Can I get %rsp with inline assembly?

Question

This question is related to this question: How can I access arguments 7+ using inline assembly?

I understand that accessing the pointers myself is non-standard, potentially unsafe, and not portable.

That being said, I'd like to get the stack pointer %rsp and transfer its value into a C variable. Is it possible to do this with the following command?

int main()
{
  void extra_args;
  __asm__("mov %%rsp, %0": "=m"(extra_args));
  return 0;
}

I've tried this, but it doesn't seem to point to the right place. Does the above command actually transfer %rsp to the extra_args variable?