How and When a frame is constructed

[kceiwH]

Hi,

When I read GDB source codes, I don't understand how a frame is  
constructed, and when.
As far as I know, get_current_frame is used to get a current frame.  
When current_frame is NULL, it will construct a sentinel_frame and  
unwind it to get current_frame. But I don't know where current_frame  
will be updated except get_current_frame. The

I also guess that the frame stack will be updated when the inferior is  
running, stepped. But I still don't find the code to update the frame  
stack.

I'm wondering when the first frame is constructed and pushed to frame  
stack and how to push to and pop from frame stack.

Thanks in advance.

Best Wishes,

Mao