1

I'm writing a macOS launch daemon (in C++). It works for the most part, except that I am noticing some random crashes via SIGABRT (in the log.)

I've asked earlier about the use of the Core Graphics framework, which it turns out was not allowed in a launch daemon. I removed it, but I still get very rare crashes.

Thus I'm wondering which of the following frameworks am I allowed to use from a launch daemon?

enter image description here

Improve this question

1 Answer 1

Reset to default
2

There's no technical restriction on specific frameworks you're allowed or not allowed to link. Merely linking a library will not cause crashes. Some APIs need to be called from specific contexts, such as much of Core Graphics only making sense if a process is part of a windowing session, as you've found out. But even there, some of the off-screen drawing APIs which don't interface with the windowing system directly ought to work from a Launch Daemon context.

If you're seeing crashes calling specific APIs, you'll need to investigate those one by one. If you're seeing "random" crashes, I suspect you have a bug (memory overwrite? multithreading error?) in your code. Note that Apple's toolchain provides a bunch of different diagnostic options which you can use even when not running in the debugger, such as ASan, UBSan, and Thread Sanitizer. These may help you track down issues.

As you've specifically tagged this question with the IOKit tag and it's shown in your screenshot, I'll add that using IOKit from a launch daemon is very common. It's even possible to use IOKit matching events as a launch trigger for the daemon using XPC events/xpc_set_event_stream_handler - this way, your daemon can be started on-demand when a specific (type of) device is connected, for example. (As opposed to starting up at every system boot and consuming system resources even if it's not needed or doing anything.)

Improve this answer
Sign up to request clarification or add additional context in comments.

9 Comments

It's hard to say where the issue originates. All I can tell is that the crash happens in one of the functions that frees memory from the heap (inside a system call.) So something corrupts that memory. It doesn't happen all the time. It almost always comes from within CFDateFormatterCreateStringWithDate when it tries to release memory. But there's really nothing in my date formatter code that could lead to it. I can see further down the callstack of that function that it crashes trying to free some internal memory. As for where that memory gets clobbered up - that's a million dollar question.
Yeah, sounds like a memory clobber or object over-release. Addess Sanitiser is definitely worth a try for debugging this. You can also try enabling zombie objects. Oh, and if you aren't already using it, the clang compiler's static analysis can also catch some kinds of retain/release bugs and a bunch of other types of issues. Product -> Analyze in Xcode.
My guess is that I'm calling some system API from one of those libraries when I'm not supposed to, or if it releases internal buffer too early, that I didn't envision or called right. I don't have sufficient knowledge of macOS to figure out what exactly I'm not supposed to do in a launch daemon. My approach now is to eliminate some group of functions and see if the crashes continue. But that is fraught with danger because the absence of crashes in that case doesn't mean that I found the source - they could just be quite rare because of some internal race condition. Like it happened yesterday.
To be honest, given the information you've provided, I would not assume the code is crashing because it's running as a launch daemon. It seems equally if not more likely this is just a straight up programming error.
XPC is a message-based IPC mechanism built on Mach messages. You can use it to send almost anything between 2 processes; you'll need to register the port name(s) of the "server" in its launchd plist so the clients can find it.
|

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.