PC Online 1996 December/PCO1296.ISO/filesbbs/content/perl-faq.txt

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Subject: comp.lang.perl.* FAQ 0/5 - Introduction
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Archive-name: perl-faq/part0
Version: $Id: perl-intro,v 2.8 1995/05/15 15:43:59 spp Exp spp $
Posting-Frequency: bi-weekly
Last Edited: Thu Jan 11 00:52:58 1996 by spp (Stephen P Potter) on syrinx.psa.com

Hear ye, hear ye.  This most honourable session on Perl Frequently Asked
Questions is hereby called to order.  Please stand and show your respect
for those that have gone before us.  Let us honour Larry Wall*, the father
of Perl who unselfishly spends vast amounts of time reviewing this document
for veracity and accuracy.  Let us also honour Tom Christiansen* without
whom this document would not exist.  Finally, let us collectively honour
the members of comp.lang.perl.misc who have offered much advice and
corrections to this list.

This article details the contents of the most Frequently Asked Questions in
comp.lang.perl.misc, a Usenet newsgroup devoted to the Perl programming
language.  There are five pieces following this, the general information
questions in part1, part2 and part3 and the programming ones in part4 and
part5.

If this is your first exposure to Perl, please read this document and the
perl(1) man page before asking questions in comp.lang.perl.misc.  If you're
using v4 perl, that page contains all you need to know (or at least enough
to get started).  If you're using v5 perl, that page will show you where to
look for specific information.  When we refer to perlmod(1), it means the
"perlmod" man page in section "1" of the manual, just as Foo(3pm), that
means it's the "Foo" man page in section "3pm" (perl modules) of the
library.  The perl install does NOT automatically install the module man
pages for you, however.

Hopefully the questions herein are asked enough that considerable net
bandwidth can be saved by looking here before asking.  Also, hopefully
there is enough information contained here that someone who has never
heard of Perl can read this and at least have some sort of idea as to what
Perl is.

Some questions in this group aren't really about Perl, but rather about
system-specific issues.  You might also consult the Most Frequently Asked
Questions list in comp.unix.questions for answers to this type of question.

The current version of perl is 5.001, perl 5.000 emerged into the world on
16 October, 1994.  The previous non-beta version was 4.036 (version 4,
patchlevel 36).  Many of these questions were written for perl4, however a
lot of perl5 information has also been added.  Perl5 only features will be
clearly marked as such, so as not to cause confusion for those still using
perl4.  You should upgrade to perl5 as soon as possible though (see below).

This list was initially written, and still hacked upon, by Tom
Christiansen*.  However, due to his erratic schedule, it is currently
maintained by Stephen P Potter*.  First person singular pronouns, when not
in quoted postings, generally are Tom talking.

This document, and all its parts, are Copyright (c) 1994/1995, Stephen P
Potter and Tom Christiansen, [e-mail].  All rights reservered.
Permisson to distribute this collection, in part or full, via electronic
means (emailed, posted or archived) or printed copy are granted providing
that no charges are involved, reasonable attempt is made to use the most
current version, and all credits and copyright notices are retained.
Requests for other distribution rights, including incorporation in
commercial products, such as books, magazine articles, or CD-ROMs should be
made to [e-mail].

This FAQ is archived on ftp.cis.ufl.edu [128.227.100.198] in the file
pub/perl/doc/FAQ, as well as on rtfm.mit.edu [18.181.0.24] in
/pub/usenet/comp.lang.perl.*.  If you have any suggested additions or
corrections to this article, please send them to <[e-mail]>.


Part 1 - General Information and Availability
1.1) What is Perl?
1.2) What are perl4 and perl5, are there any differences?
1.3) What features does perl5 provide over perl4?
1.4) Where can I get docs on perl5?
1.5) Will perl5 break my perl4 scripts?
1.6) When will Perl stabilize?
1.7) What's the difference between "perl" and "Perl"?
1.8) Is it a perl program or a perl script?
1.9) Is perl difficult to learn?
1.10) Should I program everything in perl?
1.11) How does perl compare with other scripting languages, like Tcl,
        Python or REXX?
1.12) Where can I get Perl over the Internet (FTP)?
1.13) How can I get Perl via email?
1.14) How can I get Perl via UUCP?
1.15) Are there other ways of getting perl?
1.16) Has perl been ported to machine FOO?
1.17) How do I get perl to compile on Solaris?
1.18) How do I get perl to compile on a NeXT?
1.19) What extensions are available for Perl and where can I get them?
1.20) What is dbperl and where can I get it?
1.21) Which DBM should I use?
1.22) Is there an SNMP aware perl?
1.23) Is there an ISO or ANSI certified version of Perl?


Part 2 - Informational Sources
2.1) Is there a USENET group for perl?
2.2) Have any books or magazine articles been published about perl?
2.3) When will the Camel and Llama books be updated?
2.4) What FTP resources are available?
2.5) What WWW/gopher resources are available?
2.6) Can people who don't have access to USENET get comp.lang.perl.misc?
2.7) Are archives of comp.lang.perl.* available?
2.8) Is there a WAIS server for comp.lang.perl.*?
2.9) What other sources of information about Perl or training are available?
2.10) Where can I get training classes on Perl?
2.11) What companies ship or use perl?
2.12) Is there commercial, third-party support for perl?
2.13) What is a JAPH?  What does "Will hack perl for ..." mean?
2.14) Where can I get a collection of Larry Wall witticisms?
2.15) What are the known bugs?
2.16) Where should I post bugs?
2.17) Where should I post source code?
2.18) Where can I learn about object-orienting Perl programming?
2.19) Where can I learn about linking C with Perl? [h2xs, xsubpp]
2.20) What is perl.com?
2.21) What do the asterisks (*) throughout the FAQ stand for?


Part 3 - Programming Aids
3.1) How do I use perl interactively?
3.2) Is there a perl profiler?
3.3) Is there a yacc for perl?
3.4) Is there a pretty printer (similar to indent(1)) for perl?
3.5) How can I convert my perl scripts directly to C or compile them into
     binary form?
3.6) Where can I get a perl mode for emacs?
3.7) Is there a perl shell?
3.8) How can I use curses with perl?
3.9) How can I use X or Tk with perl?
3.10) Can I dynamically load C user routines?
3.11) What is undump and where can I get it?
3.12) How can I get '#!perl' to work under MS-DOS?
3.13) Can I write useful perl programs on the command line?
3.14) What's a "closure"?


Part 4 - General Programming, Regular Expressions (Regexp) and I/O
4.1) What are all these $@%*<> signs and how do I know when to use them?
4.2) Why do Perl operators have different precedence than C operators?
4.3) What's the difference between dynamic and static (lexical) scoping?
4.4) What's the difference between deep and shallow binding?
4.5) How can I manipulate fixed-record-length files?
4.6) How can I make a file handle local to a subroutine?
4.7) How can I sleep or alarm for under a second?
4.8) How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
4.9) How can I catch signals?
4.10) Why isn't my octal data interpretted correctly?
4.11) How can I compare two date strings?
4.12) How can I find the Julian Day?
4.13) Does perl have a round function?  What about ceil() and floor()?
4.14) What's the fastest way to code up a given task in perl?
4.15) Do I always/never have to quote my strings or use semicolons?
4.16) What is variable suicide and how can I prevent it?
4.17) What does "Malformed command links" mean?
4.18) How can I set up a footer format to be used with write()?
4.19) Why does my program keep growing in size?
4.20) Can I do RPC?
4.21) Why doesn't my sockets program work under System V (Solaris)?  What
        does the error message "Protocol not supported" mean?
4.22) How can I quote a variable to use in a regexp?
4.23) How can I change the first N letters of a string?
4.24) How can I count the number of occurrences of a substring within a
        string?
4.25) Can I use Perl regular expressions to match balanced text?
4.26) What does it mean that regexps are greedy?  How can I get around it?
4.27) How do I use a regular expression to strip C style comments from a
        file?
4.28) How can I split a [character] delimited string except when inside
        [character]?
4.29) Why doesn't "local($foo) = <FILE>;" work right?
4.30) How can I detect keyboard input without reading it?
4.31) How can I read a single character from the keyboard under UNIX and DOS?
4.32) How can I get input from the keyboard without it echoing to the
        screen?
4.33) Is there any easy way to strip blank space from the beginning/end of
        a string?
4.34) How can I output my numbers with commas added?
4.35) How do I expand tags in a string?
4.36) What's wrong with grep in a void context?


Part 5 - Arrays and Shell and External Program Interactions
5.1) What is the difference between $array[1] and @array[1]?
5.2) How can I make an array of arrays or other recursive data types?
5.3) How can I make an array of structures containing various data types?
5.4) How can I extract just the unique elements of an array?
5.5) How can I tell whether an array contains a certain element?
5.6) How can I sort an associative array by value instead of by key?
5.7) How can I know how many entries are in an associative array?
5.8) What's the difference between "delete" and "undef" with %arrays?
5.9) Why don't backticks work as they do in shells?
5.10) Why does my converted awk/sed/sh script run more slowly in perl?
5.11) How can I call my system's unique C functions from perl?
5.12) Where do I get the include files to do ioctl() or syscall()?  [h2ph]
5.13) Why do setuid perl scripts complain about kernel problems?
5.14) How can I open a pipe both to and from a command?
5.15) How can I capture STDERR from an external command?
5.16) Why doesn't open() return an error when a pipe open fails?
5.17) Why can't my script read from STDIN after I gave it ^D (EOF)?
5.18) How can I translate tildes (~) in a filename?
5.19) How can I convert my shell script to perl?
5.20) Can I use perl to run a telnet or ftp session?
5.21) Why do I sometimes get an "Argument list to long" when I use <*>?
5.22) How do I do a "tail -f" in perl?
5.23) Is there a way to hide perl's command line from programs such as "ps"?
5.24) I {changed directory, modified my environment} in a perl script.  How
      come the change disappeared when I exited the script?  How do I get
      my changes to be visible?
5.25) How can I pass a filehandle to a function, or make a list of
      filehandles?
5.26) How can I open a file with a leading ">" or trailing blanks?
5.27) How can I tell if a variable is tainted?
--
Stephen P Potter        Pencom Systems Administration              Beaching It
[e-mail]    Pager: 1-800-759-8888, 547-9561     Work: 703-860-2222
  Cthulhu for President in '96: When You're Tired of the Lesser of Two Evils
--
Stephen P Potter        Pencom Systems Administration              Beaching It
[e-mail]    Pager: 1-800-759-8888, 547-9561     Work: 703-860-2222
"I don't care whether people actually like Perl, just so long as they *think*
    they like it...  ;-)"    -Larry Wall

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Subject: comp.lang.perl.* FAQ 1/5 - Availability
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Archive-name: perl-faq/part1
Version: $Id: part1,v 2.9 1995/05/15 15:44:17 spp Exp spp $
Posting-Frequency: bi-weekly
Last Edited: Thu Jan 11 00:53:46 1996 by spp (Stephen P Potter) on syrinx.psa.com

This posting contains answers to general information and availability
questions.  The following questions are answered in this posting:


1.1) What is Perl?

    Perl is a compiled scripting language written by Larry Wall*.

    Here's the beginning of the description from the perl(1) man page:

    Perl is an interpreted language optimized for scanning arbi-
    trary  text  files,  extracting  information from those text
    files, and printing reports based on that information.  It's
    also  a good language for many system management tasks.  The
    language is intended to be practical  (easy  to  use,  effi-
    cient,  complete)  rather  than  beautiful  (tiny,  elegant,
    minimal).  It combines (in  the  author's  opinion,  anyway)
    some  of the best features of C, sed, awk, and sh, so people
    familiar with those languages should have little  difficulty
    with  it.  (Language historians will also note some vestiges
    of csh, Pascal,  and  even  BASIC-PLUS.)  Expression  syntax
    corresponds  quite  closely  to C expression syntax.  Unlike
        most Unix utilities, perl does  not  arbitrarily  limit  the
    size  of your data--if you've got the memory, perl can slurp
        in your whole file as a  single  string.   Recursion  is  of
    unlimited  depth.   And  the hash tables used by associative
        arrays grow as necessary to  prevent  degraded  performance.
    Perl  uses sophisticated pattern matching techniques to scan
        large amounts of data very quickly.  Although optimized  for
    scanning  text, perl can also deal with binary data, and can
        make dbm files look like associative arrays  (where  dbm  is
    available).   Setuid  perl scripts are safer than C programs
        through a dataflow tracing  mechanism  which  prevents  many
    stupid  security  holes.   If  you have a problem that would
        ordinarily use sed or awk or sh, but it exceeds their  capa-
    bilities  or must run a little faster, and you don't want to
        write the silly thing in C, then perl may be for you.  There
    are  also  translators to turn your sed and awk scripts into
        perl scripts.  OK, enough hype.


1.2) What are perl4 and perl5, are there any differences?

    Perl4 and perl5 are different versions of the language.  Perl4 was the
    previous release, and perl5 is "Perl: The Next Generation."
    Perl5 is, essentially, a complete rewrite of the perl source code
    from the ground up.  It has been modularized, object oriented,
    tweaked, trimmed, and optimized until it almost doesn't look like
    the old code.  However, the interface is mostly the same, and
    compatibility with previous releases is very high.


1.3) What features does perl5 provide over perl4?

    If you get the newest source (from any of the main FTP sites), you will
    find a directory full of man pages (possibly to be installed as section
    1p and 3pm) that discuss the differences, new features, old
    incompatibilies and much more.  Here, however, are some highlights as
    to the new features and old incompatibilities.

    * Enhanced Usability:  Perl code can now be written in a much more
    legible style.  Regular expressions have been enhanced to allow
    minimal matches, conditionals, and much more.  Cryptic variable
    names (although still supported) have been aliased to new
        nmemonics, using the "English" module, allowing old scripts to run
        and new scripts to be readable.  Error messages and optional
        warnings are more informative and will catch many common mistakes.
        See the perldiag(1) man page, which contains pithy prose from Larry
        Wall* on each and every possible muttering perl might spout at you.
    * Simplified Grammar:  The new yacc grammar is one half the size of
    the old one.  Many of the arbitrary grammar rules have been
    regularized.  The number of reserved words has been cut by 2/3.
    * Lexical Scoping:  Perl variables may now be declared within a
    lexical scope, similar to C's "auto" variables.  This is a
    great improvement on efficiency and contributes to better
    privacy.  See the my() entry in perlfunc(1).
    * Arbitrarily nested data structures:  Full fledged multidimensional
    arrays.  Any scalar value, including an array element, may now
    contain a reference to any other variable or subroutine.
    Easily created anonymous variables and subroutines.  See
        perlref(1).
    * Modularity and Reusability:  The Perl library is now defined in
    terms of modules which can be easily shared among various
    packages.  Packages can import any or all of a module's
    published interface.  See perlmod(1), perlsub(1), and
        Exporter(3pm).
    * Object-oriented programming:  A package can function as a class.
    Dynamic multiple inheritance and virtual methods are supported
    in a straight-forward manner with little new syntax.  Filehandles
    are now treated as objects.  See perlobj(1), perlmod(1), and
        FileHandle(3pm).
    * Embeddable and Extensible:  Perl can be easily embedded in C/C++
    applications, and can either call or be called by your routines
        through a documented interface.  The XS preprocessor is provided to
        make it easy to glue your C/C++ routines into Perl.  Dynamic
        loading of modules is supported.  See perlapi(1), perlcall(1), and
        DynaLoader(3pm).
    * POSIX compliant:  A major new module is the POSIX module, which
    provides access to all available POSIX routines and definitions.
        Seee POSIX(3pm).
    * Package constructors and destructors:  The new BEGIN and END blocks
        provide means to capture control as a package is being compiled and
        after the program exits.  As a degenerate case, they work just like
        awk's BEGIN and END when you use the -p or -n switches.  See
        perlmod(1).
    * Multiple simultaneous DBM implementations:  A perl program now has
        access to DBM, NDBM, SDBM, GDBM and Berkeley DB files in the same
        script.  The dbmopen interface has been generalized to allow any
        variable to be tied to an object class which defines its access
        methods.  tie/untie now preferable to dbmopen/dbmclose.  See the
        tie() entry in perlfunc(1) and the DB_File(3pm) man pages.
    * Subroutine definitions may now be autoloaded:  The AUTOLOAD mechanism
        allows any arbitrary semantics to undefined subroutine calls.  See
        the section on Autoloading in the perlsub(1) manpage.
    * Regular Expression Enhancements:  Qualifiers may be followed by a "?"
    to signify that they should be non-greedy.  A "?" directly after
    an opening paren indicates non backreference grouping and the next
    character determines the purpose of the match (?:a|b|c) will match
    any of a b or c without producing a backreference, (?=stuff) does
    a non-eating look ahead to assure that the next thing is stuff,
    (?!nonsense) looks ahead to assure that the next thing must not
    be "nonsense".  Embedded whitespace and comments for readability.
    A consistent extensibility mechanism has been added that is
    upwardly compatible with all old regexps.  Variables may now be
    interpolated literally into a pattern with \Q or the quotemeta
    function, which works like \U but backwhacks non-alphanumerics.
    New m and s "flags" for pattern matching force multi- or
    single-line matching.  The "s" makes "." match "\n".  \A and
    \Z anchor matches to the beginning and end of a string and ignore
    multiline semantics.  \G matches where the previous m//g or s///g
    left off.
    * The -w (warnings) switch is much more informative.
    * References and Objects (see t/op/ref.t) for examples.
    * => is a synonym for comma and helps group paired arguments, such
    as initializers for associative arrays and named arguments to
    subroutines.
    * All functions, even predeclared subroutines, are treated as list
        operators or unary operators.  Parens are optional.
    * Flattened interpreter:  Compare perl4's eval.c with perl5's pp.c.
    Compare perl4's 900 line interpreter look with perl5's one line.
    * eval is now treated like a subroutine call, meaning (among other
    things) you can return from it.
    * format value lists may be spread over multiple lines with a do {}
    block.
    * flags on the #! line are interpreted even if the script wasn't
    invoked directly.
    * ?: is now an lvalue.
    * list context now propogates to the right side of && and ||, and
    as the 2nd and 3rd arguments of ?:
    * preferred package delimiter now :: rather than '.
    * new "and" and "or" operators, like && and || but with a lower
    precedence than comma, so they work better with list operators.
    * New functions abs(), chr(), uc(), ucfirst(), lc(), and lcfirst()
    * require(number) checks to see that the version is at least that
    version
    * qw//, which is equivalent to split(' ', q//)
    * assignment of a reference to a glob replaces the single element
    of the glob corresponding to the reference type:
        *foo = \$bar, * foo = \&bletch;
    * filehandle methods are supported:
    output_autoflush STDOUT 1;
    * Autoload stubs can now call the replacement subroutine with
    goto &realsub.
    * Subroutines can be defined lazily in any package by declaring
    an AUTOLOAD routine, which will be called if a non-existent
    subroutine is called in that package.
    * "use" and "no" subsume many features.  "use Module LIST" is
    short for "BEGIN { require Module; import Module LIST }"
    "no" is identical, except that it calls "unimport" instead.
    "use integer" and variations of "use strict [vars,refs,subs]"
    were implemented through new modules.

(Thanks to Tom Christiansen* for this section)


1.4) Where can I get docs on perl5?

    The complete perl documentation is available with the Perl
    distribution, or can be accessed from the following sites.
    Note that the PerlDoc ps file is 240 pages long!!

    Marked Up (HTML) format:
        http://www.metronet.com/0/perlinfo/perl5/manual/perl.html
        http://web.nexor.co.uk/perl/perl.html                    (Europe)

    PostScript:
    ftp://ftp.cis.ufl.edu/pub/perl/doc/PerlDoc.ps.gz
        ftp://ftp.uu.net/languages/perl/PerlDoc.ps.gz
        ftp://www.metronet.com/pub/perl/perl5/manual/PerlDoc.ps.gz
        ftp://ftp.zrz.tu-berlin.de/pub/unix/perl/PerlDoc.ps.gz  (Europe)
        ftp://ftp.cs.ruu.nl/pub/PERL/perl5.0/doc/PerlDoc.ps.gz  (Europe)
        ftp://sungear.mame.mu.oz.au/pub/perl/doc/PerlDoc.ps.gz  (Oz)
        unable to access as of 7/15/95

    TeXinfo (Emacs) Format:
        ftp://www.metronet.com/pub/perl/perl5/manual/perl5-info.tar.gz


1.5) Will perl5 break my perl4 scripts?

    In general, no.  However, certain bad old practices have become highly
    frowned upon.  The following are the most important of the known
    incompatibilities between perl4 and perl5.  See perltrap(1) for more
    details.

    * "@" ***ALWAYS*** interpolates in double quoted strings.  Non-array
    "@"s must be escaped:
        Mail("[e-mail]") needs to be
        Mail("foo\@bar.com");
    The compiler catches this.
    * "open FILE || die" needs to be "open(FILE) || die".  The compiler
      forgives you for this, but won't stop complaining until you fix it.
    * Barewords are no longer (necessarily) strings: they will actually
      call the function (if it existed when that code was compiled)
      instead of returning a string of that value.  Check your
      signal handlers.  The 'use strict subs' pragma (see strict(3pm))
      will help you with this.
    * "shift @x + 20" needs to be "shift(@x) + 20" because of precedence,
      and likewise "$y = scalar keys %foo + 30" needs to be instead
      "$y = scalar keys(%foo) + 30".
    * The internal symbol table is called %{PACKAGE::} for any given
      package.  It used to be %{_PACKAGE}.
    * You may no longer (attempt to) write to read-only variables, like $1,
      or assign to a substr() past the end of a string.
    * Various deprecated practices elicit warning messages.
    * The package delimiter has been changed from ' to ::.  Use of ' is
      deprecated, but still works.  Use of :: may break scripts if you
      aren't careful (especially if you are working with colon delimited
      data files, like /etc/passwd)


1.6) When will Perl stabilize?

