| 1 | =head1 NAME |
| 2 | |
| 3 | perlfaq3 - Programming Tools ($Revision: 1.48 $, $Date: 2005/04/22 19:04:48 $) |
| 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 |
| 46 | shell that combines the interactive nature of a Unix shell with the |
| 47 | power of Perl. The goal is a full featured shell that behaves as |
| 48 | expected for normal shell activity and uses Perl syntax and |
| 49 | functionality for control-flow statements and other things. |
| 50 | You can get psh at http://www.focusresearch.com/gregor/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 |
| 59 | from the source distribution is simplistic and uninteresting, but |
| 60 | may still 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 |
| 65 | installed distributions, although it can take awhile to do |
| 66 | its magic. The standard library which comes with Perl just |
| 67 | shows up as "Perl" (although you can get those with |
| 68 | Module::CoreList). |
| 69 | |
| 70 | use ExtUtils::Installed; |
| 71 | |
| 72 | my $inst = ExtUtils::Installed->new(); |
| 73 | my @modules = $inst->modules(); |
| 74 | |
| 75 | If you want a list of all of the Perl module filenames, you |
| 76 | can use File::Find::Rule. |
| 77 | |
| 78 | use File::Find::Rule; |
| 79 | |
| 80 | my @files = File::Find::Rule->file()->name( '*.pm' )->in( @INC ); |
| 81 | |
| 82 | If you do not have that module, you can do the same thing |
| 83 | with File::Find which is part of the standard library. |
| 84 | |
| 85 | use File::Find; |
| 86 | my @files; |
| 87 | |
| 88 | find sub { push @files, $File::Find::name if -f _ && /\.pm$/ }, |
| 89 | @INC; |
| 90 | |
| 91 | print join "\n", @files; |
| 92 | |
| 93 | If you simply need to quickly check to see if a module is |
| 94 | available, you can check for its documentation. If you can |
| 95 | read the documentation the module is most likely installed. |
| 96 | If you cannot read the documentation, the module might not |
| 97 | have any (in rare cases). |
| 98 | |
| 99 | prompt% perldoc Module::Name |
| 100 | |
| 101 | You can also try to include the module in a one-liner to see if |
| 102 | perl finds it. |
| 103 | |
| 104 | perl -MModule::Name -e1 |
| 105 | |
| 106 | =head2 How do I debug my Perl programs? |
| 107 | |
| 108 | Have you tried C<use warnings> or used C<-w>? They enable warnings |
| 109 | to detect dubious practices. |
| 110 | |
| 111 | Have you tried C<use strict>? It prevents you from using symbolic |
| 112 | references, makes you predeclare any subroutines that you call as bare |
| 113 | words, and (probably most importantly) forces you to predeclare your |
| 114 | variables with C<my>, C<our>, or C<use vars>. |
| 115 | |
| 116 | Did you check the return values of each and every system call? The operating |
| 117 | system (and thus Perl) tells you whether they worked, and if not |
| 118 | why. |
| 119 | |
| 120 | open(FH, "> /etc/cantwrite") |
| 121 | or die "Couldn't write to /etc/cantwrite: $!\n"; |
| 122 | |
| 123 | Did you read L<perltrap>? It's full of gotchas for old and new Perl |
| 124 | programmers and even has sections for those of you who are upgrading |
| 125 | from languages like I<awk> and I<C>. |
| 126 | |
| 127 | Have you tried the Perl debugger, described in L<perldebug>? You can |
| 128 | step through your program and see what it's doing and thus work out |
| 129 | why what it's doing isn't what it should be doing. |
| 130 | |
| 131 | =head2 How do I profile my Perl programs? |
| 132 | |
| 133 | You should get the Devel::DProf module from the standard distribution |
| 134 | (or separately on CPAN) and also use Benchmark.pm from the standard |
| 135 | distribution. The Benchmark module lets you time specific portions of |
| 136 | your code, while Devel::DProf gives detailed breakdowns of where your |
| 137 | code spends its time. |
| 138 | |
| 139 | Here's a sample use of Benchmark: |
| 140 | |
| 141 | use Benchmark; |
| 142 | |
| 143 | @junk = `cat /etc/motd`; |
| 144 | $count = 10_000; |
| 145 | |
| 146 | timethese($count, { |
| 147 | 'map' => sub { my @a = @junk; |
| 148 | map { s/a/b/ } @a; |
| 149 | return @a }, |
| 150 | 'for' => sub { my @a = @junk; |
| 151 | for (@a) { s/a/b/ }; |
| 152 | return @a }, |
| 153 | }); |
| 154 | |
| 155 | This is what it prints (on one machine--your results will be dependent |
| 156 | on your hardware, operating system, and the load on your machine): |
| 157 | |
| 158 | Benchmark: timing 10000 iterations of for, map... |
| 159 | for: 4 secs ( 3.97 usr 0.01 sys = 3.98 cpu) |
| 160 | map: 6 secs ( 4.97 usr 0.00 sys = 4.97 cpu) |
| 161 | |
| 162 | Be aware that a good benchmark is very hard to write. It only tests the |
| 163 | data you give it and proves little about the differing complexities |
| 164 | of contrasting algorithms. |
| 165 | |
| 166 | =head2 How do I cross-reference my Perl programs? |
| 167 | |
| 168 | The B::Xref module can be used to generate cross-reference reports |
| 169 | for Perl programs. |
| 170 | |
| 171 | perl -MO=Xref[,OPTIONS] scriptname.plx |
| 172 | |
| 173 | =head2 Is there a pretty-printer (formatter) for Perl? |
| 174 | |
