I am having troubles with the cos() function.

I have two threads which are calling a member function using cos(). This class is synchronized properly so that the two threads cannot be executing this code ate the same time. Anyway, cos is reentrant.
The fact is that one of these thread(always the same) generates a stack overflow in cos() when cos() is in a member function.
If the cos() call is directly in the thread's main loop, there is no problem.

It is not the first time I do this kind of operation but now it fails. What could be the source of all this?

Any idea about it will be welcome.