CODEWITHSUNDEEP
ios
Alfonso Tesauro
Alfonso Tesauro

Reputation: 1792

Is there a way to take action, thus execute code, when a iOS application crashes ? Is this possible?

Is there a way to take action, thus execute code, when an iOS application crashes? Specifically, I would like to save the core data storage. Is this possible? I would say that this is possible since, for example, Firebase has to send information online for making crashlytics work. How can this be achieved? Thanks

Upvotes: 2

Views: 440

Answers (1)

Rob Napier
Rob Napier

Reputation: 299345

Yes, but it is very difficult, and "save core data storage" would be far too much (and very dangerous, to boot).

Most crashes result from a signal (often SIGSEGV, but also SIGABRT, SIGILL or others), and you can install a signal handler to run code in that case. However, that code must be very, very carefully written because you will be in a special execution state. There are a small number of C functions you are permitted to use (see the man page for sigaction for the list). Most notably, you can't allocate memory. Allocating memory in a signal catching function can deadlock the program in a tight spinlock (done that myself when I tried to write my own crash handler in my more naive days; it's really bad).

The way that crash handlers like Crashlytics do it is that they do as little as possible during the signal handler, mostly just writing the stack trace to storage (using pre-allocated buffers). When you restart, they see that there's an unhandled stack trace from a previous run, and then they do all the complicated stuff like uploading it to a server, or displaying UI, or whatever.

But even if you could write to Core Data in the middle of a signal handler, you would never want to do that. During a signal handler, the system is in an undefined state. Various invariants may not currently hold (such as whether the object graph is consistent). The fact that you're crashing this way indicates that something illegal has happened. The last thing you should do in that state is take data that is highly untrustworthy and overwrite the good data on disk.

Upvotes: 5

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