Managing read/write socket without polling

[Peter Ammon]

I need to manage multiple socket connections to a server. Each one can
have data that needs to be read or written at any time. I would like to
be able to read and write data as it becomes available without having to
poll for it. I am targetting Mac OS X and Linux at a minimum.

The usual way to avoid polling on a read is to use select(), but what
happens if data becomes available for writing while the thread is
waiting on select() or blocking on a read? Do I have to use a separate
thread for writing? If so, are there synchronization issues I shold be
aware of for reading and writing to the same socket at the same time in
separate threads?

Would nonblocking I/O be simpler to implement instead?

Thanks for any advice,
-Peter

[Barry Margolin]

In article <becdka$seu$1@news.apple.com>,
Peter Ammon <pa44@cornell.edu> wrote:

>I need to manage multiple socket connections to a server. Each one can
>have data that needs to be read or written at any time. I would like to
>be able to read and write data as it becomes available without having to
>poll for it. I am targetting Mac OS X and Linux at a minimum.
>
>The usual way to avoid polling on a read is to use select(), but what
>happens if data becomes available for writing while the thread is
>waiting on select() or blocking on a read? Do I have to use a separate
>thread for writing? If so, are there synchronization issues I shold be
>aware of for reading and writing to the same socket at the same time in
>separate threads?

Using separate threads may be the simplest solution. There shouldn't be
any synchronization issues with this -- reading and writing a socket are
independent, unrelated operations, and the kernel should sort it out
automatically.

Another solution is to use something you can select() on, e.g. a pipe or
Unix-domain socket, as the mechanism for making data available for writing.

>Would nonblocking I/O be simpler to implement instead?

I don't think it makes much difference, since you also have the problem
when you're blocked in select().

--
Barry Margolin, [email]barry.margolin@level3.com[/email]
Level(3), Woburn, MA
*** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to newsgroups.
Please DON'T copy followups to me -- I'll assume it wasn't posted to the group.

[David Schwartz]

"Peter Ammon" <pa44@cornell.edu> wrote in message
news:becdka$seu$1@news.apple.com...

> I need to manage multiple socket connections to a server. Each one can
> have data that needs to be read or written at any time. I would like to
> be able to read and write data as it becomes available without having to
> poll for it. I am targetting Mac OS X and Linux at a minimum.

Okay.

> The usual way to avoid polling on a read is to use select(), but what
> happens if data becomes available for writing while the thread is
> waiting on select() or blocking on a read?

What do you mean by "data becomes available for writing"? Do you mean
the socket has room in its buffer for a 'write' or do you mean something
else?

> Do I have to use a separate
> thread for writing?

That might be one way.

> If so, are there synchronization issues I shold be
> aware of for reading and writing to the same socket at the same time in
> separate threads?

No, this is perfectly safe so long as the 'read' and the 'write' are
independent. But if you're going non-blocking using 'select', you don't need
two threads.

> Would nonblocking I/O be simpler to implement instead?

I thought we were talking about nonblocking I/O. How do you use 'select'
with blocking I/O?

DS

[Michael B Allen]

On Mon, 07 Jul 2003 14:19:43 -0400, Peter Ammon wrote:

> I need to manage multiple socket connections to a server. Each one can
> have data that needs to be read or written at any time. I would like to
> be able to read and write data as it becomes available without having to
> poll for it. I am targetting Mac OS X and Linux at a minimum.
>
> The usual way to avoid polling on a read is to use select(), but what
> happens if data becomes available for writing while the thread is
> waiting on select() or blocking on a read?

But if you use select() you should not be "blocking on a read". If you
mean reading from the keyboard then you should add STDIN_FILENO to your
descriptor set and use select() to handle readyness on that descriptor
as well (although specially).

> Do I have to use a separate
> thread for writing? If so, are there synchronization issues I shold be
> aware of for reading and writing to the same socket at the same time in
> separate threads?

If you are using mutiple threads you would need to syncronize their
actions with mutexes, condition variables, and semaphores yes.

> Would nonblocking I/O be simpler to implement instead?

You might do that but it's not necessarily simplier. You could use
something like libevent. Read this:

[url]http://www.kegel.com/c10k.html[/url]

[Note: I am not necessarily advocating the techniques described on this
webpage. I only refer you to it because it has good information regarding
non-blocking I/O strategies and libevent]

Mike