When asked at what point the Perl code would be frozen, Larry answered:

    Part of the redesign of Perl is to *allow* us to more or less freeze
    the language itself.  It won't totally freeze, of course, but I think
    the rate of change of the core of the language is asymptotically
    approaching 0.  In fact, as time goes on, now that we have an official
    extension mechanism, some of the things that are currently in the core
    of the language may move out (transparently) as extensions.  This has
    already happened to dbmopen().

    I've also been continuously reminding myself of what Henry Spencer
    calls "second system syndrome", in which everything under the sun gets
    added, resulting in a colossal kludge, like OS 360.  You'll find that
    the new features in Perl 5 are all pretty minimalistic.  The
    object-oriented features in particular added only one new piece of
    syntax, a C++-style method call.

    : The whole idea behind
    : Perl is to be a fast text-processing, system-maintenance, zero-startup
    : time language. If it gets to be so large and complicated that it isn't
    : fast-running and easy to use, it won't be to anyone's benefit.

    My motto from the start has been, "If it ain't broke, don't fix it."
    I've been trying very hard not to remove those features from Perl that
    make it what it is.  At the same time, a lot of streamlining has gone
    into the syntax.  The new yacc file is about half the size of the old
    one, and the number of official reserved words has been cut by 2/3.
    All built-in functions have been unified (dualified?) as either list
    operators or unary operators.

    : I really like a lot of the features in Perl, but in order for Perl to
    : be useful on a long term basis, those features have to stay put. I
    : bought the Camel book less than a year ago and it sounds like within
    : another year it will be obsolete.

    The parts of Perl that the Camel book covers have not changed all that
    much.  Most old scripts still run.  Many scripts from Perl version 1.0
    still run.  We'll certainly be revising the Camel, but the new man
    pages are split up such that it's pretty easy to ferret out the new
    info when you want it.

    We did break a few misfeatures in going to Perl 5.  It seemed like the
    first and last chance to do so.  There's a list of the
    incompatibilities in the documentation.

    : Not only is it a lot of work to recompile Perl
    : on 20+ machines periodically, but it's hard to write scripts that are
    : useful in the long term if the guts of the language keep changing.
    : (And if I keep having to buy new books. I keep hearing about new
    : features of Perl 5 that aren't documented in any of the perl 5
    : documentation that *I* can find.)

    I think you'll find a lot of folks who think that 4.036 has been a
    pretty stable platform.

    Perl 5 is a special case.  I've been working on it for years.  (This is
    part of the reason 4.036 has been so stable!)  There are many changes,
    most of them for the better, I hope.  I don't expect the transition to
    be without pain.  But that's why I stuck numbered versions out in your
    bin directory, so that you can upgrade piecemeal if you like.  And
    that's why I made the -w switch warn about many of the incompatibilities.

    And overriding all that, I've tried to keep it so that you don't have
    to know much about the new stuff to use the old stuff.  You can upgrade
    your *knowledge* piecemeal too.

    The extension mechanism is designed to take over most of the
    evolutionary role from now on.  And it's set up so that, if you don't
    have a particular extension, you know it right up at the front.

    : Are there any plans to write a Perl compiler? While interpreted Perl
    : is great for many applications, it would also be cool to be able to
    : precompile many scripts. (Yes, I know you can undump things, but
    : undump isn't provided with Perl and I haven't found a copy.) The
    : creation of a perl library and dynamically-loadable modules seems
    : like a step in that direction.

    Yes, part of the design of Perl 5 was to make it *possible* to write a
    compiler for it.  It could even be done as an extension module, I
    suppose.  Anyone looking for a master's thesis topic?

    In summary, almost every concern that you might think of has already
    been (at least) thought about.  In a perfect world, every concern
    could be addressed perfectly.  But in this world we just have to slog
    through.


1.7) What's the difference between "perl" and "Perl"?

    32!  [ ord('p') - ord('P') ]  (Shouldn't that be 42, the Answer to the
        Great Question of Life, the Universe, and Everything?  ;)

    Larry now uses "Perl" to signify the language proper and "perl" the
    implementation of it, i.e. the current interpreter.  Hence Tom's
    quip that "Nothing but perl can parse Perl."

    On the other hand, the aesthetic value of casewise parallelism in
    "awk", "sed", and "perl" as much require the lower-case version as "C",
    "Pascal", and "Perl" require the upper-case version.  It's also easier
    to type "Perl" in typeset print than to be constantly switching in
    Courier. :-)

    In other words, it doesn't matter much, especially if all you're doing
    is hearing someone talk about the language; case is hard to distinguish
    aurally.


1.8) Is it a perl program or a perl script?

    It depends on whether you are talking about the perl binary or
    something that you wrote using perl.  And, actually, even this isn't
    necessarily true.

    "Standard" UNIX terminology is (roughly) this:  programs are compiled
    into machine code once and run multiple times, scripts are translated
    (by a program) each time they are used.  However, some say that a
    program is anything written which is executed on a computer system.
    Larry considers it a program if it is set in stone and you can't change
    it, whereas if you can go in and hack at it, it's a script.  Of course,
    if you have the source code, that makes just about anything a
    script.  ;)

    In general, it probably doesn't really matter.  The terms are used
    interchangeably.  If you particularly like one or the other, use it.  If
    you want to call yourself a perl programmer, call them programs.  If
    you want to call yourself a perl scripter, call them scripts.  Randal*
    and I (at least) will call them hacks.  (See question 2.10 ;)

    Larry says that a script is what you give an actor, but a program is
    what you give an audience.


1.9) Is perl difficult to learn?

    Not at all.  Many people find Perl extremely easy to learn.  There are
    at least three main reasons for this.

    The first reason is that most of Perl has been derived from standard
    utilities, tools, and languages that you are (probably) already
    familiar with.  If you have any knowledge of the C programming language
    and standard C library, the Unix Shell, sed and awk, Perl should be
    simple and fun for you to learn.

    The second reason that Perl is easy to learn is that you only have to
    know a very small subset of Perl to be able to get useful results.  In
    fact, once you can master

        #!/usr/local/bin/perl
     print "Hello, world\n";

    you can start writing Perl scripts.  In fact, you will probably never
    have to (or be able to) know everything about Perl.  As you feel the
    need or desire to use more sophisticated features (such as C structures
    or networking), you can learn these as you go.  The learning curve for
    Perl is not a steep one, especially if you have the headstart of having
    a background in UNIX.  Rather, its learning curve is gentle and
    gradual, but it *is* admittedly rather long.

    The third reason is that you can get immediate results from your
    scripts.  Unlike a normal compiled language (like C or Pascal, for
    example), you don't have to continually recompile your program every
    time you change one little thing.  Perl allows you to experiment and
    test/debug quickly and easily.  This ease of experimentation flattens
    the learning curve even more.

    If you don't know C or UNIX at all, it'll be a steeper learning curve,
    but what you then learn from Perl will carry over into other areas,
    like using the C library, UNIX system calls, regular expressions, and
    associative arrays, just to name a few.  To know Perl is to know UNIX,
    and vice versa.


1.10) Should I program everything in Perl?

    Most definitely.  In fact, you should delete the binaries for sed, awk,
    cc, gcc, grep, rm, ls, cat... well, just delete your /bin directory.

    But seriously, of course you shouldn't.  As with any job, you should
    use the appropriate tool for the task at hand.  Just because a hammer
    will put screws into a piece of board, you probably don't want to do
    that.

    While it's true that the answer to the question "Can I do (some
    arbitrary task) in Perl?" is almost always "yes", that doesn't mean
    this is necessarily a good thing to do.  For many people, Perl serves
    as a great replacement for shell programming.  For a few people, it
    also serves as a replacement for most of what they'd do in C.  But for
    some things, Perl just isn't the optimal choice.


1.11) How does Perl compare with other scripting languages, like Tcl, Python
     or REXX?

    REXX is an interpreted programming language first seen on IBM systems.
    Python is an interpreted programming language by Guido van Rossum*.
    TCL is John Ousterhout*'s embeddable command language, designed just
    for embedded command extensions, but lately used for larger
    applications.  TCL's most intriguing feature for many people is the
    tcl/tk toolset that allows for interpreted X-based tools.  Others use
    it for its "expect" extension.

    To avoid any flamage, if you really want to know the answer to this
    question, probably the best thing to do is try to write equivalent
    code to do a set of tasks.  All three have their own newsgroups in
    which you can learn about (but hopefully not argue about) these
    languages.

    To find out more about these or other languages, you might also check
    out David Muir Sharnoff*'s posting "Catalog of Compilers, Interpreters,
    and Other Language Tools" which he posts to comp.lang.misc,
    comp.sources.d, comp.archives.admin, and news.answers newsgroups.  It's
    a comprehensive treatment of many different languages.  (Caveat lector:
    he considers Perl's syntax "unappealing".)


1.12) How can I get Perl over the Internet?

    Perl is available from any comp.sources.misc archive.  You can use an
    archie server (see the alt.sources FAQ in news.answers) to find these
    if you want.

      Version 4:
    Volume    Issues    Patchlevel and Notes
    ------    ------    ------------------------------------------------
      18    19-54    Patchlevel 3, Initial posting.
      20    56-62    Patches 4-10

      Version 5:
        Volume    Issues    Patchlevel and Notes
        ------    ------    -----------------------------------------------
          45    64-128    Initial Posting, patchlevel 0.

    Since 1993, a number of archives have sprung up specifically for Perl
    and Perl related items.  Larry maintains the official distribution
    site (for both perl4.036 and perl5) at netlabs.  Probably the largest
    archive is at the University of Florida.  In order of probability these
    sites will have the sources.

    Site    Directory and notes                        IP
    ---------------------------------------------         -------
    North America:
    ftp://ftp.netlabs.com/pub/outgoing/perl5.0/         192.94.48.152
    ftp://ftp.cis.ufl.edu/pub/perl/src/5.0/                128.227.100.198
        ftp://prep.ai.mit.edu/pub/gnu/                        18.71.0.38
        not current as of 7/15/95
        ftp://ftp.uu.net/languages/perl/                    192.48.96.9
        not current as of 7/15/95
        ftp://ftp.khoros.unm.edu/pub/perl/                  198.59.155.28
        not current as of 7/15/95
        ftp://ftp.cbi.tamucc.edu/pub/duff/Perl/                165.95.1.3
    ftp://ftp.metronet.com/pub/perl/sources/            192.245.137.1
        ftp://genetics.upenn.edu/perl5/                        128.91.200.37

    Europe:
        ftp://ftp.cs.ruu.nl/pub/PERL/perl5.0/src/        131.211.80.17
        ftp://ftp.funet.fi/pub/languages/perl/ports/perl5/  128.214.248.6
        ftp://ftp.zrz.tu-berlin.de/pub/unix/perl/           130.149.4.40
        ftp://src.doc.ic.ac.uk/packages/perl5/                146.169.17.5

    Australia:
        ftp://sungear.mame.mu.oz.au/pub/perl/src/5.0/        128.250.209.2

    South America (mirror of ftp://prep.ai.mit.edu/pub/gnu):
        ftp://ftp.inf.utfsm.cl/pub/gnu/                        146.83.198.3

    If there is a site in Asia or Japan, please tell us about it.  Thanks!

    You can also retrieve perl via non-ftp methods:

        http://src.doc.ic.ac.uk/packages/perl5/                146.169.17.5
        gopher://src.doc.ic.ac.uk/0/packages/perl5/         146.169.17.5


1.13) How can I get Perl via Email?

    The following is a list of known ftpmail sites.  Please attempt to use
    the site closest to you with the ftp archive closest to it.  Many of
    these sites already have perl on them.  For information on how to use
    one of these sites, send email containing the word "help" to the
    address.

    United States:
        Massachusetts:    [e-mail]
        New Jersey:        [e-mail]
        North Carolina:    [e-mail]

    Europe/UK:
        Germany:        [e-mail]
                [e-mail]
        UK:            [e-mail]

    Australia:        [e-mail]

    Henk P Penning* suggests that if you are in Europe you should try the
    following (if you are in Germany or the UK, you should probably use one
    of the servers listed above):

        Email: Send a message to '[e-mail]' containing:
     begin
     path your_email_address
     send help
     send PERL/perl5.0/INDEX
     end
    The path-line may be omitted if your message contains a normal
    From:-line.  You will receive a help-file and an index of the
    directory that contains the Perl stuff.

    If all else fails, mail to Larry usually suffices.


1.14) How can I get Perl via UUCP?

    There currently is no way of getting Perl via UUCP.  If anyone knows of
    a way, please contact me.  The OSU site has discontinued the service.


1.15) Are there other ways of getting perl?

    Another possibility is to use UUNET, although they charge you for it.
    You have been duly warned.  Here's the advertisement:

           Anonymous Access to UUNET's Source Archives

                 1-900-GOT-SRCS

    UUNET now provides access to its extensive collection of UNIX
    related sources to non- subscribers.  By  calling  1-900-468-7727
    and  using the login "uucp" with no password, anyone may uucp any
    of UUNET's on line source collection.  Callers will be charged 40
    cents  per  minute.   The charges will appear on their next tele-
    phone bill.

     The file uunet!/info/help contains instructions.   The  file
    uunet!/index//ls-lR.Z  contains  a  complete  list  of  the files
    available  and is  updated daily.   Files ending  in Z need to be
    uncompressed before being used.  The file uunet!~/compress.tar is
    a tar archive containing the C sources for the uncompress program.

     This service provides a  cost  effective  way  of  obtaining
    current  releases  of sources without having to maintain accounts
    with UUNET or some other service.  All modems  connected  to  the
    900  number  are  Telebit T2500 modems.  These modems support all
    standard modem speeds including PEP, V.32 (9600), V.22bis (2400),
    Bell  212a  (1200), and Bell 103 (300).  Using PEP or V.32, a 1.5
    megabyte file such as the GNU C compiler would cost $10  in  con-
    nect  charges.   The  entire  55  megabyte X Window system V11 R4
    would cost only $370 in connect time.  These costs are less  than
    the  official  tape  distribution fees and they are available now
    via modem.

              UUNET Communications Services
           3110 Fairview Park Drive, Suite 570
             Falls Church, VA 22042
             [phone] (voice)
              [phone] (fax)
                [e-mail]


1.16) Has perl been ported to machine FOO?

    Perl runs on virtually all Unix machines simply by following the hints
    file and instructions in the Configure script.  This auto-configuration
    script allows Perl to compile on a wide variety of platforms by
    modifying the machine specific parts of the code.  For most Unix
    systems, or VMS systems for v5 perl, no porting is required.  Try to
    compile Perl on your machine.  If you have problems, examine the README
    file carefully.  If all else fails, send a message to comp.lang.perl
    and crosspost to comp.sys.[whatever], there's probably someone out
    there that has already solved your problem and will be able to help you
    out.

    Perl4.036 has been ported to many non-Unix systems, although currently
    there are only a few (beta) v5 ports.  All of the following are
    mirrored at ftp://ftp.cis.ufl.edu:/pub/perl/src/.  The following are
    the (known) official distribution points.  Please contact the porters
    directly (when possible) in case of questions on these ports.

    * MS-DOS binaries and source are available at  [130.179.8.47]
          ftp://ftp.ee.umanitoba.ca/pub/msdos/perl/perl4
      There are currently half a dozen different ports for MS-DOS.
      BigPerl4 (v4) is perl4.036 compiled with the Watcom C/C++^32
      compiler (32-bit, flat-memory model C compiler) with the
      following features:
        * Up to 32MB of memory can be used.
        * Supports virtual memory.
        * Works under Windows 3.1
        * The perl debugger can be used.
        * Contains Berkeley DB support.  GDBM is no longer supported.

      Note that the latest version of BigPerl4 can also be found at
      any SimTel mirror site (ftp.ee.umanitoba.ca does not
      necessarily have the latest version), such as:

        ftp://oak.oakland.edu/SimTel/msdos/perl/

          A beta-test version of bigperl based on Perl 5.000 can be
          obtained from the following sites:

            ftp://ftp.einet.net/pub/perl5
            ftp://ftp.khoros.unm.edu/pub/perl/msdos
            ftp://ftp.ee.umanitoba.ca/pub/msdos/perl/perl5

          This beta bigperl also contains ported versions of a2p and s2p.

    * Windows/NT binaries are available from
          ftp://ftp.intergraph.com/pub/win32/perl, or at any of the major
          NT archives.  They are compiled for NT 3.5, but appears to work
          under Win95 also.  It is visible to the NT Registry.  Dean
          Troyer* is working on extending the visibility to SAM and the
          Event Log.  His current work-in-progress is available from
          ftp://pmip.dist.maricopa.edu/pub/nt.

    * Macintosh binaries and source are available from [130.59.1.40]
          ftp://nic.switch.ch/software/mac/perl.
      Version 4.1.3 is perl4.036 compiled with the MPW C compiler
        * Mac_Perl_413_src.sit.bin        Sources
        * Mac_Perl_413_tool.sit.bin        MPW Tool
        * Mac_Perl_413_appl.sit.bin        Standalone Application
      There is a mailing list for discussing Macintosh Perl.  Contact
      "[e-mail]".

      Timothy Murphy* also ported a version of perl to the Macintosh
      using Think C.  It has probably been abandoned in favour of the
      MPW port, but is still available at [134.266.81.10]
          ftp://ftp.maths.tcd.ie/pub/Mac/perl-4.035/.

          Matthias Ulrich Neeracher* is working on a perl5 port to the
          Macintosh.  A PowerPC version is available at
          ftp://err.ethz.ch/pub/neeri/MacPerlBeta.

    * OS/2 sources are also available at
          ftp://ftp.cis.ufl.edu/pub/perl/src/4.0/os2.  This appears to have
          been abandoned and added to the official distribution.  See the
          directory os2 in the perl5 sources.

    * VMS systems should be able to build directly from the standard
      distribution.

        * Amiga sources are not available from ftp.cis.ufl.edu, but can
          be found at any Aminet archive, notably:
            * ftp://ftp.wustl.edu/pub/aminet/dev/lang/
            * ftp://ftp.uni-erlangen.de/pub/aminet/dev/lang/
            * ftp://ftp.doc.ic.ac.uk/pub/aminet/dev/lang/
            * ftp://ftp.funet.fi/pub/languages/perl/ports/perl4/amiga

1.17) How do I get Perl to compile on Solaris?

    The following directions are for perl, version 4.  Perl, version 5,
    should compile more easily.  If not, send mail to The Perl Porters
    Mailing List ([e-mail])

    John Lees* reports:

        I have built perl on Solaris 2.1, 2.2 beta, and 2.2 FCS. Take
    /usr/ucb out of your path and do not use any BSD/UCB libraries.
    Only -lsocket, -lnsl, and -lm are needed. You can use the hint for
    Solaris 2.0, but the one for 2.1 is wrong. Do not use vfork. Do not
    use -I/usr/ucbinclude.  The result works fine for me, but of couse
    does not support a couple of BSDism's.

    Casper H.S. Dik* reports

        You must remove all the references to /usr/ucblib AND
        /usr/ucbinclude.  And ignore the Solaris_2.1 hints. They are wrong.
        The undefining of vfork() probably has to do with the confusion it
        gives to the compilers.  If you use cc, you mustn't compile
    util.c/tutil.c  with -O.  I only used the following libs: -lsocket
    -lnsl -lm (there is a problem with -lmalloc)

    Michael D'Errico* reports:

        If you are using Solaris 2.x, the signal handling is broken.  If
    you set up a signal handler such as 'ripper' it will be forgotten
    after the first time the signal is caught.  To fix this, you need
    to recompile Perl.  Just add '#define signal(x,y) sigset((x),(y))'
    after the '#include <signal.h>' directive in each file that it
    occurs, then make it again.


1.18) How do I get Perl to compile on a Next?

    According to Andreas Koenig*, under NeXTstep 3.2, both perl4.036 and
    perl5.000 compile with the supplied hints file.

    However, Bill Eldridge* provides this message to help get perl4.036 on
    NeXTstep 3.0 to work:

        To get perl to compile on NeXTs, you need to combine the ANSI
        and BSD headers:

        cd /usr/include
        mkdir ansibsd
        cd ansibsd
        ln -s ../ansi
        ln -s ../bsd

        Then, follow the configuration instructions for NeXTs, *replacing*
        all mention of -I/usr/include/ansi or -I/usr/include/bsd with
        -I/usr/include/ansibsd.


1.19) What extensions are available from Perl and where can I get them?

