> On 20 Feb 2017, at 16:18, Zoltan Varga <[email protected]> wrote:
>
> Hi,
>
> Where is the crash location ? The runtime implements null reference
> exception checking by catching the SIGSEGV signal, so if a nullref happens in
> managed code while running under lldb, it will look like a crash. Running
> with
> MONO_DEBUG=explicit-null-checks
> turns this behavior off.
>
Hi
Thanks for that.
And yes using MONO_DEBUG=explicit-null-checks mitigates the issue.
I can detect when LLDB is attached and set the env var as required
https://github.com/ThesaurusSoftware/Dubrovnik/commit/15b86e3e6c9381944d95488e52ecaad314a1ed81.
This issue was occurring on a background worker thread which has perhaps
complicated things.
The code is cross platform and WIndows does not raise NullReferenceException at
any point.
This may explain why (with LLDB detached) the WIndows error reporting has valid
reason for failure detail whereas the Mono implement is empty.
I have some test code for for raising NullReferenceException that I previously
used to see just how my app processes managed exceptions into Obj-C ones.
When LLDB is attached the behaviour for this test code is that LLDB does NOT
catch the SIGSEGV - which i find curious:
scenario 1
========
methodA raises an uncaught NullReferenceException.
monoResult = mono_runtime_invoke(methodA, NULL, monoArgs, &monoException);
if (monoException != NULL) {
@throw(NSExceptionFromMonoException(monoException, @{}));
}
In this case SIGSEGV is not caught by LLDB and I see
NSExceptionFromMonoException raised.
Scenario 2
========
// no monoException argument
monoResult = mono_runtime_invoke(methodA, NULL, monoArgs, NULL);
In this case the exception handler I have installed on
AppDomain_UnhandledException gets called and still SIGSEGV is not caught by
LLDB.
Is the fact that the NullReferenceException is occurring on a background worker
thread significant?
I do have a custom System.Threading.SynchronizationContext installed to route
worker thread completion code back onto the Obj-C main thread but we never get
that far in this case.
J
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