[lttng-dev] ARM C++ segfaulting - follow-up

[Robert Daniels]

> I'm attempting to use lttng userspace tracing with a C++ application on an ARM platform. I'm using
> GCC 4.8.4 on Linux 3.14 with the 2.6 release of lttng. I've compiled lttng-modules, lttng-ust, and
> lttng-tools and have been able to get a simple test working with C code. When I attempt to run the
> hello.cxx test on my target it will segfault.


I spent a little time digging into this issue and finally discovered the cause of my segfault with ARM C++ tracepoints.

There is a struct called 'lttng_event' in ust-events.h which contains an empty union 'u'.  This was the cause of my issue.  Under C, this empty union compiles to a zero byte member while under C++ it compiles to a one byte member, and in my case was four-byte aligned which caused my C++ code to have the 'cds_list_head node' offset incorrectly by four bytes.  This lead to an incorrect linked list structure which caused my issue.

Since this union is empty, I simply removed it from the struct and everything worked correctly.

I don't know the history or purpose behind this empty union so I'd like to know if this is a safe fix.  If it is I can submit a patch with the union removed.

Thanks,

Robert Daniels


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