| 175 | Perltidy is a Perl script which indents and reformats Perl scripts |
| 176 | to make them easier to read by trying to follow the rules of the |
| 177 | L<perlstyle>. If you write Perl scripts, or spend much time reading |
| 178 | them, you will probably find it useful. It is available at |
| 179 | http://perltidy.sourceforge.net |
| 180 | |
| 181 | Of course, if you simply follow the guidelines in L<perlstyle>, |
| 182 | you shouldn't need to reformat. The habit of formatting your code |
| 183 | as you write it will help prevent bugs. Your editor can and should |
| 184 | help you with this. The perl-mode or newer cperl-mode for emacs |
| 185 | can provide remarkable amounts of help with most (but not all) |
| 186 | code, and even less programmable editors can provide significant |
| 187 | assistance. Tom Christiansen and many other VI users swear by |
| 188 | the following settings in vi and its clones: |
| 189 | |
| 190 | set ai sw=4 |
| 191 | map! ^O {^M}^[O^T |
| 192 | |
| 193 | Put that in your F<.exrc> file (replacing the caret characters |
| 194 | with control characters) and away you go. In insert mode, ^T is |
| 195 | for indenting, ^D is for undenting, and ^O is for blockdenting-- |
| 196 | as it were. A more complete example, with comments, can be found at |
| 197 | http://www.cpan.org/authors/id/TOMC/scripts/toms.exrc.gz |
| 198 | |
| 199 | The a2ps http://www-inf.enst.fr/%7Edemaille/a2ps/black+white.ps.gz does |
| 200 | lots of things related to generating nicely printed output of |
| 201 | documents, as does enscript at http://people.ssh.fi/mtr/genscript/ . |
| 202 | |
| 203 | =head2 Is there a ctags for Perl? |
| 204 | |
| 205 | Recent versions of ctags do much more than older versions did. |
| 206 | EXUBERANT CTAGS is available from http://ctags.sourceforge.net/ |
| 207 | and does a good job of making tags files for perl code. |
| 208 | |
| 209 | There is also a simple one at |
| 210 | http://www.cpan.org/authors/id/TOMC/scripts/ptags.gz which may do |
| 211 | the trick. It can be easy to hack this into what you want. |
| 212 | |
| 213 | =head2 Is there an IDE or Windows Perl Editor? |
| 214 | |
| 215 | Perl programs are just plain text, so any editor will do. |
| 216 | |
| 217 | If you're on Unix, you already have an IDE--Unix itself. The UNIX |
| 218 | philosophy is the philosophy of several small tools that each do one |
| 219 | thing and do it well. It's like a carpenter's toolbox. |
| 220 | |
| 221 | If you want an IDE, check the following (in alphabetical order, not |
| 222 | order of preference): |
| 223 | |
| 224 | =over 4 |
| 225 | |
| 226 | =item Eclipse |
| 227 | |
| 228 | The Eclipse Perl Integration Project integrates Perl |
| 229 | editing/debugging with Eclipse. |
| 230 | |
| 231 | The website for the project is http://e-p-i-c.sf.net/ |
| 232 | |
| 233 | =item Komodo |
| 234 | |
| 235 | ActiveState's cross-platform (as of October 2004, that's Windows, Linux, |
| 236 | and Solaris), multi-language IDE has Perl support, including a regular expression |
| 237 | debugger and remote debugging |
| 238 | ( http://www.ActiveState.com/Products/Komodo/ ). |
| 239 | |
| 240 | =item Open Perl IDE |
| 241 | |
| 242 | ( http://open-perl-ide.sourceforge.net/ ) |
| 243 | Open Perl IDE is an integrated development environment for writing |
| 244 | and debugging Perl scripts with ActiveState's ActivePerl distribution |
| 245 | under Windows 95/98/NT/2000. |
| 246 | |
| 247 | =item OptiPerl |
| 248 | |
| 249 | ( http://www.optiperl.com/ ) is a Windows IDE with simulated CGI |
| 250 | environment, including debugger and syntax highlighting editor. |
| 251 | |
| 252 | =item PerlBuilder |
| 253 | |
| 254 | ( http://www.solutionsoft.com/perl.htm ) is an integrated development |
| 255 | environment for Windows that supports Perl development. |
| 256 | |
| 257 | =item visiPerl+ |
| 258 | |
| 259 | ( http://helpconsulting.net/visiperl/ ) |
| 260 | From Help Consulting, for Windows. |
| 261 | |
| 262 | =item Visual Perl |
| 263 | |
| 264 | ( http://www.activestate.com/Products/Visual_Perl/ ) |
| 265 | Visual Perl is a Visual Studio.NET plug-in from ActiveState. |
| 266 | |
| 267 | |
| 268 | =back |
| 269 | |
| 270 | For editors: if you're on Unix you probably have vi or a vi clone already, |
| 271 | and possibly an emacs too, so you may not need to download anything. |
| 272 | In any emacs the cperl-mode (M-x cperl-mode) gives you perhaps the |
| 273 | best available Perl editing mode in any editor. |
| 274 | |
| 275 | If you are using Windows, you can use any editor that lets |
| 276 | you work with plain text, such as NotePad or WordPad. Word |
| 277 | processors, such as Microsoft Word or WordPerfect, typically |
| 278 | do not work since they insert all sorts of behind-the-scenes |
| 279 | information, although some allow you to save files as "Text |
| 280 | Only". You can also download text editors designed |
| 281 | specifically for programming, such as Textpad |
| 282 | ( http://www.textpad.com/ ) and UltraEdit |
| 283 | ( http://www.ultraedit.com/ ), among others. |
| 284 | |
| 285 | If you are using MacOS, the same concerns apply. MacPerl |
