Make new connections when blocked in a select() call - multithreading

Question

I have 2 threads:
Thread A:
It's the select() loop. Perform socket handling for reading operations, such as accepting new connections, receiving data.
while (1) {
FD_ZERO(&fdReadSet);
numActiveSockets = 0;
for (std::unordered_map<SOCKET, TcpSocket*>::iterator it = m_sock_table.begin(); it != m_sock_table.end(); it++)
{
numActiveSockets++;
FD_SET(it->first, &fdReadSet);
}
int ret;
bool hasListen = false;
if (( ret = select(numActiveSockets, &fdReadSet, NULL, NULL, NULL)) == SOCKET_ERROR) {
printf("Select Failed, Error code = %d\n", WSAGetLastError());
return -1;
}
for (std::unordered_map<SOCKET, TcpSocket*>::iterator it = m_sock_table.begin(); it != m_sock_table.end(); it++)
{
if (FD_ISSET(it->first, &fdReadSet))
{
if (it->first == TcpSocket::m_listen_sock)
{
if (!hasListen)
{
sockaddr_in sock_addr;
int sockLength = sizeof(sock_addr);
SOCKET sock = accept(it->first, (sockaddr *) &sock_addr, &sockLength);
TcpSocket * socket = new TcpSocket();
socket->m_sock = sock;
m_sock_table[sock] = socket;
it = m_sock_table.begin();
hasListen = true;
}
}
else
{
char * buffer = it->second->GetWriteBuffer();
int numRead = recv(it->first, buffer, SOCKET_BUFFER_SIZE, 0);
if (numRead == SOCKET_ERROR)
{
int err = WSAGetLastError();
if (err == WSAECONNRESET)
{
printf("Connection [%i]: RESET Received. Closing Socket\n", it->first);
closesocket(it->first);
it = socketVector.erase(it->first); // iterator invalidated after erase
}
else
{
printf("Recv Failed. Error code = %d\n", err);
return -1;
}
}
else if (numRead == 0)//connection close
{
printf("Connection [%i]: Graceful exit. Closing Socket\n", it->first);
closesocket(it->first);
it = socketVector.erase(it->first); // iterator invalidated after erase
}
else {
/* Process received data */
}
}
}
}
}
Thread B:
Allow the application to perform connect() to establish new connections. If a connect() is successful, it will the add the returned socket to m_sock_table.
I have a socket table called m_sock_table which holds all the sockets. I use this m_sock_table to initialize the fdReadSet to be used in select().
-----------Problem-----------------
If thread A is blocked by select(), and at the same time thread B establish a new connection through connect(), the application wouldn't be able to receive data from the new connection, because fdReadset has not been updated withed the new connected socket.
What would be a good way to solve this problem? Or the design is just wrong from the start?

Answer 1

You could use a signal that doesn't do anything other than interrupting the system call:
#include <signal.h>
void do_nothing() { }
struct sigaction sa;
sa.sa_handler = do_nothing;
sigemptyset(sa.sa_mask);
#ifdef SA_INTERRUPT
sa.sa_flags = SA_INTERRUPT;
#else
sa.sa_flags = 0;
#endif
sigaction(SIGUSR1, &sa, 0);
Then, in thread B, after starting a new connection, send the signal, after making sure thread A will handle it:
/* need only be done once, but needed in every thread other than A */
sigset_t sigs;
sigemptyset(&sigs);
sigaddset(&sigs, SIGUSR1)
pthread_sigmask(SIG_BLOCK, &sigs, 0);
/* each time we create a new connection */
kill(getpid, SIGUSR1);
With the above, select will return with an EINTR error -- so check for that and loop (adding the new connection to the set).