3 perlfaq3 - Programming Tools ($Revision: 1.27 $, $Date: 1998/07/05 15:07:20 $)
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?
53 Have you tried C<use strict>?
55 Did you check the returns of each and every system call?
57 Did you read L<perltrap>?
59 Have you tried the Perl debugger, described in L<perldebug>?
61 =head2 How do I profile my Perl programs?
63 You should get the Devel::DProf module from CPAN, and also use
64 Benchmark.pm from the standard distribution. Benchmark lets you time
65 specific portions of your code, while Devel::DProf gives detailed
66 breakdowns of where your code spends its time.
68 =head2 How do I cross-reference my Perl programs?
70 The B::Xref module, shipped with the new, alpha-release Perl compiler
71 (not the general distribution prior to the 5.005 release), can be used
72 to generate cross-reference reports for Perl programs.
74 perl -MO=Xref[,OPTIONS] scriptname.plx
76 =head2 Is there a pretty-printer (formatter) for Perl?
78 There is no program that will reformat Perl as much as indent(1) will
79 do for C. The complex feedback between the scanner and the parser
80 (this feedback is what confuses the vgrind and emacs programs) makes it
81 challenging at best to write a stand-alone Perl parser.
83 Of course, if you simply follow the guidelines in L<perlstyle>, you
84 shouldn't need to reformat.
86 Your editor can and should help you with source formatting. The
87 perl-mode for emacs can provide a remarkable amount of help with most
88 (but not all) code, and even less programmable editors can provide
89 significant assistance.
91 If you are used to using vgrind program for printing out nice code to
92 a laser printer, you can take a stab at this using
93 http://www.perl.com/CPAN/doc/misc/tips/working.vgrind.entry, but the
94 results are not particularly satisfying for sophisticated code.
96 =head2 Is there a ctags for Perl?
98 There's a simple one at
99 http://www.perl.com/CPAN/authors/id/TOMC/scripts/ptags.gz which may do
102 =head2 Where can I get Perl macros for vi?
104 For a complete version of Tom Christiansen's vi configuration file,
105 see http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/toms.exrc,
106 the standard benchmark file for vi emulators. This runs best with nvi,
107 the current version of vi out of Berkeley, which incidentally can be built
108 with an embedded Perl interpreter -- see http://www.perl.com/CPAN/src/misc.
110 =head2 Where can I get perl-mode for emacs?
112 Since Emacs version 19 patchlevel 22 or so, there have been both a
113 perl-mode.el and support for the perl debugger built in. These should
114 come with the standard Emacs 19 distribution.
116 In the perl source directory, you'll find a directory called "emacs",
117 which contains a cperl-mode that color-codes keywords, provides
118 context-sensitive help, and other nifty things.
120 Note that the perl-mode of emacs will have fits with "main'foo"
121 (single quote), and mess up the indentation and hilighting. You
122 should be using "main::foo", anyway.
124 =head2 How can I use curses with Perl?
126 The Curses module from CPAN provides a dynamically loadable object
127 module interface to a curses library. A small demo can be found at the
128 directory http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/rep;
129 this program repeats a command and updates the screen as needed, rendering
130 B<rep ps axu> similar to B<top>.
132 =head2 How can I use X or Tk with Perl?
134 Tk is a completely Perl-based, object-oriented interface to the Tk toolkit
135 that doesn't force you to use Tcl just to get at Tk. Sx is an interface
136 to the Athena Widget set. Both are available from CPAN. See the
137 directory http://www.perl.com/CPAN/modules/by-category/08_User_Interfaces/
139 =head2 How can I generate simple menus without using CGI or Tk?
141 The http://www.perl.com/CPAN/authors/id/SKUNZ/perlmenu.v4.0.tar.gz
142 module, which is curses-based, can help with this.
144 =head2 What is undump?
146 See the next questions.
148 =head2 How can I make my Perl program run faster?
