Develop Reference

C# client socket for synchronous

I have a 3Rd party server where I send it data and then wait for a response.
It will first respond with ack then with the full response
I am doing something like this. Is there a better way.
//Connect
eftSocket.Connect(remoteEp);
// Encode the data string into a byte array.
byte[] msg = Encoding.ASCII.GetBytes(strRequest);
// Send the data through the socket.
int bytesSent = eftSocket.Send(msg);
//receive first time
int bytesRec = eftSocket.Receive(bytes);
//Do we get an ack,
if (bytes[0] != 6)
{
throw new Exception("Could not connect to EFT terminal");
}
//receive 2nd time
bytesRec = eftSocket.Receive(bytes);

That code looks broken. At first you have to embrace that TCP is a stream and that there are no packets. That means even if the client sends something of a given length you can never expect that you receive it in one turn. Which means if you do eftSocket.Receive(bytes) you might receive any amount of bytes which were sent from the remote, not necessarily the ack byte in the first call and the remaining stuff in a second call. The only thing that you know is that you won't receive more than bytes.Length with it. If you hand it over a significantly large buffer you might receive everything (ACK + remaining data) in one turn. Therefore you should always check the return value first. And if you expect more bytes repeat the Receive calls with the required offsets.
Then you should at first check whether your receive suceeded at all. You might receive 0, which means the remote closed the socket. If that's the case bytes[0] will yield anything which was stored in it before the Receive started.
You should also properly close your socket. It's not obvious from the code example if it happens, but if you return from the code with an Exception it might be missing.

TcpListener: Detecting a client disconnect as opposed to a client not sending any data for a while

I was looking how to detect a 'client disconnect' when using a TcpListener.
All the answers seem to be similar to this one:
TcpListener: How can I detect a client disconnect?
Basically, read from the stream and if Read() returns 0 the client had disconnected.
But that's assuming that a client disconnects after every single stream of data it sent.
We operate in environments where the TCP connect/disconnect overhead is both slow and expensive.
We establish a connection and then we send a number of requests.
Pseudocode:
client.Connect();
client.GetStatus();
client.DoSomething();
client.DoSomethingElse();
client.AndSoOn();
client.Disconnect();
Each call between Connect and Disconnect() sends a stream of data to the server.
The server knows how to analyze and process the streams.
If let the TcpListener read in a loop without ever disconnecting it reads and handles all the messages, but after the client disconnects, the server has no way of knowing that and
it will never release the client and accept new ones.
var read = client.GetStream().Read(buffer, 0, buffer.Length);
if (read > 0)
{
//Process
}
If I let the TcpListener drop the client when read == 0 it only accepts
the first stream of data only to drop the client immediately after.
Of course this means new clients can connect.
There is no artificial delay between the calls,
but in terms of computer time the time between two calls is 'huge' of course,
so there will always be a time when read == 0 even though that does not mean
the client has or should be disconnected.
var read = client.GetStream().Read(buffer, 0, buffer.Length);
if (read > 0)
{
//Process
}
else
{
break; //Always executed as soon as the first stream of data has been received
}
So I'm wondering... is there a better way to detect if the client has disconnected?

You could get the underlying socket using the NetworkStream.Socket property and use it's Receive method for reading.
Unlike NetworkStream.Read, the linked overload of Socket.Receive will block until the specified number of bytes have been read, and will only return zero if the remote host shuts down the TCP connection.
UPDATE: #jrh's comment is correct that NetworkStream.Socket is a protected property and cannot be accessed in this context. In order to get the client Socket, you could use the TcpListener.AcceptSocket method which returns the Socket object corresponding to the newly established connection.

Eren's answer solved the problem for me. In case anybody else is facing the same issue
here's some 'sample' code using the Socket.Receive method:
private void AcceptClientAndProcess()
{
try
{
client = server.Accept();
client.ReceiveTimeout = 20000;
}
catch
{
return;
}
while (true)
{
byte[] buffer = new byte[client.ReceiveBufferSize];
int read = 0;
try
{
read = client.Receive(buffer);
}
catch
{
break;
}
if (read > 0)
{
//Handle data
}
else
{
break;
}
}
if (client != null)
client.Close(5000);
}
You call AcceptClientAndProcess() in a loop somewhere.
The following line:
read = client.Receive(buffer);
will block until either
Data is received, (read > 0) in which case you can handle it
The connection has been closed properly (read = 0)
The connection has been closed abruptly (An exception is thrown)
Either of the last two situations indicate the client is no longer connected.
The try catch around the Socket.Accept() method is also required
as it may fail if the client connection is closed abruptly during the connect phase.
Note that did specify a 20 second timeout for the read operation.

The documentation for NetworkStream.Read does not reflect this, but in my experience, 'NetworkStream.Read' blocks if the port is still open and no data is available, but returns 0 if the port has been closed.
I ran into this problem from the other side, in that NetworkStream.Read does not immediately return 0 if no data is currently available. You have to use NetworkStream.DataAvailable to find out if NetworkStream.Read can read data right now.

