[amenonsen]

Sorry, I was wrong about the BSDs. FreeBSD's pipe.2 manpage clearly says that pipes are still bidirectional. https://www.freebsd.org/cgi/man.cgi?query=pipe&sektion=2

That said, it is not based on socketpair any more. sys/kern/sys_pipe.c says:

    /*
     * This file contains a high-performance replacement for the socket-based
     * pipes scheme originally used in FreeBSD/4.4Lite.  It does not support
     * all features of sockets, but does do everything that pipes normally
     * do.
     */