1PERLFAQ3(1) Perl Programmers Reference Guide PERLFAQ3(1)
2
6 perlfaq3 - Programming Tools
7
9 This section of the FAQ answers questions related to programmer tools
10 and programming support.
11 How do I do (anything)?
13 Have you looked at CPAN (see perlfaq2)? The chances are that someone
14 has already written a module that can solve your problem. Have you
15 read the appropriate manpages? Here's a brief index:
16 Basics perldata, perlvar, perlsyn, perlop, perlsub
18 Execution perlrun, perldebug
19 Functions perlfunc
20 Objects perlref, perlmod, perlobj, perltie
21 Data Structures perlref, perllol, perldsc
22 Modules perlmod, perlmodlib, perlsub
23 Regexes perlre, perlfunc, perlop, perllocale
24 Moving to perl5 perltrap, perl
25 Linking w/C perlxstut, perlxs, perlcall, perlguts, perlembed
26 Various http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz
27 (not a man-page but still useful, a collection
28 of various essays on Perl techniques)
29 A crude table of contents for the Perl manpage set is found in perltoc.
31 How can I use Perl interactively?
33 The typical approach uses the Perl debugger, described in the
34 perldebug(1) manpage, on an "empty" program, like this:
35 perl -de 42
37 Now just type in any legal Perl code, and it will be immediately
39 evaluated. You can also examine the symbol table, get stack
40 backtraces, check variable values, set breakpoints, and other
41 operations typically found in symbolic debuggers.
42 Is there a Perl shell?
44 The "psh" (Perl sh) is currently at version 1.8. The Perl Shell is a
45 shell that combines the interactive nature of a Unix shell with the
46 power of Perl. The goal is a full featured shell that behaves as
47 expected for normal shell activity and uses Perl syntax and
48 functionality for control-flow statements and other things. You can get
49 "psh" at http://sourceforge.net/projects/psh/ .
50 "Zoidberg" is a similar project and provides a shell written in perl,
52 configured in perl and operated in perl. It is intended as a login
53 shell and development environment. It can be found at
54 http://pardus-larus.student.utwente.nl/~pardus/projects/zoidberg/ or
55 your local CPAN mirror.
56 The "Shell.pm" module (distributed with Perl) makes Perl try commands
58 which aren't part of the Perl language as shell commands. "perlsh"
59 from the source distribution is simplistic and uninteresting, but may
60 still be what you want.
61 How do I find which modules are installed on my system?
63 From the command line, you can use the "cpan" command's "-l" switch:
64 $ cpan -l
66 You can also use "cpan"'s "-a" switch to create an autobundle file that
68 "CPAN.pm" understands and can use to re-install every module:
69 $ cpan -a
71 Inside a Perl program, you can use the "ExtUtils::Installed" module to
73 show all installed distributions, although it can take awhile to do its
74 magic. The standard library which comes with Perl just shows up as
75 "Perl" (although you can get those with "Module::CoreList").
76 use ExtUtils::Installed;
78 my $inst = ExtUtils::Installed->new();
80 my @modules = $inst->modules();
81 If you want a list of all of the Perl module filenames, you can use
83 "File::Find::Rule":
84 use File::Find::Rule;
86 my @files = File::Find::Rule->
88 extras({follow => 1})->
89 file()->
90 name( '*.pm' )->
91 in( @INC )
92 ;
93 If you do not have that module, you can do the same thing with
95 "File::Find" which is part of the standard library:
96 use File::Find;
98 my @files;
99 find(
101 {
102 wanted => sub {
103 push @files, $File::Find::fullname
104 if -f $File::Find::fullname && /\.pm$/
105 },
106 follow => 1,
107 follow_skip => 2,
108 },
109 @INC
110 );
111 print join "\n", @files;
113 If you simply need to quickly check to see if a module is available,
115 you can check for its documentation. If you can read the documentation
116 the module is most likely installed. If you cannot read the
117 documentation, the module might not have any (in rare cases):
118 $ perldoc Module::Name
120 You can also try to include the module in a one-liner to see if perl
122 finds it:
123 $ perl -MModule::Name -e1
125 How do I debug my Perl programs?
127 (contributed by brian d foy)
128 Before you do anything else, you can help yourself by ensuring that you
130 let Perl tell you about problem areas in your code. By turning on
131 warnings and strictures, you can head off many problems before they get
132 too big. You can find out more about these in strict and warnings.
133 #!/usr/bin/perl
135 use strict;
136 use warnings;
137 Beyond that, the simplest debugger is the "print" function. Use it to
139 look at values as you run your program:
140 print STDERR "The value is [$value]\n";
142 The "Data::Dumper" module can pretty-print Perl data structures:
144 use Data::Dumper qw( Dumper );
146 print STDERR "The hash is " . Dumper( \%hash ) . "\n";
147 Perl comes with an interactive debugger, which you can start with the
149 "-d" switch. It's fully explained in perldebug.