How do I know when an asynchronous socket read ends?

I have an asynchronous read method...
private void read(IAsyncResult ar) {
//Get the Server State Object
ServerState state = (ServerState)ar.AsyncState;
//read from the socket
int readCount = state.socket.EndReceive(ar);
//check if reading is done, move on if so, trigger another read if not
if (readCount > 0) {
//purge the buffer and start another read
state.purgeBuffer();
state.socket.BeginReceive(state.buffer, 0, ServerState.bufferSize, 0, new AsyncCallback(read), state);
}
else {
//all bytes have been read, dispatch the message
dispatch(state);
}
}
The problem that I am having is that read is only 0 if the connection is closed. How do I say, this is the end of that message and pass the data on to the dispatcher, while leaving the socket open to accept new messages.
Thank you!

You should not rely on what is in the TCP buffer. You must process the incoming bytes as a stream somewhere. You can't really know whether its complete. Only one layer above can know when the message completed.
Example:
If you read HTTP responses the HTTP header will contain the byte count which is in the HTTP body. So you know how much to read.
You only know how much to read if the data follows a certain protocol and you interprete it. Imagine you receive a file over the socket. The first thing you would receive is the file size. Without that you would never know how much to read.

You should make your messages fit a particular format so that you can distinguish when they start and when end. Even if it is a stream of data it should be sent in packets.
One option is to send length of message first and then you know how much data to expect. But problem with that is if you loose sync you can never recover and you will never know what is message length and what is its content. It is good to use some special marking sequence to know when message begins. It is is not 100% error proof (sequence might appear in data) but certainly helps and allows to recover from sync loose. This is particularly important when reading from a binary stream like socket.
Even ancient RS232 serial protocol had its frame and stop bit to know when you got all the data.

Loop until TcpClient response fully read [duplicate]

This question already has answers here:
Receiving data in TCP
(10 answers)
Closed 2 years ago.
I have written a simple TCP client and server. The problem lies with the client.
I'm having some trouble reading the entire response from the server. I must let the thread sleep to allow all the data be sent.
I've tried a few times to convert this code into a loop that runs until the server is finished sending data.
// Init & connect to client
TcpClient client = new TcpClient();
Console.WriteLine("Connecting.....");
client.Connect("192.168.1.160", 9988);
// Stream string to server
input += "\n";
Stream stm = client.GetStream();
ASCIIEncoding asen = new ASCIIEncoding();
byte[] ba = asen.GetBytes(input);
stm.Write(ba, 0, ba.Length);
// Read response from server.
byte[] buffer = new byte[1024];
System.Threading.Thread.Sleep(1000); // Huh, why do I need to wait?
int bytesRead = stm.Read(buffer, 0, buffer.Length);
response = Encoding.ASCII.GetString(buffer, 0, bytesRead);
Console.WriteLine("Response String: "+response);
client.Close();

The nature of streams that are built on top of sockets is that you have an open pipeline that transmits and receives data until the socket is closed.
However, because of the nature of client/server interactions, this pipeline isn't always guaranteed to have content on it to be read. The client and server have to agree to send content over the pipeline.
When you take the Stream abstraction in .NET and overlay it on the concept of sockets, the requirement for an agreement between the client and server still applies; you can call Stream.Read all you want, but if the socket that your Stream is connected to on the other side isn't sending content, the call will just wait until there is content.
This is why protocols exist. At their most basic level, they help define what a complete message that is sent between two parties is. Usually, the mechanism is something along the lines of:
A length-prefixed message where the number of bytes to be read is sent before the message
A pattern of characters used to mark the end of a message (this is less common depending on the content that is being sent, the more arbitrary any part of the message can be, the less likely this will be used)
That said you aren't adhering to the above; your call to Stream.Read is just saying "read 1024 bytes" when in reality, there might not be 1024 bytes to be read. If that's the case, the call to Stream.Read will block until that's been populated.
The reason the call to Thread.Sleep probably works is because by the time a second goes by, the Stream has 1024 bytes on it to read and it doesn't block.
Additionally, if you truly want to read 1024 bytes, you can't assume that the call to Stream.Read will populate 1024 bytes of data. The return value for the Stream.Read method tells you how many bytes were actually read. If you need more for your message, then you need to make additional calls to Stream.Read.
Jon Skeet wrote up the exact way to do this if you want a sample.

Try to repeat the
int bytesRead = stm.Read(buffer, 0, buffer.Length);
while bytesRead > 0. It is a common pattern for that as i remember.
Of course don't forget to pass appropriate params for buffer.

