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Ensure Objective-C Code Execution Before Cocoa Exit Applications

Suppose a cocoa application needs to perform some vital objective-c operation before it exits (for any reason, a failure or exit).

Is it possible to enforce arbitrary objective-c code in response to every failure event? ( SIGINT, SIGBUS, SIGSEGV, Etc., ignoring "uncatchables" ( SIGSTOP, SIGKILL, lightning bolt, etc.))

It would be helpful to know what your hooks are trying to do.

For example: let's say that for the application to work, it is absolutely necessary to change the value of some mutable system-wide configuration variable X. When launched, the application receives a snapshot of the current state of X, and then modifies it. When the application successfully exits, it simply restores X to its predetermined initial value before exiting. My question is this: is it possible to guarantee that X will be restored even if the application terminates with an error?

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3 answers

The short answer is no.

A longer answer is still missing, but you can minimize the possibility of not using reset a config variable with a mutable system. In the scheme:

  • When launched, your application will create a faceless background process;
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    signal(SIGBUS, signalHandler);
    signal(SIGSEGV, signalHandler);

void signalHandler(int signal)
{   
    NSMutableString *crashReport = [[NSMutableString alloc] init];
    void* callstack[128];
    int i, frames = backtrace(callstack, 128);
    char** strs = backtrace_symbols(callstack, frames);
    for (i = 0; i < frames; ++i) {
        //printf("%s\n", strs[i]);
        [crashReport appendFormat:@"%s\n",strs[i]];

    }
    free(strs);
    // write crashReport to file
    [crashReport release];
    exit(1);
}

but if the failure is included in NSMutableString or freed or in the free () function, this code will not work. (As bbum explains :
there is no way to guarantee that any given piece of code will be executed when the application terminates)

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