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1 =head1 NAME 2 3 perlfaq3 - Programming Tools ($Revision: 10127 $) 4 5 =head1 DESCRIPTION 6 7 This section of the FAQ answers questions related to programmer tools 8 and programming support. 9 10 =head2 How do I do (anything)? 11 12 Have you looked at CPAN (see L<perlfaq2>)? The chances are that 13 someone has already written a module that can solve your problem. 14 Have you read the appropriate manpages? Here's a brief index: 15 16 Basics perldata, perlvar, perlsyn, perlop, perlsub 17 Execution perlrun, perldebug 18 Functions perlfunc 19 Objects perlref, perlmod, perlobj, perltie 20 Data Structures perlref, perllol, perldsc 21 Modules perlmod, perlmodlib, perlsub 22 Regexes perlre, perlfunc, perlop, perllocale 23 Moving to perl5 perltrap, perl 24 Linking w/C perlxstut, perlxs, perlcall, perlguts, perlembed 25 Various http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz 26 (not a man-page but still useful, a collection 27 of various essays on Perl techniques) 28 29 A crude table of contents for the Perl manpage set is found in L<perltoc>. 30 31 =head2 How can I use Perl interactively? 32 33 The typical approach uses the Perl debugger, described in the 34 perldebug(1) manpage, on an "empty" program, like this: 35 36 perl -de 42 37 38 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 43 =head2 Is there a Perl shell? 44 45 The psh (Perl sh) is currently at version 1.8. The Perl Shell is a shell 46 that combines the interactive nature of a Unix shell with the power of 47 Perl. The goal is a full featured shell that behaves as expected for 48 normal shell activity and uses Perl syntax and functionality for 49 control-flow statements and other things. You can get psh at 50 http://sourceforge.net/projects/psh/ . 51 52 Zoidberg is a similar project and provides a shell written in perl, 53 configured in perl and operated in perl. It is intended as a login shell 54 and development environment. It can be found at http://zoidberg.sf.net/ 55 or your local CPAN mirror. 56 57 The Shell.pm module (distributed with Perl) makes Perl try commands 58 which aren't part of the Perl language as shell commands. perlsh from 59 the source distribution is simplistic and uninteresting, but may still 60 be what you want. 61 62 =head2 How do I find which modules are installed on my system? 63 64 You can use the ExtUtils::Installed module to show all installed 65 distributions, although it can take awhile to do its magic. The 66 standard library which comes with Perl just shows up as "Perl" (although 67 you can get those with Module::CoreList). 68 69 use ExtUtils::Installed; 70 71 my $inst = ExtUtils::Installed->new(); 72 my @modules = $inst->modules(); 73 74 If you want a list of all of the Perl module filenames, you 75 can use File::Find::Rule. 76 77 use File::Find::Rule; 78 79 my @files = File::Find::Rule->file()->name( '*.pm' )->in( @INC ); 80 81 If you do not have that module, you can do the same thing 82 with File::Find which is part of the standard library. 83 84 use File::Find; 85 my @files; 86 87 find( 88 sub { 89 push @files, $File::Find::name 90 if -f $File::Find::name && /\.pm$/ 91 }, 92 93 @INC 94 ); 95 96 print join "\n", @files; 97 98 If you simply need to quickly check to see if a module is 99 available, you can check for its documentation. If you can 100 read the documentation the module is most likely installed. 101 If you cannot read the documentation, the module might not 102 have any (in rare cases). 103 104 prompt% perldoc Module::Name 105 106 You can also try to include the module in a one-liner to see if 107 perl finds it. 108 109 perl -MModule::Name -e1 110 111 =head2 How do I debug my Perl programs? 112 113 (contributed by brian d foy) 114 115 Before you do anything else, you can help yourself by ensuring that 116 you let Perl tell you about problem areas in your code. By turning 117 on warnings and strictures, you can head off many problems before 118 they get too big. You can find out more about these in L<strict> 119 and L<warnings>. 120 121 #!/usr/bin/perl 122 use strict; 123 use warnings; 124 125 Beyond that, the simplest debugger is the C<print> function. Use it 126 to look at values as you run your program: 127 128 print STDERR "The value is [$value]\n"; 129 130 The C<Data::Dumper> module can pretty-print Perl data structures: 131 132 use Data::Dumper qw( Dumper ); 133 print STDERR "The hash is " . Dumper( \%hash ) . "\n"; 134 135 Perl comes with an interactive debugger, which you can start with the 136 C<-d> switch. It's fully explained in L<perldebug>. 137 138 If you'd like a graphical user interface and you have Tk, you can use 139 C<ptkdb>. It's on CPAN and available for free. 140 141 If you need something much more sophisticated and controllable, Leon 142 Brocard's Devel::ebug (which you can call with the -D switch as -Debug) 143 gives you the programmatic hooks into everything you need to write your 144 own (without too much pain and suffering). 