This is the second article in a two-part series about displaying information from the GNU Debugger (GDB) in a custom window while you are debugging a C or C++ program. The first article introduced GDB's Text User Interface (TUI) and showed how to create a window using the Python API. This second part finishes the example program by displaying values from GDB's history list.
Loading history values
To get started, add an extra line in the
__init__ method you created in the previous article:
self._next_history_index = 1
_next_history_index variable will be used to fetch values from GDB's history list. The value starts at 1 because the first value in GDB's value history is numbered 1. At each iteration of a loop you'll write, the
_next_history_index variable will represent the next index you need to fetch from GDB's value history.
Next, add two new methods to the
history_window class, which will do the job of fetching values from the history list:
def _add_next_history_value(self): try: value = gdb.history(self._next_history_index) string = value.format_string(pretty_arrays=False, pretty_structs=False) string = "$%d = %s" % (self._next_history_index, re.sub(r"\\s*\n\\s*", " ", string)) self._lines.append(string) self._next_history_index += 1 except: return False return True def _update(self): while self._add_next_history_value(): pass
_add_next_history_value method tries to fetch the next item from GDB's value history. If this is successful, the value is converted to a single-line string and added to the
_lines list. Finally, the method increments the
To keep this tutorial simple, the method converts each value to be represented to a single line. This conversion uses the
re.sub call, which replaces any newline characters with a single space using a regular expression. To enable the use of a regular expression, you need to add the following line to the top of the
_update method just calls
_add_next_history_value until all history values have been processed.
Finally, you need to call
_update in two places.
_update from the
__init__ method to ensure that, as soon as your window is created, all existing history values are loaded into the
_lines list. The complete
__init__ method should now look like this:
def __init__(self, tui_window): self._tui_window = tui_window self._tui_window.title = 'Value History' self._before_prompt_listener = lambda : self._before_prompt() gdb.events.before_prompt.connect(self._before_prompt_listener) self._lines =  self._next_history_index = 1 self._update()
Next, add a call to
_update from the
_before_prompt method, replacing the existing debug line. The full
_before_prompt method should now look like this:
def _before_prompt(self): self._update() self.render()
And with these changes, you have a basic, working history window. Restart GDB using the command line:
gdb -ex 'source history.py' \ -ex 'tui new-layout example_1 history 1 cmd 1 status 1' \ -ex 'layout example_1'
Figure 1 shows what the window should look like in action after entering a few commands in the command window:
Preparing for scrolling
What you have so far is great. But there is one problem. Once the window gets a lot of history values, the earlier ones are lost off the top of the window. It would be great if you could scroll back to view earlier values. So this will be the last feature you add in this tutorial.
But first, you need to rework the code a little to make it easier to add scrolling support.
Add the following methods to your
def _history_count(self): return self._next_history_index - 1 def _display_start(self): return self._max_history_start() def _max_history_start(self): count = self._history_count() height = self._tui_window.height return max(1, count - height + 1)
_history_count method returns the number of history items that have been loaded into the
_display_start method returns the index of the first history value that should be displayed in the window. You don't do much here yet, but will extend the method later.
_max_history_start returns the history value index that the code should start from (that is, the value displayed at the top of the window) so that the last known history value also appears in the window (at the bottom).
Finally, go back to the following line that is currently in your
lines = self._lines[-height:]
Replace that line with the following:
start = self._display_start() - 1 end = start + height lines = self._lines[start:end]
The function is now printing
height number of lines starting from
- 1 is required because
_display_start() returns a history index, which counts from 1, whereas
_lines is a Python list, indexed from 0.
After these changes, the
history_window should work just as it did before, and now you're ready for the final part of this tutorial.
You need to make three more small changes to add scrolling support.
First, add the following line to the
__init__ method before the call to
self._vscroll_start = None
This variable acts as a marker to indicate whether the window is scrolled. When set to
None, the window is not scrolled. Therefore, new values should be added to the end of the window and old values should disappear from the top. In contrast, when the variable is set to an integer, it indicates which history value to scroll back to. The window will then always display items starting from that index.
Next, rewrite the
_dispay_start method like this:
def _display_start(self): if self._vscroll_start is None: start = self._max_history_start() else: start = self._vscroll_start return start
_vscroll_start has been set, the window treats it as the index to start the display. If
_vscroll_start is not set, the method does things exactly as before.
Finally, add the following
vcsroll method to your
def vscroll(self, num): start = self._display_start() start += num start = max(1, start + num) max_start = self._max_history_start() if start >= max_start: self._vscroll_start = None else: self._vscroll_start = start self.render()
num argument indicates the number of lines by which GDB would like to scroll the window contents. Pressing the up or down arrow keys results in a single line change, whereas the PageUp or PageDown keys result in a larger change based on the current size of the window.
vscroll method figures out the history index for the current first line of the window, and adjusts this index by the value of
num. The method clamps this value to some sane bounds: thus, you can't scroll back before index 1 (the first GDB history index), nor should you scroll forward beyond the value of
max_start. This variable stores the index from which you can start printing items and still get the last item from the history shown within the window.
Finally, if the user has scrolled as far down as the value in
max_start, the method sets
None. This indicates that as new history values appear, they should be added to the bottom of the window, pushing older values off the top.
And with that, your window is complete. You can scroll back to view all the old history values, and forward again to view the latest history values.
There are two useful TUI window methods you haven't used in this tutorial:
hscroll method allows horizontal scrolling, just as
vscroll allows vertical scrolling. A good exercise would be to add horizontal scrolling to your history window. Currently, any long history values are truncated. It would be great if you could scroll left and right to view the full value.
click method allows basic mouse interaction with the window. You could use this method to enhance the example to display history values in their multiline format and use mouse clicks to expand or hide the full values.