Another potential problem occurs when
a server is not able to honor a request
in a message file record immediately because of a lack of
resources. The
requests must be handled in order,
and so the server tries to free the
resources it requires.
Eventually the server comes back and tries again
to process the request.
In this case it is useful to have a
"nondestructive" read, that is, to look at the request record
in the message file without deleting it.
A test is made to see if the request can be satisfied.
If it can, a regular destructive read is done and the request is
processed. If the request cannot be satisfied, it
stays on the top of the queue, where
it can be read and tested at a later time.
This feature is provided
by FCONTROL with a controlcode of 47.
It is valid for readers only, and it affects only
the following FREAD.
Note that repeated use of nondestructive
FREAD always reads the same record.
You can copy the message file to another file,
if, for example, you want to save it after it has been read.
You can
do this by accessing it with the Copy Mode option
(HPFOPEN Item #17, or FOPEN aoption (3:1)).
You should note, however, that accessing a file in Copy Mode
means that it does not have any of the
special features of message files.
For a description of the Copy Mode option, refer to Appendix A of
this manual, or the MPE/iX Intrinsics Reference Manual
(32650-90028).
