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Re: [libunwind] Some basic questions about libunwind

From: David Mosberger
Subject: Re: [libunwind] Some basic questions about libunwind
Date: Mon, 22 Nov 2004 14:37:44 -0800

>>>>> On Mon, 22 Nov 2004 16:01:58 -0600, Archie Cobbs <address@hidden> said:

  Archie> Up until now JC has had a non-conservative (liberal? :-) GC,
  Archie> i.e., it expects all references to be "real"
  Archie> references.

Ah, ok, that explains it, thanks.

  Archie> Of course it's still necessary to crawl the stack for
  Archie> constructing exception stack traces (JC knows how to map PC
  Archie> addresses to original Java method and source code line
  Archie> numbers).

Yes, but for that, there are more APIs.  For example, glibc provides
backtrace() and GCC 3.x provides _Unwind_Backtrace().

Again, I don't mind at all if libunwind is the answer, but I wanted to
point out potential alternatives that I'm aware of.

  Archie> 1. What is the state of x86 (32 bit) support in libunwind
  Archie> with respect to the above requirements?

  >> I'd say "improving".  I'm actively working on it.

  Archie> Great.. specifically, what work remains?

bug hunting & fixing.
libunwind-ptrace isn't there yet.
dynamic unwind-support isn't there yet.

  Archie> I'm already loading and relocating all the sections myself,

Why is this better than just dlopen()()ing a shared object?

  Archie> so ideally what I'd prefer to do is just say "Here
  Archie> libunwind, this is the .debug_foo section"... i.e., if
  Archie> libunwind already knows how to parse DWARF2 then it would be
  Archie> nice to reuse that capability. Would that require new
  Archie> libunwind API functions?

No, you can do that via _U_dyn_register().  Assuming your objects have
a binary search-table (they should), you'd just have to fill in a
unw_dyn_info_t structure, with "format" set to UNW_INFO_FORMAT_TABLE
and fill in the other info (such table-length and the starting/ending
instruction-pointer covered by the table).  All the DWARF2 parsing
will be done by libunwind.


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