This Week on p5p 1999/10/31

Nov 3, 1999 by Mark-Jason Dominus

I’m sorry that this report is late, but I had some serious hardware trouble at home and couldn’t work on the report until I fixed my computer. Fortunately traffic was light this week.

Notes

It is hard to keep track of everything that happens. As before, please let me know if you have any corrections or additions. Send them to mjd-perl-thisweek-YYYYMM@plover.com where YYYYMM is the current year and month.

You can subscribe to an email version of this summary by sending an empty message to p5p-digest-subscribe@plover.com.

glob case-sensitivity

This discussion continued from last week. Paul Moore said that he would try to resolve some of the issues with the new built-in globber under Windows. ( \ vs. /, what to do when the underlying filesystem is case-insensitive, etc.) Read about it.

The issues seemed to get thornier and thornier. For example, what do you do with glob("C:*")? On Unix systems, you would like it to look in the current directory for files beginning with C:. But on Win32 systems you would like it to look on the disk labeled C. Nevertheless Paul submitted a partial patch.

I looked for a remark from Sarathy, but I did not see one.

Perl under UNICOS

Jarkko has been making sure that Perl works on UNICOS, which I gather is a version of Unix that runs on Crays. But his Cray is going away, and he needs someone else to take over, or to give him access to a UNICOS machine. If you can do this, please contact him. If you don’t know how to contact him, contact me.

New perlthread man page

Dan Sugalski updated his proposed perlthread man page.

Threading and explicit unlocking

Last week’s discussion of the proposed perlthread man page split into two interesting digressions. This is the first one: At present, a lock is released when control leaves the dynamic scope in which it was first obtained.

Usually this is what you want and takes care of releasing locks at the right times. Tuomas Lukka suggested that there also be an explicit unlock function for releasing a lock prematurely.

Sarathy said he’d prefer an interface that lets you store a lock into a variable as if it were an object; then its release semantics would be the same as for any other value. It would be released when the variable was destroyed, whether that was at the end of the block or by an explicit undef. Read about it.

Threading and Regexes

Rob Cunningham reports that he and Brian Mancuso at MIT are working on fixing regexes, which do not always work properly in threaded Perl. This is obviously very important. One issue is the global variables like $1. If two threads try to write into $1 simultaneously, the result is backreference goulash. But there are a huge bunch of other global variables used internally by the regex engine for storing the current state and for getting the /egismosx flags from Perl and so on. All of these present thread hazards.

Rob: Brian reports that perl REGEXP code is nasty stuff, or we’d be done by now.

Ilya said that he was also planning on removing most of the internal global variables when he gets some time.

pack t Template

First a preamble: There is already new pack template syntax already in the development version of Perl. Normally, if you want to pack three characters of string data, you write something like pack "A3", $data. But what if you don’t know in advance how much data there will be? The new feature is that you can write pack "N/A*", $data and pack will back an N-sized byte count of the data in $data, followed by the actual data. Then you use unpack with a similar template to tell it to unpack the byte count and then to extract the appropriate amount of data from the string.

Ilya had idea for extending this so that the unpack function can actually figure out what the template is. He says he is just throwing it out for discussion, and not trying to get his patch included in the core. Ilya’s idea is to add a new unpack specifier t, which says to extract a certain number of characters from the input string, and then use those characters as a template for unpacking the rest of the string. If you write t12, then the next 12 characters of the string are the template for the rest. If you write N/t then unpack will unpack an N to yield a number n and then pretend that you wrote Tn as is usual with /. Ilya adds one last trick: / by itself is a synonym for t/t.