150 If you'd like a graphical user interface and you have "Tk", you can use
152 "ptkdb". It's on CPAN and available for free.
153 If you need something much more sophisticated and controllable, Leon
155 Brocard's "Devel::ebug" (which you can call with the "-D" switch as
156 "-Debug") gives you the programmatic hooks into everything you need to
157 write your own (without too much pain and suffering).
158 You can also use a commercial debugger such as Affrus (Mac OS X),
160 Komodo from Activestate (Windows and Mac OS X), or EPIC (most
161 platforms).
162 How do I profile my Perl programs?
164 (contributed by brian d foy, updated Fri Jul 25 12:22:26 PDT 2008)
165 The "Devel" namespace has several modules which you can use to profile
167 your Perl programs. The "Devel::DProf" module comes with Perl and you
168 can invoke it with the "-d" switch:
169 perl -d:DProf program.pl
171 After running your program under "DProf", you'll get a tmon.out file
173 with the profile data. To look at the data, you can turn it into a
174 human-readable report with the "dprofpp" program that comes with
175 "Devel::DProf".
176 dprofpp
178 You can also do the profiling and reporting in one step with the "-p"
180 switch to "dprofpp":
181 dprofpp -p program.pl
183 The "Devel::NYTProf" (New York Times Profiler) does both statement and
185 subroutine profiling. It's available from CPAN and you also invoke it
186 with the "-d" switch:
187 perl -d:NYTProf some_perl.pl
189 Like "DProf", it creates a database of the profile information that you
191 can turn into reports. The "nytprofhtml" command turns the data into an
192 HTML report similar to the "Devel::Cover" report:
193 nytprofhtml
195 CPAN has several other profilers that you can invoke in the same
197 fashion. You might also be interested in using the "Benchmark" to
198 measure and compare code snippets.
199 You can read more about profiling in Programming Perl, chapter 20, or
201 Mastering Perl, chapter 5.
202 perldebguts documents creating a custom debugger if you need to create
204 a special sort of profiler. brian d foy describes the process in The
205 Perl Journal, "Creating a Perl Debugger", http://www.ddj.com/184404522
206 , and "Profiling in Perl" http://www.ddj.com/184404580 .
207 Perl.com has two interesting articles on profiling: "Profiling Perl",
209 by Simon Cozens, http://www.perl.com/lpt/a/850 and "Debugging and
210 Profiling mod_perl Applications", by Frank Wiles,
211 http://www.perl.com/pub/a/2006/02/09/debug_mod_perl.html .
212 Randal L. Schwartz writes about profiling in "Speeding up Your Perl
214 Programs" for Unix Review,
215 http://www.stonehenge.com/merlyn/UnixReview/col49.html , and "Profiling
216 in Template Toolkit via Overriding" for Linux Magazine,
217 http://www.stonehenge.com/merlyn/LinuxMag/col75.html .
218 How do I cross-reference my Perl programs?
220 The "B::Xref" module can be used to generate cross-reference reports
221 for Perl programs.
222 perl -MO=Xref[,OPTIONS] scriptname.plx
224 Is there a pretty-printer (formatter) for Perl?
226 "Perltidy" is a Perl script which indents and reformats Perl scripts to
227 make them easier to read by trying to follow the rules of the
228 perlstyle. If you write Perl scripts, or spend much time reading them,
229 you will probably find it useful. It is available at
230 http://perltidy.sourceforge.net .
231 Of course, if you simply follow the guidelines in perlstyle, you
233 shouldn't need to reformat. The habit of formatting your code as you
234 write it will help prevent bugs. Your editor can and should help you
235 with this. The perl-mode or newer cperl-mode for emacs can provide
236 remarkable amounts of help with most (but not all) code, and even less
237 programmable editors can provide significant assistance. Tom
238 Christiansen and many other VI users swear by the following settings
239 in vi and its clones:
240 set ai sw=4
242 map! ^O {^M}^[O^T
243 Put that in your .exrc file (replacing the caret characters with
245 control characters) and away you go. In insert mode, ^T is for
246 indenting, ^D is for undenting, and ^O is for blockdenting--as it were.
247 A more complete example, with comments, can be found at
248 http://www.cpan.org/authors/id/TOMC/scripts/toms.exrc.gz
249 The a2ps http://www-inf.enst.fr/%7Edemaille/a2ps/black+white.ps.gz does
251 lots of things related to generating nicely printed output of
252 documents.
253 Is there a ctags for Perl?
255 (contributed by brian d foy)
256 Ctags uses an index to quickly find things in source code, and many
258 popular editors support ctags for several different languages,
259 including Perl.