    Some of the more popular extensions include those for windowing,
    graphics, or data base work.  Most of the major sites contain an
    archive of the extensions, usually in the ext directory.  Since the
    list of available extensions changes so often, I have opted to list
    only the sites and directories, not the individual extensions, please
    check the closest archive for more information

      ftp://ftp.cis.ufl.edu/pub/perl/ext
        (also linked at ftp://ftp.cis.ufl.edu/pub/perl/src/5.0/ext)
      ftp://ftp.khoros.unm.edu/pub/perl/extensions/
      ftp://ftp.metronet.com/pub/perlinfo/perl5/
      ftp://ftp.cs.ruu.nl/pub/PERL/perl5.0/ext
      ftp://black.ox.ac.uk/src/ALPHA/


1.20) What is dbperl and where can I get it?

    Many database-oriented extensions to Perl have been written.
    Basically, these use the usub mechanism (see the usub/ subdirectory) in
    the source distribution) to link in a database library, allowing
    embedded calls to Informix, Ingres, Interbase, Oracle and Sybase.

    Here are the authors of the various extensions:

    What            Target DB       Who
    --------        -----------     ----------------------------------------
    ?Infoperl       Informix        Kurt Andersen ([e-mail])
    Ingperl        Ingres        Tim Bunce ([e-mail]) and Ted Lemon
    Interperl       Interbase       Buzz Moschetti ([e-mail])
    Isqlperl        Informix        William Hails, [e-mail]
    Oraperl         Oracle          Kevin Stock ([e-mail])
    Pgperl        Postgres        Igor Metz ([e-mail])
    *Sqlperl        Ingres          Ted Lemon ([e-mail])
    Sybperl         Sybase          Michael Peppler ([e-mail])
    Uniperl        Unify 5.0        Rick Wargo ([e-mail])

    ? Does this one still exist?
    * Sqlperl appears to have been subsumed by Ingperl

    Buzz Moschetti* has organized a project to create a higher level
    interface to allow you to write your queries in a database-independent
    fashion.  If this type of project interests you, send mail to
    <[e-mail]> and asked to be placed on the
    "perldb-interest" mailing lists.

    Here's a bit of advertising from Buzz:

    Perl is an interpreted language with powerful string, scalar, and
    array processing features developed by Larry Wall that "nicely
    bridges the functionality gap between sh(1) and C."  Since
    relational DB operations are typically textually oriented, perl is
    particularly well-suited to manage the data flows.  The C source
    code, which is available free of charge and runs on many platforms,
    contains a user-defined function entry point that permits a
    developer to extend the basic function set of the language.  The
    DBperl Group seeks to exploit this capability by creating a
    standardized set of perl function extensions (e.g. db_fetch(),
    db_attach()) based on the SQL model for manipulating a relational
        DB, thus providing a portable perl interface to a variety of
        popular RDMS engines including Sybase, Oracle, Ingres, Informix,
        and Interbase.  In theory, any DB engine that implements a dynamic
        SQL interpreter in its HLI can be bolted onto the perl front end
        with predicatable results, although at this time backends exist
        only for the aforementioned five DB engines.

    The official archive for DBperl extensions is ftp.demon.co.uk:
    /pub/perl/db.  It's the home of the evolving DBperl API Specification.
    Here's an extract from the updated README there:

    DBI/        The home of the DBI archive. To join the DBI mailing list
                send your request to [e-mail]

    DBD/        Database Drivers for the DBI ...

    Oracle/      By Tim Bunce (not yet ready!)
    Ingres/      By Tim Bunce (not yet started!)

    mod/           Other Perl 5 Modules and Extensions ...

    Sybperl/    By Michael Peppler, [e-mail]


    perl4/         Perl 4 extensions (using the usub C interface)

       oraperl/   ORACLE 6 & 7  By Kevin Stock, [e-mail]
       sybperl/   SYBASE 4      By Michael Peppler, [e-mail]
       ingperl/   INGRES        By Tim Bunce [e-mail] and Ted Lemon
       isqlperl/  INFORMIX      By William Hails, [e-mail]
       interperl/ INTERBASE     By Buzz Moschetti, [e-mail]
       oraperl/   ORACLE 6 & 7  By Kevin Stock (sadly no longer on the net)
       sybperl/   SYBASE 4      By Michael Peppler, [e-mail]
       ingperl/   INGRES        By Tim Bunce [e-mail] and Ted Lemon
       isqlperl/  INFORMIX      By William Hails, [e-mail]
       interperl/ INTERBASE     By Buzz Moschetti, [e-mail]
       uniperl/   UNIFY 5.0     By Rick Wargo, [e-mail]
       pgperl/    POSTGRES      By Igor Metz, [e-mail]

       btreeperl/ NDBM perl extensions.   By John Conover, [e-mail]
       ctreeperl/ C-Tree perl extensions. By John Conover, [e-mail]
       duaperl/   X.500 Directory User Agent. By Eric Douglas.

    scripts/       Perl and shell scripts

       rdb/       RDB is a perl RDBMS for ASCII files. By Walt Hobbs,
                    [e-mail]
       shql/      SHQL is an interactive SQL database engine.  Written as a
                    shell script, SHQL interprets SQL commands and
                    manipulates flat files based on those commands. By
                    Bruce Momjian, [e-mail]
       xbase/     Perl scripts for accessing xBase style files (dBase III)


   refinfo/       Reference information

       sqlsyntax/ Yacc and lex syntax and C source code for SQL1 and SQL2
            from ftp.uu.net:/pub/uunet/published/oreilly/nutshell/yacclex,
            and a draft SQL3 syntax from Jeff Fried <[e-mail]>+
       formats/   Details of file formats such as Lotus 1-2-3 .WK1

    There are also a number of non SQL database interfaces for perl
    available from ftp.demon.co.uk.  These include:

    Directory    Target System    Authors and notes
    ---------    -------------    -------------------------------------------
    btreeperl    NDBM extension    John Conover ([e-mail])
    ctreeperl    CTree extension John Conover ([e-mail])
    duaperl    X.500 DUA    Eric Douglas
    rdb        RDBMS        Walt Hobbs ([e-mail])
    shql    SQL Engine    Bruce Momjian ([e-mail])


1.21) Which DBM should I use?

    As shipped, Perl (version 5) comes with interfaces for several DBM
    packages (SDBM, old DBM, NDBM, GDBM, Berkeley DBM) that are not supplied
    but either come with your system are readily accessible via FTP.  SDBM
    is guaranteed to be there.  For a comparison, see AnyDBM_File(3pm)
    and DB_File(3pm).


1.22) Is there an SNMP aware Perl?

    snmperl was written by Guy Streeter ([e-mail]), and was
    posted in late February 1993 to comp.protocols.snmp.  It can be found
    archived at one of two (known) places:

    Host liasun3.epfl.ch

    Location: /pub/net/snmp
           FILE -rw-rw-r--       3407  Aug 11 1992  snmperl.README
           FILE -rw-r--r--      17678  Aug 11 1992  snmperl.tar.Z

    Host ftp.cis.ufl.edu
    Location: /pub/perl/scripts/snmp

    Here is the gist of the README:

    This directory contains the source code to add callable C subroutines
    to perl.  The subroutines implement the SNMP functions "get",
    "getnext", and "set".  They use the freely-distributable SNMP package
    (version 1.1b) from CMU.

    USE:
      There are four subroutines defined in the callable interface:
    snmp_get, snmp_next, snmp_set, and snmp_error.

      snmp_get and snmp_next implement the GET and GETNEXT operations,
    respectively.  The first two calling arguments are the hostname and
    Community string.  The IP address of the host, as a dotted-quad ASCII
    string, may be used as the hostname.  The rest of the calling
    arguments are a list of variables.  See the CMU package documentation
    for how variables may be specified.
      snmp_set also takes hostname and Community string as arguments.  The
    remaining arguments are a list of triples consisting of variable name,
    variable type, and value.  The variable type is a string, such as
    "INTEGER" or "IpAddress".
      snmp_get, snmp_next, and snmp_set return a list containing
    alternating variables and values.  snmp_get and snmp_next will simply
    omit non-existent variables on return.  snmp_set will fail completely
    if one of the specified variables does not exist (or is read-only).
      snmp_error will return a text string containing some error
    information about the most recent snmp_get|next|set call, if it had an
    error.

    OTHER NOTES:
      I didn't find all the places where the CMU library writes to stderr
    or calls exit() directly.
      The changes I made to mib.c involve the formatting of variable values
    for return to the caller.  I took out the descriptive prefix so the
    string contains only the value.
      Enumerated types are returned as a string containing the symbolic
    representation followed in parentheses by the numeric.

    DISTRIBUTION and OWNERSHIP
      perl and the CMU SNMP package have their own statements.  Read them.
    The work I've done is free and clear.  Just don't say you wrote it if
    you didn't, and don't say I wrote it if you change it.

    Guy Streeter
    [e-mail]
    April 1, 1992 (not a joke!)


1.23) Is there an ISO or ANSI certified version of Perl?

    No.  Larry thinks it likely that he'll be certified before perl is.

--
Stephen P Potter        Pencom Systems Administration              Beaching It
[e-mail]    Pager: 1-800-759-8888, 547-9561     Work: 703-860-2222
  Cthulhu for President in '96: When You're Tired of the Lesser of Two Evils
--
Stephen P Potter        Pencom Systems Administration              Beaching It
[e-mail]    Pager: 1-800-759-8888, 547-9561     Work: 703-860-2222
"I don't care whether people actually like Perl, just so long as they *think*
    they like it...  ;-)"    -Larry Wall

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Archive-name: perl-faq/part2
Version: $Id: part2,v 2.9 1995/05/15 15:44:29 spp Exp spp $
Posting-Frequency: bi-weekly
Last Edited: Thu Jan 11 00:54:41 1996 by spp (Stephen P Potter) on syrinx.psa.com

This posting contains answers to general information questions, mostly of
about information sources.

2.1) Is there a USENET group for Perl?

    Yes there is: comp.lang.perl.misc.  This group, which currently can get
    up to 150 messages per day, contains all kinds of discussions about
    Perl; everything from bug reports to new features to the history to
    humour and trivia.  This is the best source of information about
    anything Perl related, especially what's new with Perl5.  Because of
    its vast array of topics, it functions as both a comp.lang.* style
    newsgroup (providing technical information) and also as a rec.* style
    newsgroup, kind of a support group for Perl addicts (PerlAnon?).  There
    is also the group comp.lang.perl.announce, a place specifically for
    announcements related to perl (new releases, the FAQ, new modules,
    etc).

    Larry is a frequent poster to this group as well as most (all?) of the
    seasoned Perl programmers.  Questions will be answered by some of the
    most knowledgable Perl Hackers, often within minutes of a question
    being posted (give or take distribution times).


2.2) Have any books or magazine articles been published about Perl?

    There are a number of books either available or planned.  Mostly
    chronologically, they are:

         Programming Perl (the Camel Book):
         Author: Larry Wall and Randal Schwartz
         Publisher: O'Reilly and Associates
             ISBN 0-937175-64-1  (English)
             ISBN 4-89052-384-7  (Japanese)
         ISBN 3-446-17257-2     (German)  (Programmieren in Perl)
            (translator: Hanser Verlag)

    This is probably the most well known and most useful book for 4.036 and
    earlier.  This part of O'Reilly's hugely successful "Nutshell Handbook"
    series.  Besides serving as a reference guide for Perl, it also contains
    tutorial material and is a great source of examples and cookbook
    procedures, as well as wit and wisdom, tricks and traps, pranks and
    pitfalls.  The code examples contained therein are available from
    ftp://ftp.ora.com/pub/examples/nutshell/programming_perl/perl.tar.Z or
    ftp://ftp.cis.ufl.edu/pub/perl/ora/programming_perl.  Corrections and
    additions to the book can be found in the Perl4 man page right before
    the BUGS section under the heading ERRATA AND ADDENDA.

         Learning Perl (the Llama Book):
             ISBN 1-56592-042-2      (English)
         ISBN 4-89502-678-1      (Japanese)
         ISBN 2-84177-005-2      (French)
         ISBN 3-930673-08-8      (German)

    Another of O'Reilly's "Nutshell Handbooks", by Randal Schwartz.  This
    book is a smaller, gentler introduction to perl and is based off of
    Randal's perl classes.  While in general this is a good book for
    learning perl (like it's title), early printings did contain many typos
    and don't cover some of the more interesting features of perl.  Please
    check the errata sheet at ftp.ora.com, as well as the on-line examples.

    If you can't find these books in your local technical bookstore, they
    may be ordered directly from O'Reilly by calling 1-800-998-9938 if in
    North America and 1-707-829-0515 otherwise.

    Johan Vromans* created a beautiful reference guide.  The reference
    guide comes with the Camel book in a nice, glossy format.  The LaTeX
    (source) and PostScript (ready to print) versions are available for FTP
    from ftp.cs.ruu.nl:/pub/DOC/perlref-4.036.1.tar.Z in Europe or from
    ftp.cis.ufl.edu:/pub/perl/doc/perlref-4.036.tar.gz in the United
    States.  Obsolete versions in TeX or troff may still be available, but
    these versions don't print as nicely.  See also:

        [] ftp://ftp.uu.net/languages/perl/perlref-4.036.1.tar.gz
        [] ftp://ftp.cs.ruu.nl/pub/DOC/perlref-4.036.1.tar.gz
        [] ftp://ftp.khoros.unm.edu/pub/perl/perlref-4.036.1.tar.gz

    Johan has also updated and released a reference guide based on version
    5.000.  This is available from the same places as the 4.036 guide.
    This version is also available from prep.gnu.ai.mit.edu in the /pub/gnu
    section along with the perl5 source.  It may be added to the standard
    perl5 distribution sometime after 5.002.  If you are using version
    5.000, you will want to get this version rather than the 4.036 version.

    Larry routinely carries around a camel stamp to use when autographing
    copies of his book.  If you can catch him at a conference you can
    usually get him to sign your book for you.

    Prentice Hall also has two perl books.  The first is ``Perl by
    Example'' by Ellie Quigley. (385 pages, $26.96, ISBN 0-13-122839-0) A
    perl tutorial (perl4); every feature is presented via an annotated
    example and sample output.  Reviews of this book have varied widely.
    Many new perl users have used this book with much success, while many
    "veteran" programmers have had many complaints about it.

    The second book is called ``Software Engineering with Perl'' by Carl
    Dichter and Mark Pease.  Randal Schwartz was a technical reviewer for
    this book and notes this:

    SEwP is not meant as instruction in the Perl language, but rather
    as an example of how Perl may be used to assist in the semi-formal
    software engineering development cycles.  There's a lot of Perl
    code that's fairly well commented, but most of the book describes
    software engineering methodologies.  For the perl-challenged,
    there's a *light* treatment of the language as well, but they refer
    to the llama and the camel for the real meat.

    SAMS Publishing also has a Perl book available, as part of their "Teach
    Yourself in 21 Days" series, called "Teach Yourself Perl in 21 Days".
    ISBN 0-672-30586-0 Price: $29.95, 841 Pages.  This book is the first
    book to have a section devoted to version 5.000, although it was
    written during an alpha stage and may not necessarily reflect current
    reality.

    Please note that none of the above books are perfect, all have some
    inaccurances and typos.  The two which Larry is directly associated
    with (the O'Reilly books) are probably the most technically correct,
    but also the most dated.  Carefully looking over any book you are
    considering purchasing will save you much time, money, and frustration.

    Starting in the March, 1995 edition of ``Unix Review''. Randal
    Schwartz* has been authoring a bi-monthly Perl column.  This has so
    far been an introductory tutorial.

    Larry Wall has published a 3-part article on perl in Unix World
    (August through October of 1991), and Rob Kolstad also had a 3-parter
    in Unix Review (May through July of 1990).  Tom Christiansen also has
    a brief overview article in the trade newsletter Unix Technology
    Advisor from November of 1989.  You might also investigate "The Wisdom
    of Perl" by Gordon Galligher from SunExpert magazine;  April 1991
    Volume 2 Number 4.  The Dec 92 Computer Language magazine also
    contains a cover article on Perl, "Perl: the Programmers Toolbox".

    Many other articles on Perl have been recently published.  If you
    have references, especially on-line copies, please mail them to
    the FAQ maintainer for inclusion is this notice.

    The USENIX LISA (Large Installations Systems Administration) Conference
    have for several years now included many papers of tools written in
    Perl.  Old proceedings of these conferences are available; look in
    your current issue of ";login:" or send mail to [e-mail]
    for further information.

    Japan seems to be jumping with Perl books.  If you can read japanese
    here are a few you might be interested in.  Thanks to Jeffrey Friedl*
    and Ken Lunde* for this list (NOTE: my screen cannot handle japanese
    characters, so this is all in English for the moment  NOTE2: These
    books are written in Japanese, these titles are just translations):

    Title: Welcome to Perl Country    (Perl-no Kuni-he Youkoso)
    Authors: Kaoru Maeda, Hiroshi Koyama, Yasushi Saito and Arihito
         Fuse
        Pages: 268+9                    Publisher: Science Company
        Pub. Date: April 25, 1993       ISBN: 4-7819-0697-4
        Price: 2472Y                    Author Email: [e-mail]
    Comments: Written during the time the Camel book was being
    translated.  A useful introduction, but uses jperl (Japanese Perl)
    which is not necessarily compatible.

    Title: How to Write Perl        (Perl Shohou)
    Author: Toshiyuki Masui
        Pages: 352                      Publisher: ASCII Corporation
        Pub. Date: July 1, 1993            ISBN: 4-7561-0281-6
        Price: 3200Y                    Author Email: [e-mail]
    Comments: More advanced than "Welcome.." and not meant as an
    introduction.  Uses the standard perl and has examples for handling
    Japanese text.

    Title: Introduction to Perl     (Nyuumon Perl)
    Author: Shinji Kono
        Pages: 203                      Publisher: ASCII Corporation
        Date: July 11, 1994             ISBN: 4-7561-0292-1
        Price: 1800Y                    Author Email: [e-mail]
    Comments: Uses the interactive Perl debugger to explain how things
    work.

    Title: Perl Programming
    Authors: L Wall & R Schwartz    Translator: Yoshiyuki Kondo
        Pages: 637+32                    Publisher: Softbank Corporation
        Pub. Date: February 28, 1993    ISBN: 4-89052-384-7
        Price: 4500Y                    Author Email: [e-mail]
    Comments: Official Japanese translation of the Camel book,
        "Programming Perl".  Somewhat laced with translator notes to
        explain the humour.  The most useful book.  Also includes the Perl
        Quick Reference -- in Japanese!


2.3) When will the Camel and Llama books be updated?

    As of August, 1995, ORA has contracted with Stephen to handle the
    Camel update.  According to the accepted timeline, the first draft
    is to be finished by the end of April, 1996.  The tutorial sections
    are being cut some, and the book will take on much more of a reference
    style.  Don't worry, it will still contain it's distinctive humor and
    flair.

    There are no current plans to update the Llama.  For the most part,
    it serves as a good introduction for both major versions of perl.
    There may be some minor editing to it, but probably nothing major.
    If anything, it is more likely that a third book (working title:
    Learning More Perl) will be written as a tutorial for the new perl5
    paradigm.

2.4) What FTP resources are available?

    Since 1993, several ftp sites have sprung up for Perl and Perl related
    items.  The site with the biggest repository of Perl scripts right now
    seems to be ftp.cis.ufl.edu [128.227.100.198] in /pub/perl.  The
    scripts directory has an INDEX with over 400 lines in it, each
    describing what the script does.  The src directory has sources and/or
    binaries for a number of different perl ports, including MS-Dos,
    Macintosh and Windows/NT.  This is maintained by the Computing Staff at
    UF*.

    Note:  European users please use the site src.doc.ic.ac.uk
    [149.169.2.1] in /pub/computing/programming/languages/perl/
    The link speed would be a lot better for all.  Contact
    [e-mail] for more information.  It is updated
    daily.

    There are also a number of other sites.  I'll add more of them as I get
    information on them.
    [site maintainers: if you want to add a blurb here, especially if you
    have something unique, please let me know. -spp]

    The Comprehensive Perl Archive Network (CPAN) is in heavy development.
    Once the main site and its mirrors are fully operational, this answer
    will change to reflect it's existance.


2.5) What WWW/gopher resources are available?

    The World Wide Web is exploding with new Perl sites all the time.  Some
    of the more notable ones are:
    http://www.cis.ufl.edu/perl
    http://www.metronet.com/1h/perlinfo, which has a great section on
        Perl5.
    http://www.eecs.nwu.edu/perl/perl.html
        http://web.nexor.co.uk/perl/perl.html, a great site for European
        and UK users.


2.6) Can people who don't have access to USENET get comp.lang.perl.misc?

    "Perl-Users" is the mailing list version of the comp.lang.perl.misc
    newsgroup.  If you're not lucky enough to be on USENET you can post to
    comp.lang.perl.misc by sending to one of the following addresses.
    Which one will work best for you depends on which nets your site is
    hooked into.  Ask your local network guru if you're not certain.

    Internet: [e-mail]
              [e-mail]

    BitNet: Perl@Virginia

    uucp: ...!uunet!virginia!perl-users

    The Perl-Users list is bidirectionally gatewayed with the USENET
    newsgroup comp.lang.perl.misc.  This means that VIRGINIA functions as a
    reflector.  All traffic coming in from the non-USENET side is
    immediately posted to the newsgroup.  Postings from the USENET side are
    periodically digested and mailed out to the Perl-Users mailing list.  A
    digest is created and distributed at least once per day, more often if
    traffic warrants.

