Valgrind FD Error On FreeBSD 15: Cause & Solution

Experiencing the dreaded "File descriptor 9 was never created" error when using valgrind --track-fds=yes on FreeBSD 15? You're not alone! This article dives into the cause of this issue, specifically within the context of Avahi and FreeBSD's new inotify implementation, and provides a potential workaround. This can be frustrating when trying to debug memory issues or track file descriptor usage, so understanding the root cause is crucial.

Understanding the Problem

Let's break down what's happening. The error message File descriptor 9 was never created indicates that Valgrind, a powerful memory debugging and profiling tool, is detecting an attempt to use a file descriptor that it believes hasn't been properly initialized or allocated. File descriptors are integers that represent open files, sockets, or other input/output resources within a system. When a program tries to read from, write to, or otherwise manipulate a file descriptor that doesn't correspond to a valid open resource, this kind of error arises.

In the specific scenario described, this issue surfaces on FreeBSD 15 when running avahi-daemon under Valgrind with the --track-fds=yes option. Avahi is a system which facilitates service discovery on a local network. The problem seems to be linked to FreeBSD 15's native inotify implementation. Inotify is a Linux kernel subsystem that acts as a file system monitor. It reports file system events to applications. FreeBSD 15 has integrated this functionality, and it appears that Valgrind isn't yet fully compatible with this new implementation. The system is picking up inotify instead of the older kqueue mechanism.

Why does this matter? Well, kqueue is a FreeBSD-specific event notification interface, similar to epoll on Linux or IOCP on Windows. It's been the standard way for applications on FreeBSD to monitor file system events for a long time. Valgrind has likely been developed and tested extensively with kqueue. The introduction of inotify as the default mechanism appears to be causing some confusion for Valgrind's file descriptor tracking, leading to these false positive errors.

The key takeaway is that this isn't necessarily a real error in your code, or in Avahi itself. It's more likely a compatibility issue between Valgrind and FreeBSD's new inotify implementation. So, before you start tearing your hair out trying to find a non-existent bug, consider the context!

Diagnosing the Issue

So, how can you confirm that you're running into this specific problem? Here are a few things to check:

1. FreeBSD Version: Ensure you're running FreeBSD 15. The issue is specifically reported in this version. You can use the command freebsd-version to verify your system's version.
2. Valgrind Version: While the original report mentions valgrind-3.26.0, it's worth checking if newer versions of Valgrind have addressed this issue. Use valgrind --version to check. If a newer version is available, try upgrading to see if it resolves the problem.
3. Avahi Configuration: The problem seems to occur specifically with avahi-daemon. If you're using other applications that rely on file descriptor monitoring, they might also trigger the same error.
4. Valgrind Options: The --track-fds=yes option is crucial for triggering the error. This option tells Valgrind to keep track of all open file descriptors. If you don't use this option, you won't see the error, but you also won't get the file descriptor tracking benefits.
5. Error Messages: The specific error message "File descriptor 9 was never created" (or similar messages with different file descriptor numbers) is a strong indicator of this issue.

If you meet all these conditions, it's highly likely that you're experiencing the same Valgrind/inotify incompatibility issue on FreeBSD 15.

The Workaround: Switching Back to kqueue

Fortunately, there's a workaround! The original poster discovered that switching back to kqueue resolves the errors. This suggests that the issue is indeed related to the inotify implementation. While the exact steps to force Avahi to use kqueue might vary depending on how Avahi is configured on your system, the general idea is to tell Avahi to prefer kqueue over inotify for file system event monitoring. This might involve editing Avahi's configuration files or setting environment variables.

How do you actually do this? That's the tricky part, and it depends on the specific application you're using (in this case, Avahi). You'll need to consult the application's documentation to see if it provides a way to select the event notification mechanism. Look for configuration options related to kqueue, inotify, or event loops. You might also be able to influence the choice by setting environment variables that the application checks. Google and Stack Overflow will be your friend, here. Search for "Avahi kqueue" or "force kqueue".

Keep in mind this isn't a solution, but rather a workaround. It avoids the Valgrind errors, but it also means you're not using the newer inotify implementation. In the long run, the ideal solution is for Valgrind to be updated to properly support inotify on FreeBSD. For example, if you are using cmake, this can be achieved by:

SET(CMAKE_DEFINITIONS "${CMAKE_DEFINITIONS} -DAVAHI_WATCH_KQUEUE")

Implications and Considerations

While switching back to kqueue might seem like a simple fix, it's important to consider the potential implications:

• Performance: Inotify might offer performance advantages over kqueue in some scenarios. By switching back to kqueue, you could potentially be sacrificing some performance. However, in many cases, the difference will be negligible.
• Future Compatibility: As FreeBSD evolves, inotify will likely become more prevalent. Eventually, kqueue might become deprecated or less well-supported. So, relying on kqueue as a long-term solution might not be ideal. This makes it critical to monitor the evolution of Valgrind and FreeBSD's inotify support.
• Other Applications: If you're using other applications that rely on inotify, switching back to kqueue for Avahi might not affect them. However, it's something to be aware of, especially if you start encountering similar issues with other programs.

Reporting the Issue: It's crucial to report this issue to the Valgrind developers. This will help them understand the problem and prioritize a fix in a future release. Provide as much detail as possible, including your FreeBSD version, Valgrind version, the exact command you're using, and the error messages you're seeing. The more information you provide, the easier it will be for them to diagnose and resolve the problem. You can typically report issues through Valgrind's bug tracker or mailing list.

Long-Term Solution: Waiting for Valgrind Updates

The most sustainable solution is to wait for an updated version of Valgrind that properly supports inotify on FreeBSD. Keep an eye on Valgrind's release notes and changelogs to see if this issue has been addressed. Regularly updating Valgrind is generally a good practice, as it includes bug fixes, performance improvements, and support for new features and technologies.

In the meantime, the kqueue workaround should allow you to continue using Valgrind for memory debugging and file descriptor tracking on FreeBSD 15 without being bombarded by false positive errors. Just remember to keep an eye on the situation and be prepared to switch back to inotify when Valgrind is ready!

By using the suggested solution your avahi, avahi instances will stop to complain about those errors.

Conclusion

While the "File descriptor 9 was never created" error in Valgrind on FreeBSD 15 can be perplexing, understanding the root cause – the incompatibility between Valgrind and FreeBSD's inotify implementation – is the first step towards resolving it. By using the kqueue workaround, you can continue to leverage Valgrind's powerful debugging capabilities. Remember to report the issue to the Valgrind developers and stay tuned for future updates that address this compatibility issue directly. This will ensure a smoother debugging experience on FreeBSD 15 and beyond. So, keep calm, debug on, and remember that sometimes, the problem isn't your code, but rather a compatibility quirk in the tools you're using!