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This Week on perl5-porters - 20-26 January 2008

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David Landgren
January 31, 2008 14:16
This Week on perl5-porters - 20-26 January 2008
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This Week on perl5-porters - 20-26 January 2008

   "Adding new lists of things [to remember] to a language is only a good
   idea if you're making money with certification." -- Juerd Waalboer, on
   why your program shouldn't have to plead to use new functionality.

Topics of Interest

More C for Perl programmers

   The "I know Perl, how to learn C" thread continued this week with a
   number of book and and web site references given. The thread then
   veered off into a discussion of memory alignment issues, what is a
   word (in computer memory parlance) and other sundry technical arcana
   of great interest to C programmers. Many people pointed out (quite
   rightly) that K&R is still a very good read.

     after all these years

Regression with autobox

   Alexandr Ciornii discovered that "Perl_ck_subr" lost its public status
   in the API somewhere between 5.8 and 5.10. This resulted in a
   compilation failure on Windows for "autobox", the hippest module on
   the block. He noted that he could provoke the same behaviour on Linux
   if he removed the "PERL_CORE" preprocessor definition. Silence ensued.

     and I thought autobox wasn't yet core

Make Perl Y2038 safe

   Michael G. Schwern saw no reason why date operations involving results
   that pushed out beyond 2038 (and thus wrap around the 32 time_t
   quantity) should not just do The Right Thing. No takers.

     hopefully I shall be retired by then

Making perl5 crosscompilable (or adventures in "autoconf")

   Enrico Weigelt reported that he had managed to set up an
   "autoconf"-based technique for building perl. This would allow him to
   make it easier to cross-compile Perl. He had managed to build the core
   interpreter but was stuck on how to build the standard extensions.

   The porters explained that they were unlikely to move away from the
   current "metaconf" system, since it allows the source to build build
   on many non-GNU, non-POSIX hosts.

   People tried to explain how the current cross-compilation mechanism
   works but it appears that few people have any real experience in the
   matter. No "autoconf" experts were able to answer Enrico's questions
   concerning how to build extensions, either.

     A work in progress

Decade old regexp tainting bug?

   Nicholas Clark uncovered an ancient bug in the regexp code. It started
   of with a boolean value that was able to take values other than 0 and
   1, which was a nice touch. It turned out that it was then binary-or'ed
   with a bit that happened to lie way past the most significant bit of
   the datatype being used to hold the boolean. Since the bit in question
   was to indicate that the pattern was tainted, we have a bit of a
   problem on our hands.

   The first thing was able to see whether it was possible to construct a
   test case that could expose the flawed behaviour. Rick Delaney was
   first past the post with a test that demonstrated the problem, and a
   fix that produced the correct behaviour.

   Ben Morrow proposed a different test, that Abigail tweaked to show
   that the problem existed all the way back to 5.004. This was
   important, for the code the Nicholas found was traced back to change
   #267, committed to the repository in 1997. Unfortunately, the change
   was a jumbo patch that changed all sorts of things in the regexp

     ye olde bugge

Dear C gurus

   Nicholas Clark discovered that an innocuous change to perl.c created a
   "argument 'flags' might be clobbered by `longjmp' or `vfork'" warning
   and wondered what needed to be done to the code in order to make "gcc"
   happy again. Zefram and Hugo van der Sanden explained that the "flags"
   variable needed to be made "volatile". Zefram went on to explain why
   it was so, in sufficient detail to merit the badge of "resident C

"Module::Build" interim release

   Ken Williams wanted to integrate the changes that were required to get
   5.10 out the door back into the mainline "Module::Build" codebase.
   Most of the changes are test tweaks to skip troublesome issues on VMS.

   Craig Berry agreed that the VMS porters ran out of time in the run-up
   to 5.10, and it would be nice to go back and tidy up the loose ends.

     mopping up

Unshifting PL_curstack is a no-no?

