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1 | =head1 NAME |
2 | |
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3 | perlfaq3 - Programming Tools ($Revision: 1.10 $, $Date: 2001/11/19 17:09:37 $) |
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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. |
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14 | Have you read the appropriate manpages? Here's a brief index: |
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15 | |
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16 | Basics perldata, perlvar, perlsyn, perlop, perlsub |
17 | Execution perlrun, perldebug |
18 | Functions perlfunc |
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19 | Objects perlref, perlmod, perlobj, perltie |
20 | Data Structures perlref, perllol, perldsc |
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21 | Modules perlmod, perlmodlib, perlsub |
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22 | Regexes perlre, perlfunc, perlop, perllocale |
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23 | Moving to perl5 perltrap, perl |
24 | Linking w/C perlxstut, perlxs, perlcall, perlguts, perlembed |
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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) |
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28 | |
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29 | A crude table of contents for the Perl manpage set is found in L<perltoc>. |
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30 | |
31 | =head2 How can I use Perl interactively? |
32 | |
33 | The typical approach uses the Perl debugger, described in the |
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34 | perldebug(1) manpage, on an ``empty'' program, like this: |
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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 |
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41 | operations typically found in symbolic debuggers. |
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42 | |
43 | =head2 Is there a Perl shell? |
44 | |
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45 | In general, not yet. There is psh available at |
46 | |
47 | http://www.focusresearch.com/gregor/psh |
48 | |
49 | Which includes the following description: |
50 | |
51 | The Perl Shell is a shell that combines the interactive nature |
52 | of a Unix shell with the power of Perl. The goal is to eventually |
53 | have a full featured shell that behaves as expected for normal |
54 | shell activity. But, the Perl Shell will use Perl syntax and |
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55 | functionality for control-flow statements and other things. |
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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. |
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61 | |
62 | =head2 How do I debug my Perl programs? |
63 | |
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64 | Have you tried C<use warnings> or used C<-w>? They enable warnings |
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65 | to detect dubious practices. |
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66 | |
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67 | Have you tried C<use strict>? It prevents you from using symbolic |
68 | references, makes you predeclare any subroutines that you call as bare |
69 | words, and (probably most importantly) forces you to predeclare your |
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70 | variables with C<my>, C<our>, or C<use vars>. |
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71 | |
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72 | Did you check the return values of each and every system call? The operating |
73 | system (and thus Perl) tells you whether they worked, and if not |
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74 | why. |
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75 | |
92c2ed05 |
76 | open(FH, "> /etc/cantwrite") |
77 | or die "Couldn't write to /etc/cantwrite: $!\n"; |
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78 | |
92c2ed05 |
79 | Did you read L<perltrap>? It's full of gotchas for old and new Perl |
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80 | programmers and even has sections for those of you who are upgrading |
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81 | from languages like I<awk> and I<C>. |
82 | |
83 | Have you tried the Perl debugger, described in L<perldebug>? You can |
84 | step through your program and see what it's doing and thus work out |
85 | why what it's doing isn't what it should be doing. |
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86 | |
87 | =head2 How do I profile my Perl programs? |
88 | |
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89 | You should get the Devel::DProf module from the standard distribution |
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90 | (or separately on CPAN) and also use Benchmark.pm from the standard |
91 | distribution. The Benchmark module lets you time specific portions of |
92 | your code, while Devel::DProf gives detailed breakdowns of where your |
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93 | code spends its time. |
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94 | |
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95 | Here's a sample use of Benchmark: |
96 | |
97 | use Benchmark; |
98 | |
99 | @junk = `cat /etc/motd`; |
100 | $count = 10_000; |
101 | |
102 | timethese($count, { |
103 | 'map' => sub { my @a = @junk; |
104 | map { s/a/b/ } @a; |
105 | return @a |
106 | }, |
107 | 'for' => sub { my @a = @junk; |
108 | local $_; |
109 | for (@a) { s/a/b/ }; |
110 | return @a }, |
111 | }); |
112 | |
113 | This is what it prints (on one machine--your results will be dependent |
114 | on your hardware, operating system, and the load on your machine): |
115 | |
116 | Benchmark: timing 10000 iterations of for, map... |
117 | for: 4 secs ( 3.97 usr 0.01 sys = 3.98 cpu) |
118 | map: 6 secs ( 4.97 usr 0.00 sys = 4.97 cpu) |
119 | |
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120 | Be aware that a good benchmark is very hard to write. It only tests the |
