python-extension-gdb-example

A simple example showing how to build and debug a CPython extension with gdb

Summary

I came to the world of CPython extensions as a python programmer with very little C experience, so segfaults were (and still are, to a certain extent) inordinately confusing. Tutorials for setting up and using gdb on Mac involve complex codesigning and multiple restarts, so I made this simple Docker setup to help make debugging extensions with gdb straightforward.

Running the example

$ git clone https://github.com/mjbryant/python-extension-gdb-example
$ cd python-extension-gdb-example
$ make build-docker
$ make run-docker
(in-docker)$ make python
(in-docker)$ make test-gdb # prints a bunch of gdb initialization
(in-docker)(gdb) run

This will run a test script that calls the extension inside gdb. To set a breakpoint and step through the program, enter break python_extension.c:4 in the (gdb) prompt before calling run.

Running with your own extension

Using this to debug your own CPython extension isn't as straightforward, since you'll have to make sure your extension install commands and test scripts work. But you can drop into the docker container with your own directory mounted by doing MOUNT_DIR=${YOUR_DIR} make run-docker. This should at least get you to an environment with all dependencies necessary to debug your extension.

Remember to compile your extension with CFLAGS="-O0 -g" to get better visibility into variable values!

Using python2

You're not still using python2, are you? It's EOL 2020-01-01! Update! But if you really have to, you should be able to remove the 3 from the python apt-get installs and invocations and it should all work.