Linux C catching kill signal for graceful termination
I have a process using sockets, database connections and the likes. It is basically a server process relaying between sensor data and a web interface, and as such it is important to ensure the application, if killed, terminates gracefully.
How do I handle unexpected exceptions such as segfaults (at least for debugging) as well as kill signals so that I can close any connections and stop any threads running so that the process does not leave a mess of anything it is using?
Answer
Catching signals is hard. You have to be careful. Your first step is to use sigaction to install a signal handler for the desired signals.
Choose a set of signals to respond to and choose what they mean for your process. For example,
SIGTERMquits,SIGHUPrestarts,SIGUSR1reloads configuration, etc.Don't try to respond to all signals and don't try to "clean up" after signal that indicates an error in your program.
SIGKILLcan't be caught.SIGSEGV,SIGBUS, and others like them shouldn't be caught unless you have a VERY good reason. If you want to debug, then raise the ulimit for core dumps — attaching a debugger to a core image is far more effective than anything you or I could ever code up. (If you do try to clean up after aSIGSEGVor something like that, realize that the cleanup code might cause an additionalSIGSEGVand things could get bad quickly. Just avoid the whole mess and letSIGSEGVterminate your program.)How you handle the signals is tricky. If your application has a main loop (e.g.,
selectorpoll) then the signal handler can just set a flag or write a byte to a special pipe to signal the main loop to quit. You can also usesiglongjmpto jump out of a signal handler, but this is VERY difficult to get right and usually not what you want.
It's hard to recommend something without knowing how your application is structured and what it does.
Also remember that the signal handler itself should do almost nothing. Many functions are not safe to call from signal handlers.