OpenVMS Version 7.3-2 adds an optional fourth argument of type
    int to the pipe function to specify the buffer quota of the
    pipe's mailbox. In previous OpenVMS versions, the buffer quota
    was equal to the buffer size.

    DECC$PIPE_BUFFER_QUOTA lets you specify a buffer quota to use
    for the pipe function if the optional fourth argument of that
    function is omitted.

    If the optional pipe fourth argument is omitted and DECC$PIPE_
    BUFFER_QUOTA is not defined, then the buffer quota defaults to
    the buffer size, as before.

    Default: 512

    Minimum: 512

    Maximum: 2147483647