipe.pl in the source-distribution of this package.