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121 | data you give it and proves little about the differing complexities |
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122 | of contrasting algorithms. |
123 | |
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124 | =head2 How do I cross-reference my Perl programs? |
125 | |
126 | The B::Xref module, shipped with the new, alpha-release Perl compiler |
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127 | (not the general distribution prior to the 5.005 release), can be used |
128 | to generate cross-reference reports for Perl programs. |
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129 | |
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130 | perl -MO=Xref[,OPTIONS] scriptname.plx |
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131 | |
132 | =head2 Is there a pretty-printer (formatter) for Perl? |
133 | |
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134 | Perltidy is a Perl script which indents and reformats Perl scripts |
135 | to make them easier to read by trying to follow the rules of the |
136 | L<perlstyle>. If you write Perl scripts, or spend much time reading |
137 | them, you will probably find it useful. It is available at |
138 | http://perltidy.sourceforge.net |
139 | |
140 | Of course, if you simply follow the guidelines in L<perlstyle>, |
141 | you shouldn't need to reformat. The habit of formatting your code |
142 | as you write it will help prevent bugs. Your editor can and should |
143 | help you with this. The perl-mode or newer cperl-mode for emacs |
144 | can provide remarkable amounts of help with most (but not all) |
145 | code, and even less programmable editors can provide significant |
146 | assistance. Tom Christiansen and many other VI users swear by |
147 | the following settings in vi and its clones: |
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148 | |
149 | set ai sw=4 |
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150 | map! ^O {^M}^[O^T |
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151 | |
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152 | Put that in your F<.exrc> file (replacing the caret characters |
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153 | with control characters) and away you go. In insert mode, ^T is |
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154 | for indenting, ^D is for undenting, and ^O is for blockdenting-- |
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155 | as it were. A more complete example, with comments, can be found at |
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156 | http://www.cpan.org/authors/id/TOMC/scripts/toms.exrc.gz |
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157 | |
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158 | The a2ps http://www-inf.enst.fr/%7Edemaille/a2ps/black+white.ps does |
159 | lots of things related to generating nicely printed output of |
160 | documents, as does enscript at http://people.ssh.fi/mtr/genscript/. |
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161 | |
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162 | =head2 Is there a ctags for Perl? |
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163 | |
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164 | Recent versions of ctags do much more than older versions did. |
165 | EXUBERANT CTAGS is available from http://ctags.sourceforge.net/ |
166 | and does a good job of making tags files for perl code. |
167 | |
168 | There is also a simple one at |
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169 | http://www.cpan.org/authors/id/TOMC/scripts/ptags.gz which may do |
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170 | the trick. It can be easy to hack this into what you want. |
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171 | |
172 | =head2 Is there an IDE or Windows Perl Editor? |
173 | |
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174 | Perl programs are just plain text, so any editor will do. |
175 | |
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176 | If you're on Unix, you already have an IDE--Unix itself. The UNIX |
177 | philosophy is the philosophy of several small tools that each do one |
178 | thing and do it well. It's like a carpenter's toolbox. |
179 | |
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180 | If you want an IDE, check the following: |
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181 | |
182 | =over 4 |
183 | |
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184 | =item Komodo |
185 | |
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186 | ActiveState's cross-platform (as of April 2001 Windows and Linux), |
187 | multi-language IDE has Perl support, including a regular expression |
188 | debugger and remote debugging |
189 | (http://www.ActiveState.com/Products/Komodo/index.html). (Visual |
190 | Perl, a Visual Studio.NET plug-in is currently (early 2001) in beta |
191 | (http://www.ActiveState.com/Products/VisualPerl/index.html)). |
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192 | |
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193 | =item The Object System |
194 | |
195 | (http://www.castlelink.co.uk/object_system/) is a Perl web |
196 | applications development IDE, apparently for any platform |
197 | that runs Perl. |
198 | |
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199 | =item Open Perl IDE |
200 | |
201 | ( http://open-perl-ide.sourceforge.net/ ) |
202 | Open Perl IDE is an integrated development environment for writing |
203 | and debugging Perl scripts with ActiveState's ActivePerl distribution |
204 | under Windows 95/98/NT/2000. |
205 | |
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206 | =item PerlBuilder |
207 | |
208 | (http://www.solutionsoft.com/perl.htm) is an integrated development |
209 | environment for Windows that supports Perl development. |
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210 | |
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211 | =item visiPerl+ |
212 | |
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213 | ( http://helpconsulting.net/visiperl/ ) |
214 | From Help Consulting, for Windows. |
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215 | |
216 | =back |
217 | |
