Sockets and multithreading

I have an interesting (for me) problem ... There are two streams, one to grab data from the std input and send it via a socket to the server, and the other to receive data from a blocking socket. So when there is no response from the server, the recv () call waits indefinitely, right? But instead of blocking only the calling thread, it blocks the general process! Why is this happening?

boost::mutex     nvtMutex;
boost::mutex    strMutex;
boost::mutex    quitMutex;
bool        quit = false;

void *processServerOutput(void *arg)
{
    NVT *nvt = (NVT*)arg;
    while(1)
    {
        // Lock the quitMutex before trying to access to quit variable
        quitMutex.lock();
        if(quit)
        {
            quitMutex.unlock();
            pthread_exit(NULL);
        }
        else
            quitMutex.unlock();

        // Receive output from server
        nvtMutex.lock();
        nvt->receive();
        cout << Util::Instance()->iconv("koi8-r", "utf-8", nvt->getOutBuffer());
        nvtMutex.unlock();

        // Delay
        sleep(1);
    }
}

void *processUserInput(void *arg)
{
    NVT *nvt = (NVT*)arg;

    while(1)
    {
        // Get user input
        //cin.getline(str, 1023);

        sleep(3);
        strcpy(str, "hello");

        // If we type 'quit', exit from thread
        if(strcmp(str, "quit") == 0)
        {
            // Lock quit variable before trying to modify it
            quitMutex.lock();
            quit = true;
            quitMutex.unlock();

            // Exit from thread
            pthread_exit(NULL);
        }

        // Send the input to server
        nvtMutex.lock();
        nvt->writeUserCommand(Util::Instance()->iconv("utf-8", "koi8-r", str));
        nvt->send();
        nvtMutex.unlock();
    }
}