(gdbint.info.gz) Stack Frames
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7 Stack Frames
A frame is a construct that GDB uses to keep track of calling and
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
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
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:
This register was not saved.
This register was copied into another register in this frame. This
is also used for unchanged registers; they are "copied" into the
This register was saved in memory.
This register was not saved, but the unwinder can compute the
previous value some other way.
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
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