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218 | For Windows there's also the |
219 | |
220 | =over 4 |
221 | |
222 | =item CodeMagicCD |
223 | |
224 | ( http://www.codemagiccd.com/ ) Collection of various programming |
225 | tools for Windows: Perl (5.005_03), TclTk, Python, GNU programming |
226 | tools, REBOL, wxWindows toolkit, the MinGW GNU C/C++ compiler, DJGPP |
227 | GNU C/C++ compiler, Cint C interpreter, YaBasic. |
228 | |
229 | =back |
230 | |
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231 | For editors: if you're on Unix you probably have vi or a vi clone already, |
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232 | and possibly an emacs too, so you may not need to download anything. |
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233 | In any emacs the cperl-mode (M-x cperl-mode) gives you perhaps the |
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234 | best available Perl editing mode in any editor. |
235 | |
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236 | If you are using Windows, you can use any editor that lets |
237 | you work with plain text, such as NotePad or WordPad. Word |
238 | processors, such as Microsoft Word or WordPerfect, typically |
239 | do not work since they insert all sorts of behind-the-scenes |
240 | information, although some allow you to save files as "Text |
241 | Only". You can also download text editors designed |
242 | specifically for programming, such as Textpad |
243 | (http://www.textpad.com/) and UltraEdit |
244 | (http://www.ultraedit.com), among others. |
245 | |
246 | If you are using MacOS, the same concerns apply. MacPerl |
247 | (for Classic environments) comes with a simple editor. |
248 | Popular external editors are BBEdit (http://www.bbedit.com) |
249 | or Alpha (http://alpha.olm.net/). MacOS X users can use Unix |
250 | editors as well. |
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251 | |
252 | =over 4 |
253 | |
254 | =item GNU Emacs |
255 | |
256 | http://www.gnu.org/software/emacs/windows/ntemacs.html |
257 | |
258 | =item MicroEMACS |
259 | |
260 | http://members.nbci.com/uemacs/ |
261 | |
262 | =item XEmacs |
263 | |
264 | http://www.xemacs.org/Download/index.html |
265 | |
266 | =back |
267 | |
268 | or a vi clone such as |
269 | |
270 | =over 4 |
271 | |
272 | =item Elvis |
273 | |
274 | ftp://ftp.cs.pdx.edu/pub/elvis/ http://www.fh-wedel.de/elvis/ |
275 | |
276 | =item Vile |
277 | |
278 | http://vile.cx/ |
279 | |
280 | =item Vim |
281 | |
282 | http://www.vim.org/ |
283 | |
284 | win32: http://www.cs.vu.nl/%7Etmgil/vi.html |
285 | |
286 | =back |
287 | |
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288 | For vi lovers in general, Windows or elsewhere: |
289 | http://www.thomer.com/thomer/vi/vi.html. |
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290 | |
291 | nvi (http://www.bostic.com/vi/, available from CPAN in src/misc/) is |
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292 | yet another vi clone, unfortunately not available for Windows, but in |
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293 | UNIX platforms you might be interested in trying it out, firstly because |
294 | strictly speaking it is not a vi clone, it is the real vi, or the new |
295 | incarnation of it, and secondly because you can embed Perl inside it |
296 | to use Perl as the scripting language. nvi is not alone in this, |
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297 | though: at least also vim and vile offer an embedded Perl. |
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298 | |
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299 | The following are Win32 multilanguage editor/IDESs that support Perl: |
300 | |
301 | =over 4 |
302 | |
303 | =item Codewright |
304 | |
305 | http://www.starbase.com/ |
306 | |
307 | =item MultiEdit |
308 | |
309 | http://www.MultiEdit.com/ |
310 | |
311 | =item SlickEdit |
312 | |
313 | http://www.slickedit.com/ |
314 | |
315 | =back |
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316 | |
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317 | There is also a toyedit Text widget based editor written in Perl |
318 | that is distributed with the Tk module on CPAN. The ptkdb |
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319 | (http://world.std.com/~aep/ptkdb/) is a Perl/tk based debugger that |
320 | acts as a development environment of sorts. Perl Composer |
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321 | (http://perlcomposer.sourceforge.net/vperl.html) is an IDE for Perl/Tk |
322 | GUI creation. |
323 | |
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324 | In addition to an editor/IDE you might be interested in a more |
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325 | powerful shell environment for Win32. Your options include |
326 | |
327 | =over 4 |
328 | |
329 | =item Bash |
330 | |
331 | from the Cygwin package (http://sources.redhat.com/cygwin/) |
332 | |
333 | =item Ksh |
334 | |
335 | from the MKS Toolkit (http://www.mks.com/), or the Bourne shell of |
336 | the U/WIN environment (http://www.research.att.com/sw/tools/uwin/) |
337 | |
338 | =item Tcsh |
339 | |
340 | ftp://ftp.astron.com/pub/tcsh/, see also |
341 | http://www.primate.wisc.edu/software/csh-tcsh-book/ |
342 | |
343 | =item Zsh |
344 | |
345 | ftp://ftp.blarg.net/users/amol/zsh/, see also http://www.zsh.org/ |
346 | |
347 | =back |
348 | |
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349 | MKS and U/WIN are commercial (U/WIN is free for educational and |
350 | research purposes), Cygwin is covered by the GNU Public License (but |
351 | that shouldn't matter for Perl use). The Cygwin, MKS, and U/WIN all |
352 | contain (in addition to the shells) a comprehensive set of standard |
353 | UNIX toolkit utilities. |
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354 | |
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355 | If you're transferring text files between Unix and Windows using FTP |
356 | be sure to transfer them in ASCII mode so the ends of lines are |
357 | appropriately converted. |
358 | |
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359 | On Mac OS the MacPerl Application comes with a simple 32k text editor |
360 | that behaves like a rudimentary IDE. In contrast to the MacPerl Application |
