PERLFAQ3(1) Perl Programmers Reference Guide PERLFAQ3(1)
NAME
perlfaq3 - Programming Tools
DESCRIPTION
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 perlfaq2)? The chances are that someone
has already written a module that can solve your problem. Have you
read the appropriate manpages? Here's a brief index:
Basics
perldata - Perl data types
perlvar - Perl pre-defined variables
perlsyn - Perl syntax
perlop - Perl operators and precedence
perlsub - Perl subroutines
Execution
perlrun - how to execute the Perl interpreter
perldebug - Perl debugging
Functions
perlfunc - Perl builtin functions
Objects
perlref - Perl references and nested data structures
perlmod - Perl modules (packages and symbol tables)
perlobj - Perl objects
perltie - how to hide an object class in a simple variable
Data Structures
perlref - Perl references and nested data structures
perllol - Manipulating arrays of arrays in Perl
perldsc - Perl Data Structures Cookbook
Modules
perlmod - Perl modules (packages and symbol tables)
perlmodlib - constructing new Perl modules and finding existing
ones
Regexes
perlre - Perl regular expressions
perlfunc - Perl builtin functions>
perlop - Perl operators and precedence
perllocale - Perl locale handling (internationalization and
localization)
Moving to perl5
perltrap - Perl traps for the unwary
perl
Linking with C
perlxstut - Tutorial for writing XSUBs
perlxs - XS language reference manual
perlcall - Perl calling conventions from C
perlguts - Introduction to the Perl API
perlembed - how to embed perl in your C program
Various
<http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz>; (not a man-page but
still useful, a collection of various essays on Perl techniques)
A crude table of contents for the Perl manpage set is found in perltoc.
How can I use Perl interactively?
The typical approach uses the Perl debugger, described in the
perldebug(1) manpage, 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.
You can also use Devel::REPL which is an interactive shell for Perl,
commonly known as a REPL - Read, Evaluate, Print, Loop. It provides
various handy features.
How do I find which modules are installed on my system?
From the command line, you can use the "cpan" command's "-l" switch:
$ cpan -l
You can also use "cpan"'s "-a" switch to create an autobundle file that
"CPAN.pm" understands and can use to re-install every module:
$ cpan -a
Inside a Perl program, you can use the ExtUtils::Installed module to
show all installed distributions, although it can take awhile to do its
magic. The standard library which comes with Perl just shows up as
"Perl" (although you can get those with Module::CoreList).
use ExtUtils::Installed;
my $inst = ExtUtils::Installed->new();
my @modules = $inst->modules();
If you want a list of all of the Perl module filenames, you can use
File::Find::Rule:
use File::Find::Rule;
my @files = File::Find::Rule->
extras({follow => 1})->
file()->
name( '*.pm' )->
in( @INC )
;
If you do not have that module, you can do the same thing with
File::Find which is part of the standard library:
use File::Find;
my @files;
find(
{
wanted => sub {
push @files, $File::Find::fullname
if -f $File::Find::fullname && /\.pm$/
},
follow => 1,
follow_skip => 2,
},
@INC
);
print join "\n", @files;
If you simply need to check quickly to see if a module is available,
you can check for its documentation. If you can read the documentation
the module is most likely installed. If you cannot read the
documentation, the module might not have any (in rare cases):
$ perldoc Module::Name
You can also try to include the module in a one-liner to see if perl
finds it:
$ perl -MModule::Name -e1
(If you don't receive a "Can't locate ... in @INC" error message, then
Perl found the module name you asked for.)
How do I debug my Perl programs?
(contributed by brian d foy)
Before you do anything else, you can help yourself by ensuring that you
let Perl tell you about problem areas in your code. By turning on
warnings and strictures, you can head off many problems before they get
too big. You can find out more about these in strict and warnings.
#!/usr/bin/perl
use strict;
use warnings;
Beyond that, the simplest debugger is the "print" function. Use it to
look at values as you run your program:
print STDERR "The value is [$value]\n";
The Data::Dumper module can pretty-print Perl data structures:
use Data::Dumper qw( Dumper );
print STDERR "The hash is " . Dumper( \%hash ) . "\n";
Perl comes with an interactive debugger, which you can start with the
"-d" switch. It's fully explained in perldebug.
If you'd like a graphical user interface and you have Tk, you can use
"ptkdb". It's on CPAN and available for free.
