send and recv on same socket from different threads not working

Question

I read that it should be safe from different threads concurrently, but my program has some weird behaviour and I don't know what's wrong.

I have concurrent threads communicating with a client socket

1. one doing send to a socket
2. one doing select and then recv from the same socket

As I'm still sending, the client has already received the data and closed the socket. At the same time, I'm doing a select and recv on that socket, which returns 0 (since it is closed) so I close this socket. However, the send has not returned yet...and since I call close on this socket the send call fails with EBADF.

I know the client has received the data correctly since I output it after I close the socket and it is right. However, on my end, my send call is still returning an error (EBADF), so I want to fix it so it doesn't fail.

This doesn't always happen. It happens maybe 40% of the time. I don't use sleep anywhere. Am I supposed to have pauses between sends or recvs or anything?

Here's some code:

Sending:

while(true)
{
    // keep sending until send returns 0
    n = send(_sfd, bytesPtr, sentSize, 0);

    if (n == 0)
    {
        break;
    }
    else if(n<0)
    {
        cerr << "ERROR: send returned an error "<<errno<< endl; // this case is triggered
        return n;
    }

    sentSize -= n;
    bytesPtr += n;
}

Receiving:

 while(true)
{
    memset(bufferPointer,0,sizeLeft);
    n = recv(_sfd,bufferPointer,sizeLeft, 0);
    if (debug) cerr << "Receiving..."<<sizeLeft<<endl;
    if(n == 0)
    {
        cerr << "Connection closed"<<endl; // this case is triggered
        return n;
    }
    else if (n < 0)
    {
        cerr << "ERROR reading from socket"<<endl;
        return n;
    }
     bufferPointer += n;
     sizeLeft -= n;
     if(sizeLeft <= 0) break;

}

On the client, I use the same receive code, then I call close() on the socket. Then on my side, I get 0 from the receive call and also call close() on the socket Then my send fails. It still hasn't finished?! But my client already got the data!

Answer 1

Found the problem. It's with my loop. Notice that it's an infinite loop. When I don't have anymore left to send, my sentSize is 0, but I'll still loop to try to send more. At this time, the other thread has already closed this thread and so my send call for 0 bytes returns with an error.

I fixed it by changing the loop to stop looping when sentSize is 0 and it fixed the problem!