3 perlfaq3 - Programming Tools ($Revision: 1.28 $, $Date: 1998/07/16 22:08:49 $)
7 This section of the FAQ answers questions related to programmer tools
8 and programming support.
10 =head2 How do I do (anything)?
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 man pages? Here's a brief index:
16 Basics perldata, perlvar, perlsyn, perlop, perlsub
17 Execution perlrun, perldebug
19 Objects perlref, perlmod, perlobj, perltie
20 Data Structures perlref, perllol, perldsc
21 Modules perlmod, perlmodlib, perlsub
22 Regexps perlre, perlfunc, perlop, perllocale
23 Moving to perl5 perltrap, perl
24 Linking w/C perlxstut, perlxs, perlcall, perlguts, perlembed
25 Various http://www.perl.com/CPAN/doc/FMTEYEWTK/index.html
26 (not a man-page but still useful)
28 L<perltoc> provides a crude table of contents for the perl man page set.
30 =head2 How can I use Perl interactively?
32 The typical approach uses the Perl debugger, described in the
33 perldebug(1) man page, on an ``empty'' program, like this:
37 Now just type in any legal Perl code, and it will be immediately
38 evaluated. You can also examine the symbol table, get stack
39 backtraces, check variable values, set breakpoints, and other
40 operations typically found in symbolic debuggers.
42 =head2 Is there a Perl shell?
44 In general, no. The Shell.pm module (distributed with perl) makes
45 perl try commands which aren't part of the Perl language as shell
46 commands. perlsh from the source distribution is simplistic and
47 uninteresting, but may still be what you want.
49 =head2 How do I debug my Perl programs?
51 Have you used C<-w>? It enables warnings for dubious practices.
53 Have you tried C<use strict>? It prevents you from using symbolic
54 references, makes you predeclare any subroutines that you call as bare
55 words, and (probably most importantly) forces you to predeclare your
56 variables with C<my> or C<use vars>.
58 Did you check the returns of each and every system call? The operating
59 system (and thus Perl) tells you whether they worked or not, and if not
62 open(FH, "> /etc/cantwrite")
63 or die "Couldn't write to /etc/cantwrite: $!\n";
65 Did you read L<perltrap>? It's full of gotchas for old and new Perl
66 programmers, and even has sections for those of you who are upgrading
67 from languages like I<awk> and I<C>.
69 Have you tried the Perl debugger, described in L<perldebug>? You can
70 step through your program and see what it's doing and thus work out
71 why what it's doing isn't what it should be doing.
73 =head2 How do I profile my Perl programs?
75 You should get the Devel::DProf module from CPAN, and also use
76 Benchmark.pm from the standard distribution. Benchmark lets you time
77 specific portions of your code, while Devel::DProf gives detailed
78 breakdowns of where your code spends its time.
80 Here's a sample use of Benchmark:
84 @junk = `cat /etc/motd`;
88 'map' => sub { my @a = @junk;
92 'for' => sub { my @a = @junk;
98 This is what it prints (on one machine--your results will be dependent
99 on your hardware, operating system, and the load on your machine):
101 Benchmark: timing 10000 iterations of for, map...
102 for: 4 secs ( 3.97 usr 0.01 sys = 3.98 cpu)
103 map: 6 secs ( 4.97 usr 0.00 sys = 4.97 cpu)
105 =head2 How do I cross-reference my Perl programs?
107 The B::Xref module, shipped with the new, alpha-release Perl compiler
108 (not the general distribution prior to the 5.005 release), can be used
109 to generate cross-reference reports for Perl programs.
111 perl -MO=Xref[,OPTIONS] scriptname.plx
113 =head2 Is there a pretty-printer (formatter) for Perl?
115 There is no program that will reformat Perl as much as indent(1) does
116 for C. The complex feedback between the scanner and the parser (this
117 feedback is what confuses the vgrind and emacs programs) makes it
118 challenging at best to write a stand-alone Perl parser.
120 Of course, if you simply follow the guidelines in L<perlstyle>, you
121 shouldn't need to reformat. The habit of formatting your code as you
122 write it will help prevent bugs. Your editor can and should help you
123 with this. The perl-mode for emacs can provide a remarkable amount of
124 help with most (but not all) code, and even less programmable editors
125 can provide significant assistance.
127 If you are used to using I<vgrind> program for printing out nice code
128 to a laser printer, you can take a stab at this using
129 http://www.perl.com/CPAN/doc/misc/tips/working.vgrind.entry, but the
130 results are not particularly satisfying for sophisticated code.
132 =head2 Is there a ctags for Perl?
