Re: [libunwind] Signal handlers and unw_resume

[David Mosberger]

>>>>> On Mon, 23 Aug 2004 11:55:05 +0100, "Thomas Hallgren" <address@hidden> 
>>>>> said:

  Thomas> I have a question on doing a unw_resume from within a signal
  Thomas> handler. I do the following:

  Thomas> I call unw_getcontext() from within a signal handler
  Thomas> followed by an unw_init_local() and a number of unw_step()
  Thomas> calls to take me passed the signal frame and to the point
  Thomas> where the code was interrupted. After some local processing
  Thomas> I then call unw_resume() just prior to where the signal
  Thomas> handler return would be.

  Thomas> Then it struck me. Using this approach I effectively skip
  Thomas> any clean-up processing that normally would occur in the
  Thomas> caller of the signal handler. Is that a problem? Or is it an
  Thomas> unspoken law that such processing should never occur in a
  Thomas> well written OS?

I think it's a can of worms.  Enough so that there is a regular
setjmp() and a setjmp_unwind() (defined by C++ ABI).  The former
doesn't do any cleanups, whereas the latter does.  Your situation
sound similar.

Note that NPTL takes advantage of C cleanup handlers so the problem is
not limited to C++ (other other languages with exception handling).

I suspect it would be safer (but also much slower), to invoke handlers
if you encounter them during unwinding.

        --david