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361 | the MPW Perl tool can make use of the MPW Shell itself as an editor (with |
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362 | no 32k limit). |
363 | |
364 | =over 4 |
365 | |
366 | =item BBEdit and BBEdit Lite |
367 | |
368 | are text editors for Mac OS that have a Perl sensitivity mode |
369 | (http://web.barebones.com/). |
370 | |
371 | =item Alpha |
372 | |
373 | is an editor, written and extensible in Tcl, that nonetheless has |
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374 | built in support for several popular markup and programming languages |
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375 | including Perl and HTML (http://alpha.olm.net/). |
376 | |
377 | =back |
378 | |
379 | Pepper and Pe are programming language sensitive text editors for Mac |
380 | OS X and BeOS respectively (http://www.hekkelman.com/). |
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381 | |
382 | =head2 Where can I get Perl macros for vi? |
383 | |
384 | For a complete version of Tom Christiansen's vi configuration file, |
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385 | see http://www.cpan.org/authors/Tom_Christiansen/scripts/toms.exrc.gz , |
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386 | the standard benchmark file for vi emulators. The file runs best with nvi, |
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387 | the current version of vi out of Berkeley, which incidentally can be built |
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388 | with an embedded Perl interpreter--see http://www.cpan.org/src/misc. |
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389 | |
390 | =head2 Where can I get perl-mode for emacs? |
391 | |
392 | Since Emacs version 19 patchlevel 22 or so, there have been both a |
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393 | perl-mode.el and support for the Perl debugger built in. These should |
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394 | come with the standard Emacs 19 distribution. |
395 | |
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396 | In the Perl source directory, you'll find a directory called "emacs", |
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397 | which contains a cperl-mode that color-codes keywords, provides |
398 | context-sensitive help, and other nifty things. |
399 | |
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400 | Note that the perl-mode of emacs will have fits with C<"main'foo"> |
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401 | (single quote), and mess up the indentation and highlighting. You |
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402 | are probably using C<"main::foo"> in new Perl code anyway, so this |
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403 | shouldn't be an issue. |
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404 | |
405 | =head2 How can I use curses with Perl? |
406 | |
407 | The Curses module from CPAN provides a dynamically loadable object |
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408 | module interface to a curses library. A small demo can be found at the |
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409 | directory http://www.cpan.org/authors/Tom_Christiansen/scripts/rep; |
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410 | this program repeats a command and updates the screen as needed, rendering |
411 | B<rep ps axu> similar to B<top>. |
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412 | |
413 | =head2 How can I use X or Tk with Perl? |
414 | |
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415 | Tk is a completely Perl-based, object-oriented interface to the Tk toolkit |
416 | that doesn't force you to use Tcl just to get at Tk. Sx is an interface |
417 | to the Athena Widget set. Both are available from CPAN. See the |
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418 | directory http://www.cpan.org/modules/by-category/08_User_Interfaces/ |
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419 | |
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420 | Invaluable for Perl/Tk programming are the Perl/Tk FAQ at |
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421 | http://w4.lns.cornell.edu/%7Epvhp/ptk/ptkTOC.html , the Perl/Tk Reference |
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422 | Guide available at |
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423 | http://www.cpan.org/authors/Stephen_O_Lidie/ , and the |
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424 | online manpages at |
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425 | http://www-users.cs.umn.edu/%7Eamundson/perl/perltk/toc.html . |
92c2ed05 |
426 | |
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427 | =head2 How can I generate simple menus without using CGI or Tk? |
428 | |
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429 | The http://www.cpan.org/authors/id/SKUNZ/perlmenu.v4.0.tar.gz |
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430 | module, which is curses-based, can help with this. |
431 | |
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432 | =head2 How can I make my Perl program run faster? |
433 | |
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434 | The best way to do this is to come up with a better algorithm. This |
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435 | can often make a dramatic difference. Jon Bentley's book |
92c2ed05 |
436 | ``Programming Pearls'' (that's not a misspelling!) has some good tips |
437 | on optimization, too. Advice on benchmarking boils down to: benchmark |
438 | and profile to make sure you're optimizing the right part, look for |
439 | better algorithms instead of microtuning your code, and when all else |
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440 | fails consider just buying faster hardware. You will probably want to |
441 | read the answer to the earlier question ``How do I profile my Perl programs?'' |
442 | if you haven't done so already. |
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443 | |
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444 | A different approach is to autoload seldom-used Perl code. See the |
