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Here's a brief index: + + Objects perlref, perlmod, perlobj, perltie + Data Structures perlref, perllol, perldsc + Modules perlmod, perlsub + Regexps perlre, perlfunc, perlop + Moving to perl5 perltrap, perl + Linking w/C perlxstut, perlxs, perlcall, perlguts, perlembed + Various http://www.perl.com/CPAN/doc/FMTEYEWTK/index.html + (not a man-page but still useful) + +L<perltoc> provides a crude table of contents for the perl man page set. + +=head2 How can I use Perl interactively? + +The typical approach uses the Perl debugger, described in the +perldebug(1) man page, on an "empty" program, like this: + + perl -de 42 + +Now just type in any legal Perl code, and it will be immediately +evaluated. You can also examine the symbol table, get stack +backtraces, check variable values, set breakpoints, and other +operations typically found in symbolic debuggers + +=head2 Is there a Perl shell? + +In general, no. The Shell.pm module (distributed with perl) makes +perl try commands which aren't part of the Perl language as shell +commands. perlsh from the source distribution is simplistic and +uninteresting, but may still be what you want. + +=head2 How do I debug my Perl programs? + +Have you used C<-w>? + +Have you tried C<use strict>? + +Did you check the returns of each and every system call? + +Did you read L<perltrap>? + +Have you tried the Perl debugger, described in L<perldebug>? + +=head2 How do I profile my Perl programs? + +You should get the Devel::DProf module from CPAN, and also use +Benchmark.pm from the standard distribution. Benchmark lets you time +specific portions of your code, while Devel::DProf gives detailed +breakdowns of where your code spends its time. + +=head2 How do I cross-reference my Perl programs? + +The B::Xref module, shipped with the new, alpha-release Perl compiler +(not the general distribution), can be used to generate +cross-reference reports for Perl programs. + + perl -MO=Xref[,OPTIONS] foo.pl + +=head2 Is there a pretty-printer (formatter) for Perl? + +There is no program that will reformat Perl as much as indent(1) will +do for C. The complex feedback between the scanner and the parser +(this feedback is what confuses the vgrind and emacs programs) makes it +challenging at best to write a stand-alone Perl parser. + +Of course, if you simply follow the guidelines in L<perlstyle>, you +shouldn't need to reformat. + +Your editor can and should help you with source formatting. The +perl-mode for emacs can provide a remarkable amount of help with most +(but not all) code, and even less programmable editors can provide +significant assistance. + +If you are using to using vgrind program for printing out nice code to +a laser printer, you can take a stab at this using +http://www.perl.com/CPAN/doc/misc/tips/working.vgrind.entry, but the +results are not particularly satisfying for sophisticated code. + +=head2 Is there a ctags for Perl? + +There's a simple one at +http://www.perl.com/CPAN/authors/id/TOMC/scripts/ptags.gz which may do +the trick. + +=head2 Where can I get Perl macros for vi? + +For a complete version of Tom Christiansen's vi configuration file, +see ftp://ftp.perl.com/pub/vi/toms.exrc, the standard benchmark file +for vi emulators. This runs best with nvi, the current version of vi +out of Berkeley, which incidentally can be built with an embedded Perl +interpreter -- see http://www.perl.com/CPAN/src/misc . + +=head2 Where can I get perl-mode for emacs? + +Since Emacs version 19 patchlevel 22 or so, there have been both a +perl-mode.el and support for the perl debugger built in. These should +come with the standard Emacs 19 distribution. + +In the perl source directory, you'll find a directory called "emacs", +which contains a cperl-mode that color-codes keywords, provides +context-sensitive help, and other nifty things. + +Note that the perl-mode of emacs will have fits with "main'foo" +(single quote), and mess up the indentation and hilighting. You +should be using "main::foo", anyway. + +=head2 How can I use curses with Perl? + +The Curses module from CPAN provides a dynamically loadable object +module interface to a curses library. + +=head2 How can I use X or Tk with Perl? + +Tk is a completely Perl-based, object-oriented interface to the Tk +toolkit that doesn't force you to use Tcl just to get at Tk. Sx is an +interface to the Athena Widget set. Both are available from CPAN. + +=head2 How can I generate simple menus without using CGI or Tk? + +The http://www.perl.com/CPAN/authors/id/SKUNZ/perlmenu.v4.0.tar.gz +module, which is curses-based, can help with this. + +=head2 Can I dynamically load C routines into Perl? + +If your system architecture supports it, then the standard perl +on your system should also provide you with this via the +DynaLoader module. Read L<perlxstut> for details. + +=head2 What is undump? + +See the next questions. + +=head2 How can I make my Perl program run faster? + +The best way to do this is to come up with a better algorithm. +This can often make a dramatic difference. Chapter 8 in the Camel +has some efficiency tips in it you might want to look at. + +Other approaches include autoloading seldom-used Perl code. See the +AutoSplit and AutoLoader modules in the standard distribution for +that. Or you could locate the bottleneck and think about writing just +that part in C, the way we used to take bottlenecks in C code and +write them in assembler. Similar to rewriting in C is the use of +modules that have critical sections written in C (for instance, the +PDL module from CPAN). + +In some cases, it may be worth it to use the backend compiler to +produce byte code (saving compilation time) or compile into C, which +will certainly save compilation time and sometimes a small amount (but +not much) execution time. See the question about compiling your Perl +programs. + +If you're currently linking your perl executable to a shared libc.so, +you can often gain a 10-25% performance benefit by rebuilding it to +link with a static libc.a instead. This will make a bigger perl +executable, but your Perl programs (and programmers) may thank you for +it. See the F<INSTALL> file in the source distribution for more +information. + +Unsubstantiated reports allege that Perl interpreters that use sfio +outperform those that don't (for IO intensive applications). To try +this, see the F<INSTALL> file in the source distribution, especially +the "Selecting File IO mechanisms" section. + +The undump program was an old attempt to speed up your Perl program +by storing the already-compiled form to disk. This is no longer +a viable option, as it only worked on a few architectures, and +wasn't a good solution anyway. + +=head2 How can I make my Perl program take less memory? + +When it comes to time-space tradeoffs, Perl nearly always prefers to +throw memory at a problem. Scalars in Perl use more memory than +strings in C, arrays take more that, and hashes use even more. While +there's still a lot to be done, recent releases have been addressing +these issues. For example, as of 5.004, duplicate hash keys are +shared amongst all hashes using them, so require no reallocation. + +In some cases, using substr() or vec() to simulate arrays can be +highly beneficial. For example, an array of a thousand booleans will +take at least 20,000 bytes of space, but it can be turned into one +125-byte bit vector for a considerable memory savings. The standard +Tie::SubstrHash module can also help for certain types of data +structure. If you're working with specialist data structures +(matrices, for instance) modules that implement these in C may use +less memory than equivalent Perl modules. + +Another thing to try is learning whether your Perl was compiled with +the system malloc or with Perl's built-in malloc. Whichever one it +is, try using the other one and see whether this makes a difference. +Information about malloc is in the F<INSTALL> file in the source +distribution. You can find out whether you are using perl's malloc by +typing C<perl -V:usemymalloc>. + +=head2 Is it unsafe to return a pointer to local data? + +No, Perl's garbage collection system takes care of this. + + sub makeone { + my @a = ( 1 .. 10 ); + return \@a; + } + + for $i ( 1 .. 10 ) { + push @many, makeone(); + } + + print $many[4][5], "\n"; + + print "@many\n"; + +=head2 How can I free an array or hash so my program shrinks? + +You can't. Memory the system allocates to a program will never be +returned to the system. That's why long-running programs sometimes +re-exec themselves. + +However, judicious use of my() on your variables will help make sure +that they go out of scope so that Perl can free up their storage for +use in other parts of your program. (NB: my() variables also execute +about 10% faster than globals.) A global variable, of course, never +goes out of scope, so you can't get its space automatically reclaimed, +although undef()ing and/or delete()ing it will achieve the same effect. +In general, memory allocation and de-allocation isn't something you can +or should be worrying about much in Perl, but even this capability +(preallocation of data types) is in the works. + +=head2 How can I make my CGI script more efficient? + +Beyond the normal measures described to make general Perl programs +faster or smaller, a CGI program has additional issues. It may be run +several times per second. Given that each time it runs it will need +to be re-compiled and will often allocate a megabyte or more of system +memory, this can be a killer. Compiling into C B<isn't going to help +you> because the process start-up overhead is where the bottleneck is. + +There are at least two popular ways to avoid this overhead. One +solution involves running the Apache HTTP server (available from +http://www.apache.org/) with either of the mod_perl or mod_fastcgi +plugin modules. With mod_perl and the Apache::* modules (from CPAN), +httpd will run with an embedded Perl interpreter which pre-compiles +your script and then executes it within the same address space without +forking. The Apache extension also gives Perl access to the internal +server API, so modules written in Perl can do just