| 286 | (for Classic environments) comes with a simple editor. |
| 287 | Popular external editors are BBEdit ( http://www.bbedit.com/ ) |
| 288 | or Alpha ( http://www.his.com/~jguyer/Alpha/Alpha8.html ). MacOS X users can |
| 289 | use Unix editors as well. |
| 290 | |
| 291 | =over 4 |
| 292 | |
| 293 | =item GNU Emacs |
| 294 | |
| 295 | http://www.gnu.org/software/emacs/windows/ntemacs.html |
| 296 | |
| 297 | =item MicroEMACS |
| 298 | |
| 299 | http://www.microemacs.de/ |
| 300 | |
| 301 | =item XEmacs |
| 302 | |
| 303 | http://www.xemacs.org/Download/index.html |
| 304 | |
| 305 | =item Jed |
| 306 | |
| 307 | http://space.mit.edu/~davis/jed/ |
| 308 | |
| 309 | =back |
| 310 | |
| 311 | or a vi clone such as |
| 312 | |
| 313 | =over 4 |
| 314 | |
| 315 | =item Elvis |
| 316 | |
| 317 | ftp://ftp.cs.pdx.edu/pub/elvis/ http://www.fh-wedel.de/elvis/ |
| 318 | |
| 319 | =item Vile |
| 320 | |
| 321 | http://dickey.his.com/vile/vile.html |
| 322 | |
| 323 | =item Vim |
| 324 | |
| 325 | http://www.vim.org/ |
| 326 | |
| 327 | =back |
| 328 | |
| 329 | For vi lovers in general, Windows or elsewhere: |
| 330 | |
| 331 | http://www.thomer.com/thomer/vi/vi.html |
| 332 | |
| 333 | nvi ( http://www.bostic.com/vi/ , available from CPAN in src/misc/) is |
| 334 | yet another vi clone, unfortunately not available for Windows, but in |
| 335 | UNIX platforms you might be interested in trying it out, firstly because |
| 336 | strictly speaking it is not a vi clone, it is the real vi, or the new |
| 337 | incarnation of it, and secondly because you can embed Perl inside it |
| 338 | to use Perl as the scripting language. nvi is not alone in this, |
| 339 | though: at least also vim and vile offer an embedded Perl. |
| 340 | |
| 341 | The following are Win32 multilanguage editor/IDESs that support Perl: |
| 342 | |
| 343 | =over 4 |
| 344 | |
| 345 | =item Codewright |
| 346 | |
| 347 | http://www.borland.com/codewright/ |
| 348 | |
| 349 | =item MultiEdit |
| 350 | |
| 351 | http://www.MultiEdit.com/ |
| 352 | |
| 353 | =item SlickEdit |
| 354 | |
| 355 | http://www.slickedit.com/ |
| 356 | |
| 357 | =back |
| 358 | |
| 359 | There is also a toyedit Text widget based editor written in Perl |
| 360 | that is distributed with the Tk module on CPAN. The ptkdb |
| 361 | ( http://world.std.com/~aep/ptkdb/ ) is a Perl/tk based debugger that |
| 362 | acts as a development environment of sorts. Perl Composer |
| 363 | ( http://perlcomposer.sourceforge.net/ ) is an IDE for Perl/Tk |
| 364 | GUI creation. |
| 365 | |
| 366 | In addition to an editor/IDE you might be interested in a more |
| 367 | powerful shell environment for Win32. Your options include |
| 368 | |
| 369 | =over 4 |
| 370 | |
| 371 | =item Bash |
| 372 | |
| 373 | from the Cygwin package ( http://sources.redhat.com/cygwin/ ) |
| 374 | |
| 375 | =item Ksh |
| 376 | |
| 377 | from the MKS Toolkit ( http://www.mks.com/ ), or the Bourne shell of |
| 378 | the U/WIN environment ( http://www.research.att.com/sw/tools/uwin/ ) |
| 379 | |
| 380 | =item Tcsh |
| 381 | |
| 382 | ftp://ftp.astron.com/pub/tcsh/ , see also |
| 383 | http://www.primate.wisc.edu/software/csh-tcsh-book/ |
| 384 | |
| 385 | =item Zsh |
| 386 | |
| 387 | ftp://ftp.blarg.net/users/amol/zsh/ , see also http://www.zsh.org/ |
| 388 | |
| 389 | =back |
| 390 | |
| 391 | MKS and U/WIN are commercial (U/WIN is free for educational and |
| 392 | research purposes), Cygwin is covered by the GNU Public License (but |
| 393 | that shouldn't matter for Perl use). The Cygwin, MKS, and U/WIN all |
| 394 | contain (in addition to the shells) a comprehensive set of standard |
| 395 | UNIX toolkit utilities. |
| 396 | |
| 397 | If you're transferring text files between Unix and Windows using FTP |
| 398 | be sure to transfer them in ASCII mode so the ends of lines are |
| 399 | appropriately converted. |
| 400 | |
| 401 | On Mac OS the MacPerl Application comes with a simple 32k text editor |
| 402 | that behaves like a rudimentary IDE. In contrast to the MacPerl Application |
| 403 | the MPW Perl tool can make use of the MPW Shell itself as an editor (with |
| 404 | no 32k limit). |
| 405 | |
| 406 | =over 4 |
| 407 | |
| 408 | =item Affrus |
| 409 | |
| 410 | is a full Perl development environment with full debugger support |
| 411 | ( http://www.latenightsw.com ). |
| 412 | |
| 413 | =item Alpha |
| 414 | |
| 415 | is an editor, written and extensible in Tcl, that nonetheless has |
| 416 | built in support for several popular markup and programming languages |
| 417 | including Perl and HTML ( http://www.his.com/~jguyer/Alpha/Alpha8.html ). |
| 418 | |
| 419 | =item BBEdit and BBEdit Lite |
| 420 | |
| 421 | are text editors for Mac OS that have a Perl sensitivity mode |
| 422 | ( http://web.barebones.com/ ). |
| 423 | |
| 424 | |
| 425 | =back |
| 426 | |
| 427 | Pepper and Pe are programming language sensitive text editors for Mac |
| 428 | OS X and BeOS respectively ( http://www.hekkelman.com/ ). |
| 429 | |
| 430 | =head2 Where can I get Perl macros for vi? |
| 431 | |
| 432 | For a complete version of Tom Christiansen's vi configuration file, |
