Should I be able to run java(c) in gdb?

[Will Hawkins]

Andrews,

Thank you SO much for your quick and incredibly thorough responses!
This is going to make my debugging that much easier. Now, when I
actually fix the problem, I will tell you a) why I am debugging
java/javac and b) thank you again for your help!

I hope that you have a great afternoon and great rest of the week!

Will

On Thu, Dec 22, 2016 at 4:01 PM, Andrew Dinn <adinn at redhat.com> wrote:
> On 22/12/16 20:44, Andrew Haley wrote:
>> On 22/12/16 20:14, Will Hawkins wrote:
>>
>>> I am attempting to run java/javac under gdb. Everything works a-okay
>>> when I run those binaries from the command line. It works okay under
>>> strace, too. When I run them under gdb, though, I get a segfault every
>>> time. Is there a reason this happens? Is there some set of flags that
>>> I should be passing to java/javac when I want to run it under gdb?
>>
>> Type:
>>
>>   handle 11 nostop noprint
>>
>> and all will be well.
>
> 11 . . . also known as SIGSEGV i.e. the signal sent when a segmentation
> violation (access to an unmapped virtual memory address) occurs. The
> above command tells gdb not to stop if that happens and let the running
> program deal with it.
>
> You are probably wondering why the JVM would ever try to access an
> unmapped memory address. Well, this sometimes without it actually being
> an error.
>
> Internally the JVM can choose to represent the Java value null with an
> address with an unmapped address (typically 0). When a program tries to
> access an object field and it turns out that the object reference it is
> trying to read from is null then this results in a SEGV signal being
> raised. The JVM catches it with a registered handler, checks that the
> cause was actually a null reference and raises a NullPointerException,
> unwinding the stack back to a catch block handler or aborting the thread
> if it is not caught.
>
> A JVM doesn't have to this but -- it could just check every object
> reference before trying to access an object. However for almost all
> programs omitting the check and catching an exception when the access
> fails gives better performance.
>
> Also, note that the JVM knows when it is performing an object access so
> it only ever patches up a SEGV signal when the access is to the address
> which represents null. If it catches a SEGV signal in any other
> circumstance it bails out, writing an error log and dumping a core file.
>
> regards,
>
>
> Andrew Dinn
> -----------
> (Yeah, that other Andrew)