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Re: 1.5.19: changes have broken Qt3

On Tue, May 23, 2006 at 08:13:24PM +0200, Ralf Habacker wrote:
>Hash: SHA1
>Dave Korn schrieb:
>> On 23 May 2006 18:10, Ralf Habacker wrote:
>>   Oh no, not this old saw again!
>>> C:\cygwin\home\Habacker\src\pthreads-snap-2005-03-08\tests>strace
>>> mutex1n | grep  C0000005
>>> - --- Process 4872, exception C0000005 at 610B1005
>>>   155   78759 [main] mutex1n 4872 _cygtls::handle_exceptions: In
>>> cygwin_except_handler exc 0xC0000005 at 0x610B1005 sp 0x22CC00
>> [ snip many more ].
>>> I identified the problem to the function pthread_mutexattr_init address
>>> 0x610b1005 in which a null pointer (%eax below) causes this seg fault
>>   Yes, but it's wrapped in an exception handler.  That's why you get to see an
>> strace error message, rather than having the process exit.  Names like
>> "cygwin_except_handler" and "_cygtls::handle_exceptions" should have given you
>> some clues about this
>Hmmh, I have learned to fix obviously problems instead let it handle by
>an exception handler, which has several disadvantages.
>1. It costs additional runtime. In the mentioned designer i count 1653
>internal exceptions, which are caused by the null pointer issue.
>2. 70% of my strace output (1000 lines) are polluted by this internal
>exceptions messages, which could be avoided by changing only on lines of
>code. Do you think that this is the effort worth ?
>Are there more problems with this simple patch ?

The obvious problem is that you have provided an analysis of the common
code path.  If the standard code path does not usually involve a NULL
pointer then your patch introduces a statistically unneeded test, i.e.,
in your test case there are NNN internal exceptions but you haven't
stated that your test case is the standard way that these functions
would be used.  If the standard code path involves non-NULL pointers
then adding your test would mean a net slowdown.

Also, since the current code is supposed to deal with the problem
without your patch, your patch can't be considered as anything other
than a band-aid until the reason for the problem is actually understood.


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