| 433 | see http://www.cpan.org/authors/Tom_Christiansen/scripts/toms.exrc.gz , |
| 434 | the standard benchmark file for vi emulators. The file runs best with nvi, |
| 435 | the current version of vi out of Berkeley, which incidentally can be built |
| 436 | with an embedded Perl interpreter--see http://www.cpan.org/src/misc/ . |
| 437 | |
| 438 | =head2 Where can I get perl-mode for emacs? |
| 439 | |
| 440 | Since Emacs version 19 patchlevel 22 or so, there have been both a |
| 441 | perl-mode.el and support for the Perl debugger built in. These should |
| 442 | come with the standard Emacs 19 distribution. |
| 443 | |
| 444 | In the Perl source directory, you'll find a directory called "emacs", |
| 445 | which contains a cperl-mode that color-codes keywords, provides |
| 446 | context-sensitive help, and other nifty things. |
| 447 | |
| 448 | Note that the perl-mode of emacs will have fits with C<"main'foo"> |
| 449 | (single quote), and mess up the indentation and highlighting. You |
| 450 | are probably using C<"main::foo"> in new Perl code anyway, so this |
| 451 | shouldn't be an issue. |
| 452 | |
| 453 | =head2 How can I use curses with Perl? |
| 454 | |
| 455 | The Curses module from CPAN provides a dynamically loadable object |
| 456 | module interface to a curses library. A small demo can be found at the |
| 457 | directory http://www.cpan.org/authors/Tom_Christiansen/scripts/rep.gz ; |
| 458 | this program repeats a command and updates the screen as needed, rendering |
| 459 | B<rep ps axu> similar to B<top>. |
| 460 | |
| 461 | =head2 How can I use X or Tk with Perl? |
| 462 | |
| 463 | Tk is a completely Perl-based, object-oriented interface to the Tk toolkit |
| 464 | that doesn't force you to use Tcl just to get at Tk. Sx is an interface |
| 465 | to the Athena Widget set. Both are available from CPAN. See the |
| 466 | directory http://www.cpan.org/modules/by-category/08_User_Interfaces/ |
| 467 | |
| 468 | Invaluable for Perl/Tk programming are the Perl/Tk FAQ at |
| 469 | http://phaseit.net/claird/comp.lang.perl.tk/ptkFAQ.html , the Perl/Tk Reference |
| 470 | Guide available at |
| 471 | http://www.cpan.org/authors/Stephen_O_Lidie/ , and the |
| 472 | online manpages at |
| 473 | http://www-users.cs.umn.edu/%7Eamundson/perl/perltk/toc.html . |
| 474 | |
| 475 | =head2 How can I make my Perl program run faster? |
| 476 | |
| 477 | The best way to do this is to come up with a better algorithm. This |
| 478 | can often make a dramatic difference. Jon Bentley's book |
| 479 | I<Programming Pearls> (that's not a misspelling!) has some good tips |
| 480 | on optimization, too. Advice on benchmarking boils down to: benchmark |
| 481 | and profile to make sure you're optimizing the right part, look for |
| 482 | better algorithms instead of microtuning your code, and when all else |
| 483 | fails consider just buying faster hardware. You will probably want to |
| 484 | read the answer to the earlier question "How do I profile my Perl |
| 485 | programs?" if you haven't done so already. |
| 486 | |
| 487 | A different approach is to autoload seldom-used Perl code. See the |
| 488 | AutoSplit and AutoLoader modules in the standard distribution for |
| 489 | that. Or you could locate the bottleneck and think about writing just |
| 490 | that part in C, the way we used to take bottlenecks in C code and |
| 491 | write them in assembler. Similar to rewriting in C, modules that have |
| 492 | critical sections can be written in C (for instance, the PDL module |
| 493 | from CPAN). |
| 494 | |
| 495 | If you're currently linking your perl executable to a shared |
| 496 | I<libc.so>, you can often gain a 10-25% performance benefit by |
| 497 | rebuilding it to link with a static libc.a instead. This will make a |
| 498 | bigger perl executable, but your Perl programs (and programmers) may |
| 499 | thank you for it. See the F<INSTALL> file in the source distribution |
| 500 | for more information. |
| 501 | |
| 502 | The undump program was an ancient attempt to speed up Perl program by |
| 503 | storing the already-compiled form to disk. This is no longer a viable |
| 504 | option, as it only worked on a few architectures, and wasn't a good |
| 505 | solution anyway. |
| 506 | |
| 507 | =head2 How can I make my Perl program take less memory? |
| 508 | |
| 509 | When it comes to time-space tradeoffs, Perl nearly always prefers to |
| 510 | throw memory at a problem. Scalars in Perl use more memory than |
| 511 | strings in C, arrays take more than that, and hashes use even more. While |
| 512 | there's still a lot to be done, recent releases have been addressing |
| 513 | these issues. For example, as of 5.004, duplicate hash keys are |
| 514 | shared amongst all hashes using them, so require no reallocation. |
| 515 | |
| 516 | In some cases, using substr() or vec() to simulate arrays can be |
| 517 | highly beneficial. For example, an array of a thousand booleans will |
| 518 | take at least 20,000 bytes of space, but it can be turned into one |
| 519 | 125-byte bit vector--a considerable memory savings. The standard |
