Socket Send/receive simultaneously

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can any one point out the reason why u cant send and receive on a socket at the same time ?

to my understanding there are 2 streams one to push and one to pull

if you attempt to send/receive simultaneously you will get wasealready error

what is the reason that the socket throws wasealready error (10035)
does it have any thing to do with the ack window the receiving side sends back ?
as if to keep the line open for the window ?

Posted 11-Feb-11 4:44am

otzap

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Nish Nishant · Answer 1 · 2011-02-11T04:54:00

Solution 1

You can send and receive on the same socket at the same time (via multiple threads). But the send and receive may not actually occur simultaneously, since one operation may block the other from starting until it's done. That is how it's in Winsock. The .NET wrappers/implementation may or may not permit this - it depends on which class you are using (you will need to look at the implementation using some tool like Reflector to see if this is permitted).

Posted 11-Feb-11 4:54am

Nish Nishant

Comments

Sergey Alexandrovich Kryukov 11-Feb-11 13:36pm

It's correct (a 5), but since we agree that it is not literally "at the same time", but the same socket can sometimes read, sometimes write, the answer is more general, not only for different thread. However, the possibility to work with the sockets in different threads is very important.
--SA

Nish Nishant 11-Feb-11 17:54pm

Thanks Kryukov.

Sergey Alexandrovich Kryukov 11-Feb-11 13:39pm

A just referenced you answer in mine, an important point
--SA

Paulo Zemek · Answer 2 · 2011-02-11T07:40:00

In fact, you can send and receive at the same time. What you can't have is two threads sending over the same TCP/IP socket or two threads receiving at the same TCP/IP socket.

I use this a lot in my remoting framework. See:
I am sending a file to the client (so I keep calling write).
The client, for some reason, wants to cancel the file, but don't want to disconnect (my application does many things). So, how can it inform such situation to the server?

One of the ways is:
Write some bytes. Read for a response. Write more bytes. But, in this case, I need to read between writes.

Another approach is:
I do a while(canSend) and write bytes.
From another thread, I read the same socket. If I receive a "cancellation message", I set canSend to false.

Note that I am only simplifying the example. But if you need any asynchronous message, you may need to send and receive at the same time.

**** I searched for WSAEALREADY: In my understanding, it only happens in non-blocking sockets, so I think it will be better to know exactly how you are using it. To be honest, I always used blocking sockets and created my own threads to access them (so a thread can be blocked until it receives a response).

Sergey Alexandrovich Kryukov · Answer 3 · 2011-02-11T07:34:00

Solution 2

What do you mean "at the same time", exactly? At least the same socket can be used for reading and writing.

But it's quite typical when a socket is used to interlace read and right. It's very usual protocol when a socket reads a message, then sends a confirmation or something like that.

Also, Nishant's note about different thread is very important. Sockets are thread-safe, but not just so. Do you know that historically sockets were designed primarily as the inter-process communication primitive to be used on the same machine? So they are a real IPC tool.

—SA

Posted 11-Feb-11 7:34am

Sergey Alexandrovich Kryukov

Updated 11-Feb-11 17:46pm

v3

Comments

Sergey Alexandrovich Kryukov 11-Feb-11 13:40pm

Who was so clever to vote "1"? I'm just curious...
--SA

Nish Nishant 11-Feb-11 17:55pm

Compensated with a 5 vote :-)

Sergey Alexandrovich Kryukov 11-Feb-11 23:47pm

I see, thanks a lot.
However, Paulo makes me to be not so sure about "at the same time", see my comments to his Answer.
So I modified my statement a bit.
--SA