Figure 2.10 shows the high level sequence diagram for GDB in response to the backtrace command. This sequence shows the behavior for the first call to backtrace after control has returned to GDB.

Figure 2.10.  High level sequence diagram for the GDB backtrace command

The main command function is backtrace_command, which uses print_frame_info to print the name of each function on the stack with its arguments.

The first frame is already known from the program counter and stack pointer of the stopped target, so is printed out by print_frame. That will ultimately use the current target's to_xfer_partial function to get the local argument values.

Since this is the first backtrace after the program stopped, the stack pointer and program counter are each obtained from the sentinel frame using get_func_type. print_frame is then called for each frame in turn as the stack is unwound until there are no more stack frames. The information in each frame is built up using the architecture's frame sniffers.

It is useful to look at print_frame in more detail. Figure 2.11 shows the sequence diagram for the second series of calls to the print_frame function when handling the GDB backtrace command, used to print out the stack frame.

Figure 2.11.  Sequence diagram for the GDB print_frame function used by the backtrace command

The information about the function on the stack frame can be obtained from the program counter and stack pointer associated with the stack frame. These are obtained by calls to the gdbarch_unwind_pc and gdbarch_unwind_sp functions.

Then for each argument, its value must be printed out. The symbol table debug data will identify the arguments, and enough information for GDB to work out if the value is on the stack or in a register. The frame sniffer function to get registers from the stack frame (in this example arch_frame_prev_register) is used to get the values of any registers as appropriate.

The precise sequence of calls depends on the functions in the stack frame, the arguments they have, and whether those arguments are in registers or on the stack.