Can I designate a specific register to use instead of Thread Local Storage?

I want to use Windows' Vectored Exception Handlers, but there's no way to pass critical information to the handler.

This can be solved by using Thread Local Storage, but I'm looking for a better solution.

Because Vectored Exception Handlers allow you to read the contents of the registers as they were at the time that the exception occurred, it would seem that storing a pointer to an information structure (or just the information itself) in a register would make sense.

While this is trivial in assembly code, C doesn't have any easy way to designate a specific register for you to use for this purpose.

Is there a way to implement this in C safely?

> it would seem that storing a pointer to an information structure (or just the information itself) in a register would make sense.
>
> While this is trivial in assembly code, C doesn't have any easy way to designate a specific register for you to use for this purpose.
>
> Is there a way to implement this in C safely?

The C language specification has nothing whatever to say about CPU registers (and no, the register keyword does not contradict that). Therefore, no, there is no way to implement it in (strictly conforming) C (alone), safely or otherwise.

But you could potentially write a helper function in assembly that you could call from C. Or many C implementations support inline assembly of some variety. And some architectures may even support helpful features such as mapping CPU registers to addresses, which would also serve your purpose. But all such mechanisms rely on another language, and / or C extensions, and / or implementation-specific behavior. They are in no way portable, and how safe any of them are must be evaluated on a case by case basis.