145 146 You can also use a commercial debugger such as Affrus (Mac OS X), Komodo 147 from Activestate (Windows and Mac OS X), or EPIC (most platforms). 148 149 =head2 How do I profile my Perl programs? 150 151 You should get the Devel::DProf module from the standard distribution 152 (or separately on CPAN) and also use Benchmark.pm from the standard 153 distribution. The Benchmark module lets you time specific portions of 154 your code, while Devel::DProf gives detailed breakdowns of where your 155 code spends its time. 156 157 Here's a sample use of Benchmark: 158 159 use Benchmark; 160 161 @junk = `cat /etc/motd`; 162 $count = 10_000; 163 164 timethese($count, { 165 'map' => sub { my @a = @junk; 166 map { s/a/b/ } @a; 167 return @a }, 168 'for' => sub { my @a = @junk; 169 for (@a) { s/a/b/ }; 170 return @a }, 171 }); 172 173 This is what it prints (on one machine--your results will be dependent 174 on your hardware, operating system, and the load on your machine): 175 176 Benchmark: timing 10000 iterations of for, map... 177 for: 4 secs ( 3.97 usr 0.01 sys = 3.98 cpu) 178 map: 6 secs ( 4.97 usr 0.00 sys = 4.97 cpu) 179 180 Be aware that a good benchmark is very hard to write. It only tests the 181 data you give it and proves little about the differing complexities 182 of contrasting algorithms. 183 184 =head2 How do I cross-reference my Perl programs? 185 186 The B::Xref module can be used to generate cross-reference reports 187 for Perl programs. 188 189 perl -MO=Xref[,OPTIONS] scriptname.plx 190 191 =head2 Is there a pretty-printer (formatter) for Perl? 192 193 Perltidy is a Perl script which indents and reformats Perl scripts 194 to make them easier to read by trying to follow the rules of the 195 L<perlstyle>. If you write Perl scripts, or spend much time reading 196 them, you will probably find it useful. It is available at 197 http://perltidy.sourceforge.net 198 199 Of course, if you simply follow the guidelines in L<perlstyle>, 200 you shouldn't need to reformat. The habit of formatting your code 201 as you write it will help prevent bugs. Your editor can and should 202 help you with this. The perl-mode or newer cperl-mode for emacs 203 can provide remarkable amounts of help with most (but not all) 204 code, and even less programmable editors can provide significant 205 assistance. Tom Christiansen and many other VI users swear by 206 the following settings in vi and its clones: 207 208 set ai sw=4 209 map! ^O {^M}^[O^T 210 211 Put that in your F<.exrc> file (replacing the caret characters 212 with control characters) and away you go. In insert mode, ^T is 213 for indenting, ^D is for undenting, and ^O is for blockdenting--as 214 it were. A more complete example, with comments, can be found at 215 http://www.cpan.org/authors/id/TOMC/scripts/toms.exrc.gz 216 217 The a2ps http://www-inf.enst.fr/%7Edemaille/a2ps/black+white.ps.gz does 218 lots of things related to generating nicely printed output of 219 documents. 220 221 =head2 Is there a ctags for Perl? 222 223 (contributed by brian d foy) 224 225 Ctags uses an index to quickly find things in source code, and many 226 popular editors support ctags for several different languages, 227 including Perl. 228 229 Exuberent ctags supports Perl: http://ctags.sourceforge.net/ 230 231 You might also try pltags: http://www.mscha.com/pltags.zip 232 233 =head2 Is there an IDE or Windows Perl Editor? 234 235 Perl programs are just plain text, so any editor will do. 236 237 If you're on Unix, you already have an IDE--Unix itself. The UNIX 238 philosophy is the philosophy of several small tools that each do one 239 thing and do it well. It's like a carpenter's toolbox. 240 241 If you want an IDE, check the following (in alphabetical order, not 242 order of preference): 243 244 =over 4 245 246 =item Eclipse 247 248 http://e-p-i-c.sf.net/ 249 250 The Eclipse Perl Integration Project integrates Perl 251 editing/debugging with Eclipse. 252 253 =item Enginsite 254 255 http://www.enginsite.com/ 256 257 Perl Editor by EngInSite is a complete integrated development 258 environment (IDE) for creating, testing, and debugging Perl scripts; 259 the tool runs on Windows 9x/NT/2000/XP or later. 260 261 =item Komodo 262 263 http://www.ActiveState.com/Products/Komodo/ 264 265 ActiveState's cross-platform (as of October 2004, that's Windows, Linux, 266 and Solaris), multi-language IDE has Perl support, including a regular expression 267 debugger and remote debugging. 268 269 =item Open Perl IDE 270 271 http://open-perl-ide.sourceforge.net/ 272 273 Open Perl IDE is an integrated development environment for writing 274 and debugging Perl scripts with ActiveState's ActivePerl distribution 275 under Windows 95/98/NT/2000. 276 277 =item OptiPerl 278 279 http://www.optiperl.com/ 280 281 OptiPerl is a Windows IDE with simulated CGI environment, including 282 debugger and syntax highlighting editor. 283 284 =item PerlBuilder 285 286 http://www.solutionsoft.com/perl.htm 287 288 PerlBuidler is an integrated development environment for Windows that 289 supports Perl development. 