Frame filters are Python objects that manipulate the visibility of a frame or frames when a backtrace (see Backtrace) is printed by GDB.
Only commands that print a backtrace, or, in the case of GDB/MI commands (see GDB/MI), those that return a collection of frames are affected. The commands that work with frame filters are:
backtrace (see The backtrace command),
(see The -stack-list-frames command),
-stack-list-variables (see The
see The -stack-list-arguments command) and
-stack-list-locals (see The
A frame filter works by taking an iterator as an argument, applying
actions to the contents of that iterator, and returning another
iterator (or, possibly, the same iterator it was provided in the case
where the filter does not perform any operations). Typically, frame
filters utilize tools such as the Python’s
itertools module to
work with and create new iterators from the source iterator.
Regardless of how a filter chooses to apply actions, it must not alter
the underlying GDB frame or frames, or attempt to alter the
call-stack within GDB. This preserves data integrity within
GDB. Frame filters are executed on a priority basis and care
should be taken that some frame filters may have been executed before,
and that some frame filters will be executed after.
An important consideration when designing frame filters, and well worth reflecting upon, is that frame filters should avoid unwinding the call stack if possible. Some stacks can run very deep, into the tens of thousands in some cases. To search every frame when a frame filter executes may be too expensive at that step. The frame filter cannot know how many frames it has to iterate over, and it may have to iterate through them all. This ends up duplicating effort as GDB performs this iteration when it prints the frames. If the filter can defer unwinding frames until frame decorators are executed, after the last filter has executed, it should. See Frame Decorator API, for more information on decorators. Also, there are examples for both frame decorators and filters in later chapters. See Writing a Frame Filter, for more information.
The Python dictionary
gdb.frame_filters contains key/object
pairings that comprise a frame filter. Frame filters in this
dictionary are called
global frame filters, and they are
available when debugging all inferiors. These frame filters must
register with the dictionary directly. In addition to the
global dictionary, there are other dictionaries that are loaded
with different inferiors via auto-loading (see Python Auto-loading). The two other areas where frame filter dictionaries
can be found are:
gdb.Progspace which contains a
frame_filters dictionary attribute, and each
object which also contains a
When a command is executed from GDB that is compatible with
frame filters, GDB combines the
gdb.Progspace and all
gdb.Objfile dictionaries currently
loaded. All of the
gdb.Objfile dictionaries are combined, as
several frames, and thus several object files, might be in use.
GDB then prunes any frame filter whose
False. This pruned list is then sorted according
priority attribute in each filter.
Once the dictionaries are combined, pruned and sorted, GDB
creates an iterator which wraps each frame in the call stack in a
FrameDecorator object, and calls each filter in order. The
output from the previous filter will always be the input to the next
filter, and so on.
Frame filters have a mandatory interface which each frame filter must implement, defined here:
GDB will call this method on a frame filter when it has reached the order in the priority list for that filter.
For example, if there are four frame filters:
Name Priority Filter1 5 Filter2 10 Filter3 100 Filter4 1
The order that the frame filters will be called is:
Filter3 -> Filter2 -> Filter1 -> Filter4
Note that the output from
Filter3 is passed to the input of
Filter2, and so on.
filter method is passed a Python iterator. This iterator
contains a sequence of frame decorators that wrap each
gdb.Frame, or a frame decorator that wraps another frame
decorator. The first filter that is executed in the sequence of frame
filters will receive an iterator entirely comprised of default
FrameDecorator objects. However, after each frame filter is
executed, the previous frame filter may have wrapped some or all of
the frame decorators with their own frame decorator. As frame
decorators must also conform to a mandatory interface, these
decorators can be assumed to act in a uniform manner (see Frame Decorator API).
This method must return an object conforming to the Python iterator protocol. Each item in the iterator must be an object conforming to the frame decorator interface. If a frame filter does not wish to perform any operations on this iterator, it should return that iterator untouched.
This method is not optional. If it does not exist, GDB will raise and print an error.
name attribute must be Python string which contains the
name of the filter displayed by GDB (see Frame Filter Management). This attribute may contain any combination of letters
or numbers. Care should be taken to ensure that it is unique. This
attribute is mandatory.
enabled attribute must be Python boolean. This attribute
indicates to GDB whether the frame filter is enabled, and
should be considered when frame filters are executed. If
True, then the frame filter will be executed
when any of the backtrace commands detailed earlier in this chapter
are executed. If
False, then the frame
filter will not be executed. This attribute is mandatory.
priority attribute must be Python integer. This attribute
controls the order of execution in relation to other frame filters.
There are no imposed limits on the range of
priority other than
it must be a valid integer. The higher the
the sooner the frame filter will be executed in relation to other
frame filters. Although
priority can be negative, it is
recommended practice to assume zero is the lowest priority that a
frame filter can be assigned. Frame filters that have the same
priority are executed in unsorted order in that priority slot. This
attribute is mandatory. 100 is a good default priority.