Thread: question about pthreads and sigsegv

Originally Posted by Elkvis

if I am executing in a thread, and the thread causes segfault, caught by the signal handler, can I call pthread_self() and have it return the thread id of the faulting thread? if not, how can I determine what thread faulted so I can kill it?

I think it sounds like a recipe for disaster. SIGSEGV is catchable but the behavior after doing so is undefined unless the signal was generated by raise() or kill(). It sounds like you are trying to cover up bugs in your program -- the better solution is to find and squash these bugs so that you're not getting SIGSEGV in the first place.

If the thread is executing code which is out of your direct control, you really should be running it in a separate process anyway.

Originally Posted by Elkvis

I don't actually ever see this kind of activity happening, but I'm considering converting my multi-process server program over to a single-process, multi-threaded model, and I want to be prepared for bad things to happen if they do.

The fundamental problem is that signals behave strangely under pthreads. Signals are not delivered to the thread which triggered them. Instead, they are delivered to whichever thread has the signal unblocked. If all threads have the signal unblocked, then any thread at all could receive the signal, not just the one which triggered it. In other words, pthread_self() may not be referring to the thread which caused the SIGSEGV.

There is no solution to this. In order to ensure that only the faulting thread receives the signal, it must be the only thread with this signal unblocked. But there is no way to accomplish that, because you do not know ahead of time which thread is about to segfault.

The best you can do is to block SIGSEGV in ALL threads but one, and when this thread receives the signal, simply shut everything down gracefully.

There is a hack that could possibly do what you're looking for, but I think it's evil and I don't really want to describe it. I only mention it in case somebody else points out that it's possible. I know it is, but it sucks.