Reading errno directly after the FFI call can eliminate heap overflows, but the async exception and timer issues still seem possible.
I have played around a bit and I can't understand what's going on with this anymore. Check out this example: https://gist.github.com/abbradar/76dafcee1807c9c5ac4d. Compile it with "ghc test_c.c test.hs". I used "mask_" here to check if it can fix the problem.
There will be multiple discrepancies seen between written and read values because of threads preemption if my_errno is used. However, if you use "errno" (change #define for that) instead, everything seems good.
Anyway, it looks like getErrno and friends rely on this magical behaviour of errno -- if some other library which uses global error state like "my_errno" in example is used (I remember SDL doing that, and I have also done it myself), there should be problems without some way to temporary disable threads preemption, which I haven't found. Hence -- should I maybe post a bug report about this?