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445 | AutoSplit and AutoLoader modules in the standard distribution for |
446 | that. Or you could locate the bottleneck and think about writing just |
447 | that part in C, the way we used to take bottlenecks in C code and |
a6dd486b |
448 | write them in assembler. Similar to rewriting in C, |
449 | modules that have critical sections can be written in C (for instance, the |
68dc0745 |
450 | PDL module from CPAN). |
451 | |
452 | In some cases, it may be worth it to use the backend compiler to |
453 | produce byte code (saving compilation time) or compile into C, which |
454 | will certainly save compilation time and sometimes a small amount (but |
455 | not much) execution time. See the question about compiling your Perl |
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456 | programs for more on the compiler--the wins aren't as obvious as you'd |
457 | hope. |
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458 | |
92c2ed05 |
459 | If you're currently linking your perl executable to a shared I<libc.so>, |
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460 | you can often gain a 10-25% performance benefit by rebuilding it to |
461 | link with a static libc.a instead. This will make a bigger perl |
462 | executable, but your Perl programs (and programmers) may thank you for |
463 | it. See the F<INSTALL> file in the source distribution for more |
464 | information. |
465 | |
466 | Unsubstantiated reports allege that Perl interpreters that use sfio |
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467 | outperform those that don't (for I/O intensive applications). To try |
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468 | this, see the F<INSTALL> file in the source distribution, especially |
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469 | the ``Selecting File I/O mechanisms'' section. |
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470 | |
471 | The undump program was an old attempt to speed up your Perl program |
472 | by storing the already-compiled form to disk. This is no longer |
473 | a viable option, as it only worked on a few architectures, and |
474 | wasn't a good solution anyway. |
475 | |
476 | =head2 How can I make my Perl program take less memory? |
477 | |
478 | When it comes to time-space tradeoffs, Perl nearly always prefers to |
479 | throw memory at a problem. Scalars in Perl use more memory than |
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480 | strings in C, arrays take more than that, and hashes use even more. While |
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481 | there's still a lot to be done, recent releases have been addressing |
482 | these issues. For example, as of 5.004, duplicate hash keys are |
483 | shared amongst all hashes using them, so require no reallocation. |
484 | |
485 | In some cases, using substr() or vec() to simulate arrays can be |
486 | highly beneficial. For example, an array of a thousand booleans will |
487 | take at least 20,000 bytes of space, but it can be turned into one |
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488 | 125-byte bit vector--a considerable memory savings. The standard |
68dc0745 |
489 | Tie::SubstrHash module can also help for certain types of data |
490 | structure. If you're working with specialist data structures |
491 | (matrices, for instance) modules that implement these in C may use |
492 | less memory than equivalent Perl modules. |
493 | |
494 | Another thing to try is learning whether your Perl was compiled with |
54310121 |
495 | the system malloc or with Perl's builtin malloc. Whichever one it |
68dc0745 |
496 | is, try using the other one and see whether this makes a difference. |
497 | Information about malloc is in the F<INSTALL> file in the source |
498 | distribution. You can find out whether you are using perl's malloc by |
499 | typing C<perl -V:usemymalloc>. |
500 | |
24f1ba9b |
501 | Of course, the best way to save memory is to not do anything to waste |
502 | it in the first place. Good programming practices can go a long way |
503 | toward this: |
504 | |
505 | =over 4 |
506 | |
507 | =item * Don't slurp! |
508 | |
509 | Don't read an entire file into memory if you can process it line |
510 | by line. Or more concretely, use a loop like this: |
511 | |
512 | # |
513 | # Good Idea |
514 | # |
515 | while (<FILE>) { |
516 | # ... |
517 | } |
518 | |
519 | instead of this: |
520 | |
521 | # |
522 | # Bad Idea |
523 | # |
524 | @data = <FILE>; |
525 | foreach (@data) { |
526 | # ... |
527 | } |
528 | |
529 | When the files you're processing are small, it doesn't much matter which |
530 | way you do it, but it makes a huge difference when they start getting |
531 | larger. |
532 | |
bc06af74 |
533 | =item * Use map and grep selectively |
534 | |
535 | Remember that both map and grep expect a LIST argument, so doing this: |
536 | |
537 | @wanted = grep {/pattern/} <FILE>; |
538 | |
539 | will cause the entire file to be slurped. For large files, it's better |
540 | to loop: |
541 | |
542 | while (<FILE>) { |
543 | push(@wanted, $_) if /pattern/; |
544 | } |
545 | |
546 | =item * Avoid unnecessary quotes and stringification |
547 | |
548 | Don't quote large strings unless absolutely necessary: |
549 | |
550 | my $copy = "$large_string"; |
551 | |
552 | makes 2 copies of $large_string (one for $copy and another for the |
553 | quotes), whereas |
554 | |
555 | my $copy = $large_string; |
556 | |
557 | only makes one copy. |
558 | |
559 | Ditto for stringifying large arrays: |
560 | |
561 | { |
562 | local $, = "\n"; |
563 | print @big_array; |
564 | } |
565 | |
566 | is much more memory-efficient than either |
567 | |
568 | print join "\n", @big_array; |
569 | |
570 | or |
571 | |
572 | { |
573 | local $" = "\n"; |
574 | print "@big_array"; |
575 | } |
576 | |
577 | |
24f1ba9b |
578 | =item * Pass by reference |
579 | |
580 | Pass arrays and hashes by reference, not by value. For one thing, it's |
581 | the only way to pass multiple lists or hashes (or both) in a single |
582 | call/return. It also avoids creating a copy of all the contents. This |
583 | requires some judgment, however, because any changes will be propagated |
584 | back to the original data. If you really want to mangle (er, modify) a |