260 Exuberent ctags supports Perl: http://ctags.sourceforge.net/
262 You might also try pltags: http://www.mscha.com/pltags.zip
264 Is there an IDE or Windows Perl Editor?
266 Perl programs are just plain text, so any editor will do.
267 If you're on Unix, you already have an IDE--Unix itself. The Unix
269 philosophy is the philosophy of several small tools that each do one
270 thing and do it well. It's like a carpenter's toolbox.
271 If you want an IDE, check the following (in alphabetical order, not
273 order of preference):
274 Eclipse
276 http://e-p-i-c.sf.net/
277 The Eclipse Perl Integration Project integrates Perl
279 editing/debugging with Eclipse.
280 Enginsite
282 http://www.enginsite.com/
283 Perl Editor by EngInSite is a complete integrated development
285 environment (IDE) for creating, testing, and debugging Perl
286 scripts; the tool runs on Windows 9x/NT/2000/XP or later.
287 Komodo
289 http://www.ActiveState.com/Products/Komodo/
290 ActiveState's cross-platform (as of October 2004, that's Windows,
292 Linux, and Solaris), multi-language IDE has Perl support, including
293 a regular expression debugger and remote debugging.
294 Notepad++
296 http://notepad-plus.sourceforge.net/
297 Open Perl IDE
299 http://open-perl-ide.sourceforge.net/
300 Open Perl IDE is an integrated development environment for writing
302 and debugging Perl scripts with ActiveState's ActivePerl
303 distribution under Windows 95/98/NT/2000.
304 OptiPerl
306 http://www.optiperl.com/
307 OptiPerl is a Windows IDE with simulated CGI environment, including
309 debugger and syntax highlighting editor.
310 Padre
312 http://padre.perlide.org/
313 Padre is cross-platform IDE for Perl written in Perl using
315 wxWidgets to provide a native look and feel. It's open source under
316 the Artistic License.
317 PerlBuilder
319 http://www.solutionsoft.com/perl.htm
320 PerlBuilder is an integrated development environment for Windows
322 that supports Perl development.
323 visiPerl+
325 http://helpconsulting.net/visiperl/
326 From Help Consulting, for Windows.
328 Visual Perl
330 http://www.activestate.com/Products/Visual_Perl/
331 Visual Perl is a Visual Studio.NET plug-in from ActiveState.
333 Zeus
335 http://www.zeusedit.com/lookmain.html
336 Zeus for Window is another Win32 multi-language editor/IDE that
338 comes with support for Perl:
339 For editors: if you're on Unix you probably have vi or a vi clone
341 already, and possibly an emacs too, so you may not need to download
342 anything. In any emacs the cperl-mode (M-x cperl-mode) gives you
343 perhaps the best available Perl editing mode in any editor.
344 If you are using Windows, you can use any editor that lets you work
346 with plain text, such as NotePad or WordPad. Word processors, such as
347 Microsoft Word or WordPerfect, typically do not work since they insert
348 all sorts of behind-the-scenes information, although some allow you to
349 save files as "Text Only". You can also download text editors designed
350 specifically for programming, such as Textpad ( http://www.textpad.com/
351 ) and UltraEdit ( http://www.ultraedit.com/ ), among others.
352 If you are using MacOS, the same concerns apply. MacPerl (for Classic
354 environments) comes with a simple editor. Popular external editors are
355 BBEdit ( http://www.bbedit.com/ ) or Alpha (
356 http://www.his.com/~jguyer/Alpha/Alpha8.html ). MacOS X users can use
357 Unix editors as well.
358 GNU Emacs
360 http://www.gnu.org/software/emacs/windows/ntemacs.html
361 MicroEMACS
363 http://www.microemacs.de/
364 XEmacs
366 http://www.xemacs.org/Download/index.html
367 Jed http://space.mit.edu/~davis/jed/
369 or a vi clone such as
371 Elvis
373 ftp://ftp.cs.pdx.edu/pub/elvis/ http://www.fh-wedel.de/elvis/
374 Vile
376 http://dickey.his.com/vile/vile.html
377 Vim http://www.vim.org/
379 For vi lovers in general, Windows or elsewhere:
381 http://www.thomer.com/thomer/vi/vi.html
383 nvi ( http://www.bostic.com/vi/ , available from CPAN in src/misc/) is
385 yet another vi clone, unfortunately not available for Windows, but in
386 Unix platforms you might be interested in trying it out, firstly
387 because strictly speaking it is not a vi clone, it is the real vi, or
388 the new incarnation of it, and secondly because you can embed Perl
389 inside it to use Perl as the scripting language. nvi is not alone in
390 this, though: at least also vim and vile offer an embedded Perl.