150 The best way to do this is to come up with a better algorithm.
151 This can often make a dramatic difference. Chapter 8 in the Camel
152 has some efficiency tips in it you might want to look at.
154 Other approaches include autoloading seldom-used Perl code. See the
155 AutoSplit and AutoLoader modules in the standard distribution for
156 that. Or you could locate the bottleneck and think about writing just
157 that part in C, the way we used to take bottlenecks in C code and
158 write them in assembler. Similar to rewriting in C is the use of
159 modules that have critical sections written in C (for instance, the
160 PDL module from CPAN).
162 In some cases, it may be worth it to use the backend compiler to
163 produce byte code (saving compilation time) or compile into C, which
164 will certainly save compilation time and sometimes a small amount (but
165 not much) execution time. See the question about compiling your Perl
168 If you're currently linking your perl executable to a shared libc.so,
169 you can often gain a 10-25% performance benefit by rebuilding it to
170 link with a static libc.a instead. This will make a bigger perl
171 executable, but your Perl programs (and programmers) may thank you for
172 it. See the F<INSTALL> file in the source distribution for more
175 Unsubstantiated reports allege that Perl interpreters that use sfio
176 outperform those that don't (for IO intensive applications). To try
177 this, see the F<INSTALL> file in the source distribution, especially
178 the "Selecting File IO mechanisms" section.
180 The undump program was an old attempt to speed up your Perl program
181 by storing the already-compiled form to disk. This is no longer
182 a viable option, as it only worked on a few architectures, and
183 wasn't a good solution anyway.
185 =head2 How can I make my Perl program take less memory?
187 When it comes to time-space tradeoffs, Perl nearly always prefers to
188 throw memory at a problem. Scalars in Perl use more memory than
189 strings in C, arrays take more that, and hashes use even more. While
190 there's still a lot to be done, recent releases have been addressing
191 these issues. For example, as of 5.004, duplicate hash keys are
192 shared amongst all hashes using them, so require no reallocation.
194 In some cases, using substr() or vec() to simulate arrays can be
195 highly beneficial. For example, an array of a thousand booleans will
196 take at least 20,000 bytes of space, but it can be turned into one
197 125-byte bit vector for a considerable memory savings. The standard
198 Tie::SubstrHash module can also help for certain types of data
199 structure. If you're working with specialist data structures
200 (matrices, for instance) modules that implement these in C may use
201 less memory than equivalent Perl modules.
203 Another thing to try is learning whether your Perl was compiled with
204 the system malloc or with Perl's builtin malloc. Whichever one it
205 is, try using the other one and see whether this makes a difference.
206 Information about malloc is in the F<INSTALL> file in the source
207 distribution. You can find out whether you are using perl's malloc by
208 typing C<perl -V:usemymalloc>.
210 =head2 Is it unsafe to return a pointer to local data?
212 No, Perl's garbage collection system takes care of this.
220 push @many, makeone();
223 print $many[4][5], "\n";
227 =head2 How can I free an array or hash so my program shrinks?
229 You can't. On most operating systems, memory allocated to a program
230 can never be returned to the system. That's why long-running programs
231 sometimes re-exec themselves. Some operating systems (notably, FreeBSD)
232 allegedly reclaim large chunks of memory that is no longer used, but
233 it doesn't appear to happen with Perl (yet). The Mac appears to be the
234 only platform that will reliably (albeit, slowly) return memory to the OS.
236 However, judicious use of my() on your variables will help make sure
237 that they go out of scope so that Perl can free up their storage for
238 use in other parts of your program. (NB: my() variables also execute
239 about 10% faster than globals.) A global variable, of course, never
240 goes out of scope, so you can't get its space automatically reclaimed,
241 although undef()ing and/or delete()ing it will achieve the same effect.
242 In general, memory allocation and de-allocation isn't something you can
243 or should be worrying about much in Perl, but even this capability
244 (preallocation of data types) is in the works.