You dont know the size of data you will be reading so you have to set a mechanism to decide. One is timeout and another is using delimiters.
On your example you read whatever data from just one iteration(read) because you dont set the timeout for reading and using default value thats "0" milisecond. So you have to sleep just 1000 ms. You get same effect with using recieve time out to 1000 ms.
I think using lenght of data as prefix is not the real solution because when socket is closed by both sides, socket time-wait situation can not handled properly. Same data can be send to server and cause server to get exception . We used prefix-ending character sequence. After every read we check the data for start and end character sequence, if we cant get end characters, we call another read. But of course this works only if you have the control of server side and client side code.

In the TCP Client / Server I just wrote I generate the packet I want to send to a memory stream, then take the length of that stream and use it as a prefix when sending the data. That way the client knows how many bytes of data it's going to need to read for a full packet.

C# NetworkStream.Read oddity

Can anyone point out the flaw in this code? I'm retrieving some HTML with TcpClient. NetworkStream.Read() never seems to finish when talking to an IIS server. If I go use the Fiddler proxy instead, it works fine, but when talking directly to the target server the .read() loop won't exit until the connection exceptions out with an error like "the remote server has closed the connection".
internal TcpClient Client { get; set; }
/// bunch of other code here...
try
{
NetworkStream ns = Client.GetStream();
StreamWriter sw = new StreamWriter(ns);
sw.Write(request);
sw.Flush();
byte[] buffer = new byte[1024];
int read=0;
try
{
while ((read = ns.Read(buffer, 0, buffer.Length)) > 0)
{
response.AppendFormat("{0}", Encoding.ASCII.GetString(buffer, 0, read));
}
}
catch //(SocketException se)
{
}
finally
{
Close();
}
Update
In the debugger, I can see the entire response coming through immediately and being appended to my StringBuilder (response). It just appears that the connection isn't being closed when the server is done sending the response, or my code isn't detecting it.
Conclusion
As has been said here, it's best to take advantage of the offerings of the protocol (in the case of HTTP, the Content-Length header) to determine when a transaction is complete. However, I've found that not all pages have content-length set. So, I'm now using a hybrid solution:
For ALL transactions, set the request's Connection header to "close", so that the server is discouraged from keeping the socket open. This improves the chances that the server will close the connection when it is through responding to your request.
If Content-Length is set, use it to determine when a request is complete.
Else, set the NetworkStream's RequestTimeout property to a large, but reasonable, value like 1 second. Then, loop on NetworkStream.Read() until either a) the timeout occurs, or b) you read fewer bytes than you asked for.
Thanks to everyone for their excellent and detailed responses.

Contrary to what the documentation for NetworkStream.Read implies, the stream obtained from a TcpClient does not simply return 0 for the number of bytes read when there is no data available - it blocks.
If you look at the documentation for TcpClient, you will see this line:
The TcpClient class provides simple methods for connecting, sending, and receiving stream data over a network in synchronous blocking mode.
Now my guess is that if your Read call is blocking, it's because the server has decided not to send any data back. This is probably because the initial request is not getting through properly.
My first suggestion would be to eliminate the StreamWriter as a possible cause (i.e. buffering/encoding nuances), and write directly to the stream using the NetworkStream.Write method. If that works, make sure that you're using the correct parameters for the StreamWriter.
My second suggestion would be not to depend on the result of a Read call to break the loop. The NetworkStream class has a DataAvailable property that is designed for this. The correct way to write a receive loop is:
NetworkStream netStream = client.GetStream();
int read = 0;
byte[] buffer = new byte[1024];
StringBuilder response = new StringBuilder();
do
{
read = netStream.Read(buffer, 0, buffer.Length);
response.Append(Encoding.ASCII.GetString(buffer, 0, read));
}
while (netStream.DataAvailable);

Read the response until you reach a double CRLF. What you now have is the Response headers.
Parse the headers to read the Content-Length header which will be the count of bytes left in the response.
Here is a regular expression that can catch the Content-Length header.
David's Updated Regex
Content-Length: (?<1>\d+)\r\n
Content-Length
Note
If the server does not properly set this header I would not use it.

Not sure if this is helpful or not but with HTTP 1.1 the underlying connection to the server might not be closed so maybe the stream doesn't get closed either? The idea being that you can reuse the connection to send a new request. I think you have to use the content-length. Alternatively use the WebClient or WebRequest classes instead.

I may be wrong, but it looks like your call to Write is writing (under the hood) to the stream ns (via StreamWriter). Later, you're reading from the same stream (ns). I don't quite understand why are you doing this?
Anyway, you may need to use Seek on the stream, to move to the location where you want to start reading. I'd guess that it seeks to the end after writing. But as I said, I'm not really sure if this is a useful answer!

Two Suggestions...
Have you tried using the DataAvailable property of NetworkStream? It should return true if there is data to be read from the stream.
while (ns.DataAvailable)
{
//Do stuff here
}
Another option would be to change the ReadTimeOut to a low value so you don't end up blocking for a long time. It can be done like this:
ns.ReadTimeOut=100;