134 There's a simple one at
135 http://www.perl.com/CPAN/authors/id/TOMC/scripts/ptags.gz which may do
138 =head2 Where can I get Perl macros for vi?
140 For a complete version of Tom Christiansen's vi configuration file,
141 see http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/toms.exrc,
142 the standard benchmark file for vi emulators. This runs best with nvi,
143 the current version of vi out of Berkeley, which incidentally can be built
144 with an embedded Perl interpreter -- see http://www.perl.com/CPAN/src/misc.
146 =head2 Where can I get perl-mode for emacs?
148 Since Emacs version 19 patchlevel 22 or so, there have been both a
149 perl-mode.el and support for the perl debugger built in. These should
150 come with the standard Emacs 19 distribution.
152 In the perl source directory, you'll find a directory called "emacs",
153 which contains a cperl-mode that color-codes keywords, provides
154 context-sensitive help, and other nifty things.
156 Note that the perl-mode of emacs will have fits with C<"main'foo">
157 (single quote), and mess up the indentation and hilighting. You
158 should be using C<"main::foo"> in new Perl code anyway, so this
159 shouldn't be an issue.
161 =head2 How can I use curses with Perl?
163 The Curses module from CPAN provides a dynamically loadable object
164 module interface to a curses library. A small demo can be found at the
165 directory http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/rep;
166 this program repeats a command and updates the screen as needed, rendering
167 B<rep ps axu> similar to B<top>.
169 =head2 How can I use X or Tk with Perl?
171 Tk is a completely Perl-based, object-oriented interface to the Tk toolkit
172 that doesn't force you to use Tcl just to get at Tk. Sx is an interface
173 to the Athena Widget set. Both are available from CPAN. See the
174 directory http://www.perl.com/CPAN/modules/by-category/08_User_Interfaces/
176 Invaluable for Perl/Tk programming are: the Perl/Tk FAQ at
177 http://w4.lns.cornell.edu/~pvhp/ptk/ptkTOC.html , the Perl/Tk Reference
179 http://www.perl.com/CPAN-local/authors/Stephen_O_Lidie/ , and the
181 http://www-users.cs.umn.edu/~amundson/perl/perltk/toc.html .
183 =head2 How can I generate simple menus without using CGI or Tk?
185 The http://www.perl.com/CPAN/authors/id/SKUNZ/perlmenu.v4.0.tar.gz
186 module, which is curses-based, can help with this.
188 =head2 What is undump?
190 See the next questions.
192 =head2 How can I make my Perl program run faster?
194 The best way to do this is to come up with a better algorithm. This
195 can often make a dramatic difference. Chapter 8 in the Camel has some
196 efficiency tips in it you might want to look at. Jon Bentley's book
197 ``Programming Pearls'' (that's not a misspelling!) has some good tips
198 on optimization, too. Advice on benchmarking boils down to: benchmark
199 and profile to make sure you're optimizing the right part, look for
200 better algorithms instead of microtuning your code, and when all else
201 fails consider just buying faster hardware.
203 A different approach is to autoload seldom-used Perl code. See the
204 AutoSplit and AutoLoader modules in the standard distribution for
205 that. Or you could locate the bottleneck and think about writing just
206 that part in C, the way we used to take bottlenecks in C code and
207 write them in assembler. Similar to rewriting in C is the use of
208 modules that have critical sections written in C (for instance, the
209 PDL module from CPAN).
211 In some cases, it may be worth it to use the backend compiler to
212 produce byte code (saving compilation time) or compile into C, which
213 will certainly save compilation time and sometimes a small amount (but
214 not much) execution time. See the question about compiling your Perl
215 programs for more on the compiler--the wins aren't as obvious as you'd
218 If you're currently linking your perl executable to a shared I<libc.so>,
219 you can often gain a 10-25% performance benefit by rebuilding it to
220 link with a static libc.a instead. This will make a bigger perl
221 executable, but your Perl programs (and programmers) may thank you for
222 it. See the F<INSTALL> file in the source distribution for more
225 Unsubstantiated reports allege that Perl interpreters that use sfio
226 outperform those that don't (for IO intensive applications). To try
227 this, see the F<INSTALL> file in the source distribution, especially
228 the ``Selecting File IO mechanisms'' section.
230 The undump program was an old attempt to speed up your Perl program
231 by storing the already-compiled form to disk. This is no longer
232 a viable option, as it only worked on a few architectures, and
233 wasn't a good solution anyway.
235 =head2 How can I make my Perl program take less memory?
237 When it comes to time-space tradeoffs, Perl nearly always prefers to
238 throw memory at a problem. Scalars in Perl use more memory than
239 strings in C, arrays take more that, and hashes use even more. While
240 there's still a lot to be done, recent releases have been addressing
241 these issues. For example, as of 5.004, duplicate hash keys are
242 shared amongst all hashes using them, so require no reallocation.
