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* [Bug libc/6966] New: hsearch almost always probes the same index when using the secondary hash
@ 2008-10-19 1:30 bugzilla at bsb dot me dot uk
2008-10-19 1:34 ` [Bug libc/6966] " bugzilla at bsb dot me dot uk
2008-11-01 15:39 ` drepper at redhat dot com
0 siblings, 2 replies; 4+ messages in thread
From: bugzilla at bsb dot me dot uk @ 2008-10-19 1:30 UTC (permalink / raw)
To: glibc-bugs
hsearch references Kunth vol. 3 6.4 on "open addressing" but the code does
implement the algorithm from Knuth. Instead of generating the second hash from
the first, the code generates the second hash from the first index. Because
this hash is then used to step the index, the result is that the first index
generated is (almost) always the same.
This partly defeats the purpose of using a secondary hash.
I say "almost" because it happens for all but 2 of the possible index values in
any given table. For example, in a table of size 11, all secondary probes will
start at index 10 unless the inital hash produced 9 or 10.
This was reported on news:comp.lang.c by James Dow Allen and I decided to check
on it. I have a simple patch but I am not sure of the procedure for posting
patches to glibc.
--
Summary: hsearch almost always probes the same index when using
the secondary hash
Product: glibc
Version: 2.8
Status: NEW
Severity: minor
Priority: P3
Component: libc
AssignedTo: drepper at redhat dot com
ReportedBy: bugzilla at bsb dot me dot uk
CC: glibc-bugs at sources dot redhat dot com
http://sourceware.org/bugzilla/show_bug.cgi?id=6966
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* [Bug libc/6966] hsearch almost always probes the same index when using the secondary hash
2008-10-19 1:30 [Bug libc/6966] New: hsearch almost always probes the same index when using the secondary hash bugzilla at bsb dot me dot uk
@ 2008-10-19 1:34 ` bugzilla at bsb dot me dot uk
2008-11-01 15:39 ` drepper at redhat dot com
1 sibling, 0 replies; 4+ messages in thread
From: bugzilla at bsb dot me dot uk @ 2008-10-19 1:34 UTC (permalink / raw)
To: glibc-bugs
------- Additional Comments From bugzilla at bsb dot me dot uk 2008-10-19 01:33 -------
Created an attachment (id=3006)
--> (http://sourceware.org/bugzilla/attachment.cgi?id=3006&action=view)
Suggested fix.
The patch suggests two related changes. First, that idx be set from hval
without reducing havl % htab->size. hval2 can now be derived from hval without
generating the same probe sequence all the time. Secondly, a new variable must
be used to detect the wrap-around, since hval no longer contains the initial
value of idx.
--
http://sourceware.org/bugzilla/show_bug.cgi?id=6966
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* [Bug libc/6966] hsearch almost always probes the same index when using the secondary hash
2008-10-19 1:30 [Bug libc/6966] New: hsearch almost always probes the same index when using the secondary hash bugzilla at bsb dot me dot uk
2008-10-19 1:34 ` [Bug libc/6966] " bugzilla at bsb dot me dot uk
@ 2008-11-01 15:39 ` drepper at redhat dot com
1 sibling, 0 replies; 4+ messages in thread
From: drepper at redhat dot com @ 2008-11-01 15:39 UTC (permalink / raw)
To: glibc-bugs
------- Additional Comments From drepper at redhat dot com 2008-11-01 15:37 -------
Applied to cvs.
--
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution| |FIXED
http://sourceware.org/bugzilla/show_bug.cgi?id=6966
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* [Bug libc/6966] hsearch almost always probes the same index when using the secondary hash
[not found] <bug-6966-131@http.sourceware.org/bugzilla/>
@ 2014-07-02 6:15 ` fweimer at redhat dot com
0 siblings, 0 replies; 4+ messages in thread
From: fweimer at redhat dot com @ 2014-07-02 6:15 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=6966
Florian Weimer <fweimer at redhat dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Flags| |security-
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end of thread, other threads:[~2014-07-02 6:15 UTC | newest]
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2008-10-19 1:30 [Bug libc/6966] New: hsearch almost always probes the same index when using the secondary hash bugzilla at bsb dot me dot uk
2008-10-19 1:34 ` [Bug libc/6966] " bugzilla at bsb dot me dot uk
2008-11-01 15:39 ` drepper at redhat dot com
[not found] <bug-6966-131@http.sourceware.org/bugzilla/>
2014-07-02 6:15 ` fweimer at redhat dot com
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