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That's not a bug, that's a feature.

You don't get a guarantee about where your signal is going to be delivered, and the set of things you're allowed to do in a signal handler is restricted.

See sigaction(3) for details about what you can do (and you can't do anything else). Your program is doing many things that are not allowed in a signal handler.

The correct thing to do is to signal something else in your program and return from the signal handler. An example technique for doing that is the "self pipe trick". Create a pipe and keep a handle to both ends. Read from one end in your normal I/O processing. If you get a signal, in the signal handler, write a byte to the other end of the pipe and return. When the byte is read from the pipe you know the signal has arrived and you can do extended processing safely.

Update:

As Michael Burr has pointed out, you can block particular threads from receiving particular signals using pthread_sigmask(3). However, to fix the underlying problem here you still need to not do the work in the signal handler.