290 291 =item visiPerl+ 292 293 http://helpconsulting.net/visiperl/ 294 295 From Help Consulting, for Windows. 296 297 =item Visual Perl 298 299 http://www.activestate.com/Products/Visual_Perl/ 300 301 Visual Perl is a Visual Studio.NET plug-in from ActiveState. 302 303 =item Zeus 304 305 http://www.zeusedit.com/lookmain.html 306 307 Zeus for Window is another Win32 multi-language editor/IDE 308 that comes with support for Perl: 309 310 =back 311 312 For editors: if you're on Unix you probably have vi or a vi clone 313 already, and possibly an emacs too, so you may not need to download 314 anything. In any emacs the cperl-mode (M-x cperl-mode) gives you 315 perhaps the best available Perl editing mode in any editor. 316 317 If you are using Windows, you can use any editor that lets you work 318 with plain text, such as NotePad or WordPad. Word processors, such as 319 Microsoft Word or WordPerfect, typically do not work since they insert 320 all sorts of behind-the-scenes information, although some allow you to 321 save files as "Text Only". You can also download text editors designed 322 specifically for programming, such as Textpad ( 323 http://www.textpad.com/ ) and UltraEdit ( http://www.ultraedit.com/ ), 324 among others. 325 326 If you are using MacOS, the same concerns apply. MacPerl (for Classic 327 environments) comes with a simple editor. Popular external editors are 328 BBEdit ( http://www.bbedit.com/ ) or Alpha ( 329 http://www.his.com/~jguyer/Alpha/Alpha8.html ). MacOS X users can use 330 Unix editors as well. 331 332 =over 4 333 334 =item GNU Emacs 335 336 http://www.gnu.org/software/emacs/windows/ntemacs.html 337 338 =item MicroEMACS 339 340 http://www.microemacs.de/ 341 342 =item XEmacs 343 344 http://www.xemacs.org/Download/index.html 345 346 =item Jed 347 348 http://space.mit.edu/~davis/jed/ 349 350 =back 351 352 or a vi clone such as 353 354 =over 4 355 356 =item Elvis 357 358 ftp://ftp.cs.pdx.edu/pub/elvis/ http://www.fh-wedel.de/elvis/ 359 360 =item Vile 361 362 http://dickey.his.com/vile/vile.html 363 364 =item Vim 365 366 http://www.vim.org/ 367 368 =back 369 370 For vi lovers in general, Windows or elsewhere: 371 372 http://www.thomer.com/thomer/vi/vi.html 373 374 nvi ( http://www.bostic.com/vi/ , available from CPAN in src/misc/) is 375 yet another vi clone, unfortunately not available for Windows, but in 376 UNIX platforms you might be interested in trying it out, firstly because 377 strictly speaking it is not a vi clone, it is the real vi, or the new 378 incarnation of it, and secondly because you can embed Perl inside it 379 to use Perl as the scripting language. nvi is not alone in this, 380 though: at least also vim and vile offer an embedded Perl. 381 382 The following are Win32 multilanguage editor/IDESs that support Perl: 383 384 =over 4 385 386 =item Codewright 387 388 http://www.borland.com/codewright/ 389 390 =item MultiEdit 391 392 http://www.MultiEdit.com/ 393 394 =item SlickEdit 395 396 http://www.slickedit.com/ 397 398 =back 399 400 There is also a toyedit Text widget based editor written in Perl 401 that is distributed with the Tk module on CPAN. The ptkdb 402 ( http://ptkdb.sourceforge.net/ ) is a Perl/tk based debugger that 403 acts as a development environment of sorts. Perl Composer 404 ( http://perlcomposer.sourceforge.net/ ) is an IDE for Perl/Tk 405 GUI creation. 406 407 In addition to an editor/IDE you might be interested in a more 408 powerful shell environment for Win32. Your options include 409 410 =over 4 411 412 =item Bash 413 414 from the Cygwin package ( http://sources.redhat.com/cygwin/ ) 415 416 =item Ksh 417 418 from the MKS Toolkit ( http://www.mks.com/ ), or the Bourne shell of 419 the U/WIN environment ( http://www.research.att.com/sw/tools/uwin/ ) 420 421 =item Tcsh 422 423 ftp://ftp.astron.com/pub/tcsh/ , see also 424 http://www.primate.wisc.edu/software/csh-tcsh-book/ 425 426 =item Zsh 427 428 http://www.zsh.org/ 429 430 =back 431 432 MKS and U/WIN are commercial (U/WIN is free for educational and 433 research purposes), Cygwin is covered by the GNU Public License (but 434 that shouldn't matter for Perl use). The Cygwin, MKS, and U/WIN all 435 contain (in addition to the shells) a comprehensive set of standard 436 UNIX toolkit utilities. 437 438 If you're transferring text files between Unix and Windows using FTP 439 be sure to transfer them in ASCII mode so the ends of lines are 440 appropriately converted. 441 442 On Mac OS the MacPerl Application comes with a simple 32k text editor 443 that behaves like a rudimentary IDE. In contrast to the MacPerl Application 444 the MPW Perl tool can make use of the MPW Shell itself as an editor (with 445 no 32k limit). 446 447 =over 4 448 449 =item Affrus 450 451 is a full Perl development environment with full debugger support 452 ( http://www.latenightsw.com ). 453 454 =item Alpha 455 456 is an editor, written and extensible in Tcl, that nonetheless has 457 built in support for several popular markup and programming languages 458 including Perl and HTML ( http://www.his.com/~jguyer/Alpha/Alpha8.html ). 