    All requests to be added to or deleted from this list, problems,
    questions, etc., should be sent to:

    Internet: [e-mail]
              [e-mail]

    BitNet: Perl-Req@Virginia

    uucp: ...!uunet!virginia!perl-users-request

    Coordinator: Marc Rouleau <[e-mail]>


2.7) Are archives of comp.lang.perl.misc available?

    Yes, there are.  ftp.cis.ufl.edu:/pub/perl/comp.lang.perl.*/monthly has
    an almost complete collection dating back to 12/89 (missing 08/91
    through 12/93).  They are kept as one large file for each month.

    A more sophisticated query and retrieval mechanism is desirable.
    Preferably one that allows you to retrieve article using a fast-access
    indices, keyed on at least author, date, subject, thread (as in "trn")
    and probably keywords.  Right now, the MH pick command works for this,
    but it is very slow to select on 18000 articles.

    If you have, or know where I can find, the missing sections, please let
    [e-mail] know.


2.8) Is there a WAIS server for comp.lang.perl.*?

    Yes there is.  Set your WAIS client to
    archive.orst.edu:9000/comp.lang.perl.*.  According to their
    introduction, they have a complete selection from 1989 on.

2.9) What other sources of information about Perl or training are available?

    There is a #Perl channel on IRC (Internet Relay Chat) where Tom and
    Randal have been known to hang out.  Here you can get immediate answers
    to questions from some of the most well-known Perl Hackers.

    The perl5-porters ([e-mail]) mailing list was created to
    aid in communication among the people working on perl5.  However, it
    has overgrown this function and now also handles a good deal of traffic
    about perl internals.

2.10) Where can I get training classes on Perl?

    USENIX, LISA, SUG, WCSAS, AUUG, FedUnix and Europen sponsor tutorials
    of varying lengths on Perl at the System Administration and General
    Conferences.  These public classes are typically taught by Tom
    Christiansen*.

    In part, Tom and Randal teach Perl to help keep bread on their tables
    long enough while they continue their pro bono efforts of documenting
    perl (Tom keeps writing more man pages for it :-) and expanding the
    perl toolkit through extension libraries, work which they enjoy doing
    as it's fun and helps out the whole world, but which really doesn't
    pay the bills.  Such is the nature of free(ly available) software.
    Send mail to <[e-mail]> for details and availability.

    Tom is also available to teach on-site classes, included courses on
    advanced perl and perl5.  Classes run anywhere from one day to week
    long sessions and cover a wide range of subject matter.  Classes can
    include lab time with exercises, a generally beneficial aspect.  If you
    would like more information regarding Perl classes or when the next
    public appearances are, please contact Tom directly at 1.303.444.3212.

    Randal Schwartz* provides a 2-day lecture-only and a 4-5 day lecture-lab
    course based on his popular book "Learning Perl".  For details, contact
    Randal directly via email or at 1.503.777.0095.

    Internet One provides a 2 day "Introduction to Perl"  and  2  day
    "Advanced Perl" workshop. The 50% hands-on and 50% lecture format
    allow attendees to write several programs  themselves.   Supplied
    are  the user manuals, reference copies of Larry Wall's "Program-
    ming Perl", and a UNIX directory of  all  training  examples  and
    labs. To obtain outlines, pricing, or scheduling information, use
    the following:

        o Phone: [phone]
        o Email: [e-mail]
        o See our Ad in the "SysAdmin" magazine
        o View the outlines via the Web: http://www.InternetOne.COM/


2.11) What companies use or ship Perl?

    At this time, the known list of companies that ship Perl includes at
    least the following, although some have snuck it into /usr/contrib or
    its moral equivalent:

    BSDI
    Comdisco Systems
    CONVEX Computer Corporation
    Crosspoint Solutions
        Data General
    Dell
    DRD Corporation
    IBM (SP systems)
    Intergraph
    Kubota Pacific
    Netlabs
    SGI (without taintperl)
    Univel

    Some companies ship it on their "User Contributed Software Tape",
    such as DEC and HP.  Apple Computer has shipped the MPW version of
    Macintosh Perl on one of their Developer CDs (Essentials*Tools*Objects
    #11) (and they included it under "Essentials" :-)

    Many other companies use Perl internally for purposes of tools
    development, systems administration, installation scripts, and test
    suites.  Rumor has it that the large workstation vendors (the TLA set)
    are seriously looking into shipping Perl with their standard systems
    "soon".

    People with support contracts with their vendors are actively
    encouraged to submit enhancement requests that Perl be shipped
    as part of their standard system.  It would, at the very least,
    reduce the FTP load on the Internet. :-)

    If you know of any others, please send them in.


2.12) Is there commercial, third-party support for Perl?

    Not really.  Although perl is included in the GNU distribution, at last
    check, Cygnus does not offer support for it.  However, it's unclear
    whether they've ever been offered sufficient financial incentive to do
    so.  Feel free to try.

    On the other hand, you do have comp.lang.perl.misc as a totally gratis
    support mechanism.  As long as you ask "interesting" questions, you'll
    probably get plenty of help. :-)

    While some vendors do ship Perl with their platforms, that doesn't mean
    they support it on arbitrary other platforms.  And in fact, all they'll
    probably do is forward any bug reports on to Larry.  In practice, this
    is far better support than you could hope for from nearly any vendor.

    If you purchase a product from Netlabs (the company Larry works for),
    you actually can get a support contract that includes Perl.

    The companies who won't use something unless they can pay money for it
    will be left out.  Often they're motivated by wanting someone whom they
    could sue.  If all they want is someone to help them out with Perl
    problems, there's always the net.  And if they really want to pay
    someone for that help, well, any of a number of the regular Perl
    "dignitaries" would appreciate the money.  ;-)

    If companies want "commercial support" for it badly enough, speak up --
    something might be able to be arranged.


2.13) What is a JAPH?  What does "Will hack perl for ..." mean?

    These are the "just another perl hacker" signatures that some people
    sign their postings with.  About 100 of the of the earlier ones are
    available from the various FTP sites.

    When people started running out of tricky and interesting JAPHs, some
    of them turned to writing "Will hack perl for ..." quotes.  While
    sometimes humourous, they just didn't have the flair of the JAPHs and
    have since almost completely vanished.


2.14) Where can I get a list of Larry Wall witticisms?

    Over a hundred quips by Larry, from postings of his or source code,
    can be found in many of the FTP sites or through the World Wide Web at
    "ftp://ftp.cis.ufl.edu/pub/perl/misc/lwall-quotes"


2.15) What are the known bugs?

    This is *NOT* a complete list, just some of the more common bugs that
    tend to bite people.

    5.001:
    op.c: Inconsistent parameter definition for pad_findlex - fixed
        in 5.001a, get development patches a-l.

    walk.c: redeclaration of emit_split - fixed in perl5.001a, get
        development patches a-l.

    On linux systems "make test" fails on "op/exec Failed test 5".  This
        is a known bug with bash, not perl.  You can get a new version
        of bash.

    Also on linux systems, "make test" hangs on lib/anydbm if you include
        NDBM in the extentions.  Do not include NDBM.

    Another linux problem is getting Dynamic Loading to work.  You must
        use dld-2.3.6 (the newest version at the time of writing) to
        use Dynamic Loading.

    - All versions of h2ph previous to the one supplied with perl5.001 tended
    to generate improper header files.  Something such as:

        #if __GNUC__

    was incorrectly translated into

        if ( &__GNUC__ ) {

    instead of

        if ( defined(&__GNUC__) ? &__GNUC__ : 0 ) {

    Perl5 binaries compiled on SunOS4 exhibit strange behaviour on SunOS5.
    For example, backticks do not work in the scripts.  You need to
    compile perl for both architectures, even with Binary Compatibility.


2.16) Where should I post bugs?

    Before posting about a bug, please make sure that you are using the
    most recent versions of perl (currently 4.036 and 5.001) available.
    Please also check at the major archive sites to see if there are any
    development patches available (usually named something like
    perl5.001a.patch or patch5.001a - the patch itself, or
    perl5.001a.tar.gz - a prepatched distribution).  If you are not using
    one of these versions, chances are you will be told to upgrade because
    the bug has already been fixed.

    If you are reporting a bug in perl5, the best place to send your bug
    is <[e-mail]>, which is currently just an alias for
    <[e-mail]>.  In the past, there have been problems with
    the perlbug address.  If you have problems with it, please send your
    bug directly to <[e-mail]>.  You may subscribe to the list
    in the customary fashion via mail to <[e-mail]>.
    Feel free to post your bugs to the comp.lang.perl.misc newsgroup as
    well, but do make sure they still go to the mailing list.

    If you are posting a bug with a non-Unix port, a non-standard Module
    (such as Tk, Sx, etc) please see the documentation that came with it
    to determine the correct place to post bugs.

    To enhance your chances of getting any bug you report fixed:

    1. Make sure you are using a production version of perl.  Alpha and
       Beta version problems have probably already been reported and
       fixed.

    2. Try to narrow the problem down to as small a piece of code as
       possible.  If you can get it down to 1 line of Perl then so much
       the better.

    3. Include a copy of the output from the myconfig script from the
       Perl source distribution in your posting.

2.17) Where should I post source code?

    You should post source code to whichever group is most appropriate,
    but feel free to cross-post to comp.lang.perl.misc.  If you want to
    cross-post to alt.sources, please make sure it follows their
    posting standards, including setting the Followups-To header
    line to NOT include alt.sources; see their FAQ for details.


2.18) Where can I learn about object-oriented Perl programming?

    The perlobj(1) man page is a good place to start, and then you can
    check out the excellent perlbot(1) man page written by the dean of perl
    o-o himself, Dean Roehrich.  Areas covered include the following:

        Idx  Subsections in perlobj.1          Lines
         1   NAME                                  2
         2   DESCRIPTION                          16
         3   An Object is Simply a Reference      60
         4   A Class is Simply a Package          31
         5   A Method is Simply a Subroutine      34
         6   Method Invocation                    75
         7   Destructors                          14
         8   Summary                               7

        Idx  Subsections in perlbot.1          Lines
         1   NAME                                  2
         2   INTRODUCTION                          9
         3   Instance Variables                   43
         4   Scalar Instance Variables            21
         5   Instance Variable Inheritance        35
         6   Object Relationships                 33
         7   Overriding Superclass Methods        49
         8   Using Relationship with Sdbm         45
         9   Thinking of Code Reuse              111

    The section on instance variables should prove very helpful to those
    wondering how to get data inheritance in perl.


2.19) Where can I learn about linking C with Perl? [h2xs, xsubpp]

    While it used to be deep magic, how to do this is now revealed in the
    perlapi(1), perlguts(1), and perlcall(1) man pages, which treat with
    this matter extensively.  You should also check the many extensions
    that people have written (see question 1.19), many of which do this
    very thing.


2.20) What is perl.com?

    Perl.com is just Tom's domain name, registered as dedicated to "Perl
    training and consulting".  While not a full ftp site (he hasn't got
    the bandwidth (yet)), it does have some interesting bits, most of which
    are replicated elsewhere.  It serves as a clearinghouse for certain
    perl related mailing lists.  The following aliases work:

        perl-packrats:          The archivist list
          perl-porters:           The porters list
        perlbook:               The Camel/Llama/Alpaca writing committee
        perlbugs:               The bug list (perl-porters for now)
        perlclasses:            Info on Perl training
        perlfaq:                Submissions/Errata to the Perl FAQ
                                (Tom and Steve)
        perlrefguide:           Submissions/Errata to the Perl RefGuide
                                (Johan)

2.21) What do the asterisks (*) throughout the FAQ stand for?

    To keep from cluttering up the FAQ and for easy reference all email
    addresses have been collected in this location.  For each person
    listed, I offer my thanks for their input and help.

    * Larry Wall        <[e-mail]>
    * Tom Christiansen        <[e-mail]>
    * Stephen P Potter        <[e-mail]>
    * Andreas Koenig        <[e-mail]>
    * Bill Eldridge        <[e-mail]>
    * Buzz Moschetti        <[e-mail]>
    * Casper H.S. Dik        <[e-mail]>
    * David Muir Sharnoff   <[e-mail]>
    * Dean Roehrich         <[e-mail]>
    * Dominic Giampaolo     <[e-mail]>,
    * Frederic Chauveau     <[e-mail]>
    * Gene Spafford         <[e-mail]>
    * Guido van Rossum        <[e-mail]>
    * Henk P Penning        <[e-mail]>
    * Jeff Friedl        <[e-mail]>
    * Johan Vromans        <[e-mail]>
    * John Dallman        <[e-mail]>
    * John Lees            <[e-mail]>
    * John Ousterhout        <[e-mail]>
    * Jon Biggar        <[e-mail]>
    * Ken Lunde                <[e-mail]>
    * Malcolm Beattie        <[e-mail]>
    * Matthias Neeracher    <[e-mail]>
    * Michael D'Errico        <[e-mail]>
    * Nick Ing-Simmons        <[e-mail]>
    * Randal Schwartz        <[e-mail]>
    * Roberto Salama        <[e-mail]>
    * Steven L Kunz        <[e-mail]>
    * Theodore C. Law        <[e-mail]>
    * Thomas R. Kimpton        <[e-mail]>
    * Timothy Murphy        <[e-mail]>
    * UF Computer Staff        <[e-mail]>

--
Stephen P Potter        Pencom Systems Administration              Beaching It
[e-mail]    Pager: 1-800-759-8888, 547-9561     Work: 703-860-2222
  Cthulhu for President in '96: When You're Tired of the Lesser of Two Evils
--
Stephen P Potter        Pencom Systems Administration              Beaching It
[e-mail]    Pager: 1-800-759-8888, 547-9561     Work: 703-860-2222
"I don't care whether people actually like Perl, just so long as they *think*
    they like it...  ;-)"    -Larry Wall

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Subject: comp.lang.perl.* FAQ 3/5 - Programming Aids
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Archive-name: perl-faq/part3
Version: $Id: part3,v 2.7 1995/05/15 15:44:43 spp Exp spp $
Posting-Frequency: bi-weekly
Last Edited: Thu Jan 11 00:55:42 1996 by spp (Stephen P Potter) on syrinx.psa.com

This posting contains answers to general information questions, mostly
about programming aids.


3.1) How can I use Perl interactively?

    The easiest way to do this is to run Perl under its debugger.  If you
    have no program to debug, you can invoke the debugger on an `empty'
    program like this:

        perl -de 0

    (The more positive hackers prefer "perl -de 1". :-)

    Now you can 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, and if you want to, set breakpoints
    and do the other things you can do in a symbolic debugger.


3.2) Is there a Perl profiler?

    While there isn't one included with the perl source distribution (yet)
    various folks have written packages that allow you to do at least some
    sort of profiling.  The strategy usually includes modifying the perl
    debugger to handle profiling.  Authors of these packages include

    Wayne Thompson         <[e-mail]>
    Ray Lischner         <[e-mail]>
    Kresten Krab Thorup     <[e-mail]>

    The original articles by these folks containing their profilers are
    available at ftp://convex.com/pub/perl/info/profiling.shar.

    Recently, Dean Roerich* has written a profiler for version 5 that
    likely will be distributed with the standard release.  For now, it
    should be available through any of the extension archives as
    DProf.tar.gz.


3.3) Is there a yacc for Perl?

    Yes!! It's a version of Berkeley yacc that outputs Perl code instead
    of C code!  You can get this from
    ftp://ftp.sterling.com/local/perl-byacc1.8.2.tar.Z, or send the author
    mail for details.


3.4) Is there a pretty-printer (similar to indent(1)) for Perl?

    That depends on what you mean.  If you want something that works like
    vgrind on Perl programs, then the answer is "yes, nearly".  Here's a
    vgrind entry for perl:

    PERL|perl|Perl:\
        :pb=^\d?(sub|package)\d\p\d:\
        :bb={:be=}:cb=#:ce=$:sb=":se=\e":lb=':\
        :le=\e':tl:\
        :id=_:\
        :kw=\
        if for foreach unless until while continue else elsif \
        do eval require \
        die exit \
        defined delete reset \
        goto last redo next dump \
        local undef return  \
        write format  \
        sub package

    It doesn't actually do everything right; in particular,
    things like $#, $', s#/foo##, and $foo'bar all confuse it.

    David Levine uses this:

    # perl 4.x                    David Levine <[e-mail]> 05 apr 1993
    # Derived from Tom Christiansen's perl vgrindef.  I'd like to treat all  of
    # perl's built-ins as  keywords,  but vgrind   fields are  limited  to 1024
    # characters  and the built-ins overflow that (surprise  :-).  So, I didn't
    # include the dbm*, end*, get*, msg*, sem*, set*,  and  shm* functions.   I
    # couldn't come up with an easy way to  distinguish beginnings  of literals
    # ('...') from package prefixes, so literals are not marked.
    # Be sure to:
    # 1) include whitespace between a subprogram name and its opening {
    # 2) include whitespace before a comment (so that $# doesn't get
    # interpreted as one).
    perl4:\
        :pb=^\d?(sub|package)\d\p\d:\
        :id=$%@_:\
        :bb=\e{:be=\e}:cb=\d\e#:ce=$:sb=\e":se=\e":\
        :kw=accept alarm atan2 bind binmode caller chdir chmod chop \
    chown chroot close closedir connect continue cos crypt defined delete \
    die do dump each else elsif eof eval exec exit exp fcntl fileno flock \
    for foreach fork format getc gmtime goto grep hex if include index int \
    ioctl join keys kill last length link listen local localtime log lstat \
    m mkdir next oct open opendir ord pack package pipe pop print printf \
    push q qq qx rand read readdir readlink recv redo rename require reset \
    return reverse rewinddir rindex rmdir s scalar seek seekdir select send \
    shift shutdown sin sleep socket socketpair sort splice split sprintf \
    sqrt srand stat study sub substr symlink syscall sysread system \
    syswrite tell telldir time times tr truncate umask undef unless unlink \
    unpack unshift until utime values vec wait waitpid wantarray warn while \
    write y:

    If what you mean is whether there is a program that will reformat the
    program much as indent(1) will do for C, then the answer is no.  The
    complex feedback between the scanner and the parser (as in the things
    that confuse vgrind) make it challenging at best to write a stand-alone
    Perl parser.

    Of course, if you follow the guidelines in perlstyle(1), you shouldn't
    need to reformat.


3.5) How can I convert my perl scripts directly to C or compile them into
     binary form?

    The short answer is: "No, you can't compile perl into C.  Period."

    However, having said that, it is believed that it would be possible to
    write a perl to C translator, although it is a PhD thesis waiting to
    happen.  Anyone need a good challenging thesis?

    In the way of further, detailed explication, it seems that the reasons
    people want to do this usaully break down into one or more of the
    following:

    A) speed
    B) secrecy
    C) maintainability

    SPEED:

    1)    You can't turn perl source code or perl intermediary code into
    native machine code to make it run faster, and saving the perl
    intermediary code doesn't really buy you as much as you'd like.
    If you really must, check out the undump and unexec alternatives.
    If your motivations are speed, then this may or may not help you
    much.

    You might also look into autoloading functions on the fly, which
    can greatly reduce start-up time.

    If you have a few routines that are bogging you down, you just
    possibly might wish to hand-translate just them into C, then
    dynamically load these in.  See perlapi(1) for details.  Most
    of the time, however, reorganizing your perl algorithm is the best
    way to address this.


    SOURCE-CODE SECRECY:

    2)  If you're trying to stop people from seeing what you're doing,
    you can shroud it, i.e. turn all the idents into silly stuff,
    rearrange strings, and remove redundant white space.  There's
    a program out there called ShroudIt! that works on a number of
        languages, including Perl.  Note that it is a commercial product
        though.  Contact David Webber ([e-mail]) for more
        information.

    3)  You might also look into the cryptswitch() stuff in the perl
        source, which would allow you to ship something in a form they
        can't read.  This isn't particulary well-documented.

    4)  If you're worried about them using your software without licence,
    you put some huge disclaimer at the top that says something like
    the following.  This is actually the best solution, because only a
    legal solution will really work if legality is what you're worried
    about: trying to solve legal problems with technical solutions
    is not worth the effort, and too easily circumvented.

        This is UNPUBLISHED PROPRIETARY SOURCE CODE of XYZZY, Inc.; the
        contents of this file may not be disclosed to third parties,
        copied or duplicated in any form, in whole or in part, without
        the prior written permission of XYZZY, Inc.

        Permission is hereby granted soley to the licencee for use of
        this source code in its unaltered state.  This source code may
        not be modified by licencee except under direction of XYZZY
        Inc.  Neither may this source code be given under any
        circumstances to non-licensees in any form, including source
        or binary.  Modification of this source constitutes breach of
        contract, which voids any potential pending support
        responsibilities by XYZZY Inc.  Divulging the exact or
        paraphrased contents of this source code to unlicensed parties
        either directly or indirectly constitutes violation of federal
        and international copyright and trade secret laws, and will be
        duly prosecuted to the fullest extent permitted under law.

        This software is provided by XYZZY Inc. ``as is'' and any
        express or implied warranties, including, but not limited to,
        the implied warranties of merchantability and fitness for a
        particular purpose are disclaimed.  In no event shall the
        regents or contributors be liable for any direct, indirect,
        incidental, special, exemplary, or consequential damages
        (including, but not limited to, procurement of substitute
        goods or services; loss of use, data, or profits; or business
        interruption) however caused and on any theory of liability,
        whether in contract, strict liability, or tort (including
        negligence or otherwise) arising in any way out of the use of
        this software, even if advised of the possibility of such
        damage.