585 | copy, you'll have to sacrifice the memory needed to make one. |
586 | |
587 | =item * Tie large variables to disk. |
588 | |
589 | For "big" data stores (i.e. ones that exceed available memory) consider |
590 | using one of the DB modules to store it on disk instead of in RAM. This |
591 | will incur a penalty in access time, but that's probably better that |
592 | causing your hard disk to thrash due to massive swapping. |
593 | |
594 | =back |
595 | |
68dc0745 |
596 | =head2 Is it unsafe to return a pointer to local data? |
597 | |
598 | No, Perl's garbage collection system takes care of this. |
599 | |
600 | sub makeone { |
601 | my @a = ( 1 .. 10 ); |
602 | return \@a; |
603 | } |
604 | |
605 | for $i ( 1 .. 10 ) { |
606 | push @many, makeone(); |
607 | } |
608 | |
609 | print $many[4][5], "\n"; |
610 | |
611 | print "@many\n"; |
612 | |
613 | =head2 How can I free an array or hash so my program shrinks? |
614 | |
c8db1d39 |
615 | You can't. On most operating systems, memory allocated to a program |
616 | can never be returned to the system. That's why long-running programs |
65acb1b1 |
617 | sometimes re-exec themselves. Some operating systems (notably, |
618 | FreeBSD and Linux) allegedly reclaim large chunks of memory that is no |
619 | longer used, but it doesn't appear to happen with Perl (yet). The Mac |
620 | appears to be the only platform that will reliably (albeit, slowly) |
621 | return memory to the OS. |
622 | |
623 | We've had reports that on Linux (Redhat 5.1) on Intel, C<undef |
624 | $scalar> will return memory to the system, while on Solaris 2.6 it |
625 | won't. In general, try it yourself and see. |
68dc0745 |
626 | |
627 | However, judicious use of my() on your variables will help make sure |
a6dd486b |
628 | that they go out of scope so that Perl can free up that space for |
92c2ed05 |
629 | use in other parts of your program. A global variable, of course, never |
68dc0745 |
630 | goes out of scope, so you can't get its space automatically reclaimed, |
631 | although undef()ing and/or delete()ing it will achieve the same effect. |
46fc3d4c |
632 | In general, memory allocation and de-allocation isn't something you can |
68dc0745 |
633 | or should be worrying about much in Perl, but even this capability |
634 | (preallocation of data types) is in the works. |
635 | |
636 | =head2 How can I make my CGI script more efficient? |
637 | |
638 | Beyond the normal measures described to make general Perl programs |
639 | faster or smaller, a CGI program has additional issues. It may be run |
640 | several times per second. Given that each time it runs it will need |
46fc3d4c |
641 | to be re-compiled and will often allocate a megabyte or more of system |
68dc0745 |
642 | memory, this can be a killer. Compiling into C B<isn't going to help |
46fc3d4c |
643 | you> because the process start-up overhead is where the bottleneck is. |
68dc0745 |
644 | |
92c2ed05 |
645 | There are two popular ways to avoid this overhead. One solution |
646 | involves running the Apache HTTP server (available from |
68dc0745 |
647 | http://www.apache.org/) with either of the mod_perl or mod_fastcgi |
92c2ed05 |
648 | plugin modules. |
649 | |
650 | With mod_perl and the Apache::Registry module (distributed with |
651 | mod_perl), httpd will run with an embedded Perl interpreter which |
652 | pre-compiles your script and then executes it within the same address |
653 | space without forking. The Apache extension also gives Perl access to |
654 | the internal server API, so modules written in Perl can do just about |
655 | anything a module written in C can. For more on mod_perl, see |
656 | http://perl.apache.org/ |
657 | |
65acb1b1 |
658 | With the FCGI module (from CPAN) and the mod_fastcgi |
87275199 |
659 | module (available from http://www.fastcgi.com/) each of your Perl |
660 | programs becomes a permanent CGI daemon process. |
68dc0745 |
661 | |
662 | Both of these solutions can have far-reaching effects on your system |
87275199 |
663 | and on the way you write your CGI programs, so investigate them with |
68dc0745 |
664 | care. |
665 | |
a93751fa |
666 | See http://www.cpan.org/modules/by-category/15_World_Wide_Web_HTML_HTTP_CGI/ . |
5a964f20 |
667 | |
65acb1b1 |
668 | A non-free, commercial product, ``The Velocity Engine for Perl'', |
a6dd486b |
669 | (http://www.binevolve.com/ or http://www.binevolve.com/velocigen/ ) |
670 | might also be worth looking at. It will allow you to increase the |
671 | performance of your Perl programs, running programs up to 25 times |
672 | faster than normal CGI Perl when running in persistent Perl mode or 4 |
673 | to 5 times faster without any modification to your existing CGI |
674 | programs. Fully functional evaluation copies are available from the |
675 | web site. |
c8db1d39 |
676 | |
68dc0745 |
677 | =head2 How can I hide the source for my Perl program? |
678 | |
679 | Delete it. :-) Seriously, there are a number of (mostly |
92c2ed05 |
680 | unsatisfactory) solutions with varying levels of ``security''. |
68dc0745 |
681 | |
682 | First of all, however, you I<can't> take away read permission, because |
683 | the source code has to be readable in order to be compiled and |
684 | interpreted. (That doesn't mean that a CGI script's source is |
a6dd486b |
685 | readable by people on the web, though--only by people with access to |
686 | the filesystem.) So you have to leave the permissions at the socially |
92c2ed05 |
687 | friendly 0755 level. |
68dc0745 |
688 | |
689 | Some people regard this as a security problem. If your program does |
a6dd486b |
690 | insecure things and relies on people not knowing how to exploit those |
68dc0745 |
691 | insecurities, it is not secure. It is often possible for someone to |
692 | determine the insecure things and exploit them without viewing the |
693 | source. Security through obscurity, the name for hiding your bugs |
694 | instead of fixing them, is little security indeed. |
695 | |
83df6a1d |
696 | You can try using encryption via source filters (Starting from Perl |
697 | 5.8 the Filter::Simple and Filter::Util::Call modules are included in |
698 | the standard distribution), but any decent programmer will be able to |
699 | decrypt it. You can try using the byte code compiler and interpreter |