| 520 | Tie::SubstrHash module can also help for certain types of data |
| 521 | structure. If you're working with specialist data structures |
| 522 | (matrices, for instance) modules that implement these in C may use |
| 523 | less memory than equivalent Perl modules. |
| 524 | |
| 525 | Another thing to try is learning whether your Perl was compiled with |
| 526 | the system malloc or with Perl's builtin malloc. Whichever one it |
| 527 | is, try using the other one and see whether this makes a difference. |
| 528 | Information about malloc is in the F<INSTALL> file in the source |
| 529 | distribution. You can find out whether you are using perl's malloc by |
| 530 | typing C<perl -V:usemymalloc>. |
| 531 | |
| 532 | Of course, the best way to save memory is to not do anything to waste |
| 533 | it in the first place. Good programming practices can go a long way |
| 534 | toward this: |
| 535 | |
| 536 | =over 4 |
| 537 | |
| 538 | =item * Don't slurp! |
| 539 | |
| 540 | Don't read an entire file into memory if you can process it line |
| 541 | by line. Or more concretely, use a loop like this: |
| 542 | |
| 543 | # |
| 544 | # Good Idea |
| 545 | # |
| 546 | while (<FILE>) { |
| 547 | # ... |
| 548 | } |
| 549 | |
| 550 | instead of this: |
| 551 | |
| 552 | # |
| 553 | # Bad Idea |
| 554 | # |
| 555 | @data = <FILE>; |
| 556 | foreach (@data) { |
| 557 | # ... |
| 558 | } |
| 559 | |
| 560 | When the files you're processing are small, it doesn't much matter which |
| 561 | way you do it, but it makes a huge difference when they start getting |
| 562 | larger. |
| 563 | |
| 564 | =item * Use map and grep selectively |
| 565 | |
| 566 | Remember that both map and grep expect a LIST argument, so doing this: |
| 567 | |
| 568 | @wanted = grep {/pattern/} <FILE>; |
| 569 | |
| 570 | will cause the entire file to be slurped. For large files, it's better |
| 571 | to loop: |
| 572 | |
| 573 | while (<FILE>) { |
| 574 | push(@wanted, $_) if /pattern/; |
| 575 | } |
| 576 | |
| 577 | =item * Avoid unnecessary quotes and stringification |
| 578 | |
| 579 | Don't quote large strings unless absolutely necessary: |
| 580 | |
| 581 | my $copy = "$large_string"; |
| 582 | |
| 583 | makes 2 copies of $large_string (one for $copy and another for the |
| 584 | quotes), whereas |
| 585 | |
| 586 | my $copy = $large_string; |
| 587 | |
| 588 | only makes one copy. |
| 589 | |
| 590 | Ditto for stringifying large arrays: |
| 591 | |
| 592 | { |
| 593 | local $, = "\n"; |
| 594 | print @big_array; |
| 595 | } |
| 596 | |
| 597 | is much more memory-efficient than either |
| 598 | |
| 599 | print join "\n", @big_array; |
| 600 | |
| 601 | or |
| 602 | |
| 603 | { |
| 604 | local $" = "\n"; |
| 605 | print "@big_array"; |
| 606 | } |
| 607 | |
| 608 | |
| 609 | =item * Pass by reference |
| 610 | |
| 611 | Pass arrays and hashes by reference, not by value. For one thing, it's |
| 612 | the only way to pass multiple lists or hashes (or both) in a single |
| 613 | call/return. It also avoids creating a copy of all the contents. This |
| 614 | requires some judgment, however, because any changes will be propagated |
| 615 | back to the original data. If you really want to mangle (er, modify) a |
| 616 | copy, you'll have to sacrifice the memory needed to make one. |
| 617 | |
| 618 | =item * Tie large variables to disk. |
| 619 | |
| 620 | For "big" data stores (i.e. ones that exceed available memory) consider |
| 621 | using one of the DB modules to store it on disk instead of in RAM. This |
| 622 | will incur a penalty in access time, but that's probably better than |
| 623 | causing your hard disk to thrash due to massive swapping. |
| 624 | |
| 625 | =back |
| 626 | |
| 627 | =head2 Is it safe to return a reference to local or lexical data? |
| 628 | |
| 629 | Yes. Perl's garbage collection system takes care of this so |
| 630 | everything works out right. |
| 631 | |
| 632 | sub makeone { |
| 633 | my @a = ( 1 .. 10 ); |
| 634 | return \@a; |
| 635 | } |
| 636 | |
| 637 | for ( 1 .. 10 ) { |
| 638 | push @many, makeone(); |
| 639 | } |
| 640 | |
| 641 | print $many[4][5], "\n"; |
| 642 | |
| 643 | print "@many\n"; |
| 644 | |
| 645 | =head2 How can I free an array or hash so my program shrinks? |
| 646 | |
| 647 | (contributed by Michael Carman) |
| 648 | |
| 649 | You usually can't. Memory allocated to lexicals (i.e. my() variables) |
| 650 | cannot be reclaimed or reused even if they go out of scope. It is |
| 651 | reserved in case the variables come back into scope. Memory allocated |
| 652 | to global variables can be reused (within your program) by using |
| 653 | undef()ing and/or delete(). |
| 654 | |
| 655 | On most operating systems, memory allocated to a program can never be |
| 656 | returned to the system. That's why long-running programs sometimes re- |
| 657 | exec themselves. Some operating systems (notably, systems that use |
| 658 | mmap(2) for allocating large chunks of memory) can reclaim memory that |
| 659 | is no longer used, but on such systems, perl must be configured and |
| 660 | compiled to use the OS's malloc, not perl's. |
| 661 | |