    MAINTAINABILITY:

    5)  If you just want to stop people from changing it because you're
    concerned about support issues, you can put in a big disclaimer at
    the top that says that if they touch the file they void the
    warranty, and then make them give you a size, checksum, and
    version of the file before answering any questions about it.

    If you maintain a central site that distributes software to
    internal client machines, use rdist(1) to send around a proper
    version periodically, perhaps using the -y option on the install
    to flag destinations younger than the source.

    Let it be noted than in the many, many years that Perl's author
    has been releasing and supporting freely redistributable software,
    he has NEVER ONCE been bitten by a bogus bug report generated by
    someone breaking his code because they had access to it.  Rather,
    he and many other open software provided (where open software
    means that for which the source is provided, the only truly open
    software) have saved themselves countless hours of labor thousands
    of times over because they've allowed people to inspect the source
    for themselves.  Proprietary source-code hoarding is its own
        headache.

    Thus, obscurity for the sake of maintainability would seem to be a
    red herring.

    6)  If you can't count on perl being installed at the destination
        customer, then by all means, merely ship it with your program.
        This is no hardship, since software providers are accustomed to
        shipping software in machine-specific binary form.  The basic idea
        is as simple as:

        shar /usr/local/{lib,bin,man}/perl myprog

    Just don't overwrite their own Perl installation if they have one!


3.6) Where can I get a perl-mode for emacs?

    Since Emacs version 19 patchlevel 22 or so, there has 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 several files that should help you.

    Note that the perl-mode of emacs will have fits with "main'foo" (single
    quote), and mess up the indentation and hilighting.  However, note that
    in perl5, you should be using "main::foo".  By the way, did we mention
    that you should upgrade?


3.7) Is there a Perl shell?

    Not really.  Perl is a programming language, not a command
    interpreter.  There is a very simple one called "perlsh"
    included in the Perl source distribution.  It just does this:

    $/ = '';        # set paragraph mode
    $SHlinesep = "\n";
    while ($SHcmd = <>) {
        $/ = $SHlinesep;
        eval $SHcmd; print $@ || "\n";
        $SHlinesep = $/; $/ = '';
    }

    Not very interesting, eh?

    Daniel Smith <[e-mail]> is working on an interactive Perl shell
    called SoftList.  It's currently at version 3.0b7a (beta).  SoftList
    3.0b7a has tcsh-like command line editing, can let you define a file of
    aliases so that you can run chunks of perl or UNIX commands, and so
    on.  You can pick up a copy at ftp.best.com in
    /pub/dls/SoftList-3.0b7a.gz.


3.8) How can I use curses with perl?

    In release 4 of perl, the only way to do this was was to build a
    curseperl binary by linking in your C curses library as described in
    the usub subdirectory of the perl sources.  This requires a modicum of
    work, but it will be reasonably fast since it's all in C (assuming you
    consider curses reasonably fast. :-) Programs written using this
    method require the modified curseperl, not vanilla perl, to run.
    While this is something of a disadvantage, experience indicates that
    it's better to use curseperl than to try to roll your own using
    termcap directly.

    Fortunately, in version 5, Curses is a dynamically loaded extension
    by William Setzer*.  You should be able to pick it up wherever you
    get Perl 5 from, or at least these places (expect that the version may
    change by the time you read this):

    ftp://ftp.ncsu.edu/pub/math/wsetzer/cursperl5a6.tar.gz
        ftp://ftp.metronet.com/pub/perlinfo/perl5/cursperl5a6.tar.gz
        ftp://ftp.cs.ruu.nl/pub/PERL/perl5.0/cursperl5a6.tar.gz

    For a good example of using curses with Perl, you might want to pick
    up a copy of Steven L. Kunz's* "perl menus" package ("menu.pl") via
    anonymous FTP from "ftp.iastate.edu".  It's in the directory /pub/perl
    as:

        menu.pl.v3.1.tar.Z

    menu.pl is a complete menu front-end for perl+curses and demonstrates
    a lot of things (plus it is useful to boot if you want full-screen
    menu selection ability).  It provides full-screen menu selection
    ability for three menu styles (single-selection, multiple-selection,
    and "radio-button").  The "perl menus" package also includes routines
    for full-screen data entry.  A "template" concept is implemented to
    create a simple (yet flexible) perl interface for building data-entry
    screens for registration, database, or other record-oriented tasks.

    menu.pl is supported on Perl4/curseperl and Perl5/Curses.  Complete
    user documentation is provided along with several demos and "beginner
    applications".  A menu utility module is provided that is a collection
    of useful Perl curses routines (such as "pop-up query boxes) that may
    be called from your applications.

    Another possibility is to use Henk Penning's cterm package, a curses
    emulation library written in perl.  cterm is actually a separate
    program with which you communicate via a pipe.  It is available from
    ftp.cs.ruu.nl [131.211.80.17] via anonymous ftp. in the directory
    pub/PERL.  You may also acquire the package via email in compressed,
    uuencoded form by sending a message to [e-mail]
    containing these lines:

    begin
    send PERL/cterm.shar.Z
    end

    See the question on retrieving perl via mail for more information on
    how to retrieve other items of interest from the mail server
    there.


3.9) How can I use X or Tk with Perl?

    Right now, you have several choices.  If you are still using perl4, use
    the WAFE or STDWIN packages, or try to make your own usub binding.

    However, if you've upgraded to version 5, you have several exciting
    possibilities, with more popping up each day.  Right now, Tk and Sx
    are the best known such extensions.

    If you like the tk package, you should get the Tk extension kit,
    written by Nick Ing-Simmons*.  The official distribution point is at

        ftp://ftp.wpi.edu/perl5/private/Tk-b8.tar.gz

    but many of the major archive sites now have it in their /ext{entions}
    directory also.  Depending upon your location, you may be better off
    checking there.  Also, understand that the version number may have
    changed by the time you read this.

    This package replaced the tkperl5 project, by Malcolm Beattie*, which
    was based on an older version of Tk, 3.6 as compared to the current
    4.X.  This package was also known as nTk (new Tk) while it was in the
    alpha stages, but has been changed to just Tk now that it is in beta.
    Also, be advised that you need at least perl5.001 (preferably 5.002,
    when it becomes available) and the official unofficial patches.

    You may also use the old Sx package, (Athena & Xlib), written by
    originally written by by Dominic Giampaolo*, then and rewritten for Sx
    by Frederic Chauveau*.  It's available from these sites:

        ftp://ftp.pasteur.fr/pub/Perl/Sx.tar.gz
        ftp://ftp.khoros.unm.edu/pub/perl/extensions/Sx.tar.gz
        ftp://ftp.metronet.com/pub/perlinfo/perl5/Sx.tar.gz

    STDWIN is a library written by Guido van Rossum* (author of the Python
    programming language) that is portable between Mac, Dos and X11.  One
    could write a Perl agent to speak to this STDWIN server.

    WAFE is a package that implements a symbolic interface to the Athena
    widgets (X11R5). A typical Wafe application consists in our framework
    of two parts: the front-end (we call it Wafe for Widget[Athena]front
    end) and an application program running typically as a separate
    process.  The application program can be implemented in an arbitrary
    programming language and talks to the front-end via stdio.  Since Wafe
    (the front-end) was developed using the extensible TCL shell (cite John
    Ousterhout), an application program can dynamically submit requests to
    the front-end to build up the graphical user interface; the
    application can even down-load application specific procedures into
    the front-end.  The distribution contains sample application programs
    in Perl, GAWK, Prolog, TCL, and C talking to the same Wafe binary.
    Many of the demo applications are implemented in Perl.  Wafe 0.9 can
    be obtained via anonymous ftp from
    ftp.wu-wien.ac.at[137.208.3.5]:pub/src/X11/wafe-0.9.tar.Z

    Alternatively, you could use wish from tcl.

    #!/usr/local/bin/perl
    #####################################################################
    #  An example of calling wish as a subshell under Perl and
    #  interactively communicating with it through sockets.
    #
    #  The script is directly based on Gustaf Neumann's perlwafe script.
    #
    #  Dov Grobgeld [e-mail]
    #  1993-05-17
    #####################################################################

    $wishbin = "/usr/local/bin/wish";

    die "socketpair unsuccessful: $!!\n" unless socketpair(W0,WISH,1,1,0);
    if ($pid=fork) {
        select(WISH); $| = 1;
        select(STDOUT);

    # Create some TCL procedures
        print WISH 'proc echo {s} {puts stdout $s; flush stdout}',"\n";

    # Create the widgets
    print WISH <<TCL;
    # This is a comment "inside" wish

    frame .f -relief raised -border 1 -bg green
    pack append . .f {top fill expand}

    button .f.button-pressme -text "Press me" -command {
        echo "That's nice."
    }
    button .f.button-quit -text quit -command {
        echo "quit"
    }
    pack append .f .f.button-pressme {top fill expand} \\
               .f.button-quit {top expand}

TCL
    ;
    # Here is the main loop which receives and sends commands
    # to wish.
    while (<WISH>) {
        chop;
        print "Wish sais: <$_>\n";
        if (/^quit/) { print WISH "destroy .\n"; last; }
    }
        wait;
    } elsif (defined $pid) {
    open(STDOUT, ">&W0");
    open(STDIN, ">&W0");
    close(W0);
    select(STDOUT); $| = 1;
    exec "$wishbin --";
    } else {
    die "fork error: $!\n";
    }


3.10) Can I dynamically load C user routines?

    Yes -- dynamic loading comes with the distribution.  That means that
    you no longer need 18 different versions of fooperl floating around.
    In fact, all of perl can be stuck into a libperl.so library and
    then your /usr/local/bin/perl binary reduced to just 50k or so.
    See DynLoader(3pm) for details.

    In perl4, the answer was kinda.  One package has been released that does
    this, by Roberto Salama*.  He writes:

    Here is a version of dylperl, dynamic linker for perl. The code here is
    based on Oliver Sharp's May 1993 article in Dr. Dobbs Journal (Dynamic
    Linking under Berkeley UNIX).

          dyl.h
          dyl.c - code extracted from Oliver Sharp's article

          hash.h
          hash.c - Berkeley's hash functions, should use perl's but
               could not be bothered

       dylperl.c - perl usersubs
          user.c - userinit function

        sample.c - sample code to be dyl'ed
       sample2.c -          "
         test.pl - sample perl script that dyl's sample*.o

    The Makefile assumes that uperl.o is in /usr/local/src/perl/... You
    will probably have to change this to reflect your installation. Other
    than that, just type 'make'...

    The idea behind being able to dynamically link code into perl is that
    the linked code should become perl functions, i.e. they can be invoked
    as &foo(...).  For this to happen, the incrementally loaded code must
    use the perl stack, look at sample.c to get a better idea.

    The few functions that make up this package are outlined below.

    &dyl("file.o"): dynamically link file.o. All functions and non-static
           variables become visible from within perl. This
           function returns a pointer to an internal hash table
           corresponding to the symbol table of the newly loaded
           code.

           eg: $ht = &dyl("sample.o")

       This function can also be called with the -L and -l ld options.

           eg: $ht = &dyl(""sample2.o", "-L/usr/lib", "-lm")
               will also pick up the math library if sample.o
               accesses any symbols there.

    &dyl_find("func"): find symbol 'func' and return its symbol table entry

    &dyl_functions($ht): print the contents of the internal hash table
    &dyl_print_symbols($f): prints the contents of the symbol returned by
    dyl_find()

    There is very little documentation, maybe something to do for a future
    release.  The files sample.o, and sample2.o contain code to be
    incrementally loaded, test.pl is the test perl script.

    Comments are welcome. I submit this code for public consumption and,
    basically, am not responsible for it in any way.


3.11) What is undump and where can I get it?

    The undump program comes from the TeX distribution.  If you have TeX,
    then you may have a working undump.  If you don't, and you can't get
    one, *AND* you have a GNU emacs working on your machine that can clone
    itself, then you might try taking its unexec() function and compiling
    Perl with -DUNEXEC, which will make Perl call unexec() instead of
    abort().  You'll have to add unexec.o to the objects line in the
    Makefile.  If you succeed, post to comp.lang.perl.misc about your
    experience so others can benefit from it.

    If you have a version of undump that works with Perl, please submit
    its anon-FTP whereabouts to the FAQ maintainer.


3.12) How can I get '#!perl' to work under MS-DOS?

    John Dallman* has written a program "#!perl.exe" which will do this.
    It is available through anonymous ftp from ftp.ee.umanitoba.ca in the
    directory /pub/msdos/perl/hbp_30.zip.  This program works by finding
    the script and perl.exe, building a command line and running perl.exe
    as a child process.  For more information on this, contact John
    directly.

3.13) Can I write useful perl programs on the command line?

    Sure, if they're simple enough.  Of course, for most programs,
    you'll enter them in a file and call perl on them from your
    shell.  That way you can go into the hack/execute/debug cycle.
    But there are plenty of useful one-liner: see below.  (Things
    marked perl5 need to be run from v5.000 or better, but the
    rest don't care.)

    # what's octal value of random char (":" in this case)?
    perl -e 'printf "%#o\n", ord(shift)' ":"

    # sum first and last fields
    perl -lane 'print $F[0] + $F[1]'

    # strip high bits
    perl -pe 'tr/\200-\377/\000-\177/'

    # find text files
    perl -le 'for(@ARGV) {print if -f && -T}' *

    # trim newsrc
    perl5 -i.old -pe 's/!.*?(\d+)$/! 1-$1/' ~/.newsrc

    # cat a dbmfile
    perl -e 'dbmopen(%f,shift,undef);while(($k,$v)=each%f){print "$k:\
            $v\n"}' /etc/aliases

    # remove comments from C program
    perl5 -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
    perl5 -le '$i++ while getpwuid($i); print $i'

    # find first unused uid after 100, even with perl4
    perl -le '$i = 100; $i++ while ($x) = getpwuid($i); print $i'

    # detect pathetically insecurable systems
    perl5 -le 'use POSIX; print "INSECURE" unless sysconf(_PC_CHOWN_RESTRICTED)'

    # display reasonable manpath
    echo $PATH | perl5 -nl -072 -e '
    s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}'

    Ok, the last one was actually an obfuscate perl entry. :-)

3.14) What's a "closure"?

    (Larry wrote) This is a notion out of the Lisp world that says if you
    define an anonymous function in a particular lexical context, it
    pretends to run in that context even when it's called outside of the
    context.

    In human terms, it's a funny way of passing arguments to a subroutine
    when you define it as well as when you call it.  It's useful for
    setting up little bits of code to run later, such as callbacks.  You
    can even do object-oriented stuff with it, though Perl provides a
    different mechanism to do that already.

    You can also think of it as a way to write a subroutine template without
    using eval.

    Here's a small example of how this works:

    sub newprint {
        my $x = shift;
        return sub { my $y = shift; print "$x, $y!\n"; };
    }
    $h = newprint("Howdy");
    $g = newprint("Greetings");

    # Time passes...

    &$h("world");
    &$g("earthlings");

    This prints:

    Howdy, world!
    Greetings, earthlings!

    Note particularly that $x continues to refer to the value passed into
    newprint() *despite* the fact that the "my $x" has seemingly gone out
    of scope by the time the anonymous subroutine runs.  That's what
    closure is all about.

    This only applies to lexical variables, by the way.  Dynamic variables
    continue to work as they have always worked.  Closure is not something
    that most Perl programmers need trouble themselves about to begin with.

--
Stephen P Potter        Pencom Systems Administration              Beaching It
[e-mail]    Pager: 1-800-759-8888, 547-9561     Work: 703-860-2222
  Cthulhu for President in '96: When You're Tired of the Lesser of Two Evils
--
Stephen P Potter        Pencom Systems Administration              Beaching It
[e-mail]    Pager: 1-800-759-8888, 547-9561     Work: 703-860-2222
"I don't care whether people actually like Perl, just so long as they *think*
    they like it...  ;-)"    -Larry Wall

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Archive-name: perl-faq/part4
Version: $Id: part4,v 2.9 1995/05/15 15:46:10 spp Exp spp $
Posting-Frequency: bi-weekly
Last Edited: Thu Jan 11 00:52:25 1996 by spp (Stephen P Potter) on syrinx.psa.com

This posting contains answers to the following questions about General
Programming, Regular Expressions (Regexp) and Input/Output:


4.1) What are all these $@%*<> signs and how do I know when to use them?

    Those are type specifiers:
    $ for scalar values
    @ for indexed arrays
    % for hashed arrays (associative arrays)
    * for all types of that symbol name.  These are sometimes used like
        pointers in perl4, but perl5 uses references.
    <> are used for inputting a record from a filehandle.
        \ takes a reference to something.

    See the question on arrays of arrays for more about Perl pointers.

    While there are a few places where you don't actually need these type
    specifiers, except for files, you should always use them.  Note that
    <FILE> is NOT the type specifier for files; it's the equivalent of awk's
    getline function, that is, it reads a line from the handle FILE.  When
    doing open, close, and other operations besides the getline function on
    files, do NOT use the brackets.

    Beware of saying:
    $foo = BAR;
    Which wil be interpreted as
    $foo = 'BAR';
    and not as
    $foo = <BAR>;
    If you always quote your strings, you'll avoid this trap.

    Normally, files are manipulated something like this (with appropriate
    error checking added if it were production code):

    open (FILE, ">/tmp/foo.$$");
    print FILE "string\n";
    close FILE;

    If instead of a filehandle, you use a normal scalar variable with file
    manipulation functions, this is considered an indirect reference to a
    filehandle.  For example,

    $foo = "TEST01";
    open($foo, "file");

    After the open, these two while loops are equivalent:

    while (<$foo>) {}
    while (<TEST01>) {}

    as are these two statements:

    close $foo;
    close TEST01;

    but NOT to this:

    while (<$TEST01>) {} # error
        ^
        ^ note spurious dollar sign

    This is another common novice mistake; often it's assumed that

    open($foo, "output.$$");

    will fill in the value of $foo, which was previously undefined.  This
    just isn't so -- you must set $foo to be the name of a filehandle
    before you attempt to open it.

    Often people request:

    : How about changing perl syntax to be more like awk or C?  I $$mean @less
    : $-signs = <and> &other *special \%characters?

    Larry's answer is:

    Then it would be less like the shell.  :-)

    You'll be pleased to know that I've been trying real hard to get
    rid of unnecessary punctuation in Perl 5.  You'll be displeased to
    know that I don't think noun markers like $ and @ unnecessary.
    Not only do they function like case markers do in human language,
    but they are automatically distinguished within interpolative
    contexts, and the user doesn't have to worry about different
    syntactic treatments for variable references within or without
    such a context.

    But the & prefix on verbs is now optional, just as "do" is in
    English.  I do hope you do understand what I mean.

    For example, you used to have to write this:

        &california || &bust;

    It can now be written more cleanly like this:

        california or bust;

    Strictly speaking, of course, $ and @ aren't case markers, but
    number markers.  English has mandatory number markers, and people
    get upset when they doesn't agree.

    It were just convenient in Perl (for the shellish interplative
    reasons mentioned above) to pull the markers out to the front of
    each noun phrase.  Most people seems to like it that way.  It
    certainly seem to make more sense than putting them on the end,
    like most varieties of BASIC does.


4.2) How come Perl operators have different precedence than C operators?

    Actually, they don't; all C operators have the same precedence in Perl as
    they do in C.  The problem is with a class of functions called list
    operators, e.g. print, chdir, exec, system, and so on.  These are somewhat
    bizarre in that they have different precedence depending on whether you
    look on the left or right of them.  Basically, they gobble up all things
    on their right.  For example,

    unlink $foo, "bar", @names, "others";

    will unlink all those file names.  A common mistake is to write:

    unlink "a_file" || die "snafu";

    The problem is that this gets interpreted as

    unlink("a_file" || die "snafu");

    To avoid this problem, you can always make them look like function calls
    or use an extra level of parentheses:

    unlink("a_file")  || die "snafu";
    (unlink "a_file") || die "snafu";

    In perl5, there are low precedence "and", "or", and "not" operators,
    which bind less tightly than comma.  This allows you to write:

    unlink $foo, "bar", @names, "others"     or die "snafu";

    Sometimes you actually do care about the return value:

    unless ($io_ok = print("some", "list")) { }

    Yes, print() returns I/O success.  That means

    $io_ok = print(2+4) * 5;

    returns 5 times whether printing (2+4) succeeded, and
    print(2+4) * 5;
    returns the same 5*io_success value and tosses it.

    See the perlop(1) man page's section on Precedence for more gory details,
    and be sure to use the -w flag to catch things like this.

    One very important thing to be aware of is that if you start thinking
    of Perl's $, @, %, and & as just flavored versions of C's * operator,
    you're going to be sorry.  They aren't really operators, per se, and
    even if you do think of them that way.  In C, if you write

    *x[i]

    then the brackets will bind more tightly than the star, yielding

    *(x[i])

    But in perl, they DO NOT!  That's because the ${}, @{}, %{}, and &{}
    notations (and I suppose the *{} one as well for completeness) aren't
    actually operators.  If they were, you'd be able to write them as *()
    and that's not feasible.  Instead of operators whose precedence is
    easily understandable, they are instead figments of yacc's grammar.
    This means that:

    $$x[$i]

    is really

    {$$x}[$i]

    (by which I actually mean)

    ${$x}[$i]

    and not

    ${$x[$i]}

    See the difference?  If not, check out perlref(1) for gory details.


