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The Question is:
I need some help on Mailboxes. Process A creates a read/write mailbox with I/O
channel defined in variable chanA. Process B creates a read/write mailbox with
I/O channel defined in variable chanB.
Process A does a
sys$qio(0, chanA,IO$_WRITEVBLK, ....
Process B issues a read
sys$qio(efn, chanB, IO$_READVBLK
what I don't understand is how Process B can read the stuff written by Process
A as they both have different event flags and channle numbers !! My code works
I just can't understand what is happening.
The Answer is :
Process event flags and process channels are process-specific
constructs, and have no relation to the process event flags and
the process channels used in other processes.
An example of using mailboxes is available at:
http://www.openvms.compaq.com/freeware/srh_examples/mbxdemo.c
The OpenVMS Wizard would recommend one reader process per
mailbox, with zero or more processes writing to the mailbox.
This approach completely avoids the "turn-around" problems
that are involved with half-duplex communications via a
single mailbox.
Do not use event flag zero. Allocate the required event flags
using lib$get_ef or (if you are on a sufficiently recent OpenVMS
version, you can use the "do not care" event flag EFN$C_ENF
(defined in $EFNDEF/EFNDEF.H). Read up on $qiow and $synch.
Always explicitly synchronize I/O completion -- see topic (1661)
for a list of some common programming errors and various
programming suggestions. Also please acquire and read the
OpenVMS Programming Concepts manual -- the FAQ has a pointer
to the on-line version of this manual.
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