unw_getcontext man page
unw_getcontext -- get initial machine-state
int unw_getcontext(unw_context_t *ucp);
The unw_getcontext() routine initializes the context structure pointed to by ucp with the machine-state of the call-site. The exact set of registers stored by unw_getcontext() is platform-specific, but, in general, at least all preserved (“callee-saved”) and all frame-related registers, such as the stack-pointer, will be stored.
This routine is normally implemented as a macro and applications should not attempt to take its address.
On IA-64, unw_context_t has a layout that is compatible with that of ucontext_t and such structures can be initialized with getcontext() instead of unw_getcontext(). However, the reverse is not true and it is not safe to use structures initialized by unw_getcontext() in places where a structure initialized by getcontext() is expected. The reason for this asymmetry is that unw_getcontext() is optimized for maximum performance and does not, for example, save the signal mask.
On successful completion, unw_getcontext() returns 0. Otherwise, a value of -1 is returned.
Thread and Signal Safety
unw_getcontext() is thread-safe as well as safe to use from a signal handler.