4.3) What's the difference between dynamic and static (lexical) scoping?
     What are my() and local()?

    [NOTE: This question refers to perl5 only.  There is no my() in perl4]
    Scoping refers to visibility of variables.  A dynamic variable is
    created via local() and is just a local value for a global variable,
    whereas a lexical variable created via my() is more what you're
    expecting from a C auto.  (See also "What's the difference between
    deep and shallow binding.")  In general, we suggest you use lexical
    variables wherever possible, as they're faster to access and easier to
    understand.   The "use strict vars" pragma will enforce that all
    variables are either lexical, or full classified by package name.  We
    strongly suggest that you develop your code with "use strict;" and the
    -w flag.  (When using formats, however, you will still have to use
    dynamic variables.)  Here's an example of the difference:

        #!/usr/local/bin/perl
        $myvar = 10;
        $localvar = 10;

        print "Before the sub call - my: $myvar, local: $localvar\n";
        &sub1();

        print "After the sub call - my: $myvar, local: $localvar\n";

        exit(0);

        sub sub1 {
            my $myvar;
            local $localvar;

            $myvar = 5;     # Only in this block
            $localvar = 20; # Accessible to children

            print "Inside first sub call - my: $myvar, local: $localvar\n";

            &sub2();
        }

        sub sub2 {
            print "Inside second sub - my: $myvar, local: $localvar\n";
        }

    Notice that the variables declared with my() are visible only within
    the scope of the block which names them.  They are not visible outside
    of this block, not even in routines or blocks that it calls.  local()
    variables, on the other hand, are visible to routines that are called
    from the block where they are declared.  Neither is visible after the
    end (the final closing curly brace) of the block at all.

    Oh, lexical variables are only available in perl5.  Have we mentioned
    yet that you might consider upgrading? :-)


4.4) What's the difference between deep and shallow binding?

    5.000 answer:

    This only matters when you're making subroutines yourself, at least
    so far.   This will give you shallow binding:

    {
          my $x = time;
          $coderef = sub { $x };
        }

    When you call &$coderef(), it will get whatever dynamic $x happens
    to be around when invoked.  However, you can get the other behaviour
    this way:

    {
          my $x = time;
          $coderef = eval "sub { \$x }";
        }

    Now you'll access the lexical variable $x which is set to the
    time the subroutine was created.  Note that the difference in these
    two behaviours can be considered a bug, not a feature, so you should
    in particular not rely upon shallow binding, as it will likely go
    away in the future.  See perlref(1).

    5.001 Answer:

    Perl will always give deep binding to functions, so you don't need the
    eval hack anymore.   Furthermore,  functions and even formats
    lexically declared nested within another lexical scope have access to
    that scope.

    require 5.001;

    sub mkcounter {
        my $start = shift;
        return sub {
        return ++$start;
        }
    }
    $f1 = mkcounter(10);
    $f2 = mkcounter(20);
    print &$f1(), &$f2();
    11 21
    print &$f1(), &$f2(), &$f1();
    12 22 13

    See the question on "What's a closure?"


4.5) How can I manipulate fixed-record-length files?

    The most efficient way is using pack and unpack.  This is faster than
    using substr.  Here is a sample chunk of code to break up and put back
    together again some fixed-format input lines, in this case, from ps.

    # sample input line:
    #   15158 p5  T      0:00 perl /mnt/tchrist/scripts/now-what
    $ps_t = 'A6 A4 A7 A5 A*';
    open(PS, "ps|");
    $_ = <PS>; print;
    while (<PS>) {
        ($pid, $tt, $stat, $time, $command) = unpack($ps_t, $_);
        for $var ('pid', 'tt', 'stat', 'time', 'command' ) {
        print "$var: <", eval "\$$var", ">\n";
        }
        print 'line=', pack($ps_t, $pid, $tt, $stat, $time, $command),  "\n";
    }


4.6) How can I make a file handle local to a subroutine?

    You must use the type-globbing *VAR notation.  Here is some code to
    cat an include file, calling itself recursively on nested local
    include files (i.e. those with #include "file", not #include <file>):

    sub cat_include {
        local($name) = @_;
        local(*FILE);
        local($_);

        warn "<INCLUDING $name>\n";
        if (!open (FILE, $name)) {
        warn "can't open $name: $!\n";
        return;
        }
        while (<FILE>) {
        if (/^#\s*include "([^"]*)"/) {
            &cat_include($1);
        } else {
            print;
        }
        }
        close FILE;
    }


4.7) How can I call alarm() or usleep() from Perl?

    If you want finer granularity than 1 second (as usleep() provides) and
    have itimers and syscall() on your system, you can use the following.
    You could also use select().

    It takes a floating-point number representing how long to delay until
    you get the SIGALRM, and returns a floating- point number representing
    how much time was left in the old timer, if any.  Note that the C
    function uses integers, but this one doesn't mind fractional numbers.

    # alarm; send me a SIGALRM in this many seconds (fractions ok)
    # tom christiansen <[e-mail]>
    sub alarm {
    require 'syscall.ph';
    require 'sys/time.ph';

    local($ticks) = @_;
    local($in_timer,$out_timer);
    local($isecs, $iusecs, $secs, $usecs);

    local($itimer_t) = 'L4'; # should be &itimer'typedef()

    $secs = int($ticks);
    $usecs = ($ticks - $secs) * 1e6;

    $out_timer = pack($itimer_t,0,0,0,0);
    $in_timer  = pack($itimer_t,0,0,$secs,$usecs);

    syscall(&SYS_setitimer, &ITIMER_REAL, $in_timer, $out_timer)
        && die "alarm: setitimer syscall failed: $!";

    ($isecs, $iusecs, $secs, $usecs) = unpack($itimer_t,$out_timer);
    return $secs + ($usecs/1e6);
    }


4.8) How can I do an atexit() or setjmp()/longjmp() in Perl?  (Exception handling)

    Perl's exception-handling mechanism is its eval operator.  You
    can use eval as setjmp and die as longjmp.  Here's an example
    of Larry's for timed-out input, which in C is often implemented
    using setjmp and longjmp:

      $SIG{ALRM} = TIMEOUT;
      sub TIMEOUT { die "restart input\n" }

      do { eval { &realcode } } while $@ =~ /^restart input/;

      sub realcode {
          alarm 15;
          $ans = <STDIN>;
          alarm 0;
      }

   Here's an example of Tom's for doing atexit() handling:

    sub atexit { push(@_exit_subs, @_) }

    sub _cleanup { unlink $tmp }

    &atexit('_cleanup');

    eval <<'End_Of_Eval';  $here = __LINE__;
    # as much code here as you want
    End_Of_Eval

    $oops = $@;  # save error message

    # now call his stuff
    for (@_exit_subs) { &$_() }

    $oops && ($oops =~ s/\(eval\) line (\d+)/$0 .
        " line " . ($1+$here)/e, die $oops);

    You can register your own routines via the &atexit function now.  You
    might also want to use the &realcode method of Larry's rather than
    embedding all your code in the here-is document.  Make sure to leave
    via die rather than exit, or write your own &exit routine and call
    that instead.   In general, it's better for nested routines to exit
    via die rather than exit for just this reason.

    In Perl5, it is easy to set this up because of the automatic processing
    of per-package END functions.  These work much like they would in awk.
    See perlfunc(1), perlmod(1) and perlrun(1).

    Eval is also quite useful for testing for system dependent features,
    like symlinks, or using a user-input regexp that might otherwise
    blowup on you.


4.9) How do I catch signals in perl?

    Perl allows you to trap signals using the %SIG associative array.
    Using the signals you want to trap as the key, you can assign a
    subroutine to that signal.  The %SIG array will only contain those
    values which the programmer defines.  Therefore, you do not have to
    assign all signals.  For example, to exit cleanly from a ^C:

    $SIG{'INT'} = 'CLEANUP';
    sub CLEANUP {
        print "\n\nCaught Interrupt (^C), Aborting\n";
        exit(1);
    }

    There are two special "routines" for signals called DEFAULT and IGNORE.
    DEFAULT erases the current assignment, restoring the default value of
    the signal.  IGNORE causes the signal to be ignored.  In general, you
    don't need to remember these as you can emulate their functionality
    with standard programming features.  DEFAULT can be emulated by
    deleting the signal from the array and IGNORE can be emulated by any
    undeclared subroutine.

    In 5.001, the $SIG{__WARN__} and $SIG{__DIE__} handlers may be used to
    intercept die() and warn().  For example, here's how you could promote
    unitialized variables to trigger a fatal rather merely complaining:

    #!/usr/bin/perl -w
    require 5.001;
    $SIG{__WARN__} = sub {
        if ($_[0] =~ /uninit/) {
        die $@;
        } else {
        warn $@;
        }
    };


4.10) Why doesn't Perl interpret my octal data octally?

    Perl only understands octal and hex numbers as such when they occur
    as literals in your program.  If they are read in from somewhere and
    assigned, then no automatic conversion takes place.  You must
    explicitly use oct() or hex() if you want this kind of thing to happen.
    Actually, oct() knows to interpret both hex and octal numbers, while
    hex only converts hexadecimal ones.  For example:

    {
        print "What mode would you like? ";
        $mode = <STDIN>;
        $mode = oct($mode);
        unless ($mode) {
        print "You can't really want mode 0!\n";
        redo;
        }
        chmod $mode, $file;
    }

    Without the octal conversion, a requested mode of 755 would turn
    into 01363, yielding bizarre file permissions of --wxrw--wt.

    If you want something that handles decimal, octal and hex input,
    you could follow the suggestion in the man page and use:

    $val = oct($val) if $val =~ /^0/;


4.11) How can I compare two date strings?

    If the dates are in an easily parsed, predetermined format, then you
    can break them up into their component parts and call &timelocal from
    the distributed perl library.  If the date strings are in arbitrary
    formats, however, it's probably easier to use the getdate program from
    the Cnews distribution, since it accepts a wide variety of dates.  Note
    that in either case the return values you will really be comparing will
    be the total time in seconds as returned by time().

    Here's a getdate function for perl that's not very efficient; you can
    do better than this by sending it many dates at once or modifying
    getdate to behave better on a pipe.  Beware the hardcoded pathname.

    sub getdate {
        local($_) = shift;

        s/-(\d{4})$/+$1/ || s/\+(\d{4})$/-$1/;
        # getdate has broken timezone sign reversal!

        $_ = `/usr/local/lib/news/newsbin/getdate '$_'`;
        chop;
        $_;
    }

    You can also get the  GetDate extension module that's actually the C
    code linked into perl from wherever fine Perl extensions are given
    away.  It's about 50x faster.  If you can't find it elsewhere, I
    usually keep a copy on perl.com for ftp, since I (Tom) ported it.

    Richard Ohnemus <[e-mail]> actually has a getdate.y for
    use with the Perl yacc (see question 3.3 "Is there a yacc for Perl?").

    You might also consider using these:

    date.pl        - print dates how you want with the sysv +FORMAT method
    date.shar      - routines to manipulate and calculate dates
    ftp-chat2.shar - updated version of ftpget. includes library and demo
             programs
    getdate.shar   - returns number of seconds since epoch for any given
             date
    ptime.shar     - print dates how you want with the sysv +FORMAT method

    You probably want 'getdate.shar'... these and other files can be ftp'd
    from the /pub/perl/scripts directory on ftp.cis.ufl.edu. See the README
    file in the /pub/perl directory for time and the European mirror site
    details.


4.12) How can I find the Julian Day?

    Here's an example of a Julian Date function provided by Thomas R.
    Kimpton*.

    #!/usr/local/bin/perl

    @theJulianDate = ( 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 );

    #************************************************************************
    #****   Return 1 if we are after the leap day in a leap year.       *****
    #************************************************************************

    sub leapDay
    {
        my($year,$month,$day) = @_;

        if (year % 4) {
        return(0);
        }

        if (!(year % 100)) {             # years that are multiples of 100
                                     # are not leap years
        if (year % 400) {            # unless they are multiples of 400
            return(0);
        }
        }
        if (month < 2) {
            return(0);
        } elsif ((month == 2) && (day < 29)) {
        return(0);
        } else {
        return(1);
        }
    }

    #************************************************************************
    #****   Pass in the date, in seconds, of the day you want the       *****
    #****   julian date for.  If your localtime() returns the year day  *****
    #****   return that, otherwise figure out the julian date.          *****
    #************************************************************************

    sub julianDate
    {
        my($dateInSeconds) = @_;
        my($sec, $min, $hour, $mday, $mon, $year, $wday, $yday);

        ($sec, $min, $hour, $mday, $mon, $year, $wday, $yday) =
        localtime($dateInSeconds);
        if (defined($yday)) {
        return($yday+1);
        } else {
        return($theJulianDate[$mon] + $mday + &leapDay($year,$mon,$mday));
        }

    }

    print "Today's julian date is: ",&julianDate(time),"\n";


4.13) Does perl have a round function?  What about ceil() and floor()?

    Perl does not have an explicit round function.  However, it is very
    simple to create a rounding function.  Since the int() function simply
    removes the decimal value and returns the integer portion of a number,
    you can use

        sub round {
            my($number) = shift;

            return int($number + .5);
        }

    If you examine what this function is doing, you will see that any
    number greater than .5 will be increased to the next highest integer,
    and any number less than .5 will remain the current integer, which has
    the same effect as rounding.

    A slightly better solution, one which handles negative numbers as well,
    might be to change the return (above) to:

        return int($number + .5 * ($number <=> 0));

    which will modify the .5 to be either positive or negative, based on
    the number passed into it.

    If you wish to round to a specific significant digit, you can use the
    printf function (or sprintf, depending upon the situation), which does
    proper rounding automatically.  See the perlfunc man page for more
    information on the (s)printf function.

    Version 5 includes a POSIX module which defines the standard C math
    library functions, including floor() and ceil().  floor($num) returns
    the largest integer not greater than $num, while ceil($num) returns the
    smallest integer not less than $num.  For example:

        #!/usr/local/bin/perl
        use POSIX qw(ceil floor);

        $num = 42.4;  # The Answer to the Great Question (on a Pentium)!

        print "Floor returns: ", floor($num), "\n";
        print "Ceil returns:  ", ceil($num), "\n";

    Which prints:

        Floor returns: 42
        Ceil returns:  43


4.14) What's the fastest way to code up a given task in perl?

    Post it to comp.lang.perl.misc and ask Tom or Randal a question about
    it.  ;)

    Because Perl so lends itself to a variety of different approaches for
    any given task, a common question is which is the fastest way to code a
    given task.  Since some approaches can be dramatically more efficient
    that others, it's sometimes worth knowing which is best.
    Unfortunately, the implementation that first comes to mind, perhaps as
    a direct translation from C or the shell, often yields suboptimal
    performance.  Not all approaches have the same results across different
    hardware and software platforms.  Furthermore, legibility must
    sometimes be sacrificed for speed.

    While an experienced perl programmer can sometimes eye-ball the code
    and make an educated guess regarding which way would be fastest,
    surprises can still occur.  So, in the spirit of perl programming
    being an empirical science, the best way to find out which of several
    different methods runs the fastest is simply to code them all up and
    time them. For example:

    $COUNT = 10_000; $| = 1;

    print "method 1: ";

        ($u, $s) = times;
        for ($i = 0; $i < $COUNT; $i++) {
        # code for method 1
        }
        ($nu, $ns) = times;
        printf "%8.4fu %8.4fs\n", ($nu - $u), ($ns - $s);

    print "method 2: ";

        ($u, $s) = times;
        for ($i = 0; $i < $COUNT; $i++) {
        # code for method 2
        }
        ($nu, $ns) = times;
        printf "%8.4fu %8.4fs\n", ($nu - $u), ($ns - $s);

    Perl5 includes a new module called Benchmark.pm.  You can now simplify
    the code to use the Benchmarking, like so:

        use Benchmark;

            timethese($count, {
                Name1 => '...code for method 1...',
                Name2 => '...code for method 2...',
                ... });

    It will output something that looks similar to this:

        Benchmark: timing 100 iterations of Name1, Name2...
                Name1:  2 secs (0.50 usr 0.00 sys = 0.50 cpu)
                Name2:  1 secs (0.48 usr 0.00 sys = 0.48 cpu)


    For example, the following code will show the time difference between
    three different ways of assigning the first character of a string to
    a variable:

    use Benchmark;
    timethese(100000, {
        'regex1' => '$str="ABCD"; $str =~ s/^(.)//; $ch = $1',
        'regex2' => '$str="ABCD"; $str =~ s/^.//; $ch = $&',
        'substr' => '$str="ABCD"; $ch=substr($str,0,1); substr($str,0,1)="",
    });

    The results will be returned like this:

    Benchmark: timing 100000 iterations of regex1, regex2, substr...
       regex1: 11 secs (10.80 usr   0.00 sys =  10.80 cpu)
       regex2: 10 secs (10.23 usr   0.00 sys =  10.23 cpu)
       substr:  7 secs ( 5.62 usr    0.00 sys =   5.62 cpu)

    For more specific tips, see the section on Efficiency in the
    ``Other Oddments'' chapter at the end of the Camel Book.


4.15) Do I always/never have to quote my strings or use semicolons?

    You don't have to quote strings that can't mean anything else in the
    language, like identifiers with any upper-case letters in them.
    Therefore, it's fine to do this:

    $SIG{INT} = Timeout_Routine;
    or

    @Days = (Sun, Mon, Tue, Wed, Thu, Fri, Sat, Sun);

    but you can't get away with this:

    $foo{while} = until;

    in place of

    $foo{'while'} = 'until';

    The requirements on semicolons have been increasingly relaxed.  You no
    longer need one at the end of a block, but stylistically, you're better
    to use them if you don't put the curly brace on the same line:

    for (1..10) { print }

    is ok, as is

    @nlist = sort { $a <=> $b } @olist;

    but you probably shouldn't do this:

    for ($i = 0; $i < @a; $i++) {
        print "i is $i\n"  # <-- oops!
    }

    because you might want to add lines later, and anyway, it looks
    funny. :-)

    Actually, I lied.  As of 5.001, there are two autoquoting contexts:

    This            is like this
    ------------            ---------------
    $foo{line}        $foo{"line"}
    bar => stuff        "bar" => stuff


4.16) What is variable suicide and how can I prevent it?

    Variable suicide is a nasty side effect of dynamic scoping and the way
    variables are passed by reference.  If you say

    $x = 17;
    &munge($x);
    sub munge {
        local($x);
        local($myvar) = $_[0];
        ...
    }

    Then you have just clobbered $_[0]!  Why this is occurring is pretty
    heavy wizardry: the reference to $x stored in $_[0] was temporarily
    occluded by the previous local($x) statement (which, you're recall,
    occurs at run-time, not compile-time).  The work around is simple,
    however: declare your formal parameters first:

    sub munge {
        local($myvar) = $_[0];
        local($x);
        ...
    }

    That doesn't help you if you're going to be trying to access @_
    directly after the local()s.  In this case, careful use of the package
    facility is your only recourse.

    Another manifestation of this problem occurs due to the magical nature
    of the index variable in a foreach() loop.

    @num = 0 .. 4;
    print "num begin  @num\n";
    foreach $m (@num) { &ug }
    print "num finish @num\n";
    sub ug {
        local($m) = 42;
        print "m=$m  $num[0],$num[1],$num[2],$num[3]\n";
    }

    Which prints out the mysterious:

    num begin  0 1 2 3 4
    m=42  42,1,2,3
    m=42  0,42,2,3
    m=42  0,1,42,3
    m=42  0,1,2,42
    m=42  0,1,2,3
    num finish 0 1 2 3 4

    What's happening here is that $m is an alias for each element of @num.
    Inside &ug, you temporarily change $m.  Well, that means that you've
    also temporarily changed whatever $m is an alias to!!  The only
    workaround is to be careful with global variables, using packages,
    and/or just be aware of this potential in foreach() loops.

    The perl5 static autos via "my" do not exhibit this problem.


4.17) What does "Malformed command links" mean?

    This is a bug in 4.035.  While in general it's merely a cosmetic
    problem, it often comanifests with a highly undesirable coredumping
    problem.  Programs known to be affected by the fatal coredump include
    plum and pcops.  This bug has been fixed since 4.036.  It did not
    resurface in 5.001.


4.18) How can I set up a footer format to be used with write()?

    While the $^ variable contains the name of the current header format,
    there is no corresponding mechanism to automatically do the same thing
    for a footer.  Not knowing how big a format is going to be until you
    evaluate it is one of the major problems.

    If you have a fixed-size footer, you can get footers by checking for
    line left on page ($-) before each write, and printing the footer
    yourself if necessary.

    Another strategy is to open a pipe to yourself, using open(KID, "|-")
    and always write()ing to the KID, who then postprocesses its STDIN to
    rearrange headers and footers however you like.  Not very convenient,
    but doable.

    See the perlform(1) man page for other tricks.


4.19) Why does my Perl program keep growing in size?

    This is caused by a strange occurrence that Larry has dubbed "feeping
    creaturism".  Larry is always adding one more feature, always getting
    Perl to handle one more problem.  Hence, it keeps growing.  Once you've
    worked with perl long enough, you will probably start to do the same
    thing.  You will then notice this problem as you see your scripts
    becoming larger and larger.

