perlfaq3 - Programming Tools ($Revision: 1.7 $, $Date:
2004/04/07 21:33:08 $)
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, perlvar, perlsyn, perlop, perlsub
Execution perlrun, perldebug
Functions perlfunc
Objects perlref, perlmod, perlobj, perltie
Data Structures perlref, perllol, perldsc
Modules perlmod, perlmodlib, perlsub
Regexes perlre, perlfunc, perlop, perllocale
Moving to perl5 perltrap, perl
Linking w/C perlxstut, perlxs, perlcall,
perlguts, perlembed
Various http://www.cpan.org/misc/old-
doc/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.
Is there a Perl shell?
The psh (Perl sh) is currently at version 1.8. The Perl
Shell is a shell that combines the interactive nature of a
Unix shell with the power of Perl. The goal is a full featured
shell that behaves as expected for normal shell
activity and uses Perl syntax and functionality for control-flow
statements and other things. You can get psh at
http://www.focusresearch.com/gregor/psh/ .
Zoidberg is a similar project and provides a shell written
in perl, configured in perl and operated in perl. It is
intended as a login shell and development environment. It
can be found at http://zoidberg.sf.net/ or your local CPAN
mirror.
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 find which modules are installed on my system?
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->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 sub { push @files, $File::Find::name if -f _ &&
/.pm$/ },
@INC;
print join "0, @files;
If you simply need to quickly check 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).
prompt% 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
How do I debug my Perl programs?
Have you tried "use warnings" or used "-w"? They enable
warnings to detect dubious practices.
Have you tried "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 "my",
"our", or "use vars".
Did you check the return values of each and every system
call? The operating system (and thus Perl) tells you
whether they worked, and if not why.
open(FH, "> /etc/cantwrite")
or die "Couldn't write to /etc/cantwrite: $!0;
Did you read 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 awk and C.
Have you tried the Perl debugger, described in 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.
How do I profile my Perl programs?
You should get the Devel::DProf module from the standard
distribution (or separately on CPAN) and also use Benchmark.pm
from the standard distribution. The Benchmark
module 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;
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)
Be aware that a good benchmark is very hard to write. It
only tests the data you give it and proves little about
the differing complexities of contrasting algorithms.
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?
Perltidy is a Perl script 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 scripts, or
spend much time reading them, you will probably find it
useful. It is available at http://perltidy.source-
forge.net
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 perlmode
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
The a2ps http://www-inf.enst.fr/%7Edemaille/a2ps/black+white.ps.gz
does lots of things related
to generating nicely printed output of documents, as does
enscript at http://people.ssh.fi/mtr/genscript/ .
Is there a ctags for Perl?
Recent versions of ctags do much more than older versions
did. EXUBERANT CTAGS is available from
http://ctags.sourceforge.net/ and does a good job of making
tags files for perl code.
There is also a simple one at
http://www.cpan.org/authors/id/TOMC/scripts/ptags.gz which
may do the trick. It can be easy to hack this into what
you want.
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:
Komodo
ActiveState's cross-platform (as of April 2001 Windows
and Linux), multi-language IDE has Perl support,
including a regular expression debugger and remote
debugging ( http://www.ActiveState.com/Prod-
ucts/Komodo/index.html ). (Visual Perl, a Visual Studio.NET
plug-in is currently (early 2001) in beta (
http://www.ActiveState.com/Products/Visu-
alPerl/index.html )).
The Object System
( http://www.castlelink.co.uk/object_system/ ) is a
Perl web applications development IDE, apparently for
any platform that runs Perl.
Open Perl IDE
( 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.
PerlBuilder
( http://www.solutionsoft.com/perl.htm ) is an integrated
development environment for Windows that supports
Perl development.
visiPerl+
( http://helpconsulting.net/visiperl/ ) From Help Consulting,
for Windows.
OptiPerl
( http://www.optiperl.com/ ) is a Windows IDE with
simulated CGI environment, including debugger and syntax
highlighting editor.
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 cperlmode
(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.bbe-
dit.com/ ) or Alpha ( http://www.kelehers.org/alpha/ ).
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
Elvis
ftp://ftp.cs.pdx.edu/pub/elvis/
http://www.fh-wedel.de/elvis/
Vile
http://dickey.his.com/vile/vile.html
Vim http://www.vim.org/
For vi lovers in general, Windows or elsewhere:
http://www.thomer.com/thomer/vi/vi.html
nvi ( http://www.bostic.com/vi/ , available from CPAN in
src/misc/) is yet another vi clone, unfortunately not
available for Windows, but in UNIX platforms you might be
interested in trying it out, firstly because strictly
speaking it is not a vi clone, it is the real vi, or the
new incarnation of it, and secondly because you can embed
Perl inside it to use Perl as the scripting language. nvi
is not alone in this, though: at least also vim and vile
offer an embedded Perl.