Now what is the point of all this? The string can carry instructions for unpacking itself. For example, suppose you want to deliver the four strings "a", "bc", "def", "ghij". You would like to send these along with the template A1 A2 A3 A4. If you sent the single string "A1 A2 A3 A4abcdefghij" then the receiver could unpack this with a template of t11. Unfortunately, they still need to know that the template itself will be 11 characters long, but you can fix that. Add A211 at the begginng of the string, and have the receiver use a template of t2/t. The t2 says to get a 2-byte template, that’s the A2, and then to unpack the following data according to that template, so it gets the 11. Then it uses the 11 as a byte count for the following t. Unfortunately, the receiver still has to know that the initial template is t2/t. But after some further transformations it turns out that if you use the template t with the string "/A4t2/tA211A1 A2 A3 A4abcdefghij" then the receiver needs to know nothing about the data format, and can retrieve all the data. There are some other parts of the proposal for embedding references into teplates. The entire proposal is here if you want to see it.

Greg McCarroll: i look forward to the first CGI questions on comp.lang.pack.misc.

Several people said that the thought it was too complicated, or that they did not see the point, or that they would like to see a real-world example. (Ilya has not provided one.) Joshua Pritikin made what I thought was the most cogent comment: Why not just include Storable in the core distribution?

Happy Birthday CPAN!

CPAN first went online at 14:28:58 26 Oct 1995. Thanks you, Jarkko!

Elaine Ashton: Only 4 years! One wonders what the next 4 years will bring.

Local Address in LWP

People have been asking Gisle for a way to default the LocalAddr parameter for LWP. That is, they want to be able to specify a default for the local address to which an outgoing LWP socket is bound. Gisle could have added this as a new feature of LWP, but he thought it would be more generally useful to put it directly into IO::Socket::INET. He submitted a patch to that module that defaults the LocalAddr parameter from an environment variable if it is not explicitly set.

There was some discussion here, but it seemed to me that it missed the point of what Gisle was trying to do.

Return of ref prototype

Last week Jeff Pinyan posted a complaint about the behavior of a function prototyped with (;$). He wants print f arg1, arg2 to be parsed as if he had written print f(arg1), arg2. At present, Perl aborts, complaining that f got two arguments and expected at most one. Discussion of this got sidetracked last week.

Mike Guy pointed out that this problem also occurs when you are trying to write a function that behaves like rand: The prototype of rand is supposedly ($), but if you create a function myrand with that prototype, then print myrand, myrand; aborts with a syntax error although print rand, rand; works.

Prototypes were added to Perl so that user functions could get the syntax benefits that the built-in functions enjoyed. But some functions still can’t be imitated with prototypes. In addition to ref and rand, neither of printf or tie can be so imitated.

$^O

Andy Dougherty is patching Configure to have it find out what sort of Linux it is running on, if it is is running on Linux. This might solve Tom’s problem from last week.

sort improvements

Peter Haworth submitted an improved version of his patch for sort. He says he has benchmarked the new sort with several trivial comparator functions and performance is not bad at all. (If it were slower, you would expect to see the greatest difference with a trivial comparator.) You still cannot use an XSUB as a sort comparator function, but Peter is working on that. Reread what I said last week.

The patch.

Shell.pm enhancements.

Jenda Krynicky wants to enhance Shell.pm.

[Shell.pm] presently lets you write a function call echo("hello", "world!") and if there is no echo function already defined, it will invoke the shell’s echo command. It also has a new constructor that returns a reference to a fnuction that invokes a shell command. Jenda wants to be able to give the constructor some extra parameters to tell it to throw away the STDERR and to be able to pre-supply arguments to the function.

Jenda wanted to get some comments about this proposal before getting started on it, but nobody seemed to have anything to say about it.

Time Zone Output

Todd Olson complained that there was no easy way to obtain the current time zone in numeric format. (For example, -0400 instead of EDT or -0700 instead of PST. He points out that it would be wasteful to write a function to compute this value: The value must be inside there somewhere already, because it is used to compute localtime(). Todd wants someone to add another %-escape to the POSIX module’s strftime function that will format and display the time zone in numeric format. However, he did not provide a patch.

Python Consortium Forms

Randal Schwartz reposted an announcement about a new Python Consortium.

Yikes

Sarathy did not say `yikes’ this week.

Various

A large collection of bug reports, bug fixes, non-bug reports, questions, answers, and a small amount of flamage and spam.

Until next week I remain, your humble and obedient servant,

Mark-Jason Dominus

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