I'm doing some novice work with ucontext and threading. I'm printing out certain register values just to practice doing it.

However, I'm confused about one of the register values in Linux, REG_UESP. For instance, I know that REG_EIP contains the address of the instruction pointer, and REG_ESP contains the address of the stack pointer.

I can't figure out what REG_UESP is, though. Is it an unsigned version of ESP? Is it the address of the last element popped off the stack?

When I print it out right now, it gives an address of 0. Does this mean it points to the first memory address, or that its value is null?

The UESP is the user stack pointer, which is only relevant when there has been a change in privilege level. If there hasn't been a change in privilege level, then this will not have been set. There is actually only one real stack pointer register, but the thread context has to keep the stack pointer from the user thread when it switches to a new stack pointer after the change in privilege level.

Note also that from the point of view of a stack pointer, '0' points to the end of virtual memory, not the beginning, since the stack pointer will predecrement. But this wouldn't be a normal value in most x86 contexts.

Hmmm... I don't understand this last bit, Neon... You seem to be suggesting that the CPU does something other than simply copy the user-state SP value verbatim into this register as it performs the switch.