459 460 =item BBEdit and BBEdit Lite 461 462 are text editors for Mac OS that have a Perl sensitivity mode 463 ( http://web.barebones.com/ ). 464 465 466 =back 467 468 Pepper and Pe are programming language sensitive text editors for Mac 469 OS X and BeOS respectively ( http://www.hekkelman.com/ ). 470 471 =head2 Where can I get Perl macros for vi? 472 473 For a complete version of Tom Christiansen's vi configuration file, 474 see http://www.cpan.org/authors/Tom_Christiansen/scripts/toms.exrc.gz , 475 the standard benchmark file for vi emulators. The file runs best with nvi, 476 the current version of vi out of Berkeley, which incidentally can be built 477 with an embedded Perl interpreter--see http://www.cpan.org/src/misc/ . 478 479 =head2 Where can I get perl-mode for emacs? 480 481 Since Emacs version 19 patchlevel 22 or so, there have been both a 482 perl-mode.el and support for the Perl debugger built in. These should 483 come with the standard Emacs 19 distribution. 484 485 In the Perl source directory, you'll find a directory called "emacs", 486 which contains a cperl-mode that color-codes keywords, provides 487 context-sensitive help, and other nifty things. 488 489 Note that the perl-mode of emacs will have fits with C<"main'foo"> 490 (single quote), and mess up the indentation and highlighting. You 491 are probably using C<"main::foo"> in new Perl code anyway, so this 492 shouldn't be an issue. 493 494 =head2 How can I use curses with Perl? 495 496 The Curses module from CPAN provides a dynamically loadable object 497 module interface to a curses library. A small demo can be found at the 498 directory http://www.cpan.org/authors/Tom_Christiansen/scripts/rep.gz ; 499 this program repeats a command and updates the screen as needed, rendering 500 B<rep ps axu> similar to B<top>. 501 502 =head2 How can I write a GUI (X, Tk, Gtk, etc.) in Perl? 503 X<GUI> X<Tk> X<Wx> X<WxWidgets> X<Gtk> X<Gtk2> X<CamelBones> X<Qt> 504 505 (contributed by Ben Morrow) 506 507 There are a number of modules which let you write GUIs in Perl. Most 508 GUI toolkits have a perl interface: an incomplete list follows. 509 510 =over 4 511 512 =item Tk 513 514 This works under Unix and Windows, and the current version doesn't 515 look half as bad under Windows as it used to. Some of the gui elements 516 still don't 'feel' quite right, though. The interface is very natural 517 and 'perlish', making it easy to use in small scripts that just need a 518 simple gui. It hasn't been updated in a while. 519 520 =item Wx 521 522 This is a Perl binding for the cross-platform wxWidgets toolkit 523 L<http://www.wxwidgets.org>. It works under Unix, Win32 and Mac OS X, 524 using native widgets (Gtk under Unix). The interface follows the C++ 525 interface closely, but the documentation is a little sparse for someone 526 who doesn't know the library, mostly just referring you to the C++ 527 documentation. 528 529 =item Gtk and Gtk2 530 531 These are Perl bindings for the Gtk toolkit L<http://www.gtk.org>. The 532 interface changed significantly between versions 1 and 2 so they have 533 separate Perl modules. It runs under Unix, Win32 and Mac OS X (currently 534 it requires an X server on Mac OS, but a 'native' port is underway), and 535 the widgets look the same on every plaform: i.e., they don't match the 536 native widgets. As with Wx, the Perl bindings follow the C API closely, 537 and the documentation requires you to read the C documentation to 538 understand it. 539 540 =item Win32::GUI 541 542 This provides access to most of the Win32 GUI widgets from Perl. 543 Obviously, it only runs under Win32, and uses native widgets. The Perl 544 interface doesn't really follow the C interface: it's been made more 545 Perlish, and the documentation is pretty good. More advanced stuff may 546 require familiarity with the C Win32 APIs, or reference to MSDN. 547 548 =item CamelBones 549 550 CamelBones L<http://camelbones.sourceforge.net> is a Perl interface to 551 Mac OS X's Cocoa GUI toolkit, and as such can be used to produce native 552 GUIs on Mac OS X. It's not on CPAN, as it requires frameworks that 553 CPAN.pm doesn't know how to install, but installation is via the 554 standard OSX package installer. The Perl API is, again, very close to 555 the ObjC API it's wrapping, and the documentation just tells you how to 556 translate from one to the other. 557 558 =item Qt 559 560 There is a Perl interface to TrollTech's Qt toolkit, but it does not 561 appear to be maintained. 562 563 =item Athena 564 565 Sx is an interface to the Athena widget set which comes with X, but 566 again it appears not to be much used nowadays. 567 568 =back 569 570 =head2 How can I make my Perl program run faster? 