about anything a +module written in C can. With the FCGI module (from CPAN), a Perl +executable compiled with sfio (see the F<INSTALL> file in the +distribution) and the mod_fastcgi module (available from +http://www.fastcgi.com/) each of your perl scripts becomes a permanent +CGI daemon processes. + +Both of these solutions can have far-reaching effects on your system +and on the way you write your CGI scripts, so investigate them with +care. + +=head2 How can I hide the source for my Perl program? + +Delete it. :-) Seriously, there are a number of (mostly +unsatisfactory) solutions with varying levels of "security". + +First of all, however, you I<can't> take away read permission, because +the source code has to be readable in order to be compiled and +interpreted. (That doesn't mean that a CGI script's source is +readable by people on the web, though.) So you have to leave the +permissions at the socially friendly 0755 level. + +Some people regard this as a security problem. If your program does +insecure things, and relies on people not knowing how to exploit those +insecurities, it is not secure. It is often possible for someone to +determine the insecure things and exploit them without viewing the +source. Security through obscurity, the name for hiding your bugs +instead of fixing them, is little security indeed. + +You can try using encryption via source filters (Filter::* from CPAN). +But crackers might be able to decrypt it. You can try using the +byte-code compiler and interpreter described below, but crackers might +be able to de-compile it. You can try using the native-code compiler +described below, but crackers might be able to disassemble it. These +pose varying degrees of difficulty to people wanting to get at your +code, but none can definitively conceal it (this is true of every +language, not just Perl). + +If you're concerned about people profiting from your code, then the +bottom line is that nothing but a restrictive licence will give you +legal security. License your software and pepper it with threatening +statements like "This is unpublished proprietary software of XYZ Corp. +Your access to it does not give you permission to use it blah blah +blah." We are not lawyers, of course, so you should see a lawyer if +you want to be sure your licence's wording will stand up in court. + +=head2 How can I compile my Perl program into byte-code or C? + +Malcolm Beattie has written a multifunction backend compiler, +available from CPAN, that can do both these things. It is as of +Feb-1997 in late alpha release, which means it's fun to play with if +you're a programmer but not really for people looking for turn-key +solutions. + +I<Please> understand that merely compiling into C does not in and of +itself guarantee that your code will run very much faster. That's +because except for lucky cases where a lot of native type inferencing +is possible, the normal Perl run time system is still present and thus +will still take just as long to run and be just as big. Most programs +save little more than compilation time, leaving execution no more than +10-30% faster. A few rare programs actually benefit significantly +(like several times faster), but this takes some tweaking of your +code. + +Malcolm will be in charge of the 5.005 release of Perl itself +to try to unify and merge his compiler and multithreading work into +the main release. + +You'll probably be astonished to learn that the current version of the +compiler generates a compiled form of your script whose executable is +just as big as the original perl executable, and then some. That's +because as currently written, all programs are prepared for a full +eval() statement. You can tremendously reduce this cost by building a +shared libperl.so library and linking against that. See the +F<INSTALL> podfile in the perl source distribution for details. If +you link your main perl binary with this, it will make it miniscule. +For example, on one author's system, /usr/bin/perl is only 11k in +size! + +=head2 How can I get '#!perl' to work on [MSDOS,NT,...]? + +For OS/2 just use + + extproc perl -S -your_switches + +as the first line in C<*.cmd> file (C<-S> due to a bug in cmd.exe's +`extproc' handling). For DOS one should first invent a corresponding +batch file, and codify it in C<ALTERNATIVE_SHEBANG> (see the +F<INSTALL> file in the source distribution for more information). + +The Win95/NT installation, when using the Activeware port of Perl, +will modify the Registry to associate the .pl extension with the perl +interpreter. If you install another port, or (eventually) build your +own Win95/NT Perl using WinGCC, then you'll have to modify the +Registry yourself. + +Macintosh perl scripts will have the the appropriate Creator and +Type, so that double-clicking them will invoke the perl application. + +I<IMPORTANT!