700 | described below, but the curious might still be able to de-compile it. |
701 | You can try using the native-code compiler described below, but |
702 | crackers might be able to disassemble it. These pose varying degrees |
703 | of difficulty to people wanting to get at your code, but none can |
704 | definitively conceal it (true of every language, not just Perl). |
68dc0745 |
705 | |
706 | If you're concerned about people profiting from your code, then the |
d92eb7b0 |
707 | bottom line is that nothing but a restrictive license will give you |
68dc0745 |
708 | legal security. License your software and pepper it with threatening |
92c2ed05 |
709 | statements like ``This is unpublished proprietary software of XYZ Corp. |
68dc0745 |
710 | Your access to it does not give you permission to use it blah blah |
92c2ed05 |
711 | blah.'' We are not lawyers, of course, so you should see a lawyer if |
d92eb7b0 |
712 | you want to be sure your license's wording will stand up in court. |
68dc0745 |
713 | |
54310121 |
714 | =head2 How can I compile my Perl program into byte code or C? |
68dc0745 |
715 | |
716 | Malcolm Beattie has written a multifunction backend compiler, |
5e3006a4 |
717 | available from CPAN, that can do both these things. It is included |
718 | in the perl5.005 release, but is still considered experimental. |
719 | This means it's fun to play with if you're a programmer but not |
720 | really for people looking for turn-key solutions. |
68dc0745 |
721 | |
92c2ed05 |
722 | Merely compiling into C does not in and of itself guarantee that your |
723 | code will run very much faster. That's because except for lucky cases |
724 | where a lot of native type inferencing is possible, the normal Perl |
a6dd486b |
725 | run-time system is still present and so your program will take just as |
92c2ed05 |
726 | long to run and be just as big. Most programs save little more than |
727 | compilation time, leaving execution no more than 10-30% faster. A few |
a6dd486b |
728 | rare programs actually benefit significantly (even running several times |
92c2ed05 |
729 | faster), but this takes some tweaking of your code. |
68dc0745 |
730 | |
68dc0745 |
731 | You'll probably be astonished to learn that the current version of the |
732 | compiler generates a compiled form of your script whose executable is |
733 | just as big as the original perl executable, and then some. That's |
734 | because as currently written, all programs are prepared for a full |
735 | eval() statement. You can tremendously reduce this cost by building a |
92c2ed05 |
736 | shared I<libperl.so> library and linking against that. See the |
87275199 |
737 | F<INSTALL> podfile in the Perl source distribution for details. If |
d92eb7b0 |
738 | you link your main perl binary with this, it will make it minuscule. |
92c2ed05 |
739 | For example, on one author's system, F</usr/bin/perl> is only 11k in |
68dc0745 |
740 | size! |
741 | |
5a964f20 |
742 | In general, the compiler will do nothing to make a Perl program smaller, |
a6dd486b |
743 | faster, more portable, or more secure. In fact, it can make your |
744 | situation worse. The executable will be bigger, your VM system may take |
5a964f20 |
745 | longer to load the whole thing, the binary is fragile and hard to fix, |
746 | and compilation never stopped software piracy in the form of crackers, |
747 | viruses, or bootleggers. The real advantage of the compiler is merely |
748 | packaging, and once you see the size of what it makes (well, unless |
749 | you use a shared I<libperl.so>), you'll probably want a complete |
5e3006a4 |
750 | Perl install anyway. |
5a964f20 |
751 | |
65acb1b1 |
752 | =head2 How can I compile Perl into Java? |
753 | |
a6dd486b |
754 | You can also integrate Java and Perl with the |
65acb1b1 |
755 | Perl Resource Kit from O'Reilly and Associates. See |
a6dd486b |
756 | http://www.oreilly.com/catalog/prkunix/ . |
757 | |
758 | Perl 5.6 comes with Java Perl Lingo, or JPL. JPL, still in |
759 | development, allows Perl code to be called from Java. See jpl/README |
760 | in the Perl source tree. |
65acb1b1 |
761 | |
92c2ed05 |
762 | =head2 How can I get C<#!perl> to work on [MS-DOS,NT,...]? |
68dc0745 |
763 | |
764 | For OS/2 just use |
765 | |
766 | extproc perl -S -your_switches |
767 | |
768 | as the first line in C<*.cmd> file (C<-S> due to a bug in cmd.exe's |
46fc3d4c |
769 | `extproc' handling). For DOS one should first invent a corresponding |
a6dd486b |
770 | batch file and codify it in C<ALTERNATIVE_SHEBANG> (see the |
68dc0745 |
771 | F<INSTALL> file in the source distribution for more information). |
772 | |
92c2ed05 |
773 | The Win95/NT installation, when using the ActiveState port of Perl, |
774 | will modify the Registry to associate the C<.pl> extension with the |
d92eb7b0 |
775 | perl interpreter. If you install another port, perhaps even building |
776 | your own Win95/NT Perl from the standard sources by using a Windows port |
d702ae42 |
777 | of gcc (e.g., with cygwin or mingw32), then you'll have to modify |
d92eb7b0 |
778 | the Registry yourself. In addition to associating C<.pl> with the |
779 | interpreter, NT people can use: C<SET PATHEXT=%PATHEXT%;.PL> to let them |
780 | run the program C<install-linux.pl> merely by typing C<install-linux>. |
68dc0745 |
781 | |
87275199 |
782 | Macintosh Perl programs will have the appropriate Creator and |
783 | Type, so that double-clicking them will invoke the Perl application. |
68dc0745 |
784 | |
785 | I<IMPORTANT!>: Whatever you do, PLEASE don't get frustrated, and just |
786 | throw the perl interpreter into your cgi-bin directory, in order to |
87275199 |
787 | get your programs working for a web server. This is an EXTREMELY big |
68dc0745 |
788 | security risk. Take the time to figure out how to do it correctly. |
789 | |
87275199 |
790 | =head2 Can I write useful Perl programs on the command line? |
68dc0745 |
791 | |
792 | Yes. Read L<perlrun> for more information. Some examples follow. |
793 | (These assume standard Unix shell quoting rules.) |
794 | |
795 | # sum first and last fields |
5a964f20 |
796 | perl -lane 'print $F[0] + $F[-1]' * |
68dc0745 |
797 | |
798 | # identify text files |
799 | perl -le 'for(@ARGV) {print if -f && -T _}' * |
800 | |
5a964f20 |
801 | # remove (most) comments from C program |
68dc0745 |
