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
http://xrl.us/bevz4
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
http://xrl.us/bevz8
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
http://xrl.us/bev2a
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
http://xrl.us/bev2e
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
engine.
ye olde bugge
http://xrl.us/bev2i
http://public.activestate.com/cgi-bin/perlbrowse/p/267
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
guru".
http://xrl.us/bev2k
"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
http://xrl.us/bev2n
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
http://xrl.us/bev2t
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
http://xrl.us/bev2v
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
before.
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.
http://xrl.us/bev2x
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
http://xrl.us/bev23
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.
http://xrl.us/bev25
"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*
http://xrl.us/bev27
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
http://xrl.us/bev29
"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
blead.
nest scopes with impunity
http://xrl.us/bev3b
"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
http://xrl.us/bev3d
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
should.
http://xrl.us/bev3j
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".
http://xrl.us/bev3m
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
http://xrl.us/bev3q
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
http://xrl.us/bev3y
http://xrl.us/bev34
http://xrl.us/bev4a
He then took a couple of stabs at making a warning about a volatile
declaration go away. Unapplied.
http://xrl.us/bev32
http://xrl.us/bev36
Steven Schubiger did some consting goodness of his own, and added a
bit to util.c. Applied.
it's all good
http://xrl.us/bev4g
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
tests.
Smoke [5.8.8] 33008 FAIL(F) MSWin32 WinXP/.Net SP2 (x86/2 cpu)
http://xrl.us/bev4i
Smoke [5.11.0] 33018 FAIL(F) MSWin32 WinXP/.Net SP2 (x86/2 cpu)
http://xrl.us/bev4k
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)
http://xrl.us/bev4n
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
http://xrl.us/bev4z
"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
operations.
Probably not a major deal, but probably something that needs to be
documented.
playing it safe
http://xrl.us/bev47
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.
http://xrl.us/bev49
"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.
http://xrl.us/bev5b
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.
http://xrl.us/bev5d
"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
meta-characters.
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
http://xrl.us/bev5f
"-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.
http://xrl.us/bev5h
"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.
http://xrl.us/bev5m
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.
http://xrl.us/bev5o
Perl5 Bug Summary
317 new + 1482 open = 1799 (11 created this week)
http://xrl.us/bev5q
http://rt.perl.org/rt3/NoAuth/perl5/Overview.html
This is the BBC
PadWalker
A recent change (#33030) by Nicholas Clark broke PadWalker. But
PadWalker being what it is, no-one was really surprised.
http://xrl.us/bev3o
In Brief
Jerry D. Hedden made the "installperl" target not complain about
finding "perl" in the build directory.
http://xrl.us/bev3u
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
http://xrl.us/bevzs
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.
http://xrl.us/bevzw
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"
http://xrl.us/bev2c
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
http://xrl.us/bev2g
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.
http://xrl.us/bev2p
Nicholas Clark made t/op/inc.t happy again, regarding the overflowing
of an integer following an increment (change #33049)
http://xrl.us/bevz2
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
http://xrl.us/bev2r
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".
http://xrl.us/bev2z
Abigail added some regression tests to t/cmd/for.t to ensure that "for
reverse .." does not break one day.
http://xrl.us/bev38
Jerry D. Hedden wrote a patch tp suppress imprecision warnings in
t/op/64bitint.t. Applied by Nicholas.
http://xrl.us/bev4c
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
http://xrl.us/bev4e
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 xrl.us short linkifier has no
error checking, and it spewed garbage into last week's summary.
the code dies screaming
http://xrl.us/bevzq
last week's
http://xrl.us/bev3h
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