| 662 | In general, memory allocation and de-allocation isn't something you can |
| 663 | or should be worrying about much in Perl. |
| 664 | |
| 665 | See also "How can I make my Perl program take less memory?" |
| 666 | |
| 667 | =head2 How can I make my CGI script more efficient? |
| 668 | |
| 669 | Beyond the normal measures described to make general Perl programs |
| 670 | faster or smaller, a CGI program has additional issues. It may be run |
| 671 | several times per second. Given that each time it runs it will need |
| 672 | to be re-compiled and will often allocate a megabyte or more of system |
| 673 | memory, this can be a killer. Compiling into C B<isn't going to help |
| 674 | you> because the process start-up overhead is where the bottleneck is. |
| 675 | |
| 676 | There are two popular ways to avoid this overhead. One solution |
| 677 | involves running the Apache HTTP server (available from |
| 678 | http://www.apache.org/ ) with either of the mod_perl or mod_fastcgi |
| 679 | plugin modules. |
| 680 | |
| 681 | With mod_perl and the Apache::Registry module (distributed with |
| 682 | mod_perl), httpd will run with an embedded Perl interpreter which |
| 683 | pre-compiles your script and then executes it within the same address |
| 684 | space without forking. The Apache extension also gives Perl access to |
| 685 | the internal server API, so modules written in Perl can do just about |
| 686 | anything a module written in C can. For more on mod_perl, see |
| 687 | http://perl.apache.org/ |
| 688 | |
| 689 | With the FCGI module (from CPAN) and the mod_fastcgi |
| 690 | module (available from http://www.fastcgi.com/ ) each of your Perl |
| 691 | programs becomes a permanent CGI daemon process. |
| 692 | |
| 693 | Both of these solutions can have far-reaching effects on your system |
| 694 | and on the way you write your CGI programs, so investigate them with |
| 695 | care. |
| 696 | |
| 697 | See http://www.cpan.org/modules/by-category/15_World_Wide_Web_HTML_HTTP_CGI/ . |
| 698 | |
| 699 | =head2 How can I hide the source for my Perl program? |
| 700 | |
| 701 | Delete it. :-) Seriously, there are a number of (mostly |
| 702 | unsatisfactory) solutions with varying levels of "security". |
| 703 | |
| 704 | First of all, however, you I<can't> take away read permission, because |
| 705 | the source code has to be readable in order to be compiled and |
| 706 | interpreted. (That doesn't mean that a CGI script's source is |
| 707 | readable by people on the web, though--only by people with access to |
| 708 | the filesystem.) So you have to leave the permissions at the socially |
| 709 | friendly 0755 level. |
| 710 | |
| 711 | Some people regard this as a security problem. If your program does |
| 712 | insecure things and relies on people not knowing how to exploit those |
| 713 | insecurities, it is not secure. It is often possible for someone to |
| 714 | determine the insecure things and exploit them without viewing the |
| 715 | source. Security through obscurity, the name for hiding your bugs |
| 716 | instead of fixing them, is little security indeed. |
| 717 | |
| 718 | You can try using encryption via source filters (Starting from Perl |
| 719 | 5.8 the Filter::Simple and Filter::Util::Call modules are included in |
| 720 | the standard distribution), but any decent programmer will be able to |
| 721 | decrypt it. You can try using the byte code compiler and interpreter |
| 722 | described below, but the curious might still be able to de-compile it. |
| 723 | You can try using the native-code compiler described below, but |
| 724 | crackers might be able to disassemble it. These pose varying degrees |
| 725 | of difficulty to people wanting to get at your code, but none can |
| 726 | definitively conceal it (true of every language, not just Perl). |
| 727 | |
| 728 | It is very easy to recover the source of Perl programs. You simply |
| 729 | feed the program to the perl interpreter and use the modules in |
| 730 | the B:: hierarchy. The B::Deparse module should be able to |
| 731 | defeat most attempts to hide source. Again, this is not |
| 732 | unique to Perl. |
| 733 | |
| 734 | If you're concerned about people profiting from your code, then the |
| 735 | bottom line is that nothing but a restrictive license will give you |
| 736 | legal security. License your software and pepper it with threatening |
| 737 | statements like "This is unpublished proprietary software of XYZ Corp. |
| 738 | Your access to it does not give you permission to use it blah blah |
| 739 | blah." We are not lawyers, of course, so you should see a lawyer if |
| 740 | you want to be sure your license's wording will stand up in court. |
| 741 | |
| 742 | =head2 How can I compile my Perl program into byte code or C? |
| 743 | |
| 744 | (contributed by brian d foy) |
| 745 | |
| 746 | In general, you can't do this. There are some things that may work |
| 747 | for your situation though. People usually ask this question |
| 748 | because they want to distribute their works without giving away |
| 749 | the source code, and most solutions trade disk space for convenience. |
