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From Martin Sebor <se...@roguewave.com>
Subject Re: stdcxx 4.2.0/4.1.3 binary incompatibility on Linux
Date Wed, 17 Oct 2007 23:03:03 GMT
Travis Vitek wrote:
> So the problem is that the size of the __string_ref has changed, right?

Yes. More precisely, that is one part of the problem (the other
part is the pthread functions that are inlined in user code and
that expect to get a mutex and not random garbage).

> Specifically, this block of code causes the removal of the per-string
> mutex from __string_ref.
> 
> +#ifndef _RWSTD_NO_ATOMIC_OPS
> +   // disable string mutex when atomic operations are available
> +#  ifndef _RWSTD_NO_STRING_MUTEX
> +#    define _RWSTD_NO_STRING_MUTEX
> +#  endif   // _RWSTD_NO_STRING_MUTEX
> +#endif   // _RWSTD_NO_ATOMIC_OPS
> 
> An obvious option would be to just remove that block and let the user
> decide if they want to define _RWSTD_NO_STRING_MUTEX or not. If they
> define it then they must know that they are breaking binary
> compatibility with a library previously compiled without it. This
> doesn't help users compiling new source using the default configuration,
> but it does maintain compatibility.

I'm also leaning in this general direction. Keep the compatible
behavior and and let users opt into the breaking fix. I'll have
to think some more about how to control the behavior. I was
hoping to use an existing config macro without changing any
library code (except _config-gcc.h).

> 
> We could also ask that they define _RWSTD_NO_ATOMIC_OPS, but that may
> cause other binary incompatibilities elsewhere or it will at the very
> least cause performance problems in other places.

It is very tempting to go with the improved implementation by
default, but from a compatibility standpoint it would be the
wrong thing to do.

> 
> It would be nice if we could just insert an appropriately sized pad
> buffer in place of the _C_mutex member. If the methods of __string_ref
> were not inlined I'm pretty sure that this would work. Unfortunately
> _C_inc_ref() and _C_dec_ref() are inlined, so their code may be compiled
> in the user executable, so I'm not convinced that this is a viable
> option.

All the __string_ref members are trivial inline wrappers that
are fully expected to be inlined. It's possible that some of
them will not be inlined under some conditions but I expect
those to be exceeding rare.

Martin

> 
> Travis
> 
> 
> Martin Sebor wrote:
>> Okay, I've got it:
>>
>>    http://issues.apache.org/jira/browse/STDCXX-162
>>
>> Damn that was hard!
>>
>> So, what do we do? Going back to using a mutex for strings would
>> be *huge* performance hit on one of the most popular platforms
>> (if not the most popular one), but then again, keeping the status
>> quo will break binary compatibility on the (now) most popular
>> platform.
>>
>> Opinions?
>>
>> Martin
>>
>> Martin Sebor wrote:
>>> Martin Sebor wrote:
>>>> In a 12D build with the default gcc 4.1.0 on SuSE Linux Enterprise
>>>> Server 10 (x86_64), the following simple program abends with the
>>>> error below after upgrading the 4.1.3 library to 4.2.0:
>>> I've enhanced the program to replace operators new and delete
>>> and to print the value of the pointer. The enhanced test case
>>> and the output obtained from a 12D build with gcc 3.4.6 on Red
>>> Hat Enterprise Linux AS release 4 (Nahant Update 4) is below.
>>> Interestingly, the 12d (32-bit) output with Sun C++ on Solaris
>>> is fine.
>>>
>>> $ cat t.cpp && LD_LIBRARY_PATH=../lib ./t
>>> #include <cstdio>
>>> #include <cstdlib>
>>> #include <new>
>>> #include <string>
>>>
>>> int main ()
>>> {
>>>     std::string s = "a";
>>> }
>>>
>>> void* operator new (std::size_t n) throw (std::bad_alloc)
>>> {
>>>     void* const p = std::malloc (n);
>>>     std::fprintf (stdout, "operator new (%zu) ==> %#p\n", n, p);
>>>     return p;
>>> }
>>>
>>> void operator delete (void *p) throw ()
>>> {
>>>     std::fprintf (stdout, "operator delete (%#p)\n", p);
>>>     std::free (p);
>>> }
>>>
>>> void* operator new[] (std::size_t n) throw (std::bad_alloc)
>>> {
>>>     void* const p = std::malloc (n);
>>>     std::fprintf (stdout, "operator new[] (%zu) ==> %#p\n", n, p);
>>>     return p;
>>> }
>>>
>>> void operator delete[] (void *p) throw ()
>>> {
>>>     std::fprintf (stdout, "operator delete[] (%#p)\n", p);
>>>     std::free (p);
>>> }
>>>
>>> operator new (58) ==> 0x502010
>>> operator delete (0x501fe8)
>>> *** glibc detected *** free(): invalid pointer: 
>> 0x0000000000501fe8 ***
>>> Aborted
>>>
>>>


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