Frames In Python (Debugging with GDB)

23.2.2.25 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).

Function: gdb.invalidate_cached_frames

GDB internally keeps a cache of the frames that have been unwound. This function invalidates this cache.

This function should not generally be called by ordinary Python code. It is documented for the sake of completeness.

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. If the frame does not have a block – for example, if there is no debugging information for the code in question – then this will throw an exception.

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_register (register)

Return the value of register in this frame. Returns a Gdb.Value object. Throws an exception if register does not exist. The register argument must be one of the following:

A string that is the name of a valid register (e.g., 'sp' or 'rax').
A gdb.RegisterDescriptor object (see Registers In Python).
A GDB internal, platform specific number. Using these numbers is supported for historic reasons, but is not recommended as future changes to GDB could change the mapping between numbers and the registers they represent, breaking any Python code that uses the platform-specific numbers. The numbers are usually found in the corresponding platform-tdep.h file in the GDB source tree.

Using a string to access registers will be slightly slower than the other two methods as GDB must look up the mapping between name and internal register number. If performance is critical consider looking up and caching a gdb.RegisterDescriptor object.

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.