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Re: [PATCH] setup.exe SEGV on WinXP/Pro


On Fri, Aug 09, 2013 at 07:47:23PM +0200, Corinna Vinschen wrote:
>On Aug  9 12:55, Christopher Faylor wrote:
>> On Fri, Aug 09, 2013 at 11:01:32AM -0500, Thrall, Bryan wrote:
>> >Christopher Faylor wrote on 2013-08-09:
>> >> On Fri, Aug 09, 2013 at 11:07:26AM +0200, Corinna Vinschen wrote:
>> >>> On Aug  8 20:34, Achim Gratz wrote:
>> >>>> 
>> >>>> I've been having sporadic SEGV on WinXP/Pro just after the MD5 of a
>> >>>> package was checked that used to clear up after a reboot.  Today,
>> >with a
>> >>>> freshly built setup.exe this failure was now entirely reproduceable.
>> >>>> I've fixed it by reimplementing the string formatting for the MD5
>> >digest
>> >>>> using C++ stream functions.
>> >>>> 
>> >>> 
>> >>>>> From 677e2e89d1e4046c967dd1759ac53116f6643bd9 Mon Sep 17
>> >> 00:00:00 2001
>> >>>> From: Achim Gratz <Stromeko@Stromeko.DE>
>> >>>> Date: Thu, 8 Aug 2013 20:23:31 +0200
>> >>>> Subject: [PATCH] fix SEGV on WinXP/Pro
>> >>>> 
>> >>>>     * csu_util/MD5Sum.cc (MD5Sum::operator std::string() const):
>> >>>>     Reimplement using stringstream to avoid a SEGV on WinXP/Pro.
>> >>> 
>> >>> Patch applied.
>> >>> 
>> >>>> -  return std::string(hexdigest);
>> >>>     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> >>> I'm wondering if that was the problem.  This expression constructs a
>> >>> std:string and then immediately destructs it since the scope is
>> >limited
>> >>> to the end of the function (which the return statement is all about).
>> >>> Reading the value of this object in the parent function is basically
>> >>> luck, isn't it?
>> >> 
>> >> Sheesh.  Yes, that looks like the problem.  But doesn't the new code
>> >do
>> >> pretty much the same thing?
>> >> 
>> >> +  std::ostringstream hexdigest;
>> >> +  return hexdigest.str();
>> >
>> >According to this:
>> >
>> >http://stackoverflow.com/questions/275214/scope-and-return-values-in-c
>> >
>> >Returning the object should be ok because it is copied before leaving
>> >the function scope; returning a reference or pointer to the object is
>> >where you get into problems.
>> 
>> Thanks for clarifying.  Isn't that what the original code did too then?
>
>Not quite.  ostringstream::str returns string, the string constructor
>implicitely returns string&.  It's sometimes tricky to wrap the brain
>around the differences as far as the scope is concerned.

Perhaps a comment here would help future perusers of this function.

cgf


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