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Re: GCC stack backtraces


On Wed, Aug 29, 2012 at 9:21 AM, Ian Lance Taylor <iant@google.com> wrote:
> On Wed, Aug 29, 2012 at 12:43 AM, Janne Blomqvist
> <blomqvist.janne@gmail.com> wrote:
>> On Wed, Aug 29, 2012 at 10:22 AM, Ian Lance Taylor <iant@google.com> wrote:
>>> I've spent the last couple of days working on a stack backtrace library.
>>>
>>> It uses the GCC unwind interface to collect a stack trace, and parses
>>> DWARF debug info to get file/line/function information.
>> [snip]
>>> I expect to use this code not just for GCC proper, but also for libgo
>>> (currently libgo uses Go code to parse DWARF, but that is not very
>>> satisfactory as that code is only available if it has been imported into
>>> the program).  So I put it under a BSD license, although that is open
>>> for discussion.  Also in case it finds more uses elsewhere I wrote it in
>>> reasonably portable C rather than C++.
>>>
>>>
>>> Does this seem like something we could usefully add to GCC?  Does
>>> anybody see any big problems with it?
>>
>> I haven't looked at the code, but if it is async-signal-safe it could
>> be interesting for gfortran. Currently in libgfortran we have a
>> backtracing routine, originally written by FX Coudert IIRC, since
>> rewritten by yours truly a few times, that uses _Unwind_Backtrace()
>> from libgcc and then pipes the output via addr2line, if found. Since
>> it's invoked from a signal handler when the program (user program, not
>> the compiler!) crashes, it needs to be async-signal-safe. AFAIK the
>> current implementation *should* fulfill that requirement. But
>> something that would be async-signal-safe and won't need addr2line to
>> get symbolic info would be a nice improvement, in particular since
>> addr2line doesn't work on OSX, and all that piping stuff doesn't work
>> on Windows.
>
> I believe the unwinder proper is async signal safe--it just uses
> _Unwind_Backtrace.
>
> The DWARF reader calls malloc and is therefore not async-signal safe.
> It would be difficult to write an efficient DWARF reader that does not
> allocate any memory, and I'm not aware of any way to allocate memory
> that is defined to be async-signal safe.  That said, as far as I know
> mmap is async-signal safe in practice if not in theory, so one
> approach would be  to do memory allocation using mmap.
>

Using mmap instead of malloc is a good idea.  It should fix:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724


-- 
H.J.


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