244 In some cases, using substr() or vec() to simulate arrays can be
245 highly beneficial. For example, an array of a thousand booleans will
246 take at least 20,000 bytes of space, but it can be turned into one
247 125-byte bit vector for a considerable memory savings. The standard
248 Tie::SubstrHash module can also help for certain types of data
249 structure. If you're working with specialist data structures
250 (matrices, for instance) modules that implement these in C may use
251 less memory than equivalent Perl modules.
253 Another thing to try is learning whether your Perl was compiled with
254 the system malloc or with Perl's builtin malloc. Whichever one it
255 is, try using the other one and see whether this makes a difference.
256 Information about malloc is in the F<INSTALL> file in the source
257 distribution. You can find out whether you are using perl's malloc by
258 typing C<perl -V:usemymalloc>.
260 =head2 Is it unsafe to return a pointer to local data?
262 No, Perl's garbage collection system takes care of this.
270 push @many, makeone();
273 print $many[4][5], "\n";
277 =head2 How can I free an array or hash so my program shrinks?
279 You can't. On most operating systems, memory allocated to a program
280 can never be returned to the system. That's why long-running programs
281 sometimes re-exec themselves. Some operating systems (notably, FreeBSD)
282 allegedly reclaim large chunks of memory that is no longer used, but
283 it doesn't appear to happen with Perl (yet). The Mac appears to be the
284 only platform that will reliably (albeit, slowly) return memory to the OS.
286 However, judicious use of my() on your variables will help make sure
287 that they go out of scope so that Perl can free up their storage for
288 use in other parts of your program. A global variable, of course, never
289 goes out of scope, so you can't get its space automatically reclaimed,
290 although undef()ing and/or delete()ing it will achieve the same effect.
291 In general, memory allocation and de-allocation isn't something you can
292 or should be worrying about much in Perl, but even this capability
293 (preallocation of data types) is in the works.
295 =head2 How can I make my CGI script more efficient?
297 Beyond the normal measures described to make general Perl programs
298 faster or smaller, a CGI program has additional issues. It may be run
299 several times per second. Given that each time it runs it will need
300 to be re-compiled and will often allocate a megabyte or more of system
301 memory, this can be a killer. Compiling into C B<isn't going to help
302 you> because the process start-up overhead is where the bottleneck is.
304 There are two popular ways to avoid this overhead. One solution
305 involves running the Apache HTTP server (available from
306 http://www.apache.org/) with either of the mod_perl or mod_fastcgi
309 With mod_perl and the Apache::Registry module (distributed with
310 mod_perl), httpd will run with an embedded Perl interpreter which
311 pre-compiles your script and then executes it within the same address
312 space without forking. The Apache extension also gives Perl access to
313 the internal server API, so modules written in Perl can do just about
314 anything a module written in C can. For more on mod_perl, see
315 http://perl.apache.org/
317 With the FCGI module (from CPAN), a Perl executable compiled with sfio
318 (see the F<INSTALL> file in the distribution) and the mod_fastcgi
319 module (available from http://www.fastcgi.com/) each of your perl
320 scripts becomes a permanent CGI daemon process.
322 Both of these solutions can have far-reaching effects on your system
323 and on the way you write your CGI scripts, so investigate them with
326 See http://www.perl.com/CPAN/modules/by-category/15_World_Wide_Web_HTML_HTTP_CGI/ .
328 A non-free, commerical product, ``The Velocity Engine for Perl'',
329 (http://www.binevolve.com/ or http://www.binevolve.com/bine/vep) might
330 also be worth looking at. It will allow you to increase the performance
331 of your perl scripts, upto 25 times faster than normal CGI perl by
332 running in persistent perl mode, or 4 to 5 times faster without any
333 modification to your existing CGI scripts. Fully functional evaluation
334 copies are available from the web site.
336 =head2 How can I hide the source for my Perl program?
338 Delete it. :-) Seriously, there are a number of (mostly
339 unsatisfactory) solutions with varying levels of ``security''.
341 First of all, however, you I<can't> take away read permission, because
342 the source code has to be readable in order to be compiled and
343 interpreted. (That doesn't mean that a CGI script's source is
344 readable by people on the web, though, only by people with access to
345 the filesystem) So you have to leave the permissions at the socially
348 Some people regard this as a security problem. If your program does
349 insecure things, and relies on people not knowing how to exploit those
350 insecurities, it is not secure. It is often possible for someone to
351 determine the insecure things and exploit them without viewing the
352 source. Security through obscurity, the name for hiding your bugs
353 instead of fixing them, is little security indeed.
