Re: [Libunwind-devel] Upstream unw_backtrace_skip?

[Dave Watson]

On 01/30/17 10:23 AM, Milian Wolff wrote:
> On Sunday, January 29, 2017 8:59:03 PM CET Bert Wesarg wrote:
> > Hi Milian,
> > 
> > On Sat, Jan 28, 2017 at 10:20 PM, Milian Wolff <address@hidden> wrote:
> > > Hey all,
> > > 
> > > now that libunwind is becoming more alive again, could we get the
> > > unw_backtrace_skip functionality upstream? For those who are unacquainted
> > > with it:
> > > 
> > > It gives you the behavior of unw_backtrace, but skips the first N frames,
> > > see e.g.:
> > > 
> > > https://gist.github.com/mikesart/8365549
> > > 
> > > This is often required in profilers to skip frames introduced by the
> > > profiler itself.
> > 
> > I don't see the what this adds as a feature, what cant already be done
> > by the caller after calling unw_backtrace itself? As ignoring the
> > first 'n' entries in the resulting backtrace buffer has the same
> > effect and it wont get faster.
> 
> This is not about it being faster, it is just that in my opinion it is a 
> common way to use this API. I.e. to get a backtrace N frames deep, I 
> currently 
> have to preallocate a buffer of N + M frame pointers and call unw_backtrace, 
> then skip the M first pointers that go into "my" code. Vogl is doing the 
> same, 
> and I bet other users of libunwind will do so too.
> 
> So this is really more about convenience, and the minor improvement that I 
> don't need to allocate a buffer of N + M.

I can see the usefulness, but I'd lean towards not making the API
bigger for it since it is pretty trivial to add on top of the existing
API.  Possibly a good candidate for an API breaking release like a 2.0
though?

> > What would be a enhancement for the
> > user, would be an option to skip everything till the first (or last)
> > entered signal frame on the stack. As that includes only the part
> > which triggered the signal, but not the signal handler.
> 
> Note that in my case at least, I'm not triggering libunwind from a signal 
> handler. 
> 
> > BTW, why is 'unw_backtrace' the only symbol without the usual '_U'
> > prefix, it is the only one with the 'unw_' prefix.
> 
> No clue, I was wondering the same.

Yea I can't tell if there was a reason or just an oversight.  I think
changing it now would be ABI breaking, unfortunately.