perlfaq3
PERLFAQ3(1) Perl Programmers Reference Guide PERLFAQ3(1)
NAME
perlfaq3 - Programming Tools ($Revision: 1.41 $, $Date: 2004/11/03
22:45:32 $)
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, 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/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 perlde-
bug(1) manpage, on an ``empty'' program, like this:
perl -de 42
Now just type in any legal Perl code, and it will be immediately evalu-
ated. 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 functional-
ity 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://zoid-
berg.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 dis-
tributions, 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 "\n", @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 documenta-
tion, 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 ref-
erences, 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: $!\n";
Did you read perltrap? It's full of gotchas for old and new Perl pro-
grammers 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 perl-
style. If you write Perl scripts, or spend much time reading them, you
will probably find it useful. It is available at
http://perltidy.sourceforge.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 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 Chris-
tiansen 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 con-
trol 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 docu-
ments, 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. EXUBER-
ANT 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 phi-
losophy 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
The Eclipse Perl Integration Project integrates Perl editing/debug-
ging with Eclipse.
The website for the project is http://e-p-i-c.sf.net/
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 (
http://www.ActiveState.com/Products/Komodo/ ).
Open Perl IDE
( http://open-perl-ide.sourceforge.net/ ) Open Perl IDE is an inte-
grated development environment for writing and debugging Perl
scripts with ActiveState's ActivePerl distribution under Windows
95/98/NT/2000.
OptiPerl
( http://www.optiperl.com/ ) is a Windows IDE with simulated CGI
environment, including debugger and syntax highlighting editor.
PerlBuilder
( http://www.solutionsoft.com/perl.htm ) is an integrated develop-
ment environment for Windows that supports Perl development.
visiPerl+
( http://helpconsulting.net/visiperl/ ) 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.
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 per-
haps 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
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.borland.com/codewright/
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 cre-
ation.
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.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.pri-
mate.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 appropri-
ately 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 Appli-
cation 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 enivornment 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 lan-
guages 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/ ).
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_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 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 con-
text-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 mod-
ule 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, ren-
dering 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://pha-
seit.net/claird/comp.lang.perl.tk/ptkFAQ.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/%7Eamund-
son/perl/perltk/toc.html .
How can I generate simple menus without using CGI or Tk?
The http://www.cpan.org/authors/id/SKUNZ/perlmenu.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 exe-
cutable, 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) mod-
ules 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 distribu-
tion. 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 $, = "\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 con-
tents. 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) con-
sider 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 swap-
ping.
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?
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 any-
thing 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 per-
formance 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 crack-
ers might be able to disassemble it. These pose varying degrees of
difficulty to people wanting to get at your code, but none can defini-
tively 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 Media. See http://www.oreilly.com/catalog/prkunix/ .
Perl 5.6 comes with Java Perl Lingo, or JPL. JPL, still in develop-
ment, 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 Reg-
istry yourself. In addition to associating ".pl" with the interpreter,
NT people can use: "SET PATHEXT=%PATHEXT%;.PL" to let them run the pro-
gram "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 secu-
rity 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
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)
# 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 com-
mand 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 refer-
ences 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.perl-
doc.com/ , but consider upgrading your perl.)
A good book on OO on Perl is the "Object-Oriented Perl" by Damian Con-
way from Manning Publications, http://www.manning.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 mod-
ules 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_VER-
BOSE=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.
AUTHOR AND COPYRIGHT
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 v5.8.6 2004-11-05 PERLFAQ3(1)
