Detour: A C memory problem
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    x = malloc (...);
    foo ();
    bar ();
    free (x);   // CRASH

* Program crashes with the call to free.
* Program does not crash without the call to free (but leaks memory)
* foo and bar use x, but have no calls to free
* Neither foo nor bar reassigns x, although they do include x[i] = ....
* What might be going wrong?

Theory:
Either foo or bar dispatches one or many threads, which continue(s) to operate on x in the background while the program attempts to free x, resulting in a bad access on x.