If you need something much more sophisticated and controllable, Leon
Brocard's Devel::ebug (which you can call with the "-D" switch as
"-Debug") gives you the programmatic hooks into everything you need to
write your own (without too much pain and suffering).
You can also use a commercial debugger such as Affrus (Mac OS X),
Komodo from Activestate (Windows and Mac OS X), or EPIC (most
platforms).
How do I profile my Perl programs?
(contributed by brian d foy, updated Fri Jul 25 12:22:26 PDT 2008)
The "Devel" namespace has several modules which you can use to profile
your Perl programs.
The Devel::NYTProf (New York Times Profiler) does both statement and
subroutine profiling. It's available from CPAN and you also invoke it
with the "-d" switch:
perl -d:NYTProf some_perl.pl
It creates a database of the profile information that you can turn into
reports. The "nytprofhtml" command turns the data into an HTML report
similar to the Devel::Cover report:
nytprofhtml
You might also be interested in using the Benchmark to measure and
compare code snippets.
You can read more about profiling in Programming Perl, chapter 20, or
Mastering Perl, chapter 5.
perldebguts documents creating a custom debugger if you need to create
a special sort of profiler. brian d foy describes the process in The
Perl Journal, "Creating a Perl Debugger",
<http://www.ddj.com/184404522>; , and "Profiling in Perl"
<http://www.ddj.com/184404580>; .
Perl.com has two interesting articles on profiling: "Profiling Perl",
by Simon Cozens, <http://www.perl.com/lpt/a/850>; and "Debugging and
Profiling mod_perl Applications", by Frank Wiles,
<http://www.perl.com/pub/a/2006/02/09/debug_mod_perl.html>; .
Randal L. Schwartz writes about profiling in "Speeding up Your Perl
Programs" for Unix Review,
<http://www.stonehenge.com/merlyn/UnixReview/col49.html>; , and
"Profiling in Template Toolkit via Overriding" for Linux Magazine,
<http://www.stonehenge.com/merlyn/LinuxMag/col75.html>; .
How do I cross-reference my Perl programs?
The B::Xref module can be used to generate cross-reference reports for
Perl programs.
perl -MO=Xref[,OPTIONS] scriptname.plx
Is there a pretty-printer (formatter) for Perl?
Perl::Tidy comes with a perl script perltidy which indents and
reformats Perl scripts to make them easier to read by trying to follow
the rules of the perlstyle. If you write Perl, or spend much time
reading Perl, you will probably find it useful.
Of course, if you simply follow the guidelines in 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 or newer cperl-mode for emacs can provide
remarkable amounts of help with most (but not all) code, and even less
programmable editors can provide significant assistance. Tom
Christiansen and many other VI users swear by the following settings in
vi and its clones:
set ai sw=4
map! ^O {^M}^[O^T
Put that in your .exrc file (replacing the caret characters with
control characters) and away you go. In insert mode, ^T is for
indenting, ^D is for undenting, and ^O is for blockdenting--as it were.
A more complete example, with comments, can be found at
<http://www.cpan.org/authors/id/TOMC/scripts/toms.exrc.gz>;
Is there an IDE or Windows Perl Editor?
Perl programs are just plain text, so any editor will do.
If you're on Unix, you already have an IDE--Unix itself. The Unix
philosophy is the philosophy of several small tools that each do one
thing and do it well. It's like a carpenter's toolbox.
If you want an IDE, check the following (in alphabetical order, not
order of preference):
Eclipse
http://e-p-i-c.sf.net/ <http://e-p-i-c.sf.net/>;
The Eclipse Perl Integration Project integrates Perl
editing/debugging with Eclipse.
Enginsite
<http://www.enginsite.com/>;
Perl Editor by EngInSite is a complete integrated development
environment (IDE) for creating, testing, and debugging Perl
scripts; the tool runs on Windows 9x/NT/2000/XP or later.
Komodo
<http://www.ActiveState.com/Products/Komodo/>;
ActiveState's cross-platform (as of October 2004, that's Windows,
Linux, and Solaris), multi-language IDE has Perl support, including
a regular expression debugger and remote debugging.
Notepad++
http://notepad-plus.sourceforge.net/ <http://notepad-
plus.sourceforge.net/>
Open Perl IDE
http://open-perl-ide.sourceforge.net/ <http://open-perl-
ide.sourceforge.net/>
Open Perl IDE is an integrated development environment for writing
and debugging Perl scripts with ActiveState's ActivePerl
distribution under Windows 95/98/NT/2000.
