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PERLFAQ3(1)							   PERLFAQ3(1)

NAME    [Toc]    [Back]

     perlfaq3 -	Programming Tools ($Revision: 1.22 $, $Date: 1997/04/24
     22:43:42 $)

DESCRIPTION    [Toc]    [Back]

     This section of the FAQ answers questions related to programmer tools and
     programming support.

     How do I do (anything)?

     Have you looked at	CPAN (see the perlfaq2 manpage)?  The chances are that
     someone has already written a module that can solve your problem.	Have
     you read the appropriate man pages?  Here's a brief index:

	     Objects	     perlref, perlmod, perlobj,	perltie
	     Data Structures perlref, perllol, perldsc
	     Modules	     perlmod, perlmodlib, 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)

     the perltoc manpage provides a crude table	of contents for	the perl man
     page set.

     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

     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.

     How do I debug my Perl programs?

     Have you used -w?

     Have you tried use	strict?

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PERLFAQ3(1)							   PERLFAQ3(1)

     Did you check the returns of each and every system	call?

     Did you read the perltrap manpage?

     Have you tried the	Perl debugger, described in the	perldebug manpage?

     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.

     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

     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 the perlstyle manpage,
     you shouldn't need	to reformat.

     Your editor can and should	help you with source formatting.  The perlmode
 for emacs can	provide	a remarkable amount of help with most (but not
     all) code,	and even less programmable editors can provide significant

     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.

     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.

     Where can I get Perl macros for vi?

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PERLFAQ3(1)							   PERLFAQ3(1)

     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 .

     Where can I get perl-mode for emacs?

     Since Emacs version 19 patchlevel 22 or so, there have been both a	perlmode.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 contextsensitive
 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.

     How can I use curses with Perl?

     The Curses	module from CPAN provides a dynamically	loadable object	module
     interface to a curses library.

     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.

     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.

     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
     the perlxstut manpage for details.

     What is undump?

     See the next questions.

     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.

									Page 3

PERLFAQ3(1)							   PERLFAQ3(1)

     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

     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
     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 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.

     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

     Another thing to try is learning whether your Perl	was compiled with the
     system malloc or with Perl's builtin malloc.  Whichever one it is,	try
     using the other one and see whether this makes a difference.  Information
     about malloc is in	the INSTALL file in the	source distribution.  You can
     find out whether you are using perl's malloc by typing perl

									Page 4

PERLFAQ3(1)							   PERLFAQ3(1)


     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";

     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 reexec

     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.

     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	recompiled
 and will often allocate a	megabyte or more of system memory,
     this can be a killer.  Compiling into C 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

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PERLFAQ3(1)							   PERLFAQ3(1)

     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 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.

     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	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.

     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.

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PERLFAQ3(1)							   PERLFAQ3(1)

     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

     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 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!

     How can I get '#!perl' to work on [MS-DOS,NT,...]?

     For OS/2 just use

	 extproc perl -S -your_switches

     as	the first line in *.cmd	file (-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 ALTERNATIVE_SHEBANG (see the 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

     Macintosh perl scripts will have the the appropriate Creator and Type, so
     that double-clicking them will invoke the perl application.

     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.

     Can I write useful	perl programs on the command line?

									Page 7

PERLFAQ3(1)							   PERLFAQ3(1)

     Yes.  Read	the perlrun manpage 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 '

     Ok, the last one was actually an obfuscated perl entry. :-)

     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 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:

									Page 8

PERLFAQ3(1)							   PERLFAQ3(1)

       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.]

     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

	 Frequently Asked Questions about CGI Programming, by Nick Kew

	 Perl/CGI programming FAQ, by Shishir Gundavaram and Tom Christiansen

	 The WWW Security FAQ, by Lincoln Stein

	 World Wide Web	FAQ, by	Thomas Boutell

     Where can I learn about object-oriented Perl programming?

     the perltoot manpage is a good place to start, and	you can	use the
     perlobj manpage and the perlbot manpage 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/ .

     Where can I learn about linking C with Perl? [h2xs, xsubpp]

     If	you want to call C from	Perl, start with the perlxstut manpage,	moving
     on	to the perlxs manpage, the xsubpp manpage, and the perlguts manpage.
     If	you want to call Perl from C, then read	the perlembed manpage, the
     perlcall manpage, and the perlguts	manpage.  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.

									Page 9

PERLFAQ3(1)							   PERLFAQ3(1)

     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
     the perlbug manpage and send a bugreport with the output of make test
     TEST_VERBOSE=1 along with perl -V.

     When I tried to run my script, I got this message.	What does it mean?

     the perldiag manpage 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;


	 use diagnostics -verbose;

     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 the ExtUtils::MakeMaker manpage.


     Copyright (c) 1997	Tom Christiansen and Nathan Torkington.	 All rights
     reserved.	See the	perlfaq	manpage	for distribution information.

								       Page 10

PERLFAQ3(1)							   PERLFAQ3(1)

								       PPPPaaaaggggeeee 11111111
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