391 The following are Win32 multilanguage editor/IDEs that support Perl:
393 Codewright
395 http://www.borland.com/codewright/
396 MultiEdit
398 http://www.MultiEdit.com/
399 SlickEdit
401 http://www.slickedit.com/
402 ConTEXT
404 http://www.contexteditor.org/
405 There is also a toyedit Text widget based editor written in Perl that
407 is distributed with the Tk module on CPAN. The ptkdb (
408 http://ptkdb.sourceforge.net/ ) is a Perl/tk based debugger that acts
409 as a development environment of sorts. Perl Composer (
410 http://perlcomposer.sourceforge.net/ ) is an IDE for Perl/Tk GUI
411 creation.
412 In addition to an editor/IDE you might be interested in a more powerful
414 shell environment for Win32. Your options include
415 Bash
417 from the Cygwin package ( http://sources.redhat.com/cygwin/ )
418 Ksh from the MKS Toolkit ( http://www.mkssoftware.com/ ), or the Bourne
420 shell of the U/WIN environment (
421 http://www.research.att.com/sw/tools/uwin/ )
422 Tcsh
424 ftp://ftp.astron.com/pub/tcsh/ , see also
425 http://www.primate.wisc.edu/software/csh-tcsh-book/
426 Zsh http://www.zsh.org/
428 MKS and U/WIN are commercial (U/WIN is free for educational and
430 research purposes), Cygwin is covered by the GNU General Public License
431 (but that shouldn't matter for Perl use). The Cygwin, MKS, and U/WIN
432 all contain (in addition to the shells) a comprehensive set of standard
433 Unix toolkit utilities.
434 If you're transferring text files between Unix and Windows using FTP be
436 sure to transfer them in ASCII mode so the ends of lines are
437 appropriately converted.
438 On Mac OS the MacPerl Application comes with a simple 32k text editor
440 that behaves like a rudimentary IDE. In contrast to the MacPerl
441 Application the MPW Perl tool can make use of the MPW Shell itself as
442 an editor (with no 32k limit).
443 Affrus
445 is a full Perl development environment with full debugger support (
446 http://www.latenightsw.com ).
447 Alpha
449 is an editor, written and extensible in Tcl, that nonetheless has
450 built in support for several popular markup and programming
451 languages including Perl and HTML (
452 http://www.his.com/~jguyer/Alpha/Alpha8.html ).
453 BBEdit and BBEdit Lite
455 are text editors for Mac OS that have a Perl sensitivity mode (
456 http://web.barebones.com/ ).
457 Where can I get Perl macros for vi?
459 For a complete version of Tom Christiansen's vi configuration file, see
460 http://www.cpan.org/authors/Tom_Christiansen/scripts/toms.exrc.gz , the
461 standard benchmark file for vi emulators. The file runs best with nvi,
462 the current version of vi out of Berkeley, which incidentally can be
463 built with an embedded Perl interpreter--see
464 http://www.cpan.org/src/misc/ .
465 Where can I get perl-mode or cperl-mode for emacs?
467 Since Emacs version 19 patchlevel 22 or so, there have been both a
468 perl-mode.el and support for the Perl debugger built in. These should
469 come with the standard Emacs 19 distribution.
470 Note that the perl-mode of emacs will have fits with "main'foo" (single
472 quote), and mess up the indentation and highlighting. You are probably
473 using "main::foo" in new Perl code anyway, so this shouldn't be an
474 issue.
475 For CPerlMode, see http://www.emacswiki.org/cgi-bin/wiki/CPerlMode
477 How can I use curses with Perl?
479 The Curses module from CPAN provides a dynamically loadable object
480 module interface to a curses library. A small demo can be found at the
481 directory http://www.cpan.org/authors/Tom_Christiansen/scripts/rep.gz ;
482 this program repeats a command and updates the screen as needed,
483 rendering rep ps axu similar to top.
484 How can I write a GUI (X, Tk, Gtk, etc.) in Perl?
486 (contributed by Ben Morrow)
487 There are a number of modules which let you write GUIs in Perl. Most
489 GUI toolkits have a perl interface: an incomplete list follows.
490 Tk This works under Unix and Windows, and the current version doesn't
492 look half as bad under Windows as it used to. Some of the gui
493 elements still don't 'feel' quite right, though. The interface is
494 very natural and 'perlish', making it easy to use in small scripts
495 that just need a simple gui. It hasn't been updated in a while.
496 Wx This is a Perl binding for the cross-platform wxWidgets toolkit (
498 http://www.wxwidgets.org ). It works under Unix, Win32 and Mac OS
499 X, using native widgets (Gtk under Unix). The interface follows the
500 C++ interface closely, but the documentation is a little sparse for
501 someone who doesn't know the library, mostly just referring you to
502 the C++ documentation.