246 =head2 How can I make my CGI script more efficient?
248 Beyond the normal measures described to make general Perl programs
249 faster or smaller, a CGI program has additional issues. It may be run
250 several times per second. Given that each time it runs it will need
251 to be re-compiled and will often allocate a megabyte or more of system
252 memory, this can be a killer. Compiling into C B<isn't going to help
253 you> because the process start-up overhead is where the bottleneck is.
255 There are at least two popular ways to avoid this overhead. One
256 solution involves running the Apache HTTP server (available from
257 http://www.apache.org/) with either of the mod_perl or mod_fastcgi
258 plugin modules. With mod_perl and the Apache::* modules (from CPAN),
259 httpd will run with an embedded Perl interpreter which pre-compiles
260 your script and then executes it within the same address space without
261 forking. The Apache extension also gives Perl access to the internal
262 server API, so modules written in Perl can do just about anything a
263 module written in C can. With the FCGI module (from CPAN), a Perl
264 executable compiled with sfio (see the F<INSTALL> file in the
265 distribution) and the mod_fastcgi module (available from
266 http://www.fastcgi.com/) each of your perl scripts becomes a permanent
269 Both of these solutions can have far-reaching effects on your system
270 and on the way you write your CGI scripts, so investigate them with
273 See http://www.perl.com/CPAN/modules/by-category/15_World_Wide_Web_HTML_HTTP_CGI/.
275 A non-free, commerical product, 'The Velocity Engine for Perl',
276 (http://www.binevolve.com/ or http://www.binevolve.com/bine/vep) might
277 also be worth looking at. It will allow you to increase the performance
278 of your perl scripts, upto 25 times faster than normal cgi perl by
279 running in persistent perl mode, or 4 to 5 times faster without any
280 modification to your existing cgi scripts. Fully functional evaluation
281 copies are available from the web site.
283 =head2 How can I hide the source for my Perl program?
285 Delete it. :-) Seriously, there are a number of (mostly
286 unsatisfactory) solutions with varying levels of "security".
288 First of all, however, you I<can't> take away read permission, because
289 the source code has to be readable in order to be compiled and
290 interpreted. (That doesn't mean that a CGI script's source is
291 readable by people on the web, though.) So you have to leave the
292 permissions at the socially friendly 0755 level.
294 Some people regard this as a security problem. If your program does
295 insecure things, and relies on people not knowing how to exploit those
296 insecurities, it is not secure. It is often possible for someone to
297 determine the insecure things and exploit them without viewing the
298 source. Security through obscurity, the name for hiding your bugs
299 instead of fixing them, is little security indeed.
301 You can try using encryption via source filters (Filter::* from CPAN).
302 But crackers might be able to decrypt it. You can try using the byte
303 code compiler and interpreter described below, but crackers might be
304 able to de-compile it. You can try using the native-code compiler
305 described below, but crackers might be able to disassemble it. These
306 pose varying degrees of difficulty to people wanting to get at your
307 code, but none can definitively conceal it (this is true of every
308 language, not just Perl).
310 If you're concerned about people profiting from your code, then the
311 bottom line is that nothing but a restrictive licence will give you
312 legal security. License your software and pepper it with threatening
313 statements like "This is unpublished proprietary software of XYZ Corp.
314 Your access to it does not give you permission to use it blah blah
315 blah." We are not lawyers, of course, so you should see a lawyer if
316 you want to be sure your licence's wording will stand up in court.
318 =head2 How can I compile my Perl program into byte code or C?