| 750 | You probably won't see much of a speed increase either, since most |
| 751 | solutions simply bundle a Perl interpreter in the final product |
| 752 | (but see L<How can I make my Perl program run faster?>). |
| 753 | |
| 754 | The Perl Archive Toolkit (http://par.perl.org/index.cgi) is |
| 755 | Perl's analog to Java's JAR. It's freely available and on |
| 756 | CPAN (http://search.cpan.org/dist/PAR/). |
| 757 | |
| 758 | The B::* namespace, often called "the Perl compiler", but is really a |
| 759 | way for Perl programs to peek at its innards rather than create |
| 760 | pre-compiled versions of your program. However. the B::Bytecode |
| 761 | module can turn your script into a bytecode format that could be |
| 762 | loaded later by the ByteLoader module and executed as a regular Perl |
| 763 | script. |
| 764 | |
| 765 | There are also some commercial products that may work for |
| 766 | you, although you have to buy a license for them. |
| 767 | |
| 768 | The Perl Dev Kit |
| 769 | (http://www.activestate.com/Products/Perl_Dev_Kit/) from |
| 770 | ActiveState can "Turn your Perl programs into ready-to-run |
| 771 | executables for HP-UX, Linux, Solaris and Windows." |
| 772 | |
| 773 | Perl2Exe (http://www.indigostar.com/perl2exe.htm) is a |
| 774 | command line program for converting perl scripts to |
| 775 | executable files. It targets both Windows and unix |
| 776 | platforms. |
| 777 | |
| 778 | |
| 779 | =head2 How can I compile Perl into Java? |
| 780 | |
| 781 | You can also integrate Java and Perl with the |
| 782 | Perl Resource Kit from O'Reilly Media. See |
| 783 | http://www.oreilly.com/catalog/prkunix/ . |
| 784 | |
| 785 | Perl 5.6 comes with Java Perl Lingo, or JPL. JPL, still in |
| 786 | development, allows Perl code to be called from Java. See jpl/README |
| 787 | in the Perl source tree. |
| 788 | |
| 789 | =head2 How can I get C<#!perl> to work on [MS-DOS,NT,...]? |
| 790 | |
| 791 | For OS/2 just use |
| 792 | |
| 793 | extproc perl -S -your_switches |
| 794 | |
| 795 | as the first line in C<*.cmd> file (C<-S> due to a bug in cmd.exe's |
| 796 | "extproc" handling). For DOS one should first invent a corresponding |
| 797 | batch file and codify it in C<ALTERNATE_SHEBANG> (see the |
| 798 | F<dosish.h> file in the source distribution for more information). |
| 799 | |
| 800 | The Win95/NT installation, when using the ActiveState port of Perl, |
| 801 | will modify the Registry to associate the C<.pl> extension with the |
| 802 | perl interpreter. If you install another port, perhaps even building |
| 803 | your own Win95/NT Perl from the standard sources by using a Windows port |
| 804 | of gcc (e.g., with cygwin or mingw32), then you'll have to modify |
| 805 | the Registry yourself. In addition to associating C<.pl> with the |
| 806 | interpreter, NT people can use: C<SET PATHEXT=%PATHEXT%;.PL> to let them |
| 807 | run the program C<install-linux.pl> merely by typing C<install-linux>. |
| 808 | |
| 809 | Under "Classic" MacOS, a perl program will have the appropriate Creator and |
| 810 | Type, so that double-clicking them will invoke the MacPerl application. |
| 811 | Under Mac OS X, clickable apps can be made from any C<#!> script using Wil |
| 812 | Sanchez' DropScript utility: http://www.wsanchez.net/software/ . |
| 813 | |
| 814 | I<IMPORTANT!>: Whatever you do, PLEASE don't get frustrated, and just |
| 815 | throw the perl interpreter into your cgi-bin directory, in order to |
| 816 | get your programs working for a web server. This is an EXTREMELY big |
| 817 | security risk. Take the time to figure out how to do it correctly. |
| 818 | |
| 819 | =head2 Can I write useful Perl programs on the command line? |
| 820 | |
| 821 | Yes. Read L<perlrun> for more information. Some examples follow. |
| 822 | (These assume standard Unix shell quoting rules.) |
| 823 | |
| 824 | # sum first and last fields |
| 825 | perl -lane 'print $F[0] + $F[-1]' * |
| 826 | |
| 827 | # identify text files |
| 828 | perl -le 'for(@ARGV) {print if -f && -T _}' * |
| 829 | |
| 830 | # remove (most) comments from C program |
| 831 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c |
| 832 | |
| 833 | # make file a month younger than today, defeating reaper daemons |
| 834 | perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' * |
| 835 | |
| 836 | # find first unused uid |
| 837 | perl -le '$i++ while getpwuid($i); print $i' |
| 838 | |
| 839 | # display reasonable manpath |
| 840 | echo $PATH | perl -nl -072 -e ' |
| 841 | s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}' |
| 842 | |
| 843 | OK, the last one was actually an Obfuscated Perl Contest entry. :-) |
| 844 | |
| 845 | =head2 Why don't Perl one-liners work on my DOS/Mac/VMS system? |
| 846 | |
| 847 | The problem is usually that the command interpreters on those systems |
| 848 | have rather different ideas about quoting than the Unix shells under |
| 849 | which the one-liners were created. On some systems, you may have to |
| 850 | change single-quotes to double ones, which you must I<NOT> do on Unix |