503 Gtk and Gtk2
505 These are Perl bindings for the Gtk toolkit ( http://www.gtk.org ).
506 The interface changed significantly between versions 1 and 2 so
507 they have separate Perl modules. It runs under Unix, Win32 and Mac
508 OS X (currently it requires an X server on Mac OS, but a 'native'
509 port is underway), and the widgets look the same on every plaform:
510 i.e., they don't match the native widgets. As with Wx, the Perl
511 bindings follow the C API closely, and the documentation requires
512 you to read the C documentation to understand it.
513 Win32::GUI
515 This provides access to most of the Win32 GUI widgets from Perl.
516 Obviously, it only runs under Win32, and uses native widgets. The
517 Perl interface doesn't really follow the C interface: it's been
518 made more Perlish, and the documentation is pretty good. More
519 advanced stuff may require familiarity with the C Win32 APIs, or
520 reference to MSDN.
521 CamelBones
523 CamelBones ( http://camelbones.sourceforge.net ) is a Perl
524 interface to Mac OS X's Cocoa GUI toolkit, and as such can be used
525 to produce native GUIs on Mac OS X. It's not on CPAN, as it
526 requires frameworks that CPAN.pm doesn't know how to install, but
527 installation is via the standard OSX package installer. The Perl
528 API is, again, very close to the ObjC API it's wrapping, and the
529 documentation just tells you how to translate from one to the
530 other.
531 Qt There is a Perl interface to TrollTech's Qt toolkit, but it does
533 not appear to be maintained.
534 Athena
536 Sx is an interface to the Athena widget set which comes with X, but
537 again it appears not to be much used nowadays.
538 How can I make my Perl program run faster?
540 The best way to do this is to come up with a better algorithm. This
541 can often make a dramatic difference. Jon Bentley's book Programming
542 Pearls (that's not a misspelling!) has some good tips on optimization,
543 too. Advice on benchmarking boils down to: benchmark and profile to
544 make sure you're optimizing the right part, look for better algorithms
545 instead of microtuning your code, and when all else fails consider just
546 buying faster hardware. You will probably want to read the answer to
547 the earlier question "How do I profile my Perl programs?" if you
548 haven't done so already.
549 A different approach is to autoload seldom-used Perl code. See the
551 AutoSplit and AutoLoader modules in the standard distribution for that.
552 Or you could locate the bottleneck and think about writing just that
553 part in C, the way we used to take bottlenecks in C code and write them
554 in assembler. Similar to rewriting in C, modules that have critical
555 sections can be written in C (for instance, the PDL module from CPAN).
556 If you're currently linking your perl executable to a shared libc.so,
558 you can often gain a 10-25% performance benefit by rebuilding it to
559 link with a static libc.a instead. This will make a bigger perl
560 executable, but your Perl programs (and programmers) may thank you for
561 it. See the INSTALL file in the source distribution for more
562 information.
563 The undump program was an ancient attempt to speed up Perl program by
565 storing the already-compiled form to disk. This is no longer a viable
566 option, as it only worked on a few architectures, and wasn't a good
567 solution anyway.
568 How can I make my Perl program take less memory?
570 When it comes to time-space tradeoffs, Perl nearly always prefers to
571 throw memory at a problem. Scalars in Perl use more memory than
572 strings in C, arrays take more than that, and hashes use even more.
573 While there's still a lot to be done, recent releases have been
574 addressing these issues. For example, as of 5.004, duplicate hash keys
575 are shared amongst all hashes using them, so require no reallocation.
576 In some cases, using substr() or vec() to simulate arrays can be highly
578 beneficial. For example, an array of a thousand booleans will take at
579 least 20,000 bytes of space, but it can be turned into one 125-byte bit
580 vector--a considerable memory savings. The standard Tie::SubstrHash
581 module can also help for certain types of data structure. If you're
582 working with specialist data structures (matrices, for instance)
583 modules that implement these in C may use less memory than equivalent
584 Perl modules.
585 Another thing to try is learning whether your Perl was compiled with
587 the system malloc or with Perl's builtin malloc. Whichever one it is,
588 try using the other one and see whether this makes a difference.
589 Information about malloc is in the INSTALL file in the source
590 distribution. You can find out whether you are using perl's malloc by
591 typing "perl -V:usemymalloc".
592 Of course, the best way to save memory is to not do anything to waste
594 it in the first place. Good programming practices can go a long way
595 toward this:
596 · Don't slurp!
598 Don't read an entire file into memory if you can process it line by
600 line. Or more concretely, use a loop like this:
601 #
603 # Good Idea
604 #
605 while (<FILE>) {
606 # ...
607 }
608 instead of this:
610 #
612 # Bad Idea
613 #
614 @data = <FILE>;
615 foreach (@data) {
616 # ...