OptiPerl
<http://www.optiperl.com/>;
OptiPerl is a Windows IDE with simulated CGI environment, including
debugger and syntax-highlighting editor.
Padre
<http://padre.perlide.org/>;
Padre is cross-platform IDE for Perl written in Perl using
wxWidgets to provide a native look and feel. It's open source under
the Artistic License. It is one of the newer Perl IDEs.
PerlBuilder
<http://www.solutionsoft.com/perl.htm>;
PerlBuilder is an integrated development environment for Windows
that supports Perl development.
visiPerl+
<http://helpconsulting.net/visiperl/index.html>;
From Help Consulting, for Windows.
Visual Perl
<http://www.activestate.com/Products/Visual_Perl/>;
Visual Perl is a Visual Studio.NET plug-in from ActiveState.
Zeus
<http://www.zeusedit.com/lookmain.html>;
Zeus for Window is another Win32 multi-language editor/IDE that
comes with support for Perl.
For editors: if you're on Unix you probably have vi or a vi clone
already, and possibly an emacs too, so you may not need to download
anything. In any emacs the cperl-mode (M-x cperl-mode) gives you
perhaps the best available Perl editing mode in any editor.
If you are using Windows, you can use any editor that lets you work
with plain text, such as NotePad or WordPad. Word processors, such as
Microsoft Word or WordPerfect, typically do not work since they insert
all sorts of behind-the-scenes information, although some allow you to
save files as "Text Only". You can also download text editors designed
specifically for programming, such as Textpad (
<http://www.textpad.com/>; ) and UltraEdit ( <http://www.ultraedit.com/>;
), among others.
If you are using MacOS, the same concerns apply. MacPerl (for Classic
environments) comes with a simple editor. Popular external editors are
BBEdit ( <http://www.bbedit.com/>; ) or Alpha (
<http://www.his.com/~jguyer/Alpha/Alpha8.html>; ). MacOS X users can use
Unix editors as well.
GNU Emacs
<http://www.gnu.org/software/emacs/windows/ntemacs.html>;
MicroEMACS
<http://www.microemacs.de/>;
XEmacs
<http://www.xemacs.org/Download/index.html>;
Jed <http://space.mit.edu/~davis/jed/>;
or a vi clone such as
Vim <http://www.vim.org/>;
Vile
<http://dickey.his.com/vile/vile.html>;
The following are Win32 multilanguage editor/IDEs that support Perl:
Codewright
<http://www.borland.com/codewright/>;
MultiEdit
<http://www.MultiEdit.com/>;
SlickEdit
<http://www.slickedit.com/>;
ConTEXT
<http://www.contexteditor.org/>;
There is also a toyedit Text widget based editor written in Perl that
is distributed with the Tk module on CPAN. The ptkdb (
<http://ptkdb.sourceforge.net/>; ) is a Perl/Tk-based debugger that acts
as a development environment of sorts. Perl Composer (
<http://perlcomposer.sourceforge.net/>; ) is an IDE for Perl/Tk GUI
creation.
In addition to an editor/IDE you might be interested in a more powerful
shell environment for Win32. Your options include
Bash
from the Cygwin package ( <http://sources.redhat.com/cygwin/>; )
Ksh from the MKS Toolkit ( <http://www.mkssoftware.com/>; ), or the
Bourne shell of the U/WIN environment (
<http://www.research.att.com/sw/tools/uwin/>; )
Tcsh
<ftp://ftp.astron.com/pub/tcsh/>; , see also
http://www.primate.wisc.edu/software/csh-tcsh-book/
<http://www.primate.wisc.edu/software/csh-tcsh-book/>;
Zsh <http://www.zsh.org/>;
MKS and U/WIN are commercial (U/WIN is free for educational and
research purposes), Cygwin is covered by the GNU General Public License
(but that shouldn't matter for Perl use). The Cygwin, MKS, and U/WIN
all contain (in addition to the shells) a comprehensive set of standard
Unix toolkit utilities.
If you're transferring text files between Unix and Windows using FTP be
sure to transfer them in ASCII mode so the ends of lines are
appropriately converted.
On Mac OS the MacPerl Application comes with a simple 32k text editor
that behaves like a rudimentary IDE. In contrast to the MacPerl
Application the MPW Perl tool can make use of the MPW Shell itself as
an editor (with no 32k limit).
Affrus
is a full Perl development environment with full debugger support (
<http://www.latenightsw.com>; ).
