[9763] in Perl-Users-Digest
Perl-Users Digest, Issue: 3356 Volume: 8
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Wed Aug 5 09:06:21 1998
Date: Wed, 5 Aug 98 06:03:11 -0700
From: Perl-Users Digest <Perl-Users-Request@ruby.OCE.ORST.EDU>
To: Perl-Users@ruby.OCE.ORST.EDU (Perl-Users Digest)
Perl-Users Digest Wed, 5 Aug 1998 Volume: 8 Number: 3356
Today's topics:
perlfaq3 - Programming Tools (part 3 of 9) <perlfaq-suggestions@mox.perl.com>
Special: Digest Administrivia (Last modified: 12 Mar 98 (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: 5 Aug 1998 12:28:15 GMT
From: Tom Christiansen <perlfaq-suggestions@mox.perl.com>
Subject: perlfaq3 - Programming Tools (part 3 of 9)
Message-Id: <6q9j4v$25u$1@csnews.cs.colorado.edu>
+-----------------------------------------------------------------------+
| The following document was generated from its original pod using |
| the pod2text program included with the standard perl release, plus |
| small cosmetic mark-ups. The FAQ is also distributed with all Perl |
| releases as standard manpages; their latest versions can be retrieved |
| from http://language.perl.com/misc/faqs.tar.gz if you'd like. |
+-----------------------------------------------------------------------+
NAME
perlfaq3 - Programming Tools ($Revision: 1.29 $, $Date: 1998/08/05
11:57:04 $)
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 the perlfaq2 manpage)? The chances are
that someone has already written a module that can solve your
problem. Have you read the appropriate man pages? 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
Regexps perlre, perlfunc, perlop, perllocale
Moving to perl5 perltrap, perl
Linking w/C perlxstut, perlxs, perlcall, perlguts, perlembed
Various http://www.perl.com/CPAN/doc/FMTEYEWTK/index.html
(not a man-page but still useful)
the perltoc manpage provides a crude table of contents for the perl
man page set.
How can I use Perl interactively?
The typical approach uses the Perl debugger, described in the
perldebug(1) man page, 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?
In general, no. 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 debug my Perl programs?
Have you used `-w'? It enables warnings for 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' or `use vars'.
Did you check the returns of each and every system call? The
operating system (and thus Perl) tells you whether they worked or
not, and if not why.
open(FH, "> /etc/cantwrite")
or die "Couldn't write to /etc/cantwrite: $!\n";
Did you read the perltrap manpage? 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 the perldebug
manpage? 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 CPAN, and also use
Benchmark.pm from the standard distribution. Benchmark 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)
How do I cross-reference my Perl programs?
The B::Xref module, shipped with the new, alpha-release Perl
compiler (not the general distribution prior to the 5.005 release),
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?
There is no program that will reformat Perl as much as indent(1)
does for C. The complex feedback between the scanner and the parser
(this feedback is what confuses the vgrind and emacs programs)
makes it challenging at best to write a stand-alone Perl parser.
Of course, if you simply follow the guidelines in the perlstyle
manpage, 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 for emacs can provide
a remarkable amount of help with most (but not all) code, and even
less programmable editors can provide significant assistance.
If you are used to using *vgrind* program for printing out nice
code to a laser printer, you can take a stab at this using
http://www.perl.com/CPAN/doc/misc/tips/working.vgrind.entry, but
the results are not particularly satisfying for sophisticated code.
Is there a ctags for Perl?
There's a simple one at
http://www.perl.com/CPAN/authors/id/TOMC/scripts/ptags.gz which may
do the trick.
Where can I get Perl macros for vi?
For a complete version of Tom Christiansen's vi configuration file,
see
http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/toms.exrc
, the standard benchmark file for vi emulators. This 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.perl.com/CPAN/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 hilighting. You
should be 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.perl.com/CPAN/authors/Tom_Christiansen/scripts/rep; 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.perl.com/CPAN/modules/by-
category/08_User_Interfaces/
Invaluable for Perl/Tk programming are: the Perl/Tk FAQ at
http://w4.lns.cornell.edu/~pvhp/ptk/ptkTOC.html , the Perl/Tk
Reference Guide available at http://www.perl.com/CPAN-
local/authors/Stephen_O_Lidie/ , and the online manpages at
http://www-users.cs.umn.edu/~amundson/perl/perltk/toc.html .
How can I generate simple menus without using CGI or Tk?
The http://www.perl.com/CPAN/authors/id/SKUNZ/perlmenu.v4.0.tar.gz
module, which is curses-based, can help with this.