617 }
618 When the files you're processing are small, it doesn't much matter
620 which way you do it, but it makes a huge difference when they start
621 getting larger.
622 · Use map and grep selectively
624 Remember that both map and grep expect a LIST argument, so doing
626 this:
627 @wanted = grep {/pattern/} <FILE>;
629 will cause the entire file to be slurped. For large files, it's
631 better to loop:
632 while (<FILE>) {
634 push(@wanted, $_) if /pattern/;
635 }
636 · Avoid unnecessary quotes and stringification
638 Don't quote large strings unless absolutely necessary:
640 my $copy = "$large_string";
642 makes 2 copies of $large_string (one for $copy and another for the
644 quotes), whereas
645 my $copy = $large_string;
647 only makes one copy.
649 Ditto for stringifying large arrays:
651 {
653 local $, = "\n";
654 print @big_array;
655 }
656 is much more memory-efficient than either
658 print join "\n", @big_array;
660 or
662 {
664 local $" = "\n";
665 print "@big_array";
666 }
667 · Pass by reference
669 Pass arrays and hashes by reference, not by value. For one thing,
671 it's the only way to pass multiple lists or hashes (or both) in a
672 single call/return. It also avoids creating a copy of all the
673 contents. This requires some judgement, however, because any
674 changes will be propagated back to the original data. If you really
675 want to mangle (er, modify) a copy, you'll have to sacrifice the
676 memory needed to make one.
677 · Tie large variables to disk.
679 For "big" data stores (i.e. ones that exceed available memory)
681 consider using one of the DB modules to store it on disk instead of
682 in RAM. This will incur a penalty in access time, but that's
683 probably better than causing your hard disk to thrash due to
684 massive swapping.
685 Is it safe to return a reference to local or lexical data?
687 Yes. Perl's garbage collection system takes care of this so everything
688 works out right.
689 sub makeone {
691 my @a = ( 1 .. 10 );
692 return \@a;
693 }
694 for ( 1 .. 10 ) {
696 push @many, makeone();
697 }
698 print $many[4][5], "\n";
700 print "@many\n";
702 How can I free an array or hash so my program shrinks?
704 (contributed by Michael Carman)
705 You usually can't. Memory allocated to lexicals (i.e. my() variables)
707 cannot be reclaimed or reused even if they go out of scope. It is
708 reserved in case the variables come back into scope. Memory allocated
709 to global variables can be reused (within your program) by using
710 undef() and/or delete().
711 On most operating systems, memory allocated to a program can never be
713 returned to the system. That's why long-running programs sometimes re-
714 exec themselves. Some operating systems (notably, systems that use
715 mmap(2) for allocating large chunks of memory) can reclaim memory that
716 is no longer used, but on such systems, perl must be configured and
717 compiled to use the OS's malloc, not perl's.
718 In general, memory allocation and de-allocation isn't something you can
720 or should be worrying about much in Perl.
721 See also "How can I make my Perl program take less memory?"
723 How can I make my CGI script more efficient?
725 Beyond the normal measures described to make general Perl programs
726 faster or smaller, a CGI program has additional issues. It may be run
727 several times per second. Given that each time it runs it will need to
728 be re-compiled and will often allocate a megabyte or more of system
729 memory, this can be a killer. Compiling into C isn't going to help you
730 because the process start-up overhead is where the bottleneck is.
731 There are two popular ways to avoid this overhead. One solution
733 involves running the Apache HTTP server (available from
734 http://www.apache.org/ ) with either of the mod_perl or mod_fastcgi
735 plugin modules.
736 With mod_perl and the Apache::Registry module (distributed with
738 mod_perl), httpd will run with an embedded Perl interpreter which pre-
739 compiles your script and then executes it within the same address space
740 without forking. The Apache extension also gives Perl access to the
741 internal server API, so modules written in Perl can do just about
742 anything a module written in C can. For more on mod_perl, see
743 http://perl.apache.org/
744 With the FCGI module (from CPAN) and the mod_fastcgi module (available
746 from http://www.fastcgi.com/ ) each of your Perl programs becomes a
747 permanent CGI daemon process.
748 Both of these solutions can have far-reaching effects on your system
750 and on the way you write your CGI programs, so investigate them with
751 care.
752 See
754 http://www.cpan.org/modules/by-category/15_World_Wide_Web_HTML_HTTP_CGI/
755 .
756 How can I hide the source for my Perl program?
758 Delete it. :-) Seriously, there are a number of (mostly unsatisfactory)
759 solutions with varying levels of "security".
760 First of all, however, you can't take away read permission, because the
762 source code has to be readable in order to be compiled and interpreted.
763 (That doesn't mean that a CGI script's source is readable by people on
764 the web, though--only by people with access to the filesystem.) So you
765 have to leave the permissions at the socially friendly 0755 level.
