perlfaq3.pod

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=head1 NAME

perlfaq3 - Programming Tools ($Revision: 1.29 $, $Date: 1998/08/05 11:57:04 $)

=head1 DESCRIPTION

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

=head2 How do I do (anything)?

Have you looked at CPAN (see L<perlfaq2>)?  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:

	Basics	        perldata, perlvar, perlsyn, perlop, perlsub
	Execution	perlrun, perldebug
	Functions	perlfunc
	Objects		perlref, perlmod, perlobj, perltie
	Data Structures	perlref, perllol, perldsc
	Modules		perlmod, perlmodlib, perlsub
	Regexps		perlre, perlfunc, perlop, perllocale
	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>?  It enables warnings for dubious practices.

Have you tried C<use strict>?  It prevents you from using symbolic
references, makes you predeclare any subroutines that you call as bare
words, and (probably most importantly) forces you to predeclare your
variables with C<my> or C<use vars>.

Did you check the returns of each and every system call?  The operating
system (and thus Perl) tells you whether they worked or not, and if not
why.

  open(FH, "> /etc/cantwrite")
    or die "Couldn't write to /etc/cantwrite: $!\n";

Did you read L<perltrap>?  It's full of gotchas for old and new Perl
programmers, and even has sections for those of you who are upgrading
from languages like I<awk> and I<C>.

Have you tried the Perl debugger, described in L<perldebug>?  You can
step through your program and see what it's doing and thus work out
why what it's doing isn't what it should be doing.

=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.

Here's a sample use of Benchmark:

  use Benchmark;

  @junk = `cat /etc/motd`;
  $count = 10_000;

  timethese($count, {
            'map' => sub { my @a = @junk;
			   map { s/a/b/ } @a;
			   return @a
			 },
            'for' => sub { my @a = @junk;
			   local $_;
			   for (@a) { s/a/b/ };
			   return @a },
           });

This is what it prints (on one machine--your results will be dependent
on your hardware, operating system, and the load on your machine):

  Benchmark: timing 10000 iterations of for, map...
         for:  4 secs ( 3.97 usr  0.01 sys =  3.98 cpu)
         map:  6 secs ( 4.97 usr  0.00 sys =  4.97 cpu)

=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 prior to the 5.005 release), can be used
to generate cross-reference reports for Perl programs.

    perl -MO=Xref[,OPTIONS] scriptname.plx

=head2 Is there a pretty-printer (formatter) for Perl?

There is no program that will reformat Perl as much as indent(1) does
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.  The habit of formatting your code as you
write it will help prevent bugs.  Your editor can and should help you
with this.  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 used to using I<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 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/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 C<"main'foo">
(single quote), and mess up the indentation and hilighting.  You
should be using C<"main::foo"> in new Perl code anyway, so this
shouldn't be an issue.

=head2 How can I use curses with Perl?

The Curses module from CPAN provides a dynamically loadable object
module interface to a curses library.  A small demo can be found at the
directory http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/rep;
this program repeats a command and updates the screen as needed, rendering
B<rep ps axu> similar to B<top>.

=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.  See the
directory http://www.perl.com/CPAN/modules/by-category/08_User_Interfaces/

Invaluable for Perl/Tk programming are: the Perl/Tk FAQ at
http://w4.lns.cornell.edu/~pvhp/ptk/ptkTOC.html , the Perl/Tk Reference
Guide available at
http://www.perl.com/CPAN-local/authors/Stephen_O_Lidie/ , and the
online manpages at
http://www-users.cs.umn.edu/~amundson/perl/perltk/toc.html .

=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 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.  Jon Bentley's book
``Programming Pearls'' (that's not a misspelling!)  has some good tips
on optimization, too.  Advice on benchmarking boils down to: benchmark
and profile to make sure you're optimizing the right part, look for
better algorithms instead of microtuning your code, and when all else
fails consider just buying faster hardware.

A different approach is to autoload 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 for more on the compiler--the wins aren't as obvious as you'd
hope.

If you're currently linking your perl executable to a shared I<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 builtin 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.  On most operating systems, memory allocated to a program
can never be returned to the system.  That's why long-running programs
sometimes re-exec themselves.  Some operating systems (notably, FreeBSD)
allegedly reclaim large chunks of memory that is no longer used, but
it doesn't appear to happen with Perl (yet).  The Mac appears to be the
only platform that will reliably (albeit, slowly) return memory to the OS.

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.  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