355 You can try using encryption via source filters (Filter::* from CPAN),
356 but crackers might be able to decrypt it. You can try using the byte
357 code compiler and interpreter described below, but crackers might be
358 able to de-compile it. You can try using the native-code compiler
359 described below, but crackers might be able to disassemble it. These
360 pose varying degrees of difficulty to people wanting to get at your
361 code, but none can definitively conceal it (this is true of every
362 language, not just Perl).
364 If you're concerned about people profiting from your code, then the
365 bottom line is that nothing but a restrictive licence will give you
366 legal security. License your software and pepper it with threatening
367 statements like ``This is unpublished proprietary software of XYZ Corp.
368 Your access to it does not give you permission to use it blah blah
369 blah.'' We are not lawyers, of course, so you should see a lawyer if
370 you want to be sure your licence's wording will stand up in court.
372 =head2 How can I compile my Perl program into byte code or C?
374 Malcolm Beattie has written a multifunction backend compiler,
375 available from CPAN, that can do both these things. It is as of
376 Jul-1998 in late alpha release, which means it's fun to play with if
377 you're a programmer but not really for people looking for turn-key
380 Merely compiling into C does not in and of itself guarantee that your
381 code will run very much faster. That's because except for lucky cases
382 where a lot of native type inferencing is possible, the normal Perl
383 run time system is still present and so your program will take just as
384 long to run and be just as big. Most programs save little more than
385 compilation time, leaving execution no more than 10-30% faster. A few
386 rare programs actually benefit significantly (like several times
387 faster), but this takes some tweaking of your code.
389 The 5.005 release of Perl itself, whose main goal is merging the various
390 non-Unix ports back into the one Perl source, will also have preliminary
391 (strictly beta) support for Malcolm's compiler and his light-weight
392 processes (sometimes called ``threads'').
394 You'll probably be astonished to learn that the current version of the
395 compiler generates a compiled form of your script whose executable is
396 just as big as the original perl executable, and then some. That's
397 because as currently written, all programs are prepared for a full
398 eval() statement. You can tremendously reduce this cost by building a
399 shared I<libperl.so> library and linking against that. See the
400 F<INSTALL> podfile in the perl source distribution for details. If
401 you link your main perl binary with this, it will make it miniscule.
402 For example, on one author's system, F</usr/bin/perl> is only 11k in
405 In general, the compiler will do nothing to make a Perl program smaller,
406 faster, more portable, or more secure. In fact, it will usually hurt
407 all of those. The executable will be bigger, your VM system may take
408 longer to load the whole thing, the binary is fragile and hard to fix,
409 and compilation never stopped software piracy in the form of crackers,
410 viruses, or bootleggers. The real advantage of the compiler is merely
411 packaging, and once you see the size of what it makes (well, unless
412 you use a shared I<libperl.so>), you'll probably want a complete
413 Perl install anywayt.
415 =head2 How can I get C<#!perl> to work on [MS-DOS,NT,...]?
419 extproc perl -S -your_switches
421 as the first line in C<*.cmd> file (C<-S> due to a bug in cmd.exe's
422 `extproc' handling). For DOS one should first invent a corresponding
423 batch file, and codify it in C<ALTERNATIVE_SHEBANG> (see the
424 F<INSTALL> file in the source distribution for more information).
426 The Win95/NT installation, when using the ActiveState port of Perl,
427 will modify the Registry to associate the C<.pl> extension with the
428 perl interpreter. If you install another port (Gurusaramy Sarathy's
429 is the recommended Win95/NT port), or (eventually) build your own
430 Win95/NT Perl using WinGCC, then you'll have to modify the Registry
433 Macintosh perl scripts will have the the appropriate Creator and
434 Type, so that double-clicking them will invoke the perl application.
436 I<IMPORTANT!>: Whatever you do, PLEASE don't get frustrated, and just
437 throw the perl interpreter into your cgi-bin directory, in order to
438 get your scripts working for a web server. This is an EXTREMELY big
439 security risk. Take the time to figure out how to do it correctly.
441 =head2 Can I write useful perl programs on the command line?
443 Yes. Read L<perlrun> for more information. Some examples follow.
444 (These assume standard Unix shell quoting rules.)
