[Gluster-devel] Libunwind

[Jeff Darcy]

I looked into this a bit over the weekend.  Unfortunately, while libunwind *does* get function names for calls in shared libraries, it *doesn't* seem to handle calls through function pointers very well.  Imagine that original_func in your main program finds shared_lib_func via dlsym and calls it via a function pointer, and that shared_lib_func then calls callback_func back in the main program.  If we try to get a backtrace at this point, glibc will give us this:

   callback_func
   ??? (address) in shared_lib.so
   original_func

Libunwind will give us this:

   callback_func
   shared_lib_func

Notice that the last frame (original_func) is missing.  Also, neither has file names or line numbers.  I did find a method that will give us those, while also handling both shared libraries and function pointers well: invoke gdb to run against the current process, using "-batch" and "-ex" to run just the backtrace command.  We can even get backtraces for all threads instead of just one.  I looked a bit into using the relevant gdb functions as a library, but didn't get anywhere with that.  While invoking a whole new process makes this a bit too heavyweight for most cases, for the case where we're about to exit anyway (which is where this all started for me) it might be pretty useful.