[Info-vax] QIO Writes to a mailbox

[JF Mezei]

Michael Moroney wrote:

> It works as you state if and only if you specify the IO$M_NOW modifier.
> 
> Without it, writes will wait until another process issues a read for the
> same data.


But doesn't mean that the data in the user's buffer is not copied to the
mailbux device's buffer where it is queued for delivery.

It only means that the IO is not deemed complete until that message has
been read from the mailbox.

Since mailbox devices are FIFO devices capable of holding multiple
messages (depending on the declared buffer size and size of messages),
wouldn't the QIO immediatly want to claim its position in the mailbox
buffer and queue the message for delivery right away, and then wait (if
not IO$_M_NOW) for the message to get to the head of the queue and then
read ?


(I've always used QIOW with IO/M_NOW for reads, so never had such
problems as the OP posted and always assumed that messages were written
to the mailbox device's buffer right away and then read sequentially by
aother process.