Alpha
is an editor, written and extensible in Tcl, that nonetheless has
built-in support for several popular markup and programming
languages, including Perl and HTML (
<http://www.his.com/~jguyer/Alpha/Alpha8.html>; ).
BBEdit and BBEdit Lite
are text editors for Mac OS that have a Perl sensitivity mode (
<http://web.barebones.com/>; ).
Where can I get Perl macros for vi?
For a complete version of Tom Christiansen's vi configuration file, see
<http://www.cpan.org/authors/Tom_Christiansen/scripts/toms.exrc.gz>; ,
the standard benchmark file for vi emulators. The file 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.cpan.org/src/misc/>; .
Where can I get perl-mode or cperl-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.
Note that the perl-mode of emacs will have fits with "main'foo" (single
quote), and mess up the indentation and highlighting. You are probably
using "main::foo" in new Perl code anyway, so this shouldn't be an
issue.
For CPerlMode, see http://www.emacswiki.org/cgi-bin/wiki/CPerlMode
<http://www.emacswiki.org/cgi-bin/wiki/CPerlMode>;
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.cpan.org/authors/Tom_Christiansen/scripts/rep.gz>;
; this program repeats a command and updates the screen as needed,
rendering rep ps axu similar to top.
How can I write a GUI (X, Tk, Gtk, etc.) in Perl?
(contributed by Ben Morrow)
There are a number of modules which let you write GUIs in Perl. Most
GUI toolkits have a perl interface: an incomplete list follows.
Tk This works under Unix and Windows, and the current version doesn't
look half as bad under Windows as it used to. Some of the gui
elements still don't 'feel' quite right, though. The interface is
very natural and 'perlish', making it easy to use in small scripts
that just need a simple gui. It hasn't been updated in a while.
Wx This is a Perl binding for the cross-platform wxWidgets toolkit (
<http://www.wxwidgets.org>; ). It works under Unix, Win32 and Mac OS
X, using native widgets (Gtk under Unix). The interface follows the
C++ interface closely, but the documentation is a little sparse for
someone who doesn't know the library, mostly just referring you to
the C++ documentation.
Gtk and Gtk2
These are Perl bindings for the Gtk toolkit ( <http://www.gtk.org>;
). The interface changed significantly between versions 1 and 2 so
they have separate Perl modules. It runs under Unix, Win32 and Mac
OS X (currently it requires an X server on Mac OS, but a 'native'
port is underway), and the widgets look the same on every platform:
i.e., they don't match the native widgets. As with Wx, the Perl
bindings follow the C API closely, and the documentation requires
you to read the C documentation to understand it.
Win32::GUI
This provides access to most of the Win32 GUI widgets from Perl.
Obviously, it only runs under Win32, and uses native widgets. The
Perl interface doesn't really follow the C interface: it's been
made more Perlish, and the documentation is pretty good. More
advanced stuff may require familiarity with the C Win32 APIs, or
reference to MSDN.
CamelBones
CamelBones ( <http://camelbones.sourceforge.net>; ) is a Perl
interface to Mac OS X's Cocoa GUI toolkit, and as such can be used
to produce native GUIs on Mac OS X. It's not on CPAN, as it
requires frameworks that CPAN.pm doesn't know how to install, but
installation is via the standard OSX package installer. The Perl
API is, again, very close to the ObjC API it's wrapping, and the
documentation just tells you how to translate from one to the
other.
Qt There is a Perl interface to TrollTech's Qt toolkit, but it does
not appear to be maintained.
Athena
Sx is an interface to the Athena widget set which comes with X, but
again it appears not to be much used nowadays.
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. 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. You will probably want to read the answer to
the earlier question "How do I profile my Perl programs?" if you
haven't done so already.
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, modules that have critical
sections can be written in C (for instance, the PDL module from CPAN).
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.
The undump program was an ancient attempt to speed up 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 than 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--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 INSTALL file in the source
distribution. You can find out whether you are using perl's malloc by
typing "perl -V:usemymalloc".
Of course, the best way to save memory is to not do anything to waste
it in the first place. Good programming practices can go a long way
toward this:
Don't slurp!
Don't read an entire file into memory if you can process it line by
line. Or more concretely, use a loop like this:
#
# Good Idea
#
while (my $line = <$file_handle>) {
# ...
}
instead of this:
#
# Bad Idea
#
my @data = <$file_handle>;
foreach (@data) {
# ...
}
When the files you're processing are small, it doesn't much matter
which way you do it, but it makes a huge difference when they start
getting larger.
