(gdbint.info.gz) Stack Frames

 
 7 Stack Frames
 **************
 
 A frame is a construct that GDB uses to keep track of calling and
 called functions.
 
    GDB's frame model, a fresh design, was implemented with the need to
 support DWARF's Call Frame Information in mind.  In fact, the term
 "unwind" is taken directly from that specification.  Developers wishing
 to learn more about unwinders, are encouraged to read the DWARF
 specification, available from `http://www.dwarfstd.org'.
 
    GDB's model is that you find a frame's registers by "unwinding" them
 from the next younger frame.  That is, `get_frame_register' which
 returns the value of a register in frame #1 (the next-to-youngest
 frame), is implemented by calling frame #0's `frame_register_unwind'
 (the youngest frame).  But then the obvious question is: how do you
 access the registers of the youngest frame itself?
 
    To answer this question, GDB has the "sentinel" frame, the "-1st"
 frame.  Unwinding registers from the sentinel frame gives you the
 current values of the youngest real frame's registers.  If F is a
 sentinel frame, then `get_frame_type (F) == SENTINEL_FRAME'.
 
 7.1 Selecting an Unwinder
 =========================
 
 The architecture registers a list of frame unwinders (`struct
 frame_unwind'), using the functions `frame_unwind_prepend_unwinder' and
 `frame_unwind_append_unwinder'.  Each unwinder includes a sniffer.
 Whenever GDB needs to unwind a frame (to fetch the previous frame's
 registers or the current frame's ID), it calls registered sniffers in
 order to find one which recognizes the frame.  The first time a sniffer
 returns non-zero, the corresponding unwinder is assigned to the frame.
 
 7.2 Unwinding the Frame ID
 ==========================
 
 Every frame has an associated ID, of type `struct frame_id'.  The ID
 includes the stack base and function start address for the frame.  The
 ID persists through the entire life of the frame, including while other
 called frames are running; it is used to locate an appropriate `struct
 frame_info' from the cache.
 
    Every time the inferior stops, and at various other times, the frame
 cache is flushed.  Because of this, parts of GDB which need to keep
 track of individual frames cannot use pointers to `struct frame_info'.
 A frame ID provides a stable reference to a frame, even when the
 unwinder must be run again to generate a new `struct frame_info' for
 the same frame.
 
    The frame's unwinder's `this_id' method is called to find the ID.
 Note that this is different from register unwinding, where the next
 frame's `prev_register' is called to unwind this frame's registers.
 
    Both stack base and function address are required to identify the
 frame, because a recursive function has the same function address for
 two consecutive frames and a leaf function may have the same stack
 address as its caller.  On some platforms, a third address is part of
 the ID to further disambiguate frames--for instance, on IA-64 the
 separate register stack address is included in the ID.
 
    An invalid frame ID (`outer_frame_id') returned from the `this_id'
 method means to stop unwinding after this frame.
 
    `null_frame_id' is another invalid frame ID which should be used
 when there is no frame.  For instance, certain breakpoints are attached
 to a specific frame, and that frame is identified through its frame ID
 (we use this to implement the "finish" command).  Using `null_frame_id'
 as the frame ID for a given breakpoint means that the breakpoint is not
 specific to any frame.  The `this_id' method should never return
 `null_frame_id'.
 
 7.3 Unwinding Registers
 =======================
 
 Each unwinder includes a `prev_register' method.  This method takes a
 frame, an associated cache pointer, and a register number.  It returns
 a `struct value *' describing the requested register, as saved by this
 frame.  This is the value of the register that is current in this
 frame's caller.
 
    The returned value must have the same type as the register.  It may
 have any lvalue type.  In most circumstances one of these routines will
 generate the appropriate value:
 
 `frame_unwind_got_optimized'
      This register was not saved.
 
 `frame_unwind_got_register'
      This register was copied into another register in this frame.  This
      is also used for unchanged registers; they are "copied" into the
      same register.
 
 `frame_unwind_got_memory'
      This register was saved in memory.
 
 `frame_unwind_got_constant'
      This register was not saved, but the unwinder can compute the
      previous value some other way.
 
 `frame_unwind_got_address'
      Same as `frame_unwind_got_constant', except that the value is a
      target address.  This is frequently used for the stack pointer,
      which is not explicitly saved but has a known offset from this
      frame's stack pointer.  For architectures with a flat unified
      address space, this is generally the same as
      `frame_unwind_got_constant'.
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