571 572 The best way to do this is to come up with a better algorithm. This 573 can often make a dramatic difference. Jon Bentley's book 574 I<Programming Pearls> (that's not a misspelling!) has some good tips 575 on optimization, too. Advice on benchmarking boils down to: benchmark 576 and profile to make sure you're optimizing the right part, look for 577 better algorithms instead of microtuning your code, and when all else 578 fails consider just buying faster hardware. You will probably want to 579 read the answer to the earlier question "How do I profile my Perl 580 programs?" if you haven't done so already. 581 582 A different approach is to autoload seldom-used Perl code. See the 583 AutoSplit and AutoLoader modules in the standard distribution for 584 that. Or you could locate the bottleneck and think about writing just 585 that part in C, the way we used to take bottlenecks in C code and 586 write them in assembler. Similar to rewriting in C, modules that have 587 critical sections can be written in C (for instance, the PDL module 588 from CPAN). 589 590 If you're currently linking your perl executable to a shared 591 I<libc.so>, you can often gain a 10-25% performance benefit by 592 rebuilding it to link with a static libc.a instead. This will make a 593 bigger perl executable, but your Perl programs (and programmers) may 594 thank you for it. See the F<INSTALL> file in the source distribution 595 for more information. 596 597 The undump program was an ancient attempt to speed up Perl program by 598 storing the already-compiled form to disk. This is no longer a viable 599 option, as it only worked on a few architectures, and wasn't a good 600 solution anyway. 601 602 =head2 How can I make my Perl program take less memory? 603 604 When it comes to time-space tradeoffs, Perl nearly always prefers to 605 throw memory at a problem. Scalars in Perl use more memory than 606 strings in C, arrays take more than that, and hashes use even more. While 607 there's still a lot to be done, recent releases have been addressing 608 these issues. For example, as of 5.004, duplicate hash keys are 609 shared amongst all hashes using them, so require no reallocation. 610 611 In some cases, using substr() or vec() to simulate arrays can be 612 highly beneficial. For example, an array of a thousand booleans will 613 take at least 20,000 bytes of space, but it can be turned into one 614 125-byte bit vector--a considerable memory savings. The standard 615 Tie::SubstrHash module can also help for certain types of data 616 structure. If you're working with specialist data structures 617 (matrices, for instance) modules that implement these in C may use 618 less memory than equivalent Perl modules. 619 620 Another thing to try is learning whether your Perl was compiled with 621 the system malloc or with Perl's builtin malloc. Whichever one it 622 is, try using the other one and see whether this makes a difference. 623 Information about malloc is in the F<INSTALL> file in the source 624 distribution. You can find out whether you are using perl's malloc by 625 typing C<perl -V:usemymalloc>. 626 627 Of course, the best way to save memory is to not do anything to waste 628 it in the first place. Good programming practices can go a long way 629 toward this: 630 631 =over 4 632 633 =item * Don't slurp! 634 635 Don't read an entire file into memory if you can process it line 636 by line. Or more concretely, use a loop like this: 637 638 # 639 # Good Idea 640 # 641 while (<FILE>) { 642 # ... 643 } 644 645 instead of this: 646 647 # 648 # Bad Idea 649 # 650 @data = <FILE>; 651 foreach (@data) { 652 # ... 653 } 654 655 When the files you're processing are small, it doesn't much matter which 656 way you do it, but it makes a huge difference when they start getting 657 larger. 658 659 =item * Use map and grep selectively 660 661 Remember that both map and grep expect a LIST argument, so doing this: 662 663 @wanted = grep {/pattern/} <FILE>; 664 665 will cause the entire file to be slurped. For large files, it's better 666 to loop: 667 668 while (<FILE>) { 669 push(@wanted, $_) if /pattern/; 670 } 671 672 =item * Avoid unnecessary quotes and stringification 673 674 Don't quote large strings unless absolutely necessary: 675 676 my $copy = "$large_string"; 677 678 makes 2 copies of $large_string (one for $copy and another for the 679 quotes), whereas 680 681 my $copy = $large_string; 682 683 only makes one copy. 684 685 Ditto for stringifying large arrays: 686 687 { 688 local $, = "\n"; 689 print @big_array; 690 } 691 692 is much more memory-efficient than either 693 694 print join "\n", @big_array; 695 696 or 697 698 { 699 local $" = "\n"; 700 print "@big_array"; 701 } 702 703 704 =item * Pass by reference 705 706 Pass arrays and hashes by reference, not by value. For one thing, it's 707 the only way to pass multiple lists or hashes (or both) in a single 708 call/return. It also avoids creating a copy of all the contents. This 709 requires some judgement, however, because any changes will be propagated 710 back to the original data. If you really want to mangle (er, modify) a 711 copy, you'll have to sacrifice the memory needed to make one. 712 713 =item * Tie large variables to disk. 714 715 For "big" data stores (i.e. ones that exceed available memory) consider 716 using one of the DB modules to store it on disk instead of in RAM. This 717 will incur a penalty in access time, but that's probably better than 718 causing your hard disk to thrash due to massive swapping. 