[Info-vax] Large mailboxes

[David Jones]

On Thursday, November 26, 2020 at 11:30:32 AM UTC-5, Marc Van Dyck wrote:
> Unless I mis-read it, the OpenVMS documentation does not state any size 
> limitation for permanent mailboxes. There is apparently just a 
> limitation on the size of each message, but the number of messages can 
> apparently be arbitrarily high. I have made a call to $CREMBX to 
> create a mailbox for 1.000.000 messages of 200 bytes each, and VMS did 
> not complain. 
> 
> I understand from a Digital Technical Journal article that mailbox 
> space 
> is not reserved at mailbox creation time, but allocated each time a 
> new message is dropped in the mailbox. This I have been able to verify 
> by loading a mailbox and see the values in $SHOW MEM/POOL decrease 
> accordingly (loading chunks of 10.000 messages at a time...). 

The UCB field where the driver tracks the message count is only 16 bits. The
mailbox driver sources I have, circa 1999, show it doing no boundary checks
against a 65K limit for pending message count. It does, however, check for
zero message count in a few places, which could potentially be an issue if
the counter rolls over at the wrong time. I suppose the authors could imagine
a case where you'd get 65K messages before running out of buffer quota
for the mailbox.