COMPAS Crash On Arch Linux: Vector Out-of-Bounds Fix

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COMPAS Crash on Arch Linux: Vector Out-of-Bounds Fix

Hey guys, so I've been wrestling with a pesky crash in COMPAS, the binary star simulator, and I thought I'd share what I found. It's related to a vector out-of-bounds access issue in Log.cpp, which was causing COMPAS to crash on Arch Linux (and probably other similar systems) when compiled with newer GCC versions. I'm a newbie in astrophysics myself, so figuring this out was a bit of a journey, but hey, that's how we learn, right?

This article will dive into the problem, how to reproduce it, the error messages, the versions involved, and (most importantly) a possible fix. I'll break it down as simply as possible, so even if you're not a C++ guru, you should be able to follow along. Let's get started!

The Bug: Vector Out-of-Bounds in Log.cpp

Alright, let's get down to the nitty-gritty. The core of the problem lies in the Log.cpp file, specifically with how the program handles vectors. For those unfamiliar, vectors in C++ are like dynamic arrays – they can grow and shrink as needed. The error we're dealing with is a vector out-of-bounds access. This means the program is trying to access an element in the vector that doesn't exist (because the index is outside the valid range of the vector). It's like trying to grab a book from a shelf that's not there! This leads to a crash because the program is trying to read memory it's not supposed to, leading to an 'IOT instruction' and a core dump.

Here's the error message that popped up, which is a big clue:

/usr/include/c++/15.2.1/bits/stl_vector.h:1263: std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::operator[](size_type) [with _Tp = STRING_QUALIFIER; _Alloc = std::allocator<STRING_QUALIFIER>; reference = STRING_QUALIFIER&; size_type = long unsigned int]: Assertion '__n < this->size()' failed.
zsh: IOT instruction (core dumped)

This message tells us exactly where the problem is: inside the stl_vector.h file (which is part of the C++ Standard Template Library), specifically at line 1263. It also highlights that the issue happens when trying to access an element using the operator[] (the square brackets used to access elements in a vector). The assertion __n < this->size() failed, which means the index (__n) was greater than or equal to the vector's size, meaning we were trying to access an element beyond the vector's bounds.

Essentially, the code was expecting the vector to be a certain size, but it wasn't. This can happen if elements are not added to the vector when they should be, or if there's a miscalculation of the vector's size. My suspicion, which turned out to be correct, was that the code wasn't correctly accounting for all the string types it should be logging.

How to Reproduce the Crash

Reproducing this crash is relatively straightforward, which is good for testing any fixes. Here's a step-by-step guide:

  1. Compile COMPAS: First, you need to compile the COMPAS source code. Make sure you're using a system with newer GCC and the specified dependencies. As mentioned, the report was made on Manjaro Linux, which is Arch Linux-based, but this issue could occur on any system with a similar setup.
  2. Run COMPAS: After successful compilation, run the compiled COMPAS executable from your terminal with ./COMPAS.
  3. Observe the Crash: The program should start and then immediately crash, showing the error message detailed earlier.

Keep in mind that the exact behavior might depend on the specific version of COMPAS you're using, but the core issue should be the same across the affected versions. If you get the same error message, you're likely facing the same problem.

Affected Versions and Environment

Knowing the exact versions of the software and the operating system is super important when you're trying to debug problems like these. Here's the setup where the bug was observed:

  • OS: Manjaro KDE plasma with kernel 6.17.1 (stable branch).
  • GCC: 15.2.1 20250813 (This is a key factor, as the newer GCC is what seems to trigger the bug).
  • COMPAS: rel_v03.27.01 and the development branch. This means the bug potentially affects multiple COMPAS releases.
  • Dependencies: gsl v2.8, boost v1.88.0, and HDF5 v1.14.6. These are the libraries that COMPAS relies on.

This information is crucial for pinpointing the root cause. The combination of a newer GCC compiler and how the Log.cpp file handled vectors seems to be the critical factor.

The Potential Fix: Adding Missing push_back() Calls

Alright, time for the possible solution! After looking into the code, it looked like there were some missing push_back() calls. In C++, push_back() is used to add new elements to the end of a vector. It seemed the stringTypes vector in the LogfileDetails structure wasn't always being populated correctly, leading to the out-of-bounds access.

By adding the missing stringTypes.push_back() calls, the code is updated to properly store all the relevant string types, which in turn fixes the size mismatch and resolves the crash. This ensures the vector contains all expected values, preventing the out-of-bounds error.

I'm not a C++ expert, so I'm putting it out there for more experienced developers to double-check. However, this fix corrected the issue, allowing COMPAS to run without crashing.

Conclusion

So, there you have it, guys! We've covered the COMPAS crash due to a vector out-of-bounds access in Log.cpp. We saw the error message, how to reproduce it, the versions involved, and a potential fix by adding the missing push_back() calls to the stringTypes vector.

I hope this helps anyone else running into a similar issue. It's a great example of how understanding the error messages and digging into the code can lead to a solution. If you're new to C++, don't be discouraged! Bugs happen, and learning how to debug them is a super valuable skill.

If you have any questions or further insights, feel free to share them in the comments! Happy coding (and simulating binary stars)!