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Prologue Caches
All the frame sniffing functions typically examine the code at the start of the corresponding function, to determine the state of registers. The ABI will save old values and set new values of key registers at the start of each function in what is known as the function prologue.
For any particular stack frame this data does not change, so all the standard unwinding functions, in addition to receiving a pointer to the NEXT frame as their first argument, receive a pointer to a prologue cache as their second argument. This can be used to store values associated with a particular frame, for reuse on subsequent calls involving the same frame.
It is up to the user to define the structure used (it is a void * pointer) and arrange allocation and deallocation of storage. However for general use, GDB provides struct trad_frame_cache, with a set of accessor routines. This structure holds the stack and code address of THIS frame, the base address of the frame, a pointer to the struct frame_info for the NEXT frame and details of where the registers of the PREVIOUS frame may be found in THIS frame.
Typically the first time any sniffer function is called with NEXT frame, the prologue sniffer for THIS frame will be NULL. The sniffer will analyze the frame, allocate a prologue cache structure and populate it. Subsequent calls using the same NEXT frame will pass in this prologue cache, so the data can be returned with no additional analysis.