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ioctlsocket reporting 0 bytes available to read

Posted on 2011-09-15
Medium Priority
Last Modified: 2012-06-21
I'm trying to track down an issue I have with windows sockets.

I have an client and server which I use to send simple text commands data back and forth.

I have a non-blocking socket which I use to send data from my client to the server

int err = send( socket, stringToSend.c_str(), stringToSend.size(), 0 );

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I then use the following to check if there is any data on the socket to read:

if (SOCKET_ERROR != ioctlsocket( socket, FIONREAD, (DWORD*)&nLength ))
    if (0 < nLength)
         err = recv( socket, buf, nLength, 0 );

         //blah blah

        //blah blah

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Now I have a strange problem.
1. When I send the first piece of data from client to server I am able to receive a response from the server.
2. Then I try to send the smae piece of data again, but this time ioctlsocket always reports that no 0 bytes are available to read from the socket (nLength is always 0).

The strange thing is that if I use wireshark I can see that the server has actually sent data to the client.

What would cause ioctlsocket to report that nLength = 0 whenever I can see data sent via wireshark???

Question by:Wanting2LearnMan

Accepted Solution

ssnkumar earned 1000 total points
ID: 36547309
The prototype of ioctlsocket() is like this:
int ioctlsocket(__in     SOCKET s, __in     long cmd, __inout  u_long *argp);

The 3rd argument *argp is "A pointer to a parameter for cmd".
Set *argp to a nonzero value if the nonblocking mode should be enabled
set it to 0 if the nonblocking mode should be disabled.

In your case, I think you are not setting any value to it - you are actually waiting for it to return some value.
So, what might be happening is:
Firsttime when client sends data to server and receives back the response, the value stored in nLength was 0.
And it returned a non-zero positive value back.
Now, when you are calling the ioctlsocket() 2nd time, it is using the non-zero positive value that was set in the previous call.
Because of that, it has become a non-Blocking call and returning immediately.
So, try to explicitly set it to 0 before the 2nd call also and see what happens.

I am assuming that you are checking the value of err returned from ioctlsocket().
It should be 0 to indicate success.
If it is non-zero, then use WSAGetLastError to find the error code - that might throw some clue.

So, the first thing that you should try is to, set nLength to 0 before each call to ioctlsocket, like this:

nLength = 0;
if (SOCKET_ERROR != ioctlsocket( socket, FIONREAD, (DWORD*)&nLength ))
    if (0 < nLength)
         err = recv( socket, buf, nLength, 0 );

         //blah blah

        //blah blah

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LVL 35

Assisted Solution

sarabande earned 1000 total points
ID: 36548037
the blocking enabling and disabling was done with option FIONBIO and not FIONREAD.

i assume FIONREAD doesn't work properly with non-blocking sockets. you simply should do recv whcih would not block and mostly would return -1 with WSAGetLastError() is WSAEWOULDBLCK.


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