   Nicholas Clark was up to his elbows as usual, grovelling around in
   pp.c and pp_hot.c, seeing if you could shift elements off
   "PL_curstack". Presumably to make things go faster. After watching
   things blow up for a while, he concluded that what he wanted to do
   couldn't be done.

     not that I really understood anything

Binding operators, void context, and documentation

   Kaye Offer wondered why "$foo =~ /(bar)/;" in void context does not
   warn, but "$foo !~ /(bar)/;" does. Aristotle Pagaltzis and Rafael
   Garcia-Suarez did a pretty good job of explaining why thing were the
   way they were.

     it's a feature

Reviving the perl compiler

   Reini Urban announced that he had taken a look at the perl compiler
   that had been removed from the 5.10.0 release. He lavished sufficient
   care upon it to get to compile again, although there were problems
   with the test suite.

   Reini thought that the simple stack-based op-tree could be JITted
   easily into machine code and was looking at the GNU lightning library.
   He wanted to know if anyone else had had a look at this approach

   Nicholas Clark explained that overloaded or tied SVs make JITting
   *really* hard, and that some of the ops were so high-level that
   JITting them would produce copious amounts of machine instructions
   which in turn would make a mess of a CPU instruction cache.

   Joshua ben Jore pointed to Marc Lehmann's Faster project, that takes a
   Perl routine and turns it into C.

Floats to strings issue

   Jerry D. Hedden asked why "Configure" probes for alternatives to
   "sprintf" that produce identical behaviour to "sprint("%g")", and why
   not just use "sprintf" and be done with it. Andy Dougherty explained
   that some platforms, such as Solaris, have alternate functions
   available in the system C library that offer much better performance.
   In such cases, "Configure" favours them over "sprintf".

     only the best

Updating "Runops::Switch" - problem testing "OP_SAY"

   Jim Cromie discovered that "Runops::Switch" needed a tweak to
   recognise the new "say" in 5.10 and made a preliminary patch to get it
   to work. Rafael upgraded the module in any event, but Jim's patch
   reminded Jan Dubois that people should never link directly to the
   "Perl_pp_*" routines, since they are not part of the public API.

"perl5.6.2 -e 'delete $ENV{PATH}'" segfault on Solaris 10

   Ralf Hack provided a recipe for people to follow, should they be stuck
   on a modern Solaris with an old perl, and attempt to delete
   environment variables.

     one day this might happen to *you*

Win32 precision configuration

   Following on from change #33049 when warnings about loss of precision
   were tweaked, Nicholas Clark noticed that the win32/config.bc file
   failed the Don't Repeat Yourself principle, and wondered whether it
   would be possible to have it generated automatically.

   Steve Hay mumbled something about keeping things in sync and having a
   mind to write a script to do it, but not actually having got around to
   doing something about it.

     low itch factor

"struct context" now 12.5% smaller than 5.10

   Nicholas Clark was as pleased as Punch after pulling out an "IV" and a
   pointer from "struct context". And after thinking about it a bit more,
   thought of another possible restructuring to save a bit more space.
   Benjamin Smith took Nicholas's second idea and coded a patch to
   implement it, and in the process discovered another improvement that
   Nicholas missed initially.

   At the end, Nicholas applied all the discovered slimming goodness to

     nest scopes with impunity

"lc(undef)" is not "undef": bug or feature?

   Alberto Simões was a little surprised to learn that "lc(undef)"
   returns the empty string, and does not warn when doing so. Abigail
   reminded people that Perl's "undef" is not like SQL's "null" with its
   capacity to turn everything it touches to "null". Perl will turn
   "undef" into zero or the empty string as appropriate and will warn
   when it does so, if you ask for it.

   Jonathon Rockway noticed that "\L", "\u" and the like also behaved the
   same way. Michael G. Schwern bet the contents of a capture variable
   that "\L" and "lc" were implemented using the same underlying opcode,
   which was confirmed by a quick glance at toke.c.

   Rafael Garcia-Suarez added some code to make things warn, along with a
   few regression tests as change #33088. He hoped that people would bang
   on it and see if anything breaks.

     in that case

A warning I'd like to see

   David Nicol wished that a warning would be issued when map was fed a
   reference to an array, instead of an array. This reminded Aristotle
   Pagaltzis that this was exactly the thing that Mark-Jason Dominus's
   proposal a few weeks ago was designed to approach: raising a warning
   when a reference is directly stringified or numified.

