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Subject:	Re: Seg Fault in strftime
To:	cygwin AT cygwin DOT com
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From:	Jon TURNEY <jon DOT turney AT dronecode DOT org DOT uk>
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Date:	Fri, 31 Jul 2015 13:51:37 +0100
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On 31/07/2015 01:16, Michael Enright wrote:
> The tznam is set from the tmzone member and when this happens that
> member is garbage. This member is garbage POSSIBLY because of a
> configuration option in libmozjs. The calling code is in prmjtime.cpp
> fills in a struct tm from Spidermonkey's own broken-down time
> structure, 'a', and then if the configuration enables, it makes
> *another* struct tm with more fields filled in in order to get
> a.tm_zone's proper value. My guess is that the path is not enabled but
> the bits delivered to me do not disclose whether this righteous code
> path is enabled. __cygwin_gettzname is evidently compiled to expect
> the tm_zone member to exist because GDB shows it does exist.

Thanks for this investigation and analysis.

> So did any aspect of this change recently? The application and library
> were getting along okay before I did cygwin updates. The last time I
> had tried to run this code was early June, at which time I was running
> it dozens of times a day.

[1] looks like a highly relevant change and [2] is the associated discussion

It would be very helpful if you could tweak the testcase there and 
produce one which reproduces your problem.

[1] 
https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=commitdiff;h=75d5f68aabf62c42884ff935f888b12bbcd00001
[2] https://sourceware.org/ml/newlib/2015/msg00321.html

> Also it appears that the tm_zone member is an extension. I haven't
> been able to find POSIX guidance about how applications are supposed
> use struct tm in compliance in the presence of implementation-defined
> fields. POSIX example code shows a usage that does access the 'at
> least' fields. The language allows for implementation-defined fields.
> No mechanism is provided within POSIX to allow an application to
> discover additional fields and take care of them. It seems to me that
> an application can then assume that when it provides a struct tm as
> input, filling in the time and date reasonably, it is always
> sufficient to fill in the 'at least' fields and the implementation is
> the one who has to assume that the rest of the fields might not be
> filled in.

Yeah, this seems a bit of an under-specified area.


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