[Q] LinuxThreads internals....

[Bharadwaj Yadavalli <sby at ives dot lkg dot dec dot com>]

I am trying to understand the internals of LinuxThreads and
I have the following simple question:

In pthread_initialize(void):pthread.c, I notice that 
initial stack (__pthread_initial_thread_bos) is computed
by subtracting 2 * STACK_SIZE from the current stack_pointer 
before aligning the result to STACK_SIZE boundary. Why *2*
stack sizes? Can someone please explain the logic here?

Thanks,

Bharadwaj