Use map and grep selectively
Remember that both map and grep expect a LIST argument, so doing
this:
@wanted = grep {/pattern/} <$file_handle>;
will cause the entire file to be slurped. For large files, it's
better to loop:
while (<$file_handle>) {
push(@wanted, $_) if /pattern/;
}
Avoid unnecessary quotes and stringification
Don't quote large strings unless absolutely necessary:
my $copy = "$large_string";
makes 2 copies of $large_string (one for $copy and another for the
quotes), whereas
my $copy = $large_string;
only makes one copy.
Ditto for stringifying large arrays:
{
local $, = "\n";
print @big_array;
}
is much more memory-efficient than either
print join "\n", @big_array;
or
{
local $" = "\n";
print "@big_array";
}
Pass by reference
Pass arrays and hashes by reference, not by value. For one thing,
it's the only way to pass multiple lists or hashes (or both) in a
single call/return. It also avoids creating a copy of all the
contents. This requires some judgement, however, because any
changes will be propagated back to the original data. If you really
want to mangle (er, modify) a copy, you'll have to sacrifice the
memory needed to make one.
Tie large variables to disk
For "big" data stores (i.e. ones that exceed available memory)
consider using one of the DB modules to store it on disk instead of
in RAM. This will incur a penalty in access time, but that's
probably better than causing your hard disk to thrash due to
massive swapping.
Is it safe to return a reference to local or lexical data?
Yes. Perl's garbage collection system takes care of this so everything
works out right.
sub makeone {
my @a = ( 1 .. 10 );
return \@a;
}
for ( 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?
(contributed by Michael Carman)
You usually can't. Memory allocated to lexicals (i.e. my() variables)
cannot be reclaimed or reused even if they go out of scope. It is
reserved in case the variables come back into scope. Memory allocated
to global variables can be reused (within your program) by using
undef() and/or delete().
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, systems that use
mmap(2) for allocating large chunks of memory) can reclaim memory that
is no longer used, but on such systems, perl must be configured and
compiled to use the OS's malloc, not perl's.
In general, memory allocation and de-allocation isn't something you can
or should be worrying about much in Perl.
See also "How can I make my Perl program take less memory?"
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 isn't going to help you
because the process start-up overhead is where the bottleneck is.
There are three 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::Registry module (distributed with
mod_perl), 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. For more on mod_perl, see
<http://perl.apache.org/>;
With the FCGI module (from CPAN) and the mod_fastcgi module (available
from <http://www.fastcgi.com/>; ) each of your Perl programs becomes a
permanent CGI daemon process.
Finally, Plack is a Perl module and toolkit that contains PSGI
middleware, helpers and adapters to web servers, allowing you to easily
deploy scripts which can continue running, and provides flexibility
with regards to which web server you use. It can allow existing CGI
scripts to enjoy this flexibility and performance with minimal changes,
or can be used along with modern Perl web frameworks to make writing
and deploying web services with Perl a breeze.
These solutions can have far-reaching effects on your system and on the
way you write your CGI programs, so investigate them with care.
See also
http://www.cpan.org/modules/by-category/15_World_Wide_Web_HTML_HTTP_CGI/
<http://www.cpan.org/modules/by-
category/15_World_Wide_Web_HTML_HTTP_CGI/> .
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--only by people with access to the filesystem.) 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 (Starting from Perl 5.8
the Filter::Simple and Filter::Util::Call modules are included in the
standard distribution), but any decent programmer will be able to
decrypt it. You can try using the byte code compiler and interpreter
described later in perlfaq3, but the curious might still be able to de-
compile it. You can try using the native-code compiler described later,
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 (true of every language, not just Perl).
It is very easy to recover the source of Perl programs. You simply feed
the program to the perl interpreter and use the modules in the B::
hierarchy. The B::Deparse module should be able to defeat most attempts
to hide source. Again, this is not unique to Perl.
If you're concerned about people profiting from your code, then the
bottom line is that nothing but a restrictive license 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 license's wording will stand up in court.
How can I compile my Perl program into byte code or C?
(contributed by brian d foy)
In general, you can't do this. There are some things that may work for
your situation though. People usually ask this question because they
want to distribute their works without giving away the source code, and
most solutions trade disk space for convenience. You probably won't
see much of a speed increase either, since most solutions simply bundle
a Perl interpreter in the final product (but see "How can I make my
Perl program run faster?").
The Perl Archive Toolkit ( <http://par.perl.org/>; ) is Perl's analog to
Java's JAR. It's freely available and on CPAN (
<http://search.cpan.org/dist/PAR/>; ).