719 720 =back 721 722 =head2 Is it safe to return a reference to local or lexical data? 723 724 Yes. Perl's garbage collection system takes care of this so 725 everything works out right. 726 727 sub makeone { 728 my @a = ( 1 .. 10 ); 729 return \@a; 730 } 731 732 for ( 1 .. 10 ) { 733 push @many, makeone(); 734 } 735 736 print $many[4][5], "\n"; 737 738 print "@many\n"; 739 740 =head2 How can I free an array or hash so my program shrinks? 741 742 (contributed by Michael Carman) 743 744 You usually can't. Memory allocated to lexicals (i.e. my() variables) 745 cannot be reclaimed or reused even if they go out of scope. It is 746 reserved in case the variables come back into scope. Memory allocated 747 to global variables can be reused (within your program) by using 748 undef()ing and/or delete(). 749 750 On most operating systems, memory allocated to a program can never be 751 returned to the system. That's why long-running programs sometimes re- 752 exec themselves. Some operating systems (notably, systems that use 753 mmap(2) for allocating large chunks of memory) can reclaim memory that 754 is no longer used, but on such systems, perl must be configured and 755 compiled to use the OS's malloc, not perl's. 756 757 In general, memory allocation and de-allocation isn't something you can 758 or should be worrying about much in Perl. 759 760 See also "How can I make my Perl program take less memory?" 761 762 =head2 How can I make my CGI script more efficient? 763 764 Beyond the normal measures described to make general Perl programs 765 faster or smaller, a CGI program has additional issues. It may be run 766 several times per second. Given that each time it runs it will need 767 to be re-compiled and will often allocate a megabyte or more of system 768 memory, this can be a killer. Compiling into C B<isn't going to help 769 you> because the process start-up overhead is where the bottleneck is. 770 771 There are two popular ways to avoid this overhead. One solution 772 involves running the Apache HTTP server (available from 773 http://www.apache.org/ ) with either of the mod_perl or mod_fastcgi 774 plugin modules. 775 776 With mod_perl and the Apache::Registry module (distributed with 777 mod_perl), httpd will run with an embedded Perl interpreter which 778 pre-compiles your script and then executes it within the same address 779 space without forking. The Apache extension also gives Perl access to 780 the internal server API, so modules written in Perl can do just about 781 anything a module written in C can. For more on mod_perl, see 782 http://perl.apache.org/ 783 784 With the FCGI module (from CPAN) and the mod_fastcgi 785 module (available from http://www.fastcgi.com/ ) each of your Perl 786 programs becomes a permanent CGI daemon process. 787 788 Both of these solutions can have far-reaching effects on your system 789 and on the way you write your CGI programs, so investigate them with 790 care. 791 792 See http://www.cpan.org/modules/by-category/15_World_Wide_Web_HTML_HTTP_CGI/ . 793 794 =head2 How can I hide the source for my Perl program? 795 796 Delete it. :-) Seriously, there are a number of (mostly 797 unsatisfactory) solutions with varying levels of "security". 798 799 First of all, however, you I<can't> take away read permission, because 800 the source code has to be readable in order to be compiled and 801 interpreted. (That doesn't mean that a CGI script's source is 802 readable by people on the web, though--only by people with access to 803 the filesystem.) So you have to leave the permissions at the socially 804 friendly 0755 level. 805 806 Some people regard this as a security problem. If your program does 807 insecure things and relies on people not knowing how to exploit those 808 insecurities, it is not secure. It is often possible for someone to 809 determine the insecure things and exploit them without viewing the 810 source. Security through obscurity, the name for hiding your bugs 811 instead of fixing them, is little security indeed. 812 813 You can try using encryption via source filters (Starting from Perl 814 5.8 the Filter::Simple and Filter::Util::Call modules are included in 815 the standard distribution), but any decent programmer will be able to 816 decrypt it. You can try using the byte code compiler and interpreter 817 described later in L<perlfaq3>, but the curious might still be able to 818 de-compile it. You can try using the native-code compiler described 819 later, but crackers might be able to disassemble it. These pose 820 varying degrees of difficulty to people wanting to get at your code, 821 but none can definitively conceal it (true of every language, not just 822 Perl). 