What is __gxx_personality_v0 for?

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This is a second-hand question from an OS development site, but it made me curious since I couldn't find a decent explanation anywhere.

When compiling and linking a free-standing C++ program using gcc, sometimes a linker error like this occurs:

out/kernel.o:(.eh_frame+0x11): undefined reference to `__gxx_personality_v0'

This is apparently because this symbol is defined in libstdc++, which is missing in a free-standing environment. Fixing the problem simply requires defining this symbol somewhere:

void *__gxx_personality_v0;

Which is nice, but I don't like things that just magically work... So the question is, what is the purpose of this symbol?

Edit Report

A quick grep of the libstd++ code base revealed the following two usages of __gxx_personality_v0:

In libsupc++/unwind-cxx.h:

// GNU C++ personality routine, Version 0.                                      
extern "C" _Unwind_Reason_Code __gxx_personality_v0
     (int, _Unwind_Action, _Unwind_Exception_Class,
      struct _Unwind_Exception *, struct _Unwind_Context *);

In libsupc++/eh_personality.cc:

#define PERSONALITY_FUNCTION    __gxx_personality_v0
extern "C" _Unwind_Reason_Code
PERSONALITY_FUNCTION (int version,
                      _Unwind_Action actions,
                      _Unwind_Exception_Class exception_class,
                      struct _Unwind_Exception *ue_header,
                      struct _Unwind_Context *context)
{
  // ... code to handle exceptions and stuff ...
}

(Note: it's actually a little more complicated than that; there's some conditional compilation that can change some details).

So, as long as your code isn't actually using exception handling, defining the symbol as void* won't affect anything, but as soon as it does, you're going to crash - __gxx_personality_v0 is a function, not some global object, so trying to call the function is going to jump to address 0 and cause a segfault.

answered 2008-11-30T17:35:59.033

What is __gxx_personality_v0 for?

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