    Oh, wait... you meant a currently running program and its stack size.
    Mea culpa, I misunderstood you.  ;)  While there may be a real memory
    leak in the Perl source code or even whichever malloc() you're using,
    common causes are incomplete eval()s or local()s in loops.

    An eval() which terminates in error due to a failed parsing will leave
    a bit of memory unusable.

    A local() inside a loop:

    for (1..100) {
        local(@array);
    }

    will build up 100 versions of @array before the loop is done.  The
    work-around is:

    local(@array);
    for (1..100) {
        undef @array;
    }

    This local array behaviour has been fixed for perl5, but a failed
    eval() still leaks.

    One other possibility, due to the way reference counting works, is
    when you've introduced a circularity in a data structure that would
    normally go out of scope and be unreachable.  For example:

    sub oops {
        my $x;
        $x = \$x;
    }

    When $x goes out of scope, the memory can't be reclaimed, because
    there's still something point to $x (itself, in this case).  A
    full garbage collection system could solve this, but at the cost
    of a great deal of complexity in perl itself and some inevitable
    performance problems as well.  If you're making a circular data
    structure that you want freed eventually, you'll have to break the
    self-reference links yourself.


4.20) Can I do RPC in Perl?

    Yes, you can, since Perl has access to sockets.  An example of the rup
    program written in Perl can be found in the script ruptime.pl at the
    scripts archive on ftp.cis.ufl.edu.  I warn you, however, that it's not
    a pretty sight, as it's used nothing from h2ph or c2ph, so everything is
    utterly hard-wired.


4.21) Why doesn't my sockets program work under System V (Solaris)?  What
    does the error message "Protocol not supported" mean?

    Some System V based systems, notably Solaris 2.X, redefined some of the
    standard socket constants.  Since these were constant across all
    architectures, they were often hardwired into the perl code.  The
    "proper" way to deal with this is to make sure that you run h2ph
    against sys/socket.h, require that file and use the symbolic names
    (SOCK_STREAM, SOCK_DGRAM, SOCK_RAW, SOCK_RDM, and SOCK_SEQPACKET).

    Note that even though SunOS 4 and SunOS 5 are binary compatible, these
    values are different, and require a different socket.ph for each OS.

    Under version 5, you can also "use Socket" to get the proper values.


4.22) How can I quote a variable to use in a regexp?

    From the manual:

    $pattern =~ s/(\W)/\\$1/g;

    Now you can freely use /$pattern/ without fear of any unexpected meta-
    characters in it throwing off the search.  If you don't know whether a
    pattern is valid or not, enclose it in an eval to avoid a fatal run-
    time error.

    Perl5 provides a vastly improved way of doing this.  Simply use the
    new quotemeta character (\Q) within your variable.

4.23) How can I change the first N letters of a string?

    Remember that the substr() function produces an lvalue, that is, it may
    be assigned to.  Therefore, to change the first character to an S, you
    could do this:

    substr($var,0,1) = 'S';

    This assumes that $[ is 0;  for a library routine where you can't know
    $[, you should use this instead:

    substr($var,$[,1) = 'S';

    While it would be slower, you could in this case use a substitute:

    $var =~ s/^./S/;

    But this won't work if the string is empty or its first character is a
    newline, which "." will never match.  So you could use this instead:

    $var =~ s/^[^\0]?/S/;

    To do things like translation of the first part of a string, use
    substr, as in:

    substr($var, $[, 10) =~ tr/a-z/A-Z/;

    If you don't know the length of what to translate, something like this
    works:

    /^(\S+)/ && substr($_,$[,length($1)) =~ tr/a-z/A-Z/;

    For some things it's convenient to use the /e switch of the substitute
    operator:

    s/^(\S+)/($tmp = $1) =~ tr#a-z#A-Z#, $tmp/e

    although in this case, it runs more slowly than does the previous
    example.


4.24) How can I count the number of occurrences of a substring within a
    string?

    If you want a count of a certain character (X) within a string, you can
    use the tr/// function like so:

    $string="ThisXlineXhasXsomeXx'sXinXit":
    $count = ($string =~ tr/X//);
    print "There are $count Xs in the string";

    This is fine if you are just looking for a single character.  However,
    if you are trying to count multiple character substrings within a
    larger string, tr/// won't work.  What you can do is wrap a while loop
    around a pattern match.

    $string="-9 55 48 -2 23 -76 4 14 -44";
    $count++ while $string =~ /-\d+/g;
    print "There are $count negative numbers in the string";


4.25) Can I use Perl regular expressions to match balanced text?

    No, or at least, not by themselves.

    Regexps just aren't powerful enough.  Although Perl's patterns aren't
    strictly regular because they do backreferencing (the \1 notation), you
    still can't do it.  You need to employ auxiliary logic.  A simple
    approach would involve keeping a bit of state around, something
    vaguely like this (although we don't handle patterns on the same line):

    while(<>) {
        if (/pat1/) {
        if ($inpat++ > 0) { warn "already saw pat1" }
        redo;
        }
        if (/pat2/) {
        if (--$inpat < 0) { warn "never saw pat1" }
        redo;
        }
    }

    A rather more elaborate subroutine to pull out balanced and possibly
    nested single chars, like ` and ', { and }, or ( and ) can be found
    on convex.com in /pub/perl/scripts/pull_quotes.


4.26) What does it mean that regexps are greedy?  How can I get around it?

    The basic idea behind regexps being greedy is that they will match the
    maximum amount of data that they can, sometimes resulting in incorrect
    or strange answers.

    For example, I recently came across something like this:

    $_="this (is) an (example) of multiple parens";
    while ( m#\((.*)\)#g ) {
        print "$1\n";
    }

    This code was supposed to match everything between a set of
    parentheses.  The expected output was:

    is
    example

    However, the backreference ($1) ended up containing "is) an (example",
    clearly not what was intended.

    In perl4, the way to stop this from happening is to use a negated
    group.  If the above example is rewritten as follows, the results are
    correct:

    while ( m#\(([^)]*)\)#g ) {

    In perl5 there is a new minimal matching metacharacter, '?'.  This
    character is added to the normal metacharacters to modify their
    behaviour, such as "*?", "+?", or even "??".  The example would now be
    written in the following style:

    while (m#\((.*?)\)#g )

    Hint: This new operator leads to a very elegant method of stripping
    comments from C code:

    s:/\*.*?\*/::gs


4.27) How do I use a regular expression to strip C style comments from a
      file?

    Since we're talking about how to strip comments under perl5, now is a
    good time to talk about doing it in perl4.  Since comments can be
    embedded in strings, or look like function prototypes, care must be
    taken to ignore these cases.  Jeffrey Friedl* proposes the following
    two programs to strip C comments and C++ comments respectively:

    C comments:
        #!/usr/bin/perl
        $/ = undef;
        $_ = <>;

        s#/\*[^*]*\*+([^/*][^*]*\*+)*/|([^/"']*("[^"\\]*(\\[\d\D][^"\\]*)*"[^/"']*|'[^'\\]*(\\[\d\D][^'\\]*)*'[^/"']*|/+[^*/][^/"']*)*)#$2#g;
        print;

    C++ comments:
        #!/usr/local/bin/perl
        $/ = undef;
        $_ = <>;
        s#//(.*)|/\*[^*]*\*+([^/*][^*]*\*+)*/|"(\\.|[^"\\])*"|'(\\.|[^'\\])*'|[^/"']+#  $1 ? "/*$1 */" : $& #ge;
        print;

    (Yes, Jeffrey says, those are complete programs to strip comments
    correctly.)


4.28) How can I split a [character] delimited string except when inside
    [character]?

    I'm trying to split a string that is comma delimited into its different
    fields.  I could easily use split(/,/), except that I need to not split
    if the comma is inside quotes.  For example, my data file has a line
    like this:

    SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"

    Due to the restriction of the quotes, this is a fairly complex
    solution.  However, we thankfully have Jeff Friedl* to handle these for
    us.  He suggests (assuming that your data is contained in the special
    variable $_):

    undef @field;
    push(@fields, defined($1) ? $1:$3) while
        m/"([^"\\]*(\\.[^"\\]*)*)"|([^,]+)/g;


4.29) Why doesn't "local($foo) = <FILE>;" work right?

    Well, it does.  The thing to remember is that local() provides an array
    context, and that the <FILE> syntax in an array context will read all the
    lines in a file.  To work around this, use:

    local($foo);
    $foo = <FILE>;

    You can use the scalar() operator to cast the expression into a scalar
    context:

    local($foo) = scalar(<FILE>);


4.30) How can I detect keyboard input without reading it?

    You should check out the Frequently Asked Questions list in
    comp.unix.* for things like this: the answer is essentially the same.
    It's very system dependent.  Here's one solution that works on BSD
    systems:

    sub key_ready {
        local($rin, $nfd);
        vec($rin, fileno(STDIN), 1) = 1;
        return $nfd = select($rin,undef,undef,0);
    }

    Under perl5, you should look into getting the ReadKey extension from
    your regular perl archive.


4.31) How can I read a single character from the keyboard under UNIX and DOS?

    A closely related question to the no-echo question below is how to
    input a single character from the keyboard.  Again, this is a system
    dependent operation.  As with the previous question, you probably want
    to get the ReadKey extension.  The following code may or may not help
    you.  It should work on both SysV and BSD flavors of UNIX:

    $BSD = -f '/vmunix';
    if ($BSD) {
        system "stty cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system "stty", '-icanon',
        system "stty", 'eol', "\001";
    }

    $key = getc(STDIN);

    if ($BSD) {
        system "stty -cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system "stty", 'icanon';
        system "stty", 'eol', '^@'; # ascii null
    }
    print "\n";

    You could also handle the stty operations yourself for speed if you're
    going to be doing a lot of them.  This code works to toggle cbreak
    and echo modes on a BSD system:

    sub set_cbreak { # &set_cbreak(1) or &set_cbreak(0)
    local($on) = $_[0];
    local($sgttyb,@ary);
    require 'sys/ioctl.ph';
    $sgttyb_t   = 'C4 S' unless $sgttyb_t;  # c2ph: &sgttyb'typedef()

    ioctl(STDIN,&TIOCGETP,$sgttyb) || die "Can't ioctl TIOCGETP: $!";

    @ary = unpack($sgttyb_t,$sgttyb);
    if ($on) {
        $ary[4] |= &CBREAK;
        $ary[4] &= ~&ECHO;
    } else {
        $ary[4] &= ~&CBREAK;
        $ary[4] |= &ECHO;
    }
    $sgttyb = pack($sgttyb_t,@ary);

    ioctl(STDIN,&TIOCSETP,$sgttyb) || die "Can't ioctl TIOCSETP: $!";
    }

    Note that this is one of the few times you actually want to use the
    getc() function; it's in general way too expensive to call for normal
    I/O.  Normally, you just use the <FILE> syntax, or perhaps the read()
    or sysread() functions.

    For perspectives on more portable solutions, use anon ftp to retrieve
    the file /pub/perl/info/keypress from convex.com.

    Under Perl5, with William Setzer's Curses module, you can call
    &Curses::cbreak() and &Curses::nocbreak() to turn cbreak mode on and
    off.  You can then use getc() to read each character.  This should work
    under both BSD and SVR systems.  If anyone can confirm or deny
    (especially William), please contact the maintainers.

    For DOS systems, Dan Carson <[e-mail]> reports:

    To put the PC in "raw" mode, use ioctl with some magic numbers gleaned
    from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes
    across the net every so often):

    $old_ioctl = ioctl(STDIN,0,0);     # Gets device info
    $old_ioctl &= 0xff;
    ioctl(STDIN,1,$old_ioctl | 32);    # Writes it back, setting bit 5

    Then to read a single character:

    sysread(STDIN,$c,1);               # Read a single character

    And to put the PC back to "cooked" mode:

    ioctl(STDIN,1,$old_ioctl);         # Sets it back to cooked mode.


    So now you have $c.  If ord($c) == 0, you have a two byte code, which
    means you hit a special key.  Read another byte (sysread(STDIN,$c,1)),
    and that value tells you what combination it was according to this
    table:

    # PC 2-byte keycodes = ^@ + the following:

    # HEX     KEYS
    # ---     ----
    # 0F      SHF TAB
    # 10-19   ALT QWERTYUIOP
    # 1E-26   ALT ASDFGHJKL
    # 2C-32   ALT ZXCVBNM
    # 3B-44   F1-F10
    # 47-49   HOME,UP,PgUp
    # 4B      LEFT
    # 4D      RIGHT
    # 4F-53   END,DOWN,PgDn,Ins,Del
    # 54-5D   SHF F1-F10
    # 5E-67   CTR F1-F10
    # 68-71   ALT F1-F10
    # 73-77   CTR LEFT,RIGHT,END,PgDn,HOME
    # 78-83   ALT 1234567890-=
    # 84      CTR PgUp

    This is all trial and error I did a long time ago, I hope I'm reading the
    file that worked.


4.32) How can I get input from the keyboard without it echoing to the
      screen?

    Terminal echoing is generally handled directly by the shell.
    Therefore, there is no direct way in perl to turn echoing on and off.
    However, you can call the command "stty [-]echo".  The following will
    allow you to accept input without it being echoed to the screen, for
    example as a way to accept passwords (error checking deleted for
    brevity):

    print "Please enter your password: ";
        system("stty -echo");
    chop($password=<STDIN>);
    print "\n";
    system("stty echo");

    Again, under perl 5, you can use Curses and call &Curses::noecho() and
    &Curses::echo() to turn echoing off and on.  Or, there's always the
    ReadKey extension.


4.33) Is there any easy way to strip blank space from the beginning/end of
    a string?

    Yes, there is.  Using the substitution command, you can match the
    blanks and replace it with nothing.  For example, if you have the
    string "     String     " you can use this:

        s/^\s*(.*?)\s*$/$1/;     # perl5 only!

        s/^\s+|\s+$//g;            # perl4 or perl5

    or even

        s/^\s+//; s/\s+$//;

    Note however that Jeffrey Friedl* says these are only good for shortish
    strings.  For longer strings, and worse-case scenarios, they tend to
    break-down and become inefficient.

    For the longer strings, he suggests using either

        $_ = $1 if m/^\s*((.*\S)?)/;

    or

        s/^\s*((.*\S)?)\s*$/$1/;

    It should also be noted that for generally nice strings, these tend to
    be noticably slower than the simple ones above.  It is suggested that
    you use whichever one will fit your situation best, understanding that
    the first examples will work in roughly ever situation known even if
    slow at times.

4.34) How can I print out a number with commas into it?

    This one will do it for you:

    sub commify {
        local($_) = shift;
        1 while s/^(-?\d+)(\d{3})/$1,$2/;
        return $_;
    }

    $n = 23659019423.2331;
    print "GOT: ", &commify($n), "\n";

    GOT: 23,659,019,423.2331

    The reason you can't just do

    s/^(-?\d+)(\d{3})/$1,$2/g;

    Is that you have to put the comma in and then recalculate anything.
    Some substitutions need to work this way.  See the question on
    expanding tabs for another such.

4.35) How do I expand tabs in a string?

    1 while s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;

    You could have written that

    while (s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e) {
        # spin, spin, spin, ....
    }

    Placed in a function:

    sub tab_expand {
        local($_) = shift;
        1 while s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
        return $_;
    }

    This is especially important when you're working going to unpack
    an ascii string that might have tabs in it.  Otherwise you'll be
    off on the byte count.  For example:

    $NG = "/usr/local/lib/news/newsgroups";
    open(NG, "< $NG") || die "can't open $NG: $!";
    while (<NG>) {
        chop;  # chomp would be better, but it's only perl5
        # now for the darned tabs in the newsgroups file
        1 while s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
        ($ng, $desc) = unpack("A24 A*", $_);
        if (length($ng) == 24) {
        $desc =~ s/^(\S+)\s*//;
        $ng .= $1;
        }

4.36) What's wrong with grep() or map() in a void context?

    Well, nothing precisely, but it's not a good way to write
    maintainable code.  It's just fine to use grep when you want
    an answer, like

    @bignums = grep ($_ > 100, @allnums);
    @triplist = map {$_ * 3} @allnums;

    But using it in a void context like this:

    grep{ $_ *= 3, @nums);

    Is using it for its side-effects, and side-effects can be mystifying.
    There's no void grep that's not better written as a for() loop:

    for (@nums) { $_ *= 3 }

    In the same way, a ?: in a void context is considered poor form:

    fork ? wait : exec $prog;

    When you can write it this way:

    if (fork) {
        wait;
    } else {
        exec $prog;
        die "can't exec $prog: $!";
    }

    Of course, using ?: in expressions is just what it's made for,
    and just fine (but try not to nest them.).

    Remember that the most important things in almost any program are,
    and in this order:

    1. correctness
    2. maintainability
    3. efficiency

    Notice at no point did cleverness enter the picture.

    On the other hand, if you're just trying write JAPHs (aka Obfuscated
    Perl entries), or write ugly code, you would probably invert these :-)

    1. cleverness
    2. efficiency
    3. maintainability
    4. correctness

--
Stephen P Potter        Pencom Systems Administration              Beaching It
[e-mail]    Pager: 1-800-759-8888, 547-9561     Work: 703-860-2222
  Cthulhu for President in '96: When You're Tired of the Lesser of Two Evils
--
Stephen P Potter        Pencom Systems Administration              Beaching It
[e-mail]    Pager: 1-800-759-8888, 547-9561     Work: 703-860-2222
"I don't care whether people actually like Perl, just so long as they *think*
    they like it...  ;-)"    -Larry Wall

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Archive-name: perl-faq/part5
Version: $Id: part5,v 2.8 1995/05/15 15:47:16 spp Exp spp $
Posting-Frequency: bi-weekly
Last Edited: Thu Jan 11 00:57:03 1996 by spp (Stephen P Potter) on syrinx.psa.com

This posting contains answers to the following questions about Array, Shell
and External Program Interactions with Perl:


5.1) What is the difference between $array[1] and @array[1]?

    Always make sure to use a $ for single values and @ for multiple ones.
    Thus element 2 of the @foo array is accessed as $foo[1], not @foo[1],
    which is a list of length one (not a scalar), and is a fairly common
    novice mistake.  Sometimes you can get by with @foo[1], but it's
    not really doing what you think it's doing for the reason you think
    it's doing it, which means one of these days, you'll shoot yourself
    in the foot; ponder for a moment what these will really do:
        @foo[0] = `cmd args`;
        @foo[1] = <FILE>;
    Just always say $foo[1] and you'll be happier.

    This may seem confusing, but try to think of it this way:  you use the
    character of the type which you *want back*.  You could use @foo[1..3] for
    a slice of three elements of @foo, or even @foo{A,B,C} for a slice of
    of %foo.  This is the same as using ($foo[1], $foo[2], $foo[3]) and
    ($foo{A}, $foo{B}, $foo{C}) respectively.  In fact, you can even use
    lists to subscript arrays and pull out more lists, like @foo[@bar] or
    @foo{@bar}, where @bar is in both cases presumably a list of subscripts.


5.2) How can I make an array of arrays or other recursive data types?


    In Perl5, it's quite easy to declare these things.  For example

        @A = (
            [ 'ww' .. 'xx'  ],
            [ 'xx' .. 'yy'  ],
            [ 'yy' .. 'zz'  ],
            [ 'zz' .. 'zzz' ],
        );

    And now reference $A[2]->[0] to pull out "yy".  These may also nest
    and mix with tables:

        %T = (
            key0, { k0, v0, k1, v1 },
            key1, { k2, v2, k3, v3 },
            key2, { k2, v2, k3, [ 'a' .. 'z' ] },
        );

    Allowing you to reference $T{key2}->{k3}->[3] to pull out 'd'.

    Perl4 is infinitely more difficult.  Remember that Perl[0..4] isn't
    about nested data structures.  It's about flat ones, so if you're
    trying to do this, you may be going about it the wrong way or using the
    wrong tools.  You might try parallel arrays with common subscripts.

    But if you're bound and determined, you can use the multi-dimensional
    array emulation of $a{'x','y','z'}, or you can make an array of names
    of arrays and eval it.

    For example, if @name contains a list of names of arrays, you can get
    at a the j-th element of the i-th array like so:

        $ary = $name[$i];
        $val = eval "\$$ary[$j]";

    or in one line

        $val = eval "\$$name[$i][\$j]";

    You could also use the type-globbing syntax to make an array of *name
    values, which will be more efficient than eval.  Here @name hold a list
    of pointers, which we'll have to dereference through a temporary
    variable.

    For example:

        { local(*ary) = $name[$i]; $val = $ary[$j]; }

    In fact, you can use this method to make arbitrarily nested data
    structures.  You really have to want to do this kind of thing badly to
    go this far, however, as it is notationally cumbersome.

    Let's assume you just simply *have* to have an array of arrays of
    arrays.  What you do is make an array of pointers to arrays of
    pointers, where pointers are *name values described above.  You
    initialize the outermost array normally, and then you build up your
    pointers from there.  For example:

        @w = ( 'ww' .. 'xx' );
        @x = ( 'xx' .. 'yy' );
        @y = ( 'yy' .. 'zz' );
        @z = ( 'zz' .. 'zzz' );

        @ww = reverse @w;
        @xx = reverse @x;
        @yy = reverse @y;
        @zz = reverse @z;

    Now make a couple of arrays of pointers to these:

        @A = ( *w, *x, *y, *z );
        @B = ( *ww, *xx, *yy, *zz );

    And finally make an array of pointers to these arrays:

        @AAA = ( *A, *B );

    To access an element, such as AAA[i][j][k], you must do this:

        local(*foo) = $AAA[$i];
        local(*bar) = $foo[$j];
        $answer = $bar[$k];

    Similar manipulations on associative arrays are also feasible.