823 824 It is very easy to recover the source of Perl programs. You simply 825 feed the program to the perl interpreter and use the modules in 826 the B:: hierarchy. The B::Deparse module should be able to 827 defeat most attempts to hide source. Again, this is not 828 unique to Perl. 829 830 If you're concerned about people profiting from your code, then the 831 bottom line is that nothing but a restrictive license will give you 832 legal security. License your software and pepper it with threatening 833 statements like "This is unpublished proprietary software of XYZ Corp. 834 Your access to it does not give you permission to use it blah blah 835 blah." We are not lawyers, of course, so you should see a lawyer if 836 you want to be sure your license's wording will stand up in court. 837 838 =head2 How can I compile my Perl program into byte code or C? 839 840 (contributed by brian d foy) 841 842 In general, you can't do this. There are some things that may work 843 for your situation though. People usually ask this question 844 because they want to distribute their works without giving away 845 the source code, and most solutions trade disk space for convenience. 846 You probably won't see much of a speed increase either, since most 847 solutions simply bundle a Perl interpreter in the final product 848 (but see L<How can I make my Perl program run faster?>). 849 850 The Perl Archive Toolkit ( http://par.perl.org/ ) is Perl's 851 analog to Java's JAR. It's freely available and on CPAN ( 852 http://search.cpan.org/dist/PAR/ ). 853 854 There are also some commercial products that may work for you, although 855 you have to buy a license for them. 856 857 The Perl Dev Kit ( http://www.activestate.com/Products/Perl_Dev_Kit/ ) 858 from ActiveState can "Turn your Perl programs into ready-to-run 859 executables for HP-UX, Linux, Solaris and Windows." 860 861 Perl2Exe ( http://www.indigostar.com/perl2exe.htm ) is a command line 862 program for converting perl scripts to executable files. It targets both 863 Windows and unix platforms. 864 865 =head2 How can I get C<#!perl> to work on [MS-DOS,NT,...]? 866 867 For OS/2 just use 868 869 extproc perl -S -your_switches 870 871 as the first line in C<*.cmd> file (C<-S> due to a bug in cmd.exe's 872 "extproc" handling). For DOS one should first invent a corresponding 873 batch file and codify it in C<ALTERNATE_SHEBANG> (see the 874 F<dosish.h> file in the source distribution for more information). 875 876 The Win95/NT installation, when using the ActiveState port of Perl, 877 will modify the Registry to associate the C<.pl> extension with the 878 perl interpreter. If you install another port, perhaps even building 879 your own Win95/NT Perl from the standard sources by using a Windows port 880 of gcc (e.g., with cygwin or mingw32), then you'll have to modify 881 the Registry yourself. In addition to associating C<.pl> with the 882 interpreter, NT people can use: C<SET PATHEXT=%PATHEXT%;.PL> to let them 883 run the program C<install-linux.pl> merely by typing C<install-linux>. 884 885 Under "Classic" MacOS, a perl program will have the appropriate Creator and 886 Type, so that double-clicking them will invoke the MacPerl application. 887 Under Mac OS X, clickable apps can be made from any C<#!> script using Wil 888 Sanchez' DropScript utility: http://www.wsanchez.net/software/ . 889 890 I<IMPORTANT!>: Whatever you do, PLEASE don't get frustrated, and just 891 throw the perl interpreter into your cgi-bin directory, in order to 892 get your programs working for a web server. This is an EXTREMELY big 893 security risk. Take the time to figure out how to do it correctly. 894 895 =head2 Can I write useful Perl programs on the command line? 896 897 Yes. Read L<perlrun> for more information. Some examples follow. 898 (These assume standard Unix shell quoting rules.) 899 900 # sum first and last fields 901 perl -lane 'print $F[0] + $F[-1]' * 902 903 # identify text files 904 perl -le 'for(@ARGV) {print if -f && -T _}' * 905 906 # remove (most) comments from C program 907 perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c 908 909 # make file a month younger than today, defeating reaper daemons 910 perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' * 911 912 # find first unused uid 913 perl -le '$i++ while getpwuid($i); print $i' 914 915 # display reasonable manpath 916 echo $PATH | perl -nl -072 -e ' 917 s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}' 918 919 OK, the last one was actually an Obfuscated Perl Contest entry. :-) 920 921 =head2 Why don't Perl one-liners work on my DOS/Mac/VMS system? 922 923 The problem is usually that the command interpreters on those systems 924 have rather different ideas about quoting than the Unix shells under 925 which the one-liners were created. On some systems, you may have to 926 change single-quotes to double ones, which you must I<NOT> do on Unix 927 or Plan9 systems. You might also have to change a single % to a %%. 