#!perl to work on [MS-DOS,NT,...]?
perlfaq3 - Programming Tools ($Revision: 1.22 $, $Date: 2002/05/06 13:11:13 $)
This section of the FAQ answers questions related to programmer tools and programming support.
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.
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.
In general, not yet. There is psh available at
http://www.focusresearch.com/gregor/psh
Which includes the following description:
The Perl Shell is a shell that combines the interactive nature
of a Unix shell with the power of Perl. The goal is to eventually
have a full featured shell that behaves as expected for normal
shell activity. But, the Perl Shell will use Perl syntax and
functionality for control-flow statements and other things.
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.
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.
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;
local $_;
for (@a) { s/a/b/ };
return @a },
});
This is what it prints (on one machine--your results will be dependent on your hardware, operating system, and the load on your machine):
Benchmark: timing 10000 iterations of for, map...
for: 4 secs ( 3.97 usr 0.01 sys = 3.98 cpu)
map: 6 secs ( 4.97 usr 0.00 sys = 4.97 cpu)
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.
The B::Xref module can be used to generate cross-reference reports for Perl programs.
perl -MO=Xref[,OPTIONS] scriptname.plx
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 does lots of things related to generating nicely printed output of documents, as does enscript at http://people.ssh.fi/mtr/genscript/ .
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.
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:
For Windows there's also the
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 Mac OS, 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/ ). Mac OS X users can use Unix editors as well.
or a vi clone such as
http://www.vim.org/
win32: http://www.cs.vu.nl/%7Etmgil/vi.html
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:
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/vperl.html ) 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
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).
Pepper and Pe are programming language sensitive text editors for Mac OS X and BeOS respectively ( http://www.hekkelman.com/ ).
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/ .
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.
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 ; this program repeats a command and updates the screen as needed, rendering rep ps axu similar to top.
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 .
The http://www.cpan.org/authors/id/SKUNZ/perlmenu.v4.0.tar.gz module, which is curses-based, can help with this.
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).
In some cases, it may be worth it to use the backend compiler to produce byte code (saving compilation time) or compile into C, which will certainly save compilation time and sometimes a small amount (but not much) execution time. See the question about compiling your Perl programs for more on the compiler--the wins aren't as obvious as you'd hope.
If you're currently linking your perl executable to a shared 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.
Unsubstantiated reports allege that Perl interpreters that use sfio outperform those that don't (for I/O intensive applications). To try this, see the INSTALL file in the source distribution, especially the ``Selecting File I/O mechanisms'' section.
The undump program was an old attempt to speed up your Perl program by storing the already-compiled form to disk. This is no longer a viable option, as it only worked on a few architectures, and wasn't a good solution anyway.
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 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.
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/;
}
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";
}
No, Perl's garbage collection system takes care of this.
sub makeone {
my @a = ( 1 .. 10 );
return \@a;
}
for $i ( 1 .. 10 ) {
push @many, makeone();
}
print $many[4][5], "\n";
print "@many\n";
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.
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.
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).
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.
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.
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.
#!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 ALTERNATIVE_SHEBANG (see the
INSTALL 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.
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. :-)
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.]
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
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
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.
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.
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;
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.