>: Whatever you do, PLEASE don't get frustrated, and just +throw the perl interpreter into your cgi-bin directory, in order to +get your scripts working for a web server. This is an EXTREMELY big +security risk. Take the time to figure out how to do it correctly. + +=head2 Can I write useful perl programs on the command line? + +Yes. Read L<perlrun> for more information. Some examples follow. +(These assume standard Unix shell quoting rules.) + + # sum first and last fields + perl -lane 'print $F[0] + $F[-1]' + + # identify text files + perl -le 'for(@ARGV) {print if -f && -T _}' * + + # remove comments from C program + perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c + + # make file a month younger than today, defeating reaper daemons + perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' * + + # find first unused uid + perl -le '$i++ while getpwuid($i); print $i' + + # display reasonable manpath + echo $PATH | perl -nl -072 -e ' + s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}' + +Ok, the last one was actually an obfuscated perl entry. :-) + +=head2 Why don't perl one-liners work on my DOS/Mac/VMS system? + +The problem is usually that the command interpreters on those systems +have rather different ideas about quoting than the Unix shells under +which the one-liners were created. On some systems, you may have to +change single-quotes to double ones, which you must I<NOT> do on Unix +or Plan9 systems. You might also have to change a single % to a %%. + +For example: + + # Unix + perl -e 'print "Hello world\n"' + + # DOS, etc. + perl -e "print \"Hello world\n\"" + + # Mac + print "Hello world\n" + (then Run "Myscript" or Shift-Command-R) + + # VMS + perl -e "print ""Hello world\n""" + +The problem is that none of this is reliable: it depends on the command +interpreter. Under Unix, the first two often work. Under DOS, it's +entirely possible neither works. If 4DOS was the command shell, I'd +probably have better luck like this: + + perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>"" + +Under the Mac, it depends which environment you are using. The MacPerl +shell, or MPW, is much like Unix shells in its support for several +quoting variants, except that it makes free use of the Mac's non-ASCII +characters as control characters. + +I'm afraid that there is no general solution to all of this. It is a +mess, pure and simple. + +[Some of this answer was contributed by Kenneth Albanowski.] + +=head2 Where can I learn about CGI or Web programming in Perl? + +For modules, get the CGI or LWP modules from CPAN. For textbooks, +see the two especially dedicated to web stuff in the question on +books. For problems and questions related to the web, like "Why +do I get 500 Errors" or "Why doesn't it run from the browser right +when it runs fine on the command line", see these sources: + + The Idiot's Guide to Solving Perl/CGI Problems, by Tom Christiansen + http://www.perl.com/perl/faq/idiots-guide.html + + Frequently Asked Questions about CGI Programming, by Nick Kew + ftp://rtfm.mit.edu/pub/usenet/news.answers/www/cgi-faq + http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml + + Perl/CGI programming FAQ, by Shishir Gundavaram and Tom Christiansen + http://www.perl.com/perl/faq/perl-cgi-faq.html + + The WWW Security FAQ, by Lincoln Stein + http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html + + World Wide Web FAQ, by Thomas Boutell + http://www.boutell.com/faq/ + +=head2 Where can I learn about object-oriented Perl programming? + +L<perltoot> is a good place to start, and you can use L<perlobj> and +L<perlbot> for reference. Perltoot didn't come out until the 5.004 +release, but you can get a copy (in pod, html, or postscript) from +http://www.perl.com/CPAN/doc/FMTEYEWTK/ . + +=head2 Where can I learn about linking C with Perl? [h2xs, xsubpp] + +If you want to call C from Perl, start with L<perlxstut>, +moving on to L<perlxs>, L<xsubpp>, and L<perlguts>. If you want to +call Perl from C, then read L<perlembed>, L<perlcall>, and +L<perlguts>. Don't forget that you can learn a lot from looking at +how the authors of existing extension modules wrote their code and +solved their problems. + +=head2 I've read perlembed, perlguts, etc., but I can't embed perl in +my C program, what am I doing wrong? + +Download the ExtUtils::Embed kit from CPAN and run `make test'. If +the tests pass, read the pods again and again and again. If they +fail, see L<perlbug> and send a bugreport with the output of +C<make test TEST_VERBOSE=1> along with C<perl -V>. + +=head2 When I tried to run my script, I got this message. What does it +mean? + +L<perldiag> has a complete list of perl's error messages and warnings, +with explanatory text. You can also use the splain program (distributed +with perl) to explain the error messages: + + perl program 2>diag.out + splain [-v] [-p] diag.out + +or change your program to explain the messages for you: + + use diagnostics; + +or + + use diagnostics -verbose; + +=head2 What's MakeMaker? + +This module (part of the standard perl distribution) is designed to +write a Makefile for an extension module from a Makefile.PL. For more +information, see L<ExtUtils::MakeMaker>. + +=head1 AUTHOR AND COPYRIGHT + +Copyright (c) 1997 Tom Christiansen and Nathan Torkington. +All rights reserved. See L<perlfaq> for distribution information. |