   Juerd Waalboer pointed out "map" takes a list, and an arrayref is
   merely a one element list. In fact, you cannot do anything other than
   give a list to map. It just might not contain what you thought it

Extra stricture for hard refs in 5.12

   Following on from the above thread, Aristotle then restarted the
   "references should not stringify" discussion, which covered more or
   less the same ground as it did last month. Michael G. Schwern summed
   it up pretty well "there are times when you just want an object to
   stringify, usually for debugging purposes, and there are times when
   you don't, usually for production purposes."

   At the moment, no-one knows how to reconcile these differences,
   although Ben Morrow revealed a clever use of "Hash::Util::Fieldhash".

Not a pad error

   Elsewhere in his wanderings around the codebase, Nicholas Clark caught
   sight of an oddity in "pp_enteriter" (that sets up a "foreach" loop).
   He thought that be undoing an 8 year old change by Gurusamy Sarathy,
   it should be possible to provoke the bug the change was designed to
   fix. But, rather surprisingly, the code continued to work anyway.

   Dave Mitchell's work on lexical and closure cleanups for 5.10 turned
   out to simplify matters considerably which in turn allows Nicholas to
   chop out a certain amount of redundant checks.

     unexpected bonus

Patches of Interest

   Robin Barker had a look a "Devel::DProf" and noticed a certain amount
   of cruft that he was able to prune, some that had been lying around
   since 1999. Applied.

     all gone

   He then took a couple of stabs at making a warning about a volatile
   declaration go away. Unapplied.

   Steven Schubiger did some consting goodness of his own, and added a
   bit to util.c. Applied.

     it's all good

Watching the smoke signals

   Steve Hay was busy smoking Perl this week, and both 5.8 and 5.11 were
   complaining during the compilation stage and falling apart in the

     Smoke [5.8.8] 33008 FAIL(F) MSWin32 WinXP/.Net SP2 (x86/2 cpu)

     Smoke [5.11.0] 33018 FAIL(F) MSWin32 WinXP/.Net SP2 (x86/2 cpu)

   Jarkko Hietaniemi reported a failure compiling with C++ on a Tru64
   Alpha. Steve Peters couldn't find any problems when using g++, but
   thankfully Solaris's C++ compiler obligingly fell over, which allowed
   him to compose a first patch to get IPC::SysV up and running again.

   Jarkko followed up with an improved patch to do the same on Tru64.
   Rafael applied this, which then caused Solaris to break again, so
   Steve had to go back and tweak the tweak.

     Smoke [5.11.0] 33016 FAIL(XM) OSF1 V5.1 (EV6/4 cpu)

New and old bugs from RT

"state" variable not available (#49522)

   Dave Mitchell worked out what the problem was with Abigail's state
   variables, tracing it to a problem with the "Svf_PADSTALE" flag, which
   meant, in the context of state variables, that the variable had not
   been initialised, rather than having gone out of scope. A few lines of
   code, some tests, and the job was done.

     affairs of state

"strict" now uses "caller", unintended interaction with "Safe" (#50084)

   Simon Cozens reported that he had heard from the Postgresql developers
   running into trouble embedding Perl in Pg. Their simple recipe that
   worked in 5.8 no longer works in 5.10, as "strict" makes use of
   "caller" and the latter is not in the default list of permitted

   Probably not a major deal, but probably something that needs to be

     playing it safe

Bug in regcomp code leading to panic (#50114)

   "mls" provided a one-liner that produces a panic in 5.10 with a
   regular expression. The report went as far to identify the offending
   code and make a suggestion as to how it might be fixed.

"enc2xs -C" scans the current directory (#50116)

   "mls" also suggested that "enc2xs" (part of the "Encode" distribution,
   to add new encodings to perl) should not search "." when "-C" is used.

panic: attempt to copy freed scalar (#50142)

   Johan Vromans posted the nth bug report concerning a problem with an
   explicit shift of @ARGV within a subroutine. Dave Mitchell explained
   that it was a long-standing bug due to the fact that items weren't
   reference counted on the stack, and that perl really ought to start
   doing the right thing.

"File::Temp" and unsafe shell characters (#50146)

   Ed Avis was alarmed to discover that if you move into a directory
   named `rm -rf /`, ask "File::Temp" to create a file in said directory
   and open the file, you can be in a lot of trouble if it's the
   super-user that's running the script.

