detect program termination (C, Windows)

Question

I have a program that has to perform certain tasks before it finishes. The problem is that sometimes the program crashes with an exception (like database cannot be reached, etc). Now, is there any way to detect an abnormal termination and execute some code before it dies?

Thanks.

code is appreciated.

Answer 1

C doesn't really support "exceptions" as you'd see them in C++ or Java. What you should be doing is checking return codes and acting accordingly. How does your current program indicate that your database cannot be reached? I would expect that somewhere in there, it returns an error code - maybe a negative number, or maybe just something that isn't zero. It depends on the convention in use.

When you're writing C, it's really important to check return values so that you can handle exceptional cases without crashing.

As for an example, I would do something like this:

int status = 0;
status = access_database();
if (status != 0)
{
    execute_some_code_before_death();
    exit(1);
}

If the function you're calling sets errno, you can use perror to get more information about the error.

Answer 2

If it's Windows-only, then you can use SEH (SetUnhandledExceptionFilter), or VEH (AddVectoredExceptionHandler, but it's only for XP/2003 and up)

Answer 3

It depends what do you do with your "exceptions". If you handle them properly and exit from program, you can register you function to be called on exit, using atexit().

It won't work in case of real abnormal termination, like segfault.

Don't know about Windows, but on POSIX-compliant OS you can install signal handler that will catch different signals and do something about it. Of course you cannot catch SIGKILL and SIGSTOP.

Signal API is part of ANSI C since C89 so probably Windows supports it. See signal() syscall for details.

Answer 4

Sorry, not a windows programmer. But maybe

_onexit()

Registers a function to be called when program terminates.

http://msdn.microsoft.com/en-us/library/aa298513%28VS.60%29.aspx

Answer 5

First, though this is fairly obvious: You can never have a completely robust solution -- someone can always just hit the power cable to terminate your process. So you need a compromise, and you need to carefully lay out the details of that compromise.

One of the more robust solutions is putting the relevant code in a wrapper program. The wrapper program invokes your "real" program, waits for its process to terminate, and then -- unless your "real" program specifically signals that it has completed normally -- runs the cleanup code. This is fairly common for things like test harnesses, where the test program is likely to crash or abort or otherwise die in unexpected ways.

That still gives you the difficulty of what happens if someone does a TerminateProcess on your wrapper function, if that's something you need to worry about. If necessary, you could get around that by setting it up as a service in Windows and using the operating system's features to restart it if it dies. (This just changes things a little; someone could still just stop the service.) At this point, you probably are at a point where you need to signal successful completion by something persistent like creating a file.

Answer 6

There are sysinternals forum threads about protecting against end-process attempts by hooking NT Internals, but what you really want is either a watchdog or peer process (reasonable approach) or some method of intercepting catastrophic events (pretty dicey).

Edit: There are reasons why they make this difficult, but it's possible to intercept or block attempts to kill your process. I know you're just trying to clean up before exiting, but as soon as someone releases a process that can't be immediately killed, someone will ask for a method to kill it immediately, and so on. Anyhow, to go down this road, see above linked thread and search some keywords you find in there for more. hook OR filter NtTerminateProcess etc. We're talking about kernel code, device drivers, anti-virus, security, malware, rootkit stuff here. Some books to help in this area are Windows NT/2000 Native API, Undocumented Windows 2000 Secrets: A Programmer's Cookbook, Rootkits: Subverting the Windows Kernel, and, of course, Windows® Internals: Fifth Edition. This stuff is not too tough to code, but pretty touchy to get just right, and you may be introducing unexpected side-effects.

Perhaps Application Recovery and Restart Functions could be of use? Supported by Vista and Server 2008 and above.

ApplicationRecoveryCallback Callback Function Application-defined callback function used to save data and application state information in the event the application encounters an unhandled exception or becomes unresponsive.

On using SetUnhandledExceptionFilter, MSDN Social discussion advises that to make this work reliably, patching that method in-memory is the only way to be sure your filter gets called. Advises to instead wrap with __try/__except. Regardless, there is some sample code and discussion of filtering calls to SetUnhandledExceptionFilter in the article "SetUnhandledExceptionFilter" and VC8.

Also, see Windows SEH Revisited at The Awesome Factor for some sample code of AddVectoredExceptionHandler.

Answer 7

1. Win32

The Win32 API contains a way to do this via the SetUnhandledExceptionFilter function, as follows:

LONG myFunc(LPEXCEPTION_POINTERS p)
{
     printf("Exception!!!\n");     
     return EXCEPTION_EXECUTE_HANDLER;
}

int main()
{
     SetUnhandledExceptionFilter((LPTOP_LEVEL_EXCEPTION_FILTER)&myFunc);    
     // generate an exception !
     int x = 0;
     int y = 1/x;
     return 0;
}

2. POSIX/Linux

I usually do this via the signal() function and then handle the SIGSEGV signal appropriately. You can also handle the SIGTERM signal and SIGINT, but not SIGKILL (by design). You can use strace() to get a backtrace to see what caused the signal.

Answer 8

I published a articel at ddj.com about "post mortem debugging" some years ago.

It includes sources for windows and unix/linux to detect abnormal termination. By my experience though, a windows handler installed using SetUnhandledExceptionFilter is not always called. In many cases it is called, but i receive quite a few log files from customers that do not include a report from the installed handlers, where i.e. a ACCESS VIOLATION was the cause.

http://www.ddj.com/development-tools/185300443