[Bug manual/10672] qsort may not be a stable sort under memory exhaustion

[Bug manual/10672] qsort may not be a stable sort under memory exhaustion

[bugdal at aerifal dot cx]

http://sourceware.org/bugzilla/show_bug.cgi?id=10672

Rich Felker <bugdal at aerifal dot cx> changed:

           What    |Removed                     |Added
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                 CC|                            |bugdal at aerifal dot cx
          Component|libc                        |manual

--- Comment #2 from Rich Felker <bugdal at aerifal dot cx> 2011-10-23 08:17:07 UTC ---
This is a bug in the manual. The text about using the address as part of the
comparison function is not only ineffective but actually results in undefined
behavior due to violation of the requirements on comparison functions. It
should just be removed.

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[Bug manual/10672] qsort may not be a stable sort under memory exhaustion

[schwab@linux-m68k.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=10672

Andreas Schwab <schwab@linux-m68k.org> changed:

           What    |Removed                     |Added
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         AssignedTo|drepper.fsp at gmail dot    |unassigned at sourceware
                   |com                         |dot org

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[Bug manual/10672] qsort may not be a stable sort under memory exhaustion

[allachan at au1 dot ibm.com]

https://sourceware.org/bugzilla/show_bug.cgi?id=10672

paxdiablo <allachan at au1 dot ibm.com> changed:

           What    |Removed                     |Added
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                 CC|                            |allachan at au1 dot ibm.com

--- Comment #3 from paxdiablo <allachan at au1 dot ibm.com> ---
It's a well known trick that you can make an unstable sort like Quicksort into
a stable one by adding an address to the comparison key as the most minor
component.

However, this must be the STARTING address of each element and must be
populated before the sort proper starts with something like:

for (int i = 0; i < sz; i++) elem[i].startaddr = &(elem[i]);

By using the starting address as the most minor part of the key, the order of
otherwise similarly-keyed elements will be preserved.

Using the TRANSITORY address will not work and in fact will break the sort
contract as sometimes a will be less than b and sometimes vice versa, depending
on their current address in memory. So, yes, that section of the documentation
is dead wrong and should be removed or fixed.

And, in fact, qsort() is not REQUIRED to be stable as per the ISO C standard.
If you WANT a stable sort, go grab a copy of Mergesort from somewhere.

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[Bug manual/10672] qsort may not be a stable sort under memory exhaustion

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=10672

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
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              Flags|                            |security-

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[Bug manual/10672] qsort may not be a stable sort under memory exhaustion

[andersk at mit dot edu]

https://sourceware.org/bugzilla/show_bug.cgi?id=10672

--- Comment #4 from Anders Kaseorg <andersk at mit dot edu> ---
Created attachment 7665
  --> https://sourceware.org/bugzilla/attachment.cgi?id=7665&action=edit
Remove incorrect claim from manual

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[Bug manual/10672] qsort may not be a stable sort under memory exhaustion

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=10672

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |fweimer at redhat dot com

--- Comment #5 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Anders Kaseorg from comment #4)
> Created attachment 7665 [details]
> Remove incorrect claim from manual

Could you post this patch to the mailing list for review, please?

I think it is best to remove the problematic wording altogether, not attempting
to describe the past behavior of the implementation.

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[Bug manual/10672] qsort may not be a stable sort under memory exhaustion

[andersk at mit dot edu]

https://sourceware.org/bugzilla/show_bug.cgi?id=10672

--- Comment #6 from Anders Kaseorg <andersk at mit dot edu> ---
Posted: https://sourceware.org/ml/libc-alpha/2014-07/msg00002.html

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[Bug manual/10672] qsort may not be a stable sort under memory exhaustion

[neleai at seznam dot cz]

https://sourceware.org/bugzilla/show_bug.cgi?id=10672

Ondrej Bilka <neleai at seznam dot cz> changed:

           What    |Removed                     |Added
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             Status|NEW                         |RESOLVED
                 CC|                            |neleai at seznam dot cz
         Resolution|---                         |FIXED

--- Comment #7 from Ondrej Bilka <neleai at seznam dot cz> ---
fixed.

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