766 Some people regard this as a security problem. If your program does
768 insecure things and relies on people not knowing how to exploit those
769 insecurities, it is not secure. It is often possible for someone to
770 determine the insecure things and exploit them without viewing the
771 source. Security through obscurity, the name for hiding your bugs
772 instead of fixing them, is little security indeed.
773 You can try using encryption via source filters (Starting from Perl 5.8
775 the Filter::Simple and Filter::Util::Call modules are included in the
776 standard distribution), but any decent programmer will be able to
777 decrypt it. You can try using the byte code compiler and interpreter
778 described later in perlfaq3, but the curious might still be able to de-
779 compile it. You can try using the native-code compiler described later,
780 but crackers might be able to disassemble it. These pose varying
781 degrees of difficulty to people wanting to get at your code, but none
782 can definitively conceal it (true of every language, not just Perl).
783 It is very easy to recover the source of Perl programs. You simply
785 feed the program to the perl interpreter and use the modules in the B::
786 hierarchy. The B::Deparse module should be able to defeat most
787 attempts to hide source. Again, this is not unique to Perl.
788 If you're concerned about people profiting from your code, then the
790 bottom line is that nothing but a restrictive license will give you
791 legal security. License your software and pepper it with threatening
792 statements like "This is unpublished proprietary software of XYZ Corp.
793 Your access to it does not give you permission to use it blah blah
794 blah." We are not lawyers, of course, so you should see a lawyer if
795 you want to be sure your license's wording will stand up in court.
796 How can I compile my Perl program into byte code or C?
798 (contributed by brian d foy)
799 In general, you can't do this. There are some things that may work for
801 your situation though. People usually ask this question because they
802 want to distribute their works without giving away the source code, and
803 most solutions trade disk space for convenience. You probably won't
804 see much of a speed increase either, since most solutions simply bundle
805 a Perl interpreter in the final product (but see "How can I make my
806 Perl program run faster?").
807 The Perl Archive Toolkit ( http://par.perl.org/ ) is Perl's analog to
809 Java's JAR. It's freely available and on CPAN (
810 http://search.cpan.org/dist/PAR/ ).
811 There are also some commercial products that may work for you, although
813 you have to buy a license for them.
814 The Perl Dev Kit ( http://www.activestate.com/Products/Perl_Dev_Kit/ )
816 from ActiveState can "Turn your Perl programs into ready-to-run
817 executables for HP-UX, Linux, Solaris and Windows."
818 Perl2Exe ( http://www.indigostar.com/perl2exe.htm ) is a command line
820 program for converting perl scripts to executable files. It targets
821 both Windows and Unix platforms.
822 How can I get "#!perl" to work on [MS-DOS,NT,...]?
824 For OS/2 just use
825 extproc perl -S -your_switches
827 as the first line in "*.cmd" file ("-S" due to a bug in cmd.exe's
829 "extproc" handling). For DOS one should first invent a corresponding
830 batch file and codify it in "ALTERNATE_SHEBANG" (see the dosish.h file
831 in the source distribution for more information).
832 The Win95/NT installation, when using the ActiveState port of Perl,
834 will modify the Registry to associate the ".pl" extension with the perl
835 interpreter. If you install another port, perhaps even building your
836 own Win95/NT Perl from the standard sources by using a Windows port of
837 gcc (e.g., with cygwin or mingw32), then you'll have to modify the
838 Registry yourself. In addition to associating ".pl" with the
839 interpreter, NT people can use: "SET PATHEXT=%PATHEXT%;.PL" to let them
840 run the program "install-linux.pl" merely by typing "install-linux".
841 Under "Classic" MacOS, a perl program will have the appropriate Creator
843 and Type, so that double-clicking them will invoke the MacPerl
844 application. Under Mac OS X, clickable apps can be made from any "#!"
845 script using Wil Sanchez' DropScript utility:
846 http://www.wsanchez.net/software/ .
847 IMPORTANT!: Whatever you do, PLEASE don't get frustrated, and just
849 throw the perl interpreter into your cgi-bin directory, in order to get
850 your programs working for a web server. This is an EXTREMELY big
851 security risk. Take the time to figure out how to do it correctly.
852 Can I write useful Perl programs on the command line?
854 Yes. Read perlrun for more information. Some examples follow. (These
855 assume standard Unix shell quoting rules.)
