Debugging with GDB: Frames In Python

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#### 23.2.2.23 Accessing inferior stack frames from Python.

When the debugged program stops, GDB is able to analyze its call stack (see Stack frames). The `gdb.Frame` class represents a frame in the stack. A `gdb.Frame` object is only valid while its corresponding frame exists in the inferior’s stack. If you try to use an invalid frame object, GDB will throw a `gdb.error` exception (see Exception Handling).

Two `gdb.Frame` objects can be compared for equality with the `==` operator, like:

    (gdb) python print gdb.newest_frame() == gdb.selected_frame ()
    True

The following frame-related functions are available in the `gdb` module:

Function: gdb.selected_frame ()

  Return the selected frame object. (see Selecting a Frame).

Function: gdb.newest_frame ()

  Return the newest frame object for the selected thread.

Function: gdb.frame_stop_reason_string (reason)

  Return a string explaining the reason why GDB stopped unwinding frames, as expressed by the given reason code (an integer, see the `unwind_stop_reason` method further down in this section).

A `gdb.Frame` object has the following methods:

Function: Frame.is_valid ()

  Returns true if the `gdb.Frame` object is valid, false if not. A frame object can become invalid if the frame it refers to doesn’t exist anymore in the inferior. All `gdb.Frame` methods will throw an exception if it is invalid at the time the method is called.

Function: Frame.name ()

  Returns the function name of the frame, or `None` if it can’t be obtained.

Function: Frame.architecture ()

  Returns the `gdb.Architecture` object corresponding to the frame’s architecture. See Architectures In Python.

Function: Frame.type ()

  Returns the type of the frame. The value can be one of:

  `gdb.NORMAL_FRAME`

    An ordinary stack frame.

  `gdb.DUMMY_FRAME`

    A fake stack frame that was created by GDB when performing an inferior function call.

  `gdb.INLINE_FRAME`

    A frame representing an inlined function. The function was inlined into a `gdb.NORMAL_FRAME` that is older than this one.

  `gdb.TAILCALL_FRAME`

    A frame representing a tail call. See Tail Call Frames.

  `gdb.SIGTRAMP_FRAME`

    A signal trampoline frame. This is the frame created by the OS when it calls into a signal handler.

  `gdb.ARCH_FRAME`

    A fake stack frame representing a cross-architecture call.

  `gdb.SENTINEL_FRAME`

    This is like `gdb.NORMAL_FRAME`, but it is only used for the newest frame.

Function: Frame.unwind_stop_reason ()

  Return an integer representing the reason why it’s not possible to find more frames toward the outermost frame. Use `gdb.frame_stop_reason_string` to convert the value returned by this function to a string. The value can be one of:

  `gdb.FRAME_UNWIND_NO_REASON`

    No particular reason (older frames should be available).

  `gdb.FRAME_UNWIND_NULL_ID`

    The previous frame’s analyzer returns an invalid result. This is no longer used by GDB, and is kept only for backward compatibility.

  `gdb.FRAME_UNWIND_OUTERMOST`

    This frame is the outermost.

  `gdb.FRAME_UNWIND_UNAVAILABLE`

    Cannot unwind further, because that would require knowing the values of registers or memory that have not been collected.

  `gdb.FRAME_UNWIND_INNER_ID`

    This frame ID looks like it ought to belong to a NEXT frame, but we got it for a PREV frame. Normally, this is a sign of unwinder failure. It could also indicate stack corruption.

  `gdb.FRAME_UNWIND_SAME_ID`

    This frame has the same ID as the previous one. That means that unwinding further would almost certainly give us another frame with exactly the same ID, so break the chain. Normally, this is a sign of unwinder failure. It could also indicate stack corruption.

  `gdb.FRAME_UNWIND_NO_SAVED_PC`

    The frame unwinder did not find any saved PC, but we needed one to unwind further.

  `gdb.FRAME_UNWIND_MEMORY_ERROR`

    The frame unwinder caused an error while trying to access memory.

  `gdb.FRAME_UNWIND_FIRST_ERROR`

    Any stop reason greater or equal to this value indicates some kind of error. This special value facilitates writing code that tests for errors in unwinding in a way that will work correctly even if the list of the other values is modified in future GDB versions. Using it, you could write:

        reason = gdb.selected_frame().unwind_stop_reason ()
        reason_str =  gdb.frame_stop_reason_string (reason)
        if reason >=  gdb.FRAME_UNWIND_FIRST_ERROR:
            print "An error occured: %s" % reason_str

Function: Frame.pc ()

  Returns the frame’s resume address.

Function: Frame.block ()

  Return the frame’s code block. See Blocks In Python.

Function: Frame.function ()

  Return the symbol for the function corresponding to this frame. See Symbols In Python.

Function: Frame.older ()

  Return the frame that called this frame.

Function: Frame.newer ()

  Return the frame called by this frame.

Function: Frame.find_sal ()

  Return the frame’s symtab and line object. See Symbol Tables In Python.

Function: Frame.read_var (variable [, block])

  Return the value of variable in this frame. If the optional argument block is provided, search for the variable from that block; otherwise start at the frame’s current block (which is determined by the frame’s current program counter). The variable argument must be a string or a `gdb.Symbol` object; block must be a `gdb.Block` object.

Function: Frame.select ()

  Set this frame to be the selected frame. See Examining the Stack.

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