What is undump?
See the next questions.
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. Chapter 8 in the Camel has
some efficiency tips in it you might want to look at. 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.
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 is the use
of modules that have critical sections 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 IO intensive applications). To try
this, see the INSTALL file in the source distribution, especially
the ``Selecting File IO 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.
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 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 for 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'.
Is it unsafe to return a pointer to local data?
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";
How can I free an array or hash so my program shrinks?
You 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, FreeBSD) allegedly reclaim large chunks of memory that is
no longer used, but it doesn't appear to happen with Perl (yet).
The Mac appears to be the only platform that will reliably (albeit,
slowly) return memory to the OS.
However, judicious use of my() on your variables will help make
sure that they go out of scope so that Perl can free up their
storage 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), a Perl executable compiled with
sfio (see the INSTALL file in the distribution) and the mod_fastcgi
module (available from http://www.fastcgi.com/) each of your perl
scripts 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 scripts, so investigate
them with care.
See http://www.perl.com/CPAN/modules/by-
category/15_World_Wide_Web_HTML_HTTP_CGI/ .
A non-free, commerical product, ``The Velocity Engine for Perl'',
(http://www.binevolve.com/ or http://www.binevolve.com/bine/vep)
might also be worth looking at. It will allow you to increase the
performance of your perl scripts, upto 25 times faster than normal
CGI perl by running in persistent perl mode, or 4 to 5 times faster
without any modification to your existing CGI scripts. 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 (Filter::* from
CPAN), but crackers might be able to decrypt it. You can try using
the byte code compiler and interpreter described below, but
crackers might 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
(this is 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 licence 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
licence'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 (like 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 miniscule. 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 will
usually hurt all of those. 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 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 `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 (Gurusaramy Sarathy's
is the recommended Win95/NT port), or (eventually) build your own
Win95/NT Perl using WinGCC, then you'll have to modify the Registry
yourself.
Macintosh perl scripts will have the 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 scripts 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 the perlrun manpage 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 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)
# VMS
perl -e "print ""Hello world\n"""
The problem is that none of this is reliable: it depends on the
command interpreter. Under Unix, the first two often work. Under
DOS, it's entirely possible 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.
There is no general solution to all of this. It is a mess, pure and
simple. Sucks to be away from Unix, huh? :-)
[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 these sources:
WWW Security FAQ
http://www.w3.org/Security/Faq/
Web FAQ
http://www.boutell.com/faq/
CGI FAQ
http://www.webthing.com/page.cgi/cgifaq
HTTP Spec
http://www.w3.org/pub/WWW/Protocols/HTTP/
HTML Spec
http://www.w3.org/TR/REC-html40/
http://www.w3.org/pub/WWW/MarkUp/
CGI Spec
http://www.w3.org/CGI/
CGI Security FAQ
http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt
Where can I learn about object-oriented Perl programming?
the perltoot manpage is a good place to start, and you can use the
perlobj manpage and the perlbot manpage for reference. Perltoot
didn't come out until the 5.004 release, but you can get a copy (in
pod, html, or postscript) from
http://www.perl.com/CPAN/doc/FMTEYEWTK/ .
Where can I learn about linking C with Perl? [h2xs, xsubpp]
If you want to call C from Perl, start with the perlxstut manpage,
moving on to the perlxs manpage, the xsubpp manpage, and the
perlguts manpage. If you want to call Perl from C, then read the
perlembed manpage, the perlcall manpage, and the perlguts manpage.
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 the perlbug manpage and send a bugreport 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?
the perldiag manpage has a complete list of perl's error messages
and warnings, with explanatory text. 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 the ExtUtils::MakeMaker manpage.
AUTHOR AND COPYRIGHT
Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
All rights reserved.
When included as an integrated part of the Standard Distribution of
Perl or of its documentation (printed or otherwise), this works is
covered under Perl's Artistic Licence. For separate distributions
of all or part of this FAQ outside of that, see the perlfaq
manpage.
Irrespective of its distribution, all code examples here are 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.
------------------------------
Date: 12 Jul 98 21:33:47 GMT (Last modified)
From: Perl-Request@ruby.oce.orst.edu (Perl-Users-Digest Admin)
Subject: Special: Digest Administrivia (Last modified: 12 Mar 98)
Message-Id: <null>
Administrivia:
Special notice: in a few days, the new group comp.lang.perl.moderated
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End of Perl-Users Digest V8 Issue 3356
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