SO_RCVWAKEUP & SO_SNDWAKEUP

[Joel Sherrill]

Smith, Gene wrote:

Gene.. could you please put together a patch for the networking manual
that adds all of this.  It would be most appreciated.

> I am far from an expert on this but I have fairly successfully used these
> function in a complex application. The callbacks are called by the same
> criteria that the unix select function uses for ready sockets. If you have
> the Stevens book "Unix Network Programming" on page 153-154 the section
> "Under what Conditions Is a Descriptor Ready?" you will find the definitive
> list of conditions for readable and writable.
> 
> When the received bytes equals or exceeds the "low water mark" (default 1
> byte) you get a readable callback. If there are 100 bytes in the receive
> buffer and you only read 1, you will not get another callback. You will get
> another callback only after you read all 100 bytes and at least 1 more
> arrives. With a non-blocking socket you should read until read returns
> EWOULDBLOCK and then wait for the readable callback to tell you when more
> data has arrived. (Rule 1.a).
> 
> For sending when the socket connects and the free space is above the "low
> water mark" (default 4096 bytes) you will get a single writable callback.
> You don't get continuous callbacks if you don't write anything. Using
> non-blocking write socket, you can then write until write returns less than
> the amount of data sent or EWOULDBLOCK (indicating no longer writable). When
> this happens you can try to write again, but it is better to just wait for
> another writable callback to tell you when space is available to send again.
> I think you only get a writable callback when the free space transitions to
> above the "low water mark" in a kind of "edge triggered" manner and not
> automatically every time you write. (Rule 2.a)
> 
> The remaining rules handle the fact the sockets become readable and/or
> writable when connects, disconnects and errors occur, not just when data is
> received or sent.
> 
> -gene
> 
> -----Original Message-----
> From: Rolf Schroedter [mailto:Rolf.Schroedter at dlr.de]
> Sent: Tuesday, September 30, 2003 3:25 AM
> To: rtems-users at rtems.com
> Subject: SO_RCVWAKEUP & SO_SNDWAKEUP
> 
> 
> Seems I need some clarification on the SO_RCVWAKEUP & SO_SNDWAKEUP
> callbacks:
> 
> SO_RCVWAKEUP:
> Will it be called once when data arrives or repeatedly when
> data is available ? E.g. What happens, if the application
> doesn't read *all* available data from the socket, but leaves
> some bytes in the input queue ? Will there be a 2nd SO_RCVWAKEUP
> callback ?
> 
> SO_SNDWAKEUP:
> Will I have exactly one callback after each write() call,
> or repeated callbacks whenever there is space available
> in the output buffer ?
> 
> IMO that's important to know to avoid application deadlocks.
> 
> Thanks,
> Rolf.
> 
> ---------------------------------------------------------------
> Rolf Schroedter
> German Aerospace Center
> 


-- 
Joel Sherrill, Ph.D.             Director of Research & Development
joel at OARcorp.com                 On-Line Applications Research
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