   To counter this, Ed felt that "File::Temp" should ensure that anything
   it returned to client code should be filtered to strip out shellish

   Mark Overmeer pointed out that the problem doesn't exist with the
   3-arg form of "open" and that maybe the best solution was to deprecate
   the 2-arg form in 5.12.

     shell game

"-W" and spurious 'will not stay shared' message (#50160)

   Eric Promislow had some code that developed a tricky regexp with a
   "(??{...})" construct and wondered why it issued a "will not stay
   shared" warning. Dave Mitchell explained that one should always use
   package variables with the "??{...}" construct, at least until 5.12 at
   the earliest.

"pos" is much slower with "progressive match" and unicode (#50250)

   Heinz Knutzen discovered that a simple loop involving a "pos" was
   about 2000 times slower in 5.10 compared with 5.8.8. Dave Mitchell
   profiled the code and discovered that "Perl_utf_length" was soaking up
   an inordinate number of cycles. This led him to conclude that there
   was something broken in the UTF-8 length cache code.

Segfault on "perl -e 'split //, unpack "(B)*", "ab"'" (#50256)

   mauke reported this crash on 5.10.0 (and it looks like it was there in
   5.8.8 as well). No-one ventured a reason as to why.

Perl5 Bug Summary

     317 new + 1482 open = 1799 (11 created this week)

This is the BBC

       A recent change (#33030) by Nicholas Clark broke PadWalker. But
       PadWalker being what it is, no-one was really surprised.

In Brief

   Jerry D. Hedden made the "installperl" target not complain about
   finding "perl" in the build directory.

   H.Merijn Brand reported good results with his "Configure" and
   metaunits work. He was down to 16 warnings, and the promise of several
   files that could be removed from the branch once everything was
   wrapped up. Andy Dougherty rejoiced.

     and we all breathed a sigh of relief

   Andy Dougherty suggested a pre-5.005-compatible patch to "Configure"
   to get the 5.8.9 snapshot to compile on IRIX. Alas, David Cantrell
   reported no joy.

   Elsewhere in the push to bring on 5.8.9, Alexey Tourbin noticed that a
   recent change was causing tests to fail in "Term::ReadLine::Gnu"

   Steven Schubiger had a "make test" fail with copious reports of
   "undefined symbol: __stack_chk_fail_local". No-one ventured a reason
   as to what or why things blew up.

     try make distclean

   Yamashina Hio wrote some POD in English which was fine, but the same
   POD in Japanese failed to produce correct text for link references.
   No-one was able to provide any clues.

   Nicholas Clark made t/op/inc.t happy again, regarding the overflowing
   of an integer following an increment (change #33049)

   Andreas König reported that the All Perl Changes (APC) repository now
   deals with 5.10 correctly, and all sorts of tarballs of assorted
   versions of Perl are available.

     and there was much rejoicing

   Regarding the BBC and "Apache::DB", Richard Foley wondered if there
   was a parallel with the problems he was having with other modules with
   Apache on 5.10, such as "B::TerseSize".

   Abigail added some regression tests to t/cmd/for.t to ensure that "for
   reverse .." does not break one day.

   Jerry D. Hedden wrote a patch tp suppress imprecision warnings in
   t/op/64bitint.t. Applied by Nicholas.

   Jan Dubois pointed out that "socketpair()" *is* available on Win32,
   and has been for quite some time (as in, prior to 5.8).

     only the documentation was buggy

About this summary

   This summary was written by David Landgren.

   In last week's summary, I explained that Moritz Lenz was disappointed
   that a regexp would not recurse into an interpolated "qr//". In actual
   fact it does, and Moritz was really wishing that it wouldn't.
   Apologies to those confused by my confusion.

   Aristotle Pagaltzis also noticed that my short linkifier has no
   error checking, and it spewed garbage into last week's summary.

     the code dies screaming

     last week's

   Weekly summaries are published on and posted on a
   mailing list, (subscription: The
   archive is at Corrections
   and comments are welcome.

   If you found this summary useful, please consider contributing to the
   Perl Foundation to help support the development of Perl.

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