Question

I wanted to create a multi-threaded socket server using C++11 and standard linux C-Librarys.

The easiest way doing this would be opening a new thread for each incoming connection, but there must be an other way, because Apache isn’t doing this. As far as I know Apache handles more than one connection in a Thread. How to realise such a system?

I thought of creating one thread always listening for new clients and assigning this new client to a thread. But if all threads are excecuting an “select()” currently, having an infinite timeout and none of the already assigned client is doing anything, this could take a while for the client to be useable.

So the “select()” needs a timeout. Setting the timeout to 0.5ms would be nice, but I guess the workload could rise too much, couldn’t it?

Can someone of you tell me how you would realise such a system, handling more than one client for each thread?
PS: Hope my English is well enough for you to understand what I mean 😉

Answer 1

First off, have a look at using poll() instead of select(): it works better when you have large number of file descriptors used from different threads.

To get threads currently waiting in I/O out of waiting I’m aware of two methods:

1. You can send a suitable signal to the thread using pthread_kill(). The call to poll() fails and errno is set to EINTR.
2. Some systems allow a file descriptor to be obtained from a thread control device. poll()ing the corresponding file descriptor for input succeeds when the thread control device is signalled. See, e.g., Can we obtain a file descriptor for a semaphore or condition variable?.