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Date:	Mon, 26 Jan 2026 16:45:33 -0700
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Subject:	Re: Utterly perplexing change in behaviour
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From:	Brian Inglis via Cygwin <cygwin AT cygwin DOT com>
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On 2026-01-26 08:00, Eliot Moss via Cygwin wrote:
> On 1/26/2026 9:48 AM, Fergus Daly via Cygwin wrote:
>> This pertains to an external executable built within Cygwin and therefore not 
>> part of any Cygwin package so I am not expecting a "solution"
>> but any illumination whatsoever, or similar experience, would be much 
>> appreciated.
>>
>> I use a standalone static statistical executable built within Cygwin from a 
>> collection of .c files using gcc.
>> The resulting executable comprises many built-in functions and procedures 
>> ("routines").
>> Typically it would be used within the Cygwin environment where it is supported 
>> by large libraries of data files, Help files and call-able routines.
>> But it is sophisticated enough to be useful within a directory containing just 
>> two files, itself myprog.exe and cygwin1.dll.
>> So you could start it in a Command Prompt window with the command .\myprog 
>> <Enter> with no need of Cygwin's larger architecture.
>>
>> It has a built-in function Cygwin() which returns the scalar 1 within Cygwin.
>> (The identical collection of .c files may be used with gcc in Linux to build 
>> the equivalent executable for use in Linux.
>> Then the function Cygwin() returns 0. There are cases where this distinction 
>> requires to be drawn.)
>> This has been going for literally decades with trivial fixes and enhancements 
>> and surviving all advances over the years in cygwin1.dll
>> or the Linux kernel and upgrades to gcc.
>>
>> As mentioned here a while ago the only glitch in its construction is that I 
>> have to use /lib/libreadline.a from v.8.1-2 [prev] not v.8.3-1 [curr]
>> though I have not been able to pinpoint quite why. I just do the temporary 
>> workaround and then back again.
>>
>> Oh dear: sorry for this tedious preamble. Now to the point of this communication:
>>
>> Recently some common behaviours stalled, and it emerged that the function 
>> Cygwin() returned 0 not 1.
>> This is astonishing because
>> (a) I made no changes to the executable or to its environment.
>> Well, I would say that, wouldn't I but .. ..
>> (b) I could not recover the required functionality by reverting to an earlier 
>> version of cygwin1.dll;
>> (c) the wrong output occurred on an old machine where W11 was not recently 
>> updated; and
>> (d) the wrong output occurred on an even older machine running W10.
>> All previously error-free. Unaltered executable !! Finally, even more 
>> incredibly bizarre:
>> (e) I recompiled the executable twice in succession. With the same trigger, 
>> one version returned the value 1 as needed. The other again returned 0.
>>
>> I know. I know. Being of non-Cygwin origin; and with no access to the source 
>> collection; and with no oversight of my key-presses (especially at  (e))
>> this amounts to no more than a fringe anecdote. But it would be hugely 
>> encouraging to know of any similar experience of inexplicably changed behaviour
>> (especially if recent) or any kind of a hint of cause or cure.
>>
>> Thank you for wading through all this .. ..
> 
> Just a thought, prompted by decades of debugging experience ...
> 
> Could this maybe have to do with uninitialized data, particularly some local 
> variable?
> What's in a stack location would be whatever was left behind by some previous 
> function.
> Also, while globals probably default to some standard value (e.g., 0) even if not
> initialized, maybe that's not always true.  For example, I'm not sure what value 
> would
> be in thread-local storage that is not explicitly initialized.
> 
> I'm wondering if valgrind might be of assistance here in determining whether 
> reference
> to uninitialized data may be happening.  (This would be valgrind's memcheck tool.)

What approach are you using in Cygwin and Linux functions?

I would recommend basing them on uname(3p) and sys_utsname.h(0p) in 
man-pages-posix for portability.

-- 
Take care. Thanks, Brian Inglis              Calgary, Alberta, Canada

La perfection est atteinte                   Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter  not when there is no more to add
mais lorsqu'il n'y a plus rien à retrancher  but when there is no more to cut
                                 -- Antoine de Saint-Exupéry

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