802 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c |
803 | |
804 | # make file a month younger than today, defeating reaper daemons |
805 | perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' * |
806 | |
807 | # find first unused uid |
808 | perl -le '$i++ while getpwuid($i); print $i' |
809 | |
810 | # display reasonable manpath |
811 | echo $PATH | perl -nl -072 -e ' |
812 | s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}' |
813 | |
87275199 |
814 | OK, the last one was actually an Obfuscated Perl Contest entry. :-) |
68dc0745 |
815 | |
87275199 |
816 | =head2 Why don't Perl one-liners work on my DOS/Mac/VMS system? |
68dc0745 |
817 | |
818 | The problem is usually that the command interpreters on those systems |
819 | have rather different ideas about quoting than the Unix shells under |
820 | which the one-liners were created. On some systems, you may have to |
821 | change single-quotes to double ones, which you must I<NOT> do on Unix |
822 | or Plan9 systems. You might also have to change a single % to a %%. |
823 | |
824 | For example: |
825 | |
826 | # Unix |
827 | perl -e 'print "Hello world\n"' |
828 | |
46fc3d4c |
829 | # DOS, etc. |
68dc0745 |
830 | perl -e "print \"Hello world\n\"" |
831 | |
46fc3d4c |
832 | # Mac |
68dc0745 |
833 | print "Hello world\n" |
834 | (then Run "Myscript" or Shift-Command-R) |
835 | |
836 | # VMS |
837 | perl -e "print ""Hello world\n""" |
838 | |
a6dd486b |
839 | The problem is that none of these examples are reliable: they depend on the |
92c2ed05 |
840 | command interpreter. Under Unix, the first two often work. Under DOS, |
a6dd486b |
841 | it's entirely possible that neither works. If 4DOS was the command shell, |
92c2ed05 |
842 | you'd probably have better luck like this: |
68dc0745 |
843 | |
844 | perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>"" |
845 | |
46fc3d4c |
846 | Under the Mac, it depends which environment you are using. The MacPerl |
68dc0745 |
847 | shell, or MPW, is much like Unix shells in its support for several |
46fc3d4c |
848 | quoting variants, except that it makes free use of the Mac's non-ASCII |
68dc0745 |
849 | characters as control characters. |
850 | |
65acb1b1 |
851 | Using qq(), q(), and qx(), instead of "double quotes", 'single |
852 | quotes', and `backticks`, may make one-liners easier to write. |
853 | |
92c2ed05 |
854 | There is no general solution to all of this. It is a mess, pure and |
855 | simple. Sucks to be away from Unix, huh? :-) |
68dc0745 |
856 | |
857 | [Some of this answer was contributed by Kenneth Albanowski.] |
858 | |
859 | =head2 Where can I learn about CGI or Web programming in Perl? |
860 | |
861 | For modules, get the CGI or LWP modules from CPAN. For textbooks, |
862 | see the two especially dedicated to web stuff in the question on |
92c2ed05 |
863 | books. For problems and questions related to the web, like ``Why |
864 | do I get 500 Errors'' or ``Why doesn't it run from the browser right |
8305e449 |
865 | when it runs fine on the command line'', see the troubleshooting |
866 | guides and references in L<perlfaq9> or in the CGI MetaFAQ: |
68dc0745 |
867 | |
8305e449 |
868 | http://www.perl.org/CGI_MetaFAQ.html |
0f542199 |
869 | |
68dc0745 |
870 | =head2 Where can I learn about object-oriented Perl programming? |
871 | |
a6dd486b |
872 | A good place to start is L<perltoot>, and you can use L<perlobj>, |
06a5f41f |
873 | L<perlboot>, L<perltoot>, L<perltooc>, and L<perlbot> for reference. |
874 | (If you are using really old Perl, you may not have all of these, |
875 | try http://www.perldoc.com/ , but consider upgrading your perl.) |
876 | |
877 | A good book on OO on Perl is the "Object-Oriented Perl" |
878 | by Damian Conway from Manning Publications, |
879 | http://www.manning.com/Conway/index.html |
68dc0745 |
880 | |
881 | =head2 Where can I learn about linking C with Perl? [h2xs, xsubpp] |
882 | |
883 | If you want to call C from Perl, start with L<perlxstut>, |
884 | moving on to L<perlxs>, L<xsubpp>, and L<perlguts>. If you want to |
885 | call Perl from C, then read L<perlembed>, L<perlcall>, and |
886 | L<perlguts>. Don't forget that you can learn a lot from looking at |
887 | how the authors of existing extension modules wrote their code and |
888 | solved their problems. |
889 | |
890 | =head2 I've read perlembed, perlguts, etc., but I can't embed perl in |
a6dd486b |
891 | my C program; what am I doing wrong? |
68dc0745 |
892 | |
893 | Download the ExtUtils::Embed kit from CPAN and run `make test'. If |
894 | the tests pass, read the pods again and again and again. If they |
87275199 |
895 | fail, see L<perlbug> and send a bug report with the output of |
68dc0745 |
896 | C<make test TEST_VERBOSE=1> along with C<perl -V>. |
897 | |
898 | =head2 When I tried to run my script, I got this message. What does it |
899 | mean? |
900 | |
87275199 |
901 | A complete list of Perl's error messages and warnings with explanatory |
902 | text can be found in L<perldiag>. You can also use the splain program |
903 | (distributed with Perl) to explain the error messages: |
68dc0745 |
904 | |
905 | perl program 2>diag.out |
906 | splain [-v] [-p] diag.out |
907 | |
908 | or change your program to explain the messages for you: |
909 | |
910 | use diagnostics; |
911 | |
912 | or |
913 | |
914 | use diagnostics -verbose; |
915 | |
916 | =head2 What's MakeMaker? |
917 | |
87275199 |
918 | This module (part of the standard Perl distribution) is designed to |
68dc0745 |
919 | write a Makefile for an extension module from a Makefile.PL. For more |
920 | information, see L<ExtUtils::MakeMaker>. |
921 | |
922 | =head1 AUTHOR AND COPYRIGHT |
923 | |
65acb1b1 |
924 | Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. |
5a964f20 |
925 | All rights reserved. |
926 | |
5a7beb56 |
927 | This documentation is free; you can redistribute it and/or modify it |
928 | under the same terms as Perl itself. |
c8db1d39 |
929 | |
87275199 |
930 | Irrespective of its distribution, all code examples here are in the public |
c8db1d39 |
931 | domain. You are permitted and encouraged to use this code and any |
932 | derivatives thereof in your own programs for fun or for profit as you |
933 | see fit. A simple comment in the code giving credit to the FAQ would |
934 | be courteous but is not required. |