6. Auto-loading Python code

I think the idea of backtrace filters (the topic of the previous post) is a pretty cool one.  And, as I mentioned before, extending gdb with application-specific behavior is a compelling use for the Python scripting capability.

Remembering to source these snippets is a bit of a pain.  You could, of course, stick a command into your ~/.gdbinit — that is pretty easy.  I like things to be more automatic, though.  Suppose someone writes a new filter — it would be nice to get it without having to edit anything.

Naturally, we provide an automatic mechanism for loading code — or I wouldn’t be writing this, would I?

Internally, gdb has an structure called an “objfile“.  There is one of these for the inferior’s executable, and another one for each shared library that the inferior has loaded.  A new one is also created when gdb loads separate debug info (typical for distros — not so typical for your own builds).

When gdb creates a new objfile, it takes the objfile‘s file name, appends “-gdb.py“, and looks for that file.  If it exists, it is evaluated as Python code.

Here’s a simple way to see this in action.  Assuming you’ve been using the directory names I’ve used throughout this series, put the following into ~/archer/install/bin/gdb-gdb.py:

import gdb
print "hi from %s" % gdb.get_current_objfile().get_filename()

Now run gdb on itself (remember — you should still have the archer install directory in your PATH):

$ gdb gdb

I get:

hi from /home/tromey/archer/install/bin/gdb

This naming scheme is ok-ish for stuff you just built, but not so for distros.  We’ll be augmenting the search capability a bit so that we can hide the Python files away in a subdirectory of /usr/lib.  I’m not sure exactly what we’ll do here, but it shouldn’t be hard to come up with something reasonable.

Another wrinkle is that this scheme does not work transparently for statically-linked executables.  Ideally, we would have a way to automatically find these snippets even in this case.  One idea that has been mentioned a few times is to put the Python code directly into the executable.  Or, we could put the code next to the source.  Both of these ideas have some drawbacks, though.

Note that one of these files might be loaded multiple times in a given gdb session — gdb does not track which ones it has loaded.  So, I recommend that for “real” projects (something you ship, not just a local hack) you only put import commands (and a couple other idempotent operations, one of which we’ll discuss soon) into the auto-load file, and install the bulk of the Python code somewhere on sys.path.

Our next topic is something that many people have asked for over the years: application-specific pretty-printing.  And, as we’ll see, this provides another use for auto-loading of Python code.

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