If you set up a signal handler in pthreads and modify the struct 
sigcontext passed in, your changes to that struct are discarded.  For 
instance, modifying context.eip to point to a new function works as you 
would expect in a non-pthreads app, but does not work in pthreads.

Investigation shows that pthreads sets up its own signal handler that then 
copies the real struct sigcontext before passing it to the user 
sighandler.  The user sighandler's struct sigcontext never makes it back 
to the kernel.