320 Malcolm Beattie has written a multifunction backend compiler,
321 available from CPAN, that can do both these things. It is as of
322 Feb-1997 in late alpha release, which means it's fun to play with if
323 you're a programmer but not really for people looking for turn-key
326 I<Please> understand that merely compiling into C does not in and of
327 itself guarantee that your code will run very much faster. That's
328 because except for lucky cases where a lot of native type inferencing
329 is possible, the normal Perl run time system is still present and thus
330 will still take just as long to run and be just as big. Most programs
331 save little more than compilation time, leaving execution no more than
332 10-30% faster. A few rare programs actually benefit significantly
333 (like several times faster), but this takes some tweaking of your
336 The 5.005 release of Perl itself, whose main goal is merging the various
337 non-Unix ports back into the one Perl source, will also have preliminary
338 (strictly beta) support for Malcolm's compiler and his light-weight
339 processes (sometimes called "threads").
341 You'll probably be astonished to learn that the current version of the
342 compiler generates a compiled form of your script whose executable is
343 just as big as the original perl executable, and then some. That's
344 because as currently written, all programs are prepared for a full
345 eval() statement. You can tremendously reduce this cost by building a
346 shared libperl.so library and linking against that. See the
347 F<INSTALL> podfile in the perl source distribution for details. If
348 you link your main perl binary with this, it will make it miniscule.
349 For example, on one author's system, /usr/bin/perl is only 11k in
352 In general, the compiler will do nothing to make a Perl program smaller,
353 faster, more portable, or more secure. In fact, it will usually hurt
354 all of those. The executable will be bigger, your VM system may take
355 longer to load the whole thing, the binary is fragile and hard to fix,
356 and compilation never stopped software piracy in the form of crackers,
357 viruses, or bootleggers. The real advantage of the compiler is merely
358 packaging, and once you see the size of what it makes (well, unless
359 you use a shared I<libperl.so>), you'll probably want a complete
360 Perl install anywayt.
362 =head2 How can I get '#!perl' to work on [MS-DOS,NT,...]?
366 extproc perl -S -your_switches
368 as the first line in C<*.cmd> file (C<-S> due to a bug in cmd.exe's
369 `extproc' handling). For DOS one should first invent a corresponding
370 batch file, and codify it in C<ALTERNATIVE_SHEBANG> (see the
371 F<INSTALL> file in the source distribution for more information).
373 The Win95/NT installation, when using the Activeware port of Perl,
374 will modify the Registry to associate the .pl extension with the perl
375 interpreter. If you install another port, or (eventually) build your
376 own Win95/NT Perl using WinGCC, then you'll have to modify the
379 Macintosh perl scripts will have the the appropriate Creator and
380 Type, so that double-clicking them will invoke the perl application.
382 I<IMPORTANT!>: Whatever you do, PLEASE don't get frustrated, and just
383 throw the perl interpreter into your cgi-bin directory, in order to
384 get your scripts working for a web server. This is an EXTREMELY big
385 security risk. Take the time to figure out how to do it correctly.
387 =head2 Can I write useful perl programs on the command line?
389 Yes. Read L<perlrun> for more information. Some examples follow.
390 (These assume standard Unix shell quoting rules.)
392 # sum first and last fields
393 perl -lane 'print $F[0] + $F[-1]' *
395 # identify text files
396 perl -le 'for(@ARGV) {print if -f && -T _}' *
398 # remove (most) comments from C program
399 perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c
401 # make file a month younger than today, defeating reaper daemons
402 perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' *
404 # find first unused uid
405 perl -le '$i++ while getpwuid($i); print $i'
407 # display reasonable manpath
408 echo $PATH | perl -nl -072 -e '
409 s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}'
411 Ok, the last one was actually an obfuscated perl entry. :-)
413 =head2 Why don't perl one-liners work on my DOS/Mac/VMS system?
415 The problem is usually that the command interpreters on those systems
416 have rather different ideas about quoting than the Unix shells under
417 which the one-liners were created. On some systems, you may have to
418 change single-quotes to double ones, which you must I<NOT> do on Unix
419 or Plan9 systems. You might also have to change a single % to a %%.
