Introduction

Recently, I encountered regular segfaults in one of the Python applications I was working on. During my investigation, I discovered a simple yet remarkable utility called faulthandler, which is included in Python's standard library. I'd like to demonstrate how this utility can assist in diagnosing segfault issues within your Python applications.

Breaking things

Let's start with breaking some things. The small snippet below will produce a nasty segfault whenever you run it.

import ctypes

def inner():
    ctypes.string_at(0)

def outer():
    inner()

def main():
   outer()

if __name__ == '__main__':
    main()

Assuming that we have the above code in segfault.py file, the result of running it should look more or less like this:

[1]    33003 segmentation fault  python segfault.py

Unfortunately, that does not look very useful, but our new friend faulthandler can unravel more information for us.

Figuring out what happened

Using faulthandler is very simple. It can be enabled in a few ways, the simples one being setting PYTHONFAULTHANDLER environment variable.

PYTHONFAULTHANDLER=1 python segfault.py

The output should be a bit more interesting now:

PYTHONFAULTHANDLER=1 python segfault.py
Fatal Python error: Segmentation fault

Current thread 0x0000000102790580 (most recent call first):
  File "/Users/xxx/.pyenv/versions/3.9.7/lib/python3.9/ctypes/__init__.py", line 517 in string_at
  File "/Users/xxx/snippets/segfault.py", line 4 in inner
  File "/Users/xxx/snippets/segfault.py", line 7 in outer
  File "/Users/xxx/snippets/segfault.py", line 10 in main
  File "/Users/xxx/snippets/segfault.py", line 13 in <module>
[1]    33547 segmentation fault  PYTHONFAULTHANDLER=1 python segfault.py

Now we see a nice traceback that gives us great hints about the source of our segfault.

There are also other ways of enabling faulthandler, I recommend checking out docs for all the configuration options that faulthandler supports.

Summary

Segmentation faults in our apps can cause a lot of headaches. Today we learned how we can leverage faulthandler module to help us find the root causes for these segfaults. Thanks for reading and until next time!