The decc$write_eof_to_mbx function writes end-of-file message to
    the mailbox.

    For a mailbox that is not a pipe, the write function called with
    an nbytes argument value of 0 sends an end-of-file message to the
    mailbox. For a pipe, however, the only way to write an end-of-
    file message to the mailbox is to close the pipe.

    If the child's standard input is redirected to a pipe through a
    call to the decc$set_child_standard_streams function, the parent
    process can call decc$write_eof_to_mbx for this pipe to send an
    EOF message to the child. It has the same effect as if the child
    read the data from a terminal, and Ctrl/Z was pressed.

    After a call to decc$write_eof_to_mbx, the pipe can be reused
    for communication with another child, for example. This is
    the purpose of decc$write_eof_to_mbx: to allow reuse of the
    pipe instead of having to close it just to send an end-of-file
    message.