The following are Win32 multilanguage editor/IDESs that
support Perl:
Codewright
http://www.starbase.com/
MultiEdit
http://www.MultiEdit.com/
SlickEdit
http://www.slickedit.com/
There is also a toyedit Text widget based editor written
in Perl that is distributed with the Tk module on CPAN.
The ptkdb ( http://world.std.com/~aep/ptkdb/ ) 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.red-
hat.com/cygwin/ )
Ksh from the MKS Toolkit ( http://www.mks.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/
Zsh ftp://ftp.blarg.net/users/amol/zsh/ , see also
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 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).
BBEdit and BBEdit Lite
are text editors for Mac OS that have a Perl sensitivity
mode ( http://web.barebones.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://alpha.olm.net/ ).
Pepper and Pe are programming language sensitive text editors
for Mac OS X and BeOS respectively (
http://www.hekkelman.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_Chris-
tiansen/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 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 colorcodes
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
highlighting. You are probably using "main::foo" in new
Perl code anyway, so this shouldn't be an issue.
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_Chris-
tiansen/scripts/rep.gz ; this program repeats a command
and updates the screen as needed, rendering rep ps axu
similar to top.
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.cpan.org/modules/by-category/08_User_Inter-
faces/
Invaluable for Perl/Tk programming are the Perl/Tk FAQ at
http://w4.lns.cornell.edu/%7Epvhp/ptk/ptkTOC.html , the
Perl/Tk Reference Guide available at
http://www.cpan.org/authors/Stephen_O_Lidie/ , and the
online manpages at http://www-users.cs.umn.edu/%7Eamundson/perl/perltk/toc.html
.
How can I generate simple menus without using CGI or Tk?
The http://www.cpan.org/authors/id/SKUNZ/perl-
menu.v4.0.tar.gz module, which is curses-based, can help
with this.
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 (<FILE>) {
# ...
}
instead of this:
#
# Bad Idea
#
@data = <FILE>;
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>;
will cause the entire file to be slurped. For large
files, it's better to loop:
while (<FILE>) {
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 $, = "0;
print @big_array;
}
is much more memory-efficient than either
print join "0, @big_array;
or
{
local $" = "0;
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 judgment, 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], "0;
print "@many0;
How can I free an array or hash so my program shrinks?
You usually 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, 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.
However, judicious use of my() on your variables will help
make sure that they go out of scope so that Perl can free
up that space 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.
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 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::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.
Both of 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 http://www.cpan.org/modules/by-cate-
gory/15_World_Wide_Web_HTML_HTTP_CGI/ .
A non-free, commercial product, ``The Velocity Engine for
Perl'', (http://www.binevolve.com/ or
http://www.binevolve.com/velocigen/ ) might also be worth
looking at. It will allow you to increase the performance
of your Perl programs, running programs up to 25 times
faster than normal CGI Perl when running in persistent
Perl mode or 4 to 5 times faster without any modification
to your existing CGI programs. Fully functional evaluation
copies are available from the web site.
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
below, but the curious might still 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
(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?
Malcolm Beattie has written a multifunction backend compiler,
available from CPAN, that can do both these things.
It is included in the perl5.005 release, but is still considered
experimental. This means it's fun to play with if
you're a programmer but not really for people looking for
turn-key solutions.
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 so your program will 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
(even running several times faster), but this
takes some tweaking of your code.
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 minuscule. For example,
on one author's system, /usr/bin/perl is only 11k in size!
In general, the compiler will do nothing to make a Perl
program smaller, faster, more portable, or more secure.
In fact, it can make your situation worse. The executable
will be bigger, your VM system may take longer to load the
whole thing, the binary is fragile and hard to fix, and
compilation never stopped software piracy in the form of
crackers, viruses, or bootleggers. The real advantage of
the compiler is merely packaging, and once you see the
size of what it makes (well, unless you use a shared
libperl.so), you'll probably want a complete Perl install
anyway.
How can I compile Perl into Java?
You can also integrate Java and Perl with the Perl
Resource Kit from O'Reilly and Associates. See
http://www.oreilly.com/catalog/prkunix/ .
Perl 5.6 comes with Java Perl Lingo, or JPL. JPL, still
in development, allows Perl code to be called from Java.
See jpl/README in the Perl source tree.
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".
Macintosh Perl programs will have 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 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 singlequotes
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 world0'
# DOS, etc.
perl -e "print
# Mac
print "Hello world0
(then Run "Myscript" or Shift-Command-R)
# MPW
perl -e 'print "Hello world0'
# VMS
perl -e "print ""Hello world0""
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 world0Ctrl-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:
http://www.perl.org/CGI_MetaFAQ.html
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.
(If you are using really old Perl, you may not have
all of these, try http://www.perldoc.com/ , but consider
upgrading your perl.)
A good book on OO on Perl is the "Object-Oriented Perl" by
Damian Conway from Manning Publications, http://www.man-
ning.com/Conway/index.html
Where can I learn about linking C with Perl? [h2xs,
xsubpp]
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.
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?
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 ExtUtils::MakeMaker.
Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
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 v5.8.5 2002-11-06 19 [ Back ] |
