Programming

I've isolated the source of error a little bit more. It is because of using the third parameter of the signal handler for saving the context of the thread function. If I pass the parameter context to setcontext instead of saving it in thread_context and swapping back to scheduler_context the same error will occur.
If will post an explanation of this strange behaviour to the linux kernel mailing list. Maybe they have an answer for it. I'll post the reply then.

how to handle and initialize signals concerning concurrent thread programming

What do you mean? How do install the signal handlers when using pthreads or setcontext etc?

So finally I want to close this thread with my conclusion concerning this problem.
It seems to be a problem of Linux. That is what someone told me in another newsgroup. He tried to implement a user-lever thread library for Linux, too, but ran into similar problems. His suggestions was to use pthreads or another OS (it should work fine in NetBSD). He posted also a link to the GNU FAQ for the C library, where they mention that it's not possible to do context switching in the signal handler. Here is the link: Frequently Asked Questions about the GNU C Library

That's my conclusion. I hope this answers all questions concerning this thread.
Thread can be closed.

In fact, UNIX says the only guaranteed safe inter-thread communication that can be done in a signal handler is sem_post.