Detect if connection has been dropped

I'm trying to detect when a connection has been dropped.

On this way, it only detects that the connection is no longer active when the internet is back, but not when it has been dropped actually.

int loop = 1;
long int msConn;

// Main thread
void connect() {
  // ...

  while (loop) {
    if ((sizeBytes = recv(sockfd, buffer, MAX_BUFFER_SIZE - 1, 0)) == -1) {
      cout << "Error: connection lost." << endl;

      sleep(15);
      connect();
      return;
    }

    // Update timestamp
    struct timeval tp;
    gettimeofday(&tp, NULL);
    msConn = tp.tv_sec * 1000 + tp.tv_usec / 1000;
  }
}

// Heartbeat thread
void checkConnection() {
  for(;;)  {
    struct timeval tp;
    gettimeofday(&tp, NULL);
    long int ms = tp.tv_sec * 1000 + tp.tv_usec / 1000;

    if (msConn != 0) {
      if ((ms - msConn) >= 10000) {
        msConn = 0;
        close(sockfd);
      }
    }

    sleep(15);
  }
}