Creating a thread in DllMain?

You shouldn't be doing any API calls, especially for things like creating threads or windows, from DLLMain. Raymond Chen has written about this many times; here's one that's particularly relevant.

What does your thread do?

If you're trying to move stuff onto a second thread to avoid restrictions on what you can do inside DllMain, tough luck. Those are not restrictions on what DllMain can do, they are restrictions on what can be done while DllMain is running (and holds the loader lock). If your thread needs to take the loader lock, it will wait until the first thread finishes using it. If your thread didn't need the loader lock I don't see why it couldn't continue immediately... but there's no such thing as a thread that doesn't need the loader lock. Windows has to send DLL_THREAD_ATTACH messages to all DLLs before your thread can start running, which means it also calls your own DllMain, and Windows protects against re-entrancy.

There's no way around this. The thread can't start until after DLL_THREAD_ATTACH processing, and that can't happen while your first thread is inside DllMain. The only possible way around it is to start a new process (which has an independent loader lock and won't block waiting for yours).

No. You shouldn't call CreateThread (or any varation) from DllMain. Attempting to synchronize will result in a deadlock. What exactly are you trying to do?

Best Practices for Creating DLLs