There are also some commercial products that may work for you, although
you have to buy a license for them.
The Perl Dev Kit ( <http://www.activestate.com/Products/Perl_Dev_Kit/>;
) from ActiveState can "Turn your Perl programs into ready-to-run
executables for HP-UX, Linux, Solaris and Windows."
Perl2Exe ( <http://www.indigostar.com/perl2exe.htm>; ) is a command line
program for converting perl scripts to executable files. It targets
both Windows and Unix platforms.
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 "ALTERNATE_SHEBANG" (see the dosish.h file
in the source distribution for more information).
The Win95/NT installation, when using the ActiveState port of Perl,
will modify the Registry to associate the ".pl" extension with the perl
interpreter. If you install another port, perhaps even building your
own Win95/NT Perl from the standard sources by using a Windows port of
gcc (e.g., with cygwin or mingw32), then you'll have to modify the
Registry yourself. In addition to associating ".pl" with the
interpreter, NT people can use: "SET PATHEXT=%PATHEXT%;.PL" to let them
run the program "install-linux.pl" merely by typing "install-linux".
Under "Classic" MacOS, a perl program will have the appropriate Creator
and Type, so that double-clicking them will invoke the MacPerl
application. Under Mac OS X, clickable apps can be made from any "#!"
script using Wil Sanchez' DropScript utility:
<http://www.wsanchez.net/software/>; .
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 programs 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?
Yes. Read perlrun for more information. Some examples follow. (These
assume standard Unix shell quoting rules.)
# sum first and last fields
perl -lane 'print $F[0] + $F[-1]' *
# identify text files
perl -le 'for(@ARGV) {print if -f && -T _}' *
# remove (most) comments from C program
perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c
# make file a month younger than today, defeating reaper daemons
perl -e '$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 Contest 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 (including Mac OS X)
perl -e 'print "Hello world\n"'
# DOS, etc.
perl -e "print \"Hello world\n\""
# Mac Classic
print "Hello world\n"
(then Run "Myscript" or Shift-Command-R)
# MPW
perl -e 'print "Hello world\n"'
# VMS
perl -e "print ""Hello world\n"""
The problem is that none of these examples are reliable: they depend on
the command interpreter. Under Unix, the first two often work. Under
DOS, it's entirely possible that neither works. If 4DOS was the command
shell, you'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.
Using qq(), q(), and qx(), instead of "double quotes", 'single quotes',
and `backticks`, may make one-liners easier to write.
There is no general solution to all of this. It is a mess.
[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 the troubleshooting guides and references in
perlfaq9 or in the CGI MetaFAQ:
L<http://www.perl.org/CGI_MetaFAQ.html>;
Looking in to Plack and modern Perl web frameworks is highly
recommended, though; web programming in Perl has evolved a long way
from the old days of simple CGI scripts.
Where can I learn about object-oriented Perl programming?
A good place to start is perltoot, and you can use perlobj, perlboot,
perltoot, perltooc, and perlbot for reference.
A good book on OO on Perl is the "Object-Oriented Perl" by Damian
Conway from Manning Publications, or "Intermediate Perl" by Randal
Schwartz, brian d foy, and Tom Phoenix from O'Reilly Media.
Where can I learn about linking C with Perl?
If you want to call C from Perl, start with perlxstut, moving on to
perlxs, xsubpp, and perlguts. If you want to call Perl from C, then
read perlembed, perlcall, and 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.
You might not need all the power of XS. The Inline::C module lets you
put C code directly in your Perl source. It handles all the magic to
make it work. You still have to learn at least some of the perl API but
you won't have to deal with the complexity of the XS support files.
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
perlbug and send a bug report 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?
A complete list of Perl's error messages and warnings with explanatory
text can be found in perldiag. 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;
What's MakeMaker?
(contributed by brian d foy)
The ExtUtils::MakeMaker module, better known simply as "MakeMaker",
turns a Perl script, typically called "Makefile.PL", into a Makefile.
The Unix tool "make" uses this file to manage dependencies and actions
to process and install a Perl distribution.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and other
authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
Irrespective of its distribution, all code examples here are in the
public domain. You are permitted and encouraged to use this code and
any derivatives thereof in your own programs for fun or for profit as
you see fit. A simple comment in the code giving credit to the FAQ
would be courteous but is not required.
perl v5.16.3 2013-03-04 PERLFAQ3(1)
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