856 # sum first and last fields
858 perl -lane 'print $F[0] + $F[-1]' *
859 # identify text files
861 perl -le 'for(@ARGV) {print if -f && -T _}' *
862 # remove (most) comments from C program
864 perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c
865 # make file a month younger than today, defeating reaper daemons
867 perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' *
868 # find first unused uid
870 perl -le '$i++ while getpwuid($i); print $i'
871 # display reasonable manpath
873 echo $PATH | perl -nl -072 -e '
874 s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}'
875 OK, the last one was actually an Obfuscated Perl Contest entry. :-)
877 Why don't Perl one-liners work on my DOS/Mac/VMS system?
879 The problem is usually that the command interpreters on those systems
880 have rather different ideas about quoting than the Unix shells under
881 which the one-liners were created. On some systems, you may have to
882 change single-quotes to double ones, which you must NOT do on Unix or
883 Plan9 systems. You might also have to change a single % to a %%.
884 For example:
886 # Unix (including Mac OS X)
888 perl -e 'print "Hello world\n"'
889 # DOS, etc.
891 perl -e "print \"Hello world\n\""
892 # Mac Classic
894 print "Hello world\n"
895 (then Run "Myscript" or Shift-Command-R)
896 # MPW
898 perl -e 'print "Hello world\n"'
899 # VMS
901 perl -e "print ""Hello world\n"""
902 The problem is that none of these examples are reliable: they depend on
904 the command interpreter. Under Unix, the first two often work. Under
905 DOS, it's entirely possible that neither works. If 4DOS was the
906 command shell, you'd probably have better luck like this:
907 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
909 Under the Mac, it depends which environment you are using. The MacPerl
911 shell, or MPW, is much like Unix shells in its support for several
912 quoting variants, except that it makes free use of the Mac's non-ASCII
913 characters as control characters.
914 Using qq(), q(), and qx(), instead of "double quotes", 'single quotes',
916 and `backticks`, may make one-liners easier to write.
917 There is no general solution to all of this. It is a mess.
919 [Some of this answer was contributed by Kenneth Albanowski.]
921 Where can I learn about CGI or Web programming in Perl?
923 For modules, get the CGI or LWP modules from CPAN. For textbooks, see
924 the two especially dedicated to web stuff in the question on books.
925 For problems and questions related to the web, like "Why do I get 500
926 Errors" or "Why doesn't it run from the browser right when it runs fine
927 on the command line", see the troubleshooting guides and references in
928 perlfaq9 or in the CGI MetaFAQ:
929 http://www.perl.org/CGI_MetaFAQ.html
931 Where can I learn about object-oriented Perl programming?
933 A good place to start is perltoot, and you can use perlobj, perlboot,
934 perltoot, perltooc, and perlbot for reference.
935 A good book on OO on Perl is the "Object-Oriented Perl" by Damian
937 Conway from Manning Publications, or "Intermediate Perl" by Randal
938 Schwartz, brian d foy, and Tom Phoenix from O'Reilly Media.
939 Where can I learn about linking C with Perl?
941 If you want to call C from Perl, start with perlxstut, moving on to
942 perlxs, xsubpp, and perlguts. If you want to call Perl from C, then
943 read perlembed, perlcall, and perlguts. Don't forget that you can
944 learn a lot from looking at how the authors of existing extension
945 modules wrote their code and solved their problems.
946 You might not need all the power of XS. The Inline::C module lets you
948 put C code directly in your Perl source. It handles all the magic to
949 make it work. You still have to learn at least some of the perl API but
950 you won't have to deal with the complexity of the XS support files.
951 I've read perlembed, perlguts, etc., but I can't embed perl in my C
953 program; what am I doing wrong?
954 Download the ExtUtils::Embed kit from CPAN and run `make test'. If the
955 tests pass, read the pods again and again and again. If they fail, see
956 perlbug and send a bug report with the output of "make test
957 TEST_VERBOSE=1" along with "perl -V".
958 When I tried to run my script, I got this message. What does it mean?
960 A complete list of Perl's error messages and warnings with explanatory
961 text can be found in perldiag. You can also use the splain program
962 (distributed with Perl) to explain the error messages:
963 perl program 2>diag.out
965 splain [-v] [-p] diag.out
966 or change your program to explain the messages for you:
968 use diagnostics;
970 or
972 use diagnostics -verbose;
974 What's MakeMaker?
976 (contributed by brian d foy)
977 The "ExtUtils::MakeMaker" module, better known simply as "MakeMaker",
979 turns a Perl script, typically called "Makefile.PL", into a Makefile.
980 The Unix tool "make" uses this file to manage dependencies and actions
981 to process and install a Perl distribution.
982
984 Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and other
985 authors as noted. All rights reserved.
986 This documentation is free; you can redistribute it and/or modify it
988 under the same terms as Perl itself.
989 Irrespective of its distribution, all code examples here are in the
991 public domain. You are permitted and encouraged to use this code and
992 any derivatives thereof in your own programs for fun or for profit as
993 you see fit. A simple comment in the code giving credit to the FAQ
994 would be courteous but is not required.
995perl v5.12.4 2011-06-07 PERLFAQ3(1)