928 929 For example: 930 931 # Unix (including Mac OS X) 932 perl -e 'print "Hello world\n"' 933 934 # DOS, etc. 935 perl -e "print \"Hello world\n\"" 936 937 # Mac Classic 938 print "Hello world\n" 939 (then Run "Myscript" or Shift-Command-R) 940 941 # MPW 942 perl -e 'print "Hello world\n"' 943 944 # VMS 945 perl -e "print ""Hello world\n""" 946 947 The problem is that none of these examples are reliable: they depend on the 948 command interpreter. Under Unix, the first two often work. Under DOS, 949 it's entirely possible that neither works. If 4DOS was the command shell, 950 you'd probably have better luck like this: 951 952 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>"" 953 954 Under the Mac, it depends which environment you are using. The MacPerl 955 shell, or MPW, is much like Unix shells in its support for several 956 quoting variants, except that it makes free use of the Mac's non-ASCII 957 characters as control characters. 958 959 Using qq(), q(), and qx(), instead of "double quotes", 'single 960 quotes', and `backticks`, may make one-liners easier to write. 961 962 There is no general solution to all of this. It is a mess. 963 964 [Some of this answer was contributed by Kenneth Albanowski.] 965 966 =head2 Where can I learn about CGI or Web programming in Perl? 967 968 For modules, get the CGI or LWP modules from CPAN. For textbooks, 969 see the two especially dedicated to web stuff in the question on 970 books. For problems and questions related to the web, like "Why 971 do I get 500 Errors" or "Why doesn't it run from the browser right 972 when it runs fine on the command line", see the troubleshooting 973 guides and references in L<perlfaq9> or in the CGI MetaFAQ: 974 975 http://www.perl.org/CGI_MetaFAQ.html 976 977 =head2 Where can I learn about object-oriented Perl programming? 978 979 A good place to start is L<perltoot>, and you can use L<perlobj>, 980 L<perlboot>, L<perltoot>, L<perltooc>, and L<perlbot> for reference. 981 982 A good book on OO on Perl is the "Object-Oriented Perl" 983 by Damian Conway from Manning Publications, or "Intermediate Perl" 984 by Randal Schwartz, brian d foy, and Tom Phoenix from O'Reilly Media. 985 986 =head2 Where can I learn about linking C with Perl? 987 988 If you want to call C from Perl, start with L<perlxstut>, 989 moving on to L<perlxs>, L<xsubpp>, and L<perlguts>. If you want to 990 call Perl from C, then read L<perlembed>, L<perlcall>, and 991 L<perlguts>. Don't forget that you can learn a lot from looking at 992 how the authors of existing extension modules wrote their code and 993 solved their problems. 994 995 You might not need all the power of XS. The Inline::C module lets 996 you put C code directly in your Perl source. It handles all the 997 magic to make it work. You still have to learn at least some of 998 the perl API but you won't have to deal with the complexity of the 999 XS support files. 1000 1001 =head2 I've read perlembed, perlguts, etc., but I can't embed perl in my C program; what am I doing wrong? 1002 1003 Download the ExtUtils::Embed kit from CPAN and run `make test'. If 1004 the tests pass, read the pods again and again and again. If they 1005 fail, see L<perlbug> and send a bug report with the output of 1006 C<make test TEST_VERBOSE=1> along with C<perl -V>. 1007 1008 =head2 When I tried to run my script, I got this message. What does it mean? 1009 1010 A complete list of Perl's error messages and warnings with explanatory 1011 text can be found in L<perldiag>. You can also use the splain program 1012 (distributed with Perl) to explain the error messages: 1013 1014 perl program 2>diag.out 1015 splain [-v] [-p] diag.out 1016 1017 or change your program to explain the messages for you: 1018 1019 use diagnostics; 1020 1021 or 1022 1023 use diagnostics -verbose; 1024 1025 =head2 What's MakeMaker? 1026 1027 (contributed by brian d foy) 1028 1029 The C<ExtUtils::MakeMaker> module, better known simply as "MakeMaker", 1030 turns a Perl script, typically called C<Makefile.PL>, into a Makefile. 1031 The unix tool C<make> uses this file to manage dependencies and actions 1032 to process and install a Perl distribution. 1033 1034 =head1 REVISION 1035 1036 Revision: $Revision: 10127 $ 1037 1038 Date: $Date: 2007-10-27 21:40:20 +0200 (Sat, 27 Oct 2007) $ 1039 1040 See L<perlfaq> for source control details and availability. 1041 1042 =head1 AUTHOR AND COPYRIGHT 1043 1044 Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and 1045 other authors as noted. All rights reserved. 1046 1047 This documentation is free; you can redistribute it and/or modify it 1048 under the same terms as Perl itself. 1049 1050 Irrespective of its distribution, all code examples here are in the public 1051 domain. You are permitted and encouraged to use this code and any 1052 derivatives thereof in your own programs for fun or for profit as you 1053 see fit. A simple comment in the code giving credit to the FAQ would 1054 be courteous but is not required.
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