Description

   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.