[concurrency-interest] JVM crash when using Unsafe to read/write bytes in Java 8 (no crash with same code in Java 7)

Stanimir Simeonoff stanimir at riflexo.com
Thu Sep 18 07:34:03 EDT 2014


I suppose the problem occurs due to the last few (less than 8) bytes are
addressed. Probably getByte(address) won't read just a byte but a 32/64
word and mask+shift it.
Try to allocate extra 16 bytes instead of 8.


On Thu, Sep 18, 2014 at 1:38 PM, Gaëlle Guimezanes <ggu at quartetfs.com>

> Hello,
> I am sorry this is not really a concurrency problem since it can be
> reproduced using only one thread, but I didn't really know where to discuss
> this. Since many of you use Unsafe I thought you might be able to help me
> understand what's happening or direct me to the correct mailing list.
> I have a strange behaviour using Unsafe in Java 1.8.0_20.
> When I use getByte/putByte to manipulate bits, my JVM crashes (access
> violation).
> But when I use getLong/putLong for the same purpose it works correctly
> (even though I have allocated exactly the same amount of memory).
> Also, when using Java 1.7.0_67 to compile and run the same source code,
> both the getByte/putByte and the getLong/putLong approaches work correctly.
> I have attached a simple test class (DirectMemoryCrash.java) for
> reproducing the problem: you can run it with no argument for
> the getByte/putByte version, or with "useLongs" arguments for
> the getLong/putLong version.
> I also have attached crash reports :
> hs_err_pid5480.log for the crash on windows
> hs_err_pid7210.log for the crash on linux
> Thank you for your attention,
> Best regards,
> Gaëlle Guimezanes.
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> Concurrency-interest at cs.oswego.edu
> http://cs.oswego.edu/mailman/listinfo/concurrency-interest
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