Question

I’m maintaining a legacy application written in C++. It crashes every now and then and Valgrind tells me its a double delete of some object.

What are the best ways to find the bug that is causing a double delete in an application you don’t fully understand and which is too large to be rewritten ?

Please share your best tips and tricks!

Answer 1

Here’s some general suggestion’s that have helped me in that situation:

1. Turn your logging level up to full debug, if you are using a logger. Look for suspicious stuff in the output. If your app doesn’t log pointer allocations and deletes of the object/class under suspicion, it’s time to insert some cout << "class Foo constructed, ptr= " << this << endl; statements in your code (and corresponding delete/destructor prints).
2. Run valgrind with –db-attach=yes. I’ve found this very handy, if a bit tedious. Valgrind will show you a stack trace every time it detects a significant memory error or event and then ask you if you want to debug it. You may find yourself repeatedly pressing ‘n’ many many times if your app is large, but keep looking for the line of code where the object in question is first (and secondly) deleted.
3. Just scour the code. Look for construction/deletion of the object in question. Sadly, sometimes it winds up being in a 3rd party library :-(.
4. Update: Just found this out recently: Apparently gcc 4.8 and later (if you can use GCC on your system) has some new built-in features for detecting memory errors, the “address sanitizer“. Also available in the LLVM compiler system.