446 # sum first and last fields
447 perl -lane 'print $F[0] + $F[-1]' *
449 # identify text files
450 perl -le 'for(@ARGV) {print if -f && -T _}' *
452 # remove (most) comments from C program
453 perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c
455 # make file a month younger than today, defeating reaper daemons
456 perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' *
458 # find first unused uid
459 perl -le '$i++ while getpwuid($i); print $i'
461 # display reasonable manpath
462 echo $PATH | perl -nl -072 -e '
463 s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}'
465 Ok, the last one was actually an obfuscated perl entry. :-)
467 =head2 Why don't perl one-liners work on my DOS/Mac/VMS system?
469 The problem is usually that the command interpreters on those systems
470 have rather different ideas about quoting than the Unix shells under
471 which the one-liners were created. On some systems, you may have to
472 change single-quotes to double ones, which you must I<NOT> do on Unix
473 or Plan9 systems. You might also have to change a single % to a %%.
478 perl -e 'print "Hello world\n"'
481 perl -e "print \"Hello world\n\""
484 print "Hello world\n"
485 (then Run "Myscript" or Shift-Command-R)
488 perl -e "print ""Hello world\n"""
490 The problem is that none of this is reliable: it depends on the
491 command interpreter. Under Unix, the first two often work. Under DOS,
492 it's entirely possible neither works. If 4DOS was the command shell,
493 you'd probably have better luck like this:
495 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
497 Under the Mac, it depends which environment you are using. The MacPerl
498 shell, or MPW, is much like Unix shells in its support for several
499 quoting variants, except that it makes free use of the Mac's non-ASCII
500 characters as control characters.
502 There is no general solution to all of this. It is a mess, pure and
503 simple. Sucks to be away from Unix, huh? :-)
505 [Some of this answer was contributed by Kenneth Albanowski.]
507 =head2 Where can I learn about CGI or Web programming in Perl?
509 For modules, get the CGI or LWP modules from CPAN. For textbooks,
510 see the two especially dedicated to web stuff in the question on
511 books. For problems and questions related to the web, like ``Why
512 do I get 500 Errors'' or ``Why doesn't it run from the browser right
513 when it runs fine on the command line'', see these sources:
516 http://www.w3.org/Security/Faq/
519 http://www.boutell.com/faq/
522 http://www.webthing.com/page.cgi/cgifaq
525 http://www.w3.org/pub/WWW/Protocols/HTTP/
528 http://www.w3.org/TR/REC-html40/
529 http://www.w3.org/pub/WWW/MarkUp/
532 http://www.w3.org/CGI/
535 http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt
538 =head2 Where can I learn about object-oriented Perl programming?
540 L<perltoot> is a good place to start, and you can use L<perlobj> and
541 L<perlbot> for reference. Perltoot didn't come out until the 5.004
542 release, but you can get a copy (in pod, html, or postscript) from
543 http://www.perl.com/CPAN/doc/FMTEYEWTK/ .
545 =head2 Where can I learn about linking C with Perl? [h2xs, xsubpp]
547 If you want to call C from Perl, start with L<perlxstut>,
548 moving on to L<perlxs>, L<xsubpp>, and L<perlguts>. If you want to
549 call Perl from C, then read L<perlembed>, L<perlcall>, and
550 L<perlguts>. Don't forget that you can learn a lot from looking at
551 how the authors of existing extension modules wrote their code and
552 solved their problems.
554 =head2 I've read perlembed, perlguts, etc., but I can't embed perl in
555 my C program, what am I doing wrong?
557 Download the ExtUtils::Embed kit from CPAN and run `make test'. If
558 the tests pass, read the pods again and again and again. If they
559 fail, see L<perlbug> and send a bugreport with the output of
560 C<make test TEST_VERBOSE=1> along with C<perl -V>.
562 =head2 When I tried to run my script, I got this message. What does it
565 L<perldiag> has a complete list of perl's error messages and warnings,
566 with explanatory text. You can also use the splain program (distributed
567 with perl) to explain the error messages:
569 perl program 2>diag.out
570 splain [-v] [-p] diag.out
572 or change your program to explain the messages for you:
578 use diagnostics -verbose;
580 =head2 What's MakeMaker?
582 This module (part of the standard perl distribution) is designed to
583 write a Makefile for an extension module from a Makefile.PL. For more
584 information, see L<ExtUtils::MakeMaker>.
586 =head1 AUTHOR AND COPYRIGHT
588 Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
591 When included as an integrated part of the Standard Distribution
592 of Perl or of its documentation (printed or otherwise), this works is
593 covered under Perl's Artistic Licence. For separate distributions of
594 all or part of this FAQ outside of that, see L<perlfaq>.
596 Irrespective of its distribution, all code examples here are public
597 domain. You are permitted and encouraged to use this code and any
598 derivatives thereof in your own programs for fun or for profit as you
599 see fit. A simple comment in the code giving credit to the FAQ would
600 be courteous but is not required.