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PERLFAQ3(1)
NAME
perlfaq3 - Programming Tools ($Revision: 1.37 $, $Date: 2003/11/24 19:55:50
$)
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 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 "\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 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: $!\n";
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.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 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/Products/Komodo/index.html ). (Visual Perl,
a Visual Studio.NET plug-in is currently (early 2001) in beta (
http://www.ActiveState.com/Products/VisualPerl/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 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.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.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.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_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 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_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 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_Interfaces/
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/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 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 $, = "\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 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], "\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 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-category/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 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 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:
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.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 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.
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.
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