winsock 2. thread safety for simultaneous send's. tcp

Question

is it possible to have multiple threads sending on the same socket? will there be interleaving of the streams or will the socket block on the first thread (assuming tcp)? the majority of opinions i've found seems to warn against doing this for obvious fears of interleaving, but i've also found a few comments that state the opposite. are interleaving fears a carryover from winsock1 and are they well-founded for winsock2? is there a way to setup a winsock2 socket that would allow for lack of local synchronization?

two of the contrary opinions below... who's right?

comment 1

"Winsock 2 implementations should be completely thread safe. Simultaneous reads / writes on different threads should succeed, or fail with WSAEINPROGRESS, depending on the setting of the overlapped flag when the socket is created. Anyway by default, overlapped sockets are created; so you don't have to worry about it. Make sure you don't use NT SP6, if ur on SP6a, you should be ok !"

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comment 2

"The same DLL doesn't get accessed by multiple processes as of the introduction of Windows 95. Each process gets its own copy of the writable data segment for the DLL. The "all processes share" model was the old Win16 model, which is luckily quite dead and buried by now ;-)"

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looking forward to your comments! jim

~edit1~ to clarify what i mean by interleaving. thread 1 sends the msg "Hello" thread 2 sends the msg "world!". recipient receives: "Hwoel lorld!". this assumes both messages were NOT sent in a while loop. is this possible?

Answer 1

I'd really advice against doing this in any case. The send functions might send less than you tell it to for various very legit reasons, and if another thread might enter and try to also send something, you're just messing up your data.

Now, you can certainly write to a socket from several threads, but you've no longer any control over what gets on the wire unless you've proper locking at the application level.

consider sending some data:

WSASend(sock,buf,buflen,&sent,0,0,0:

the sent parameter will hold the no. of bytes actually sent - similar to the return value of the send()function. To send all the data in buf you will have to loop doing a WSASend until all all the data actually get sent.

If, say, the first WSASend sends all but the last 4 bytes, another thread might go and send something while you loop back and try to send the last 4 bytes.

With proper locking to ensure that can't happen, it should e no problem sending from several threads - I wouldn't do it anyway just for the pure hell it will be to debug when something does go wrong.

Answer 2

is it possible to have multiple threads sending on the same socket?

Yes - although, depending on implementation this can be more or less visible. First, I'll clarify where I am coming from:

1. C# / .Net 3.5
2. System.Net.Sockets.Socket

The overall visibility (i.e. required management) of threading and the headaches incurred will be directly dependent on how the socket is implemented (synchronously or asynchronously). If you go the synchronous route then you have a lot of work to manually manage connecting, sending, and receiving over multiple threads. I highly recommend that this implementation be avoided. The efforts to correctly and efficiently perform the synchronous methods in a threaded model simply are not worth the comparable efforts to implement the asynchronous methods.

I have implemented an asynchronous Tcp server in less time than it took for me to implement the threaded synchronous version. Async is much easier to debug - and if you are intent on Tcp (my favorite choice) then you really have few worries in lost messages, missing data, or whatever.

will there be interleaving of the streams or will the socket block on the first thread (assuming tcp)?

I had to research interleaved streams (from wiki) to ensure that I was accurate in my understanding of what you are asking. To further understand interleaving and mixed messages, refer to these links on wiki:

1. Real Time Messaging Protocol
2. Transmission Control Protocol

Specifically, the power of Tcp is best described in the following section:

Due to network congestion, traffic load balancing, or other unpredictable network behavior, IP packets can be lost, duplicated, or delivered out of order. TCP detects these problems, requests retransmission of lost packets, rearranges out-of-order packets, and even helps minimize network congestion to reduce the occurrence of the other problems. Once the TCP receiver has finally reassembled a perfect copy of the data originally transmitted, it passes that datagram to the application program. Thus, TCP abstracts the application's communication from the underlying networking details.

What this means is that interleaved messages will be re-ordered into their respective messages as sent by the sender. It is expected that threading is or would be involved in developing a performance-driven Tcp client/server mechanism - whether through async or sync methods.

In order to keep a socket from blocking, you can set it's Blocking property to false.

I hope this gives you some good information to work with. Heck, I even learned a little bit...