424 perl -e 'print "Hello world\n"'
427 perl -e "print \"Hello world\n\""
430 print "Hello world\n"
431 (then Run "Myscript" or Shift-Command-R)
434 perl -e "print ""Hello world\n"""
436 The problem is that none of this is reliable: it depends on the command
437 interpreter. Under Unix, the first two often work. Under DOS, it's
438 entirely possible neither works. If 4DOS was the command shell, I'd
439 probably have better luck like this:
441 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
443 Under the Mac, it depends which environment you are using. The MacPerl
444 shell, or MPW, is much like Unix shells in its support for several
445 quoting variants, except that it makes free use of the Mac's non-ASCII
446 characters as control characters.
448 I'm afraid that there is no general solution to all of this. It is a
449 mess, pure and simple.
451 [Some of this answer was contributed by Kenneth Albanowski.]
453 =head2 Where can I learn about CGI or Web programming in Perl?
455 For modules, get the CGI or LWP modules from CPAN. For textbooks,
456 see the two especially dedicated to web stuff in the question on
457 books. For problems and questions related to the web, like "Why
458 do I get 500 Errors" or "Why doesn't it run from the browser right
459 when it runs fine on the command line", see these sources:
462 http://www.w3.org/Security/Faq/
465 http://www.boutell.com/faq/
468 http://www.webthing.com/page.cgi/cgifaq
471 http://www.w3.org/pub/WWW/Protocols/HTTP/
474 http://www.w3.org/TR/REC-html40/
475 http://www.w3.org/pub/WWW/MarkUp/
478 http://www.w3.org/CGI/
481 http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt
484 =head2 Where can I learn about object-oriented Perl programming?
486 L<perltoot> is a good place to start, and you can use L<perlobj> and
487 L<perlbot> for reference. Perltoot didn't come out until the 5.004
488 release, but you can get a copy (in pod, html, or postscript) from
489 http://www.perl.com/CPAN/doc/FMTEYEWTK/ .
491 =head2 Where can I learn about linking C with Perl? [h2xs, xsubpp]
493 If you want to call C from Perl, start with L<perlxstut>,
494 moving on to L<perlxs>, L<xsubpp>, and L<perlguts>. If you want to
495 call Perl from C, then read L<perlembed>, L<perlcall>, and
496 L<perlguts>. Don't forget that you can learn a lot from looking at
497 how the authors of existing extension modules wrote their code and
498 solved their problems.
500 =head2 I've read perlembed, perlguts, etc., but I can't embed perl in
501 my C program, what am I doing wrong?
503 Download the ExtUtils::Embed kit from CPAN and run `make test'. If
504 the tests pass, read the pods again and again and again. If they
505 fail, see L<perlbug> and send a bugreport with the output of
506 C<make test TEST_VERBOSE=1> along with C<perl -V>.
508 =head2 When I tried to run my script, I got this message. What does it
511 L<perldiag> has a complete list of perl's error messages and warnings,
512 with explanatory text. You can also use the splain program (distributed
513 with perl) to explain the error messages:
515 perl program 2>diag.out
516 splain [-v] [-p] diag.out
518 or change your program to explain the messages for you:
524 use diagnostics -verbose;
526 =head2 What's MakeMaker?
528 This module (part of the standard perl distribution) is designed to
529 write a Makefile for an extension module from a Makefile.PL. For more
530 information, see L<ExtUtils::MakeMaker>.
532 =head1 AUTHOR AND COPYRIGHT
534 Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
537 When included as an integrated part of the Standard Distribution
538 of Perl or of its documentation (printed or otherwise), this works is
539 covered under Perl's Artistic Licence. For separate distributions of
540 all or part of this FAQ outside of that, see L<perlfaq>.
542 Irrespective of its distribution, all code examples here are public
543 domain. You are permitted and encouraged to use this code and any
544 derivatives thereof in your own programs for fun or for profit as you
545 see fit. A simple comment in the code giving credit to the FAQ would
546 be courteous but is not required.