    You could take a look at recurse.pl package posted by Felix Lee*, which
    lets you simulate vectors and tables (lists and associative arrays) by
    using type glob references and some pretty serious wizardry.

    In C, you're used to creating recursive datatypes for operations like
    recursive decent parsing or tree traversal.  In Perl, these algorithms
    are best implemented using associative arrays.  Take an array called
    %parent, and build up pointers such that $parent{$person} is the name
    of that person's parent.  Make sure you remember that $parent{'adam'}
    is 'adam'. :-) With a little care, this approach can be used to
    implement general graph traversal algorithms as well.


5.3) How do I make an array of structures containing various data types?

    This answer will work under perl5 only.  Did we mention that you should
    upgrade?  There is a perl4 solution, but you are using perl5 now,
    anyway, so there's no point in posting it.  Right?

    The best way to do this is to use an associative array to model your
    structure, then either a regular array (AKA list) or another
    associative array (AKA hash, table, or hash table) to store it.

        %foo = (
                   'field1'        => "value1",
                   'field2'        => "value2",
                   'field3'        => "value3",
                   ...
               );
        ...

        @all = ( \%foo, \%bar, ... );

        print $all[0]{'field1'};

    Or even

        @all = (
           {
               'field1'        => "value1",
               'field2'        => "value2",
               'field3'        => "value3",
               ...
           },
           {
               'field1'        => "value1",
               'field2'        => "value2",
               'field3'        => "value3",
               ...
           },
           ...
        )

    Note that if you want an associative array of lists, you'll want to make
    assignments like

        $t{$value} = [ @bar ];

    And with lists of associative arrays, you'll use

        %{$a[$i]} = %old;

    Study these for a while, and in an upcoming FAQ, we'll explain them fully:

        $table{'some key'}    =   @big_list_o_stuff;   # SCARY #0
        $table{'some key'}    =  \@big_list_o_stuff;   # SCARY #1
        @$table{'some key'}   =   @big_list_o_stuff;   # SCARY #2
        @{$table{'some key'}} =   @big_list_o_stuff;   # ICKY RANDALIAN CODE
        $table{'some key'}    = [ @big_list_o_stuff ]; # same, but NICE

    And while you're at it, take a look at these:

        $table{"051"}         = $some_scalar;          # SCARY #3
        $table{"0x51"}        = $some_scalar;          # ditto
        $table{051}           = $some_scalar;          # ditto
        $table{0x51}          = $some_scalar;          # ditto
        $table{51}            = $some_scalar;          # ok, i guess
        $table{"51"}          = $some_scalar;          # better

        $table{\@x}           = $some_scalar;          # SCARY #4
        $table{[@x]}          = $some_scalar;          # ditto
        $table{@x}            = $some_scalar;          # SCARY #5 (cf #0)


    See perlref(1) for details.


5.4) How can I extract just the unique elements of an array?

    There are several possible ways, depending on whether the
    array is ordered and you wish to preserve the ordering.

    a) If @in is sorted, and you want @out to be sorted:

        $prev = 'nonesuch';
        @out = grep($_ ne $prev && (($prev) = $_), @in);

       This is nice in that it doesn't use much extra memory,
       simulating uniq's behavior of removing only adjacent
       duplicates.

    b) If you don't know whether @in is sorted:

        undef %saw;
        @out = grep(!$saw{$_}++, @in);

    c) Like (b), but @in contains only small integers:

        @out = grep(!$saw[$_]++, @in);

    d) A way to do (b) without any loops or greps:

        undef %saw;
        @saw{@in} = ();
        @out = sort keys %saw;  # remove sort if undesired

    e) Like (d), but @in contains only small positive integers:

        undef @ary;
        @ary[@in] = @in;
        @out = sort @ary;


5.5) How can I tell whether an array contains a certain element?

    There are several ways to approach this.  If you are going to make
    this query many times and the values are arbitrary strings, the
    fastest way is probably to invert the original array and keep an
    associative array lying about whose keys are the first array's values.

        @blues = ('turquoise', 'teal', 'lapis lazuli');
        undef %is_blue;
        for (@blues) { $is_blue{$_} = 1; }

    Now you can check whether $is_blue{$some_color}.  It might have been
    a good idea to keep the blues all in an assoc array in the first place.

    If the values are all small integers, you could use a simple
    indexed array.  This kind of an array will take up less space:

        @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
        undef @is_tiny_prime;
        for (@primes) { $is_tiny_prime[$_] = 1; }

    Now you check whether $is_tiny_prime[$some_number].

    If the values in question are integers instead of strings, you can save
    quite a lot of space by using bit strings instead:

        @articles = ( 1..10, 150..2000, 2017 );
        undef $read;
        grep (vec($read,$_,1) = 1, @articles);

    Now check whether vec($read,$n,1) is true for some $n.


5.6) How do I sort an associative array by value instead of by key?

    You have to declare a sort subroutine to do this, or use an inline
    function.  Let's assume you want an ASCII sort on the values of the
    associative array %ary.  You could do so this way:

        foreach $key (sort by_value keys %ary) {
            print $key, '=', $ary{$key}, "\n";
        }
        sub by_value { $ary{$a} cmp $ary{$b}; }

    If you wanted a descending numeric sort, you could do this:

        sub by_value { $ary{$b} <=> $ary{$a}; }

    You can also inline your sort function, like this, at least if
    you have a relatively recent patchlevel of perl4 or are running perl5:

        foreach $key ( sort { $ary{$b} <=> $ary{$a} } keys %ary ) {
            print $key, '=', $ary{$key}, "\n";
        }

    If you wanted a function that didn't have the array name hard-wired
    into it, you could so this:

        foreach $key (&sort_by_value(*ary)) {
            print $key, '=', $ary{$key}, "\n";
        }
        sub sort_by_value {
            local(*x) = @_;
            sub _by_value { $x{$a} cmp $x{$b}; }
            sort _by_value keys %x;
        }

    If you want neither an alphabetic nor a numeric sort, then you'll
    have to code in your own logic instead of relying on the built-in
    signed comparison operators "cmp" and "<=>".

    Note that if you're sorting on just a part of the value, such as a
    piece you might extract via split, unpack, pattern-matching, or
    substr, then rather than performing that operation inside your sort
    routine on each call to it, it is significantly more efficient to
    build a parallel array of just those portions you're sorting on, sort
    the indices of this parallel array, and then to subscript your original
    array using the newly sorted indices.  This method works on both
    regular and associative arrays, since both @ary[@idx] and @ary{@idx}
    make sense.  See page 245 in the Camel Book on "Sorting an Array by a
    Computable Field" for a simple example of this.

    For example, here's an efficient case-insensitive comparison:

        @idx = ();
        for (@data) { push (@idx, "\U$_") }
        @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0..$#data];


5.7) How can I know how many entries are in an associative array?

    While the number of elements in a @foobar array is simply @foobar when
    used in a scalar, you can't figure out how many elements are in an
    associative array in an analogous fashion.  That's because %foobar in
    a scalar context returns the ratio (as a string) of number of buckets
    filled versus the number allocated.  For example, scalar(%ENV) might
    return "20/32".  While perl could in theory keep a count, this would
    break down on associative arrays that have been bound to dbm files.

    However, while you can't get a count this way, one thing you *can* use
    it for is to determine whether there are any elements whatsoever in
    the array, since "if (%table)" is guaranteed to be false if nothing
    has ever been stored in it.

    As of perl4.035, you can says

        $count = keys %ARRAY;

    keys() when used in a scalar context will return the number of keys,
    rather than the keys themselves.


5.8) What's the difference between "delete" and "undef" with %arrays?

    Pictures help...  here's the %ary table:

              keys  values
            +------+------+
            |  a   |  3   |
            |  x   |  7   |
            |  d   |  0   |
            |  e   |  2   |
            +------+------+

    And these conditions hold

            $ary{'a'}                       is true
            $ary{'d'}                       is false
            defined $ary{'d'}               is true
            defined $ary{'a'}               is true
            exists $ary{'a'}                is true (perl5 only)
            grep ($_ eq 'a', keys %ary)     is true

    If you now say

            undef $ary{'a'}

    your table now reads:


              keys  values
            +------+------+
            |  a   | undef|
            |  x   |  7   |
            |  d   |  0   |
            |  e   |  2   |
            +------+------+

    and these conditions now hold; changes in caps:

            $ary{'a'}                       is FALSE
            $ary{'d'}                       is false
            defined $ary{'d'}               is true
            defined $ary{'a'}               is FALSE
            exists $ary{'a'}                is true (perl5 only)
            grep ($_ eq 'a', keys %ary)     is true

    Notice the last two: you have an undef value, but a defined key!

    Now, consider this:

            delete $ary{'a'}

    your table now reads:

              keys  values
            +------+------+
            |  x   |  7   |
            |  d   |  0   |
            |  e   |  2   |
            +------+------+

    and these conditions now hold; changes in caps:

            $ary{'a'}                       is false
            $ary{'d'}                       is false
            defined $ary{'d'}               is true
            defined $ary{'a'}               is false
            exists $ary{'a'}                is FALSE (perl5 only)
            grep ($_ eq 'a', keys %ary)     is FALSE

    See, the whole entry is gone!


5.9) Why don't backticks work as they do in shells?

    Several reasons.  One is because backticks do not interpolate within
    double quotes in Perl as they do in shells.

    Let's look at two common mistakes:

         $foo = "$bar is `wc $file`";  # WRONG

    This should have been:

         $foo = "$bar is " . `wc $file`;

    But you'll have an extra newline you might not expect.  This
    does not work as expected:

      $back = `pwd`; chdir($somewhere); chdir($back); # WRONG

    Because backticks do not automatically eat trailing or embedded
    newlines.  The chop() function will remove the last character from
    a string.  This should have been:

          chop($back = `pwd`); chdir($somewhere); chdir($back);

    You should also be aware that while in the shells, embedding
    single quotes will protect variables, in Perl, you'll need
    to escape the dollar signs.

        Shell: foo=`cmd 'safe $dollar'`
        Perl:  $foo=`cmd 'safe \$dollar'`;


5.10) How come my converted awk/sed/sh script runs more slowly in Perl?

    The natural way to program in those languages may not make for the fastest
    Perl code.  Notably, the awk-to-perl translator produces sub-optimal code;
    see the a2p man page for tweaks you can make.

    Two of Perl's strongest points are its associative arrays and its regular
    expressions.  They can dramatically speed up your code when applied
    properly.  Recasting your code to use them can help a lot.

    How complex are your regexps?  Deeply nested sub-expressions with {n,m} or
    * operators can take a very long time to compute.  Don't use ()'s unless
    you really need them.  Anchor your string to the front if you can.

    Something like this:
        next unless /^.*%.*$/;
    runs more slowly than the equivalent:
        next unless /%/;

    Note that this:
        next if /Mon/;
        next if /Tue/;
        next if /Wed/;
        next if /Thu/;
        next if /Fri/;
    runs faster than this:
        next if /Mon/ || /Tue/ || /Wed/ || /Thu/ || /Fri/;
    which in turn runs faster than this:
        next if /Mon|Tue|Wed|Thu|Fri/;
    which runs *much* faster than:
        next if /(Mon|Tue|Wed|Thu|Fri)/;

    There's no need to use /^.*foo.*$/ when /foo/ will do.

    Remember that a printf costs more than a simple print.

    Don't split() every line if you don't have to.

    Another thing to look at is your loops.  Are you iterating through
    indexed arrays rather than just putting everything into a hashed
    array?  For example,

        @list = ('abc', 'def', 'ghi', 'jkl', 'mno', 'pqr', 'stv');

        for $i ($[ .. $#list) {
            if ($pattern eq $list[$i]) { $found++; }
        }

    First of all, it would be faster to use Perl's foreach mechanism
    instead of using subscripts:

        foreach $elt (@list) {
            if ($pattern eq $elt) { $found++; }
        }

    Better yet, this could be sped up dramatically by placing the whole
    thing in an associative array like this:

        %list = ('abc', 1, 'def', 1, 'ghi', 1, 'jkl', 1,
                 'mno', 1, 'pqr', 1, 'stv', 1 );
        $found += $list{$pattern};

    (but put the %list assignment outside of your input loop.)

    You should also look at variables in regular expressions, which is
    expensive.  If the variable to be interpolated doesn't change over the
    life of the process, use the /o modifier to tell Perl to compile the
    regexp only once, like this:

        for $i (1..100) {
            if (/$foo/o) {
                &some_func($i);
            }
        }

    Finally, if you have a bunch of patterns in a list that you'd like to
    compare against, instead of doing this:

        @pats = ('_get.*', 'bogus', '_read', '.*exit', '_write');
        foreach $pat (@pats) {
            if ( $name =~ /^$pat$/ ) {
                &some_func();
                last;
            }
        }

    If you build your code and then eval it, it will be much faster.
    For example:

        @pats = ('_get.*', 'bogus', '_read', '.*exit', '_write');
        $code = <<EOS
                while (<>) {
                    study;
EOS
        foreach $pat (@pats) {
            $code .= <<EOS
                if ( /^$pat\$/ ) {
                    &some_func();
                    next;
                }
EOS
        }
        $code .= "}\n";
        print $code if $debugging;
        eval $code;



5.11) How can I call my system's unique C functions from Perl?

    If these are system calls and you have the syscall() function, then
    you're probably in luck -- see the next question.  If you're using a
    POSIX function, and are running perl5, you're also in luck: see
    POSIX(3m).  For arbitrary library functions, however, it's not quite so
    straight-forward.  See "Where can I learn about linking C with Perl?".


5.12) Where do I get the include files to do ioctl() or syscall()? [h2ph]

    [Note: as of perl5, you probably want to just use h2xs instead, at
    least, if your system supports dynamic loading.]

    These are generated from your system's C include files using the h2ph
    script (once called makelib) from the Perl source directory.  This will
    make files containing subroutine definitions, like &SYS_getitimer, which
    you can use as arguments to your function.

    You might also look at the h2pl subdirectory in the Perl source for how to
    convert these to forms like $SYS_getitimer; there are both advantages and
    disadvantages to this.  Read the notes in that directory for details.

    In both cases, you may well have to fiddle with it to make these work; it
    depends how funny-looking your system's C include files happen to be.

    If you're trying to get at C structures, then you should take a look
    at using c2ph, which uses debugger "stab" entries generated by your
    BSD or GNU C compiler to produce machine-independent perl definitions
    for the data structures.  This allows to you avoid hardcoding
    structure layouts, types, padding, or sizes, greatly enhancing
    portability.  c2ph comes with the perl distribution.  On an SCO
    system, GCC only has COFF debugging support by default, so you'll have
    to build GCC 2.1 with DBX_DEBUGGING_INFO defined, and use -gstabs to
    get c2ph to work there.

    See the file /pub/perl/info/ch2ph on convex.com via anon ftp
    for more traps and tips on this process.


5.13) Why do setuid Perl scripts complain about kernel problems?

    This message:

    YOU HAVEN'T DISABLED SET-ID SCRIPTS IN THE KERNEL YET!
    FIX YOUR KERNEL, PUT A C WRAPPER AROUND THIS SCRIPT, OR USE -u AND UNDUMP!

    is triggered because setuid scripts are inherently insecure due to a
    kernel bug.  If your system has fixed this bug, you can compile Perl
    so that it knows this.  Otherwise, create a setuid C program that just
    execs Perl with the full name of the script.  Here's what the
    perldiag(1) man page says about this message:

        YOU HAVEN'T DISABLED SET-ID SCRIPTS IN THE KERNEL YET!
          (F) And you probably never will, since you probably don't have
          the sources to your kernel, and your vendor probably doesn't
          give a rip about what you want.  Your best bet is to use the
          wrapsuid script in the eg directory to put a setuid C wrapper
          around your script.


5.14) How do I open a pipe both to and from a command?

    In general, this is a dangerous move because you can find yourself in a
    deadlock situation.  It's better to put one end of the pipe to a file.
    For example:

        # first write some_cmd's input into a_file, then
        open(CMD, "some_cmd its_args < a_file |");
        while (<CMD>) {

        # or else the other way; run the cmd
        open(CMD, "| some_cmd its_args > a_file");
        while ($condition) {
            print CMD "some output\n";
            # other code deleted
        }
        close CMD || warn "cmd exited $?";

        # now read the file
        open(FILE,"a_file");
        while (<FILE>) {

    If you have ptys, you could arrange to run the command on a pty and
    avoid the deadlock problem.  See the chat2.pl package in the
    distributed library for ways to do this.

    At the risk of deadlock, it is theoretically possible to use a
    fork, two pipe calls, and an exec to manually set up the two-way
    pipe.  (BSD system may use socketpair() in place of the two pipes,
    but this is not as portable.)  The open2 library function distributed
    with the current perl release will do this for you.

    This assumes it's going to talk to something like adb, both writing to
    it and reading from it.  This is presumably safe because you "know"
    that commands like adb will read a line at a time and output a line at
    a time.  Programs like sort or cat that read their entire input stream
    first, however, are quite apt to cause deadlock.

    There's also an open3.pl library that handles this for stderr as well.


5.15) How can I capture STDERR from an external command?

    There are three basic ways of running external commands:

        system $cmd;
        $output = `$cmd`;
        open (PIPE, "cmd |");

    In the first case, both STDOUT and STDERR will go the same place as
    the script's versions of these, unless redirected.  You can always put
    them where you want them and then read them back when the system
    returns.  In the second and third cases, you are reading the STDOUT
    *only* of your command.  If you would like to have merged STDOUT and
    STDERR, you can use shell file-descriptor redirection to dup STDERR to
    STDOUT:

        $output = `$cmd 2>&1`;
        open (PIPE, "cmd 2>&1 |");

    Another possibility is to run STDERR into a file and read the file
    later, as in

        $output = `$cmd 2>some_file`;
        open (PIPE, "cmd 2>some_file |");

    Note that you *cannot* simply open STDERR to be a dup of STDOUT
    in your perl program and avoid calling the shell to do the redirection.
    This doesn't work:

        open(STDERR, ">&STDOUT");
        $alloutput = `cmd args`;  # stderr still escapes

    Here's a way to read from both of them and know which descriptor
    you got each line from.  The trick is to pipe only STDOUT through
    sed, which then marks each of its lines, and then sends that
    back into a merged STDOUT/STDERR stream, from which your Perl program
    then reads a line at a time:

        open (CMD,
          "(cmd args | sed 's/^/STDOUT:/') 2>&1 |");

        while (<CMD>) {
          if (s/^STDOUT://)  {
              print "line from stdout: ", $_;
          } else {
              print "line from stderr: ", $_;
          }
        }

    Be apprised that you *must* use Bourne shell redirection syntax in
    backticks, not csh!  For details on how lucky you are that perl's
    system() and backtick and pipe opens all use Bourne shell, fetch the
    file from convex.com called /pub/csh.whynot -- and you'll be glad that
    perl's shell interface is the Bourne shell.

    There's an &open3 routine out there which was merged with &open2 in
    perl5 production.


5.16) Why doesn't open return an error when a pipe open fails?

    These statements:

        open(TOPIPE, "|bogus_command") || die ...
        open(FROMPIPE, "bogus_command|") || die ...

    will not fail just for lack of the bogus_command.  They'll only
    fail if the fork to run them fails, which is seldom the problem.

    If you're writing to the TOPIPE, you'll get a SIGPIPE if the child
    exits prematurely or doesn't run.  If you are reading from the
    FROMPIPE, you need to check the close() to see what happened.

    If you want an answer sooner than pipe buffering might otherwise
    afford you, you can do something like this:

        $kid = open (PIPE, "bogus_command |");   # XXX: check defined($kid)
        (kill 0, $kid) || die "bogus_command failed";

    This works fine if bogus_command doesn't have shell metas in it, but
    if it does, the shell may well not have exited before the kill 0.  You
    could always introduce a delay:

        $kid = open (PIPE, "bogus_command </dev/null |");
        sleep 1;
        (kill 0, $kid) || die "bogus_command failed";

    but this is sometimes undesirable, and in any event does not guarantee
    correct behavior.  But it seems slightly better than nothing.

    Similar tricks can be played with writable pipes if you don't wish to
    catch the SIGPIPE.


5.##) How do I capture the exit status from an external program?

    Perl provides a builtin variable which holds the status of the last
    backtick command: $?.  Here is exactly what the perlvar page says about
    this variable:

     $?      The status returned by the last pipe close, backtick
             (``) command, or system() operator.  Note that this
             is the status word returned by the wait() system
             call, so the exit value of the subprocess is
             actually ($? >> 8).  Thus on many systems, $? & 255
             gives which signal, if any, the process died from,
             and whether there was a core dump.  (Mnemonic:
             similar to sh and ksh.)


5.17) Why can't my perl program read from STDIN after I gave it ^D (EOF) ?

    Because some stdio's set error and eof flags that need clearing.

    Try keeping around the seekpointer and go there, like this:
         $where = tell(LOG);
         seek(LOG, $where, 0);

    If that doesn't work, try seeking to a different part of the file and
    then back.  If