[Info-vax] Large mailboxes

[Marc Van Dyck]

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 same documentation also says that it is possible to obtain the
number of messages stored in a mailbox with a call to $GETJPI,
specifying the item DVI$_DEVDEPEND. This call returns a longword
of which only the two last bytes are significant, so the maximum
number of outstanding messages can be 65535.

I have been able to verify that too, with the same test as above,
the number of outstanding messages growing steadily till 60.000 and
then dropping back to 4000 or so after the next chunk of 10.000 
messages
were loaded.

So my question is, why this limitation ? Is it just because when this
interface was written, noone imagined that there could ever be a
mailbox with more than 64k outstanding messages ? Or am I really going
to break something other than this counter if I try loading more
than 64k messages ?

Many thanks in advance for your help !

-- 
Marc Van Dyck