The setpgid function is used either to join an existing process
    group or create a new process group within the session of the
    calling process. The process group ID of a session leader will
    not change.

    Upon successful completion, the process group ID of the process
    with a process ID of pid is set to pgid. As a special case, if
    pid is 0, the process ID of the calling process is used. Also, if
    pgid is 0, the process group ID of the indicated process is used.

    This function requires that long (32-bit) UID/GID support be
    enabled. See 32-Bit UID and GID Macro (Integrity servers, Alpha)
    for more information.