| 851 | or Plan9 systems. You might also have to change a single % to a %%. |
| 852 | |
| 853 | For example: |
| 854 | |
| 855 | # Unix |
| 856 | perl -e 'print "Hello world\n"' |
| 857 | |
| 858 | # DOS, etc. |
| 859 | perl -e "print \"Hello world\n\"" |
| 860 | |
| 861 | # Mac |
| 862 | print "Hello world\n" |
| 863 | (then Run "Myscript" or Shift-Command-R) |
| 864 | |
| 865 | # MPW |
| 866 | perl -e 'print "Hello world\n"' |
| 867 | |
| 868 | # VMS |
| 869 | perl -e "print ""Hello world\n""" |
| 870 | |
| 871 | The problem is that none of these examples are reliable: they depend on the |
| 872 | command interpreter. Under Unix, the first two often work. Under DOS, |
| 873 | it's entirely possible that neither works. If 4DOS was the command shell, |
| 874 | you'd probably have better luck like this: |
| 875 | |
| 876 | perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>"" |
| 877 | |
| 878 | Under the Mac, it depends which environment you are using. The MacPerl |
| 879 | shell, or MPW, is much like Unix shells in its support for several |
| 880 | quoting variants, except that it makes free use of the Mac's non-ASCII |
| 881 | characters as control characters. |
| 882 | |
| 883 | Using qq(), q(), and qx(), instead of "double quotes", 'single |
| 884 | quotes', and `backticks`, may make one-liners easier to write. |
| 885 | |
| 886 | There is no general solution to all of this. It is a mess. |
| 887 | |
| 888 | [Some of this answer was contributed by Kenneth Albanowski.] |
| 889 | |
| 890 | =head2 Where can I learn about CGI or Web programming in Perl? |
| 891 | |
| 892 | For modules, get the CGI or LWP modules from CPAN. For textbooks, |
| 893 | see the two especially dedicated to web stuff in the question on |
| 894 | books. For problems and questions related to the web, like "Why |
| 895 | do I get 500 Errors" or "Why doesn't it run from the browser right |
| 896 | when it runs fine on the command line", see the troubleshooting |
| 897 | guides and references in L<perlfaq9> or in the CGI MetaFAQ: |
| 898 | |
| 899 | http://www.perl.org/CGI_MetaFAQ.html |
| 900 | |
| 901 | =head2 Where can I learn about object-oriented Perl programming? |
| 902 | |
| 903 | A good place to start is L<perltoot>, and you can use L<perlobj>, |
| 904 | L<perlboot>, L<perltoot>, L<perltooc>, and L<perlbot> for reference. |
| 905 | (If you are using really old Perl, you may not have all of these, |
| 906 | try http://www.perldoc.com/ , but consider upgrading your perl.) |
| 907 | |
| 908 | A good book on OO on Perl is the "Object-Oriented Perl" |
| 909 | by Damian Conway from Manning Publications, |
| 910 | http://www.manning.com/Conway/index.html |
| 911 | |
| 912 | =head2 Where can I learn about linking C with Perl? [h2xs, xsubpp] |
| 913 | |
| 914 | If you want to call C from Perl, start with L<perlxstut>, |
| 915 | moving on to L<perlxs>, L<xsubpp>, and L<perlguts>. If you want to |
| 916 | call Perl from C, then read L<perlembed>, L<perlcall>, and |
| 917 | L<perlguts>. Don't forget that you can learn a lot from looking at |
| 918 | how the authors of existing extension modules wrote their code and |
| 919 | solved their problems. |
| 920 | |
| 921 | =head2 I've read perlembed, perlguts, etc., but I can't embed perl in my C program; what am I doing wrong? |
| 922 | |
| 923 | Download the ExtUtils::Embed kit from CPAN and run `make test'. If |
| 924 | the tests pass, read the pods again and again and again. If they |
| 925 | fail, see L<perlbug> and send a bug report with the output of |
| 926 | C<make test TEST_VERBOSE=1> along with C<perl -V>. |
| 927 | |
| 928 | =head2 When I tried to run my script, I got this message. What does it mean? |
| 929 | |
| 930 | A complete list of Perl's error messages and warnings with explanatory |
| 931 | text can be found in L<perldiag>. You can also use the splain program |
| 932 | (distributed with Perl) to explain the error messages: |
| 933 | |
| 934 | perl program 2>diag.out |
| 935 | splain [-v] [-p] diag.out |
| 936 | |
| 937 | or change your program to explain the messages for you: |
| 938 | |
| 939 | use diagnostics; |
| 940 | |
| 941 | or |
| 942 | |
| 943 | use diagnostics -verbose; |
| 944 | |
| 945 | =head2 What's MakeMaker? |
| 946 | |
| 947 | This module (part of the standard Perl distribution) is designed to |
| 948 | write a Makefile for an extension module from a Makefile.PL. For more |
| 949 | information, see L<ExtUtils::MakeMaker>. |
| 950 | |
| 951 | =head1 AUTHOR AND COPYRIGHT |
| 952 | |
| 953 | Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and |
| 954 | other authors as noted. All rights reserved. |
| 955 | |
| 956 | This documentation is free; you can redistribute it and/or modify it |
| 957 | under the same terms as Perl itself. |
| 958 | |
| 959 | Irrespective of its distribution, all code examples here are in the public |
| 960 | domain. You are permitted and encouraged to use this code and any |
| 961 | derivatives thereof in your own programs for fun or for profit as you |
| 962 | see fit. A simple comment in the code giving credit to the FAQ would |
| 963 | be courteous but is not required. |