PSPDFApplicationIsTerminating - detect application termination on iOS and macOS. License: MIT. Taken out of the commercial PSPDFKit PDF SDK. http://pspdfkit.com

PSPDFEnvironment.m

static _Atomic(BOOL) _applicationWillTerminate = NO;
__attribute__((constructor)) static void PSPDFInstallAppWillTerminateHandler(void) {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        [NSNotificationCenter.defaultCenter addObserverForName:UIApplicationWillTerminateNotification object:nil queue:nil usingBlock:^(NSNotification *note) {
            _applicationWillTerminate = YES;
            PSPDFLogWarning(@"Application shutdown event detected.");
        }];
    });
}

PSPDF_EXTERN BOOL PSPDFApplicationIsTerminating(void) {
    return _applicationWillTerminate;
}

@LeoNatan AFAIK, default variable access changes it with sequential consistency. atomic_store and other intrinsics can be used for a more relaxed memory access.

Finding good docs about that is hard:
https://en.cppreference.com/w/c/language/atomic
http://www.informit.com/articles/article.aspx?p=1832575&seqNum=4

The worst that can happen by specifying sequentially consistent is a performance penalty. The worst that can happen when you specify a more relaxed ordering than you meant to have is that your code is subtly wrong. Worst of all, if you test it on a strongly ordered architecture such as x86, it's likely to work — and then subtly fail when you port it to something like ARM.

Ouch! TIL indeed!
Thanks!

I got confused for a second. It's been a while since I used atomic_fetch_add/atomic_fetch_sub. Is the default relax order relaxed?

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If I understand correctly, what I really want is memory_order_seq_cst, which is then the order that is used by _Atomic(XXX), which also has a nicer syntax.