[Info-vax] QIO Writes to a mailbox

[Michael Moroney]

JF Mezei <jfmezei.spamnot at vaxination.ca> writes:

>Bob Koehler wrote:

>>    However, the behaviour inside the driver is not part of a documented
>>    API and could change with the next minor patch you install.


>Generic question about all write operations with QIO:

>There are generally 2 write option codes. IO$M_WRITEVBLK and another one
>whone nomenclature escapes me at the moment. Would those made a formal
>"documented" behaviour with regards to garanteed copying of the user
>buffer into a driver/device buffer during the SYS$QIO ?

If the documentation says the write buffer is copied away before the
SYS$QIO call returns, then yes.  Otherwise, it's anyone's guess.

>Now, for a non generic question about TCPIP :


>Consider TCPIP. When you write data, you are not garanteed that this
>data goes out in its own packet. It may be combined with some previously
>issued writes or may wait until there are other writes made. Doesn't
>that behaviour pretty much require that the driver make copies of the
>data in its internal buffers so that you can then reuse your own buffer
>to issue subsequent writes ?

No, it can just squirrel away relevant data (including the buffer 
address) regarding the I/O for use later.  The driver may be coded in such
a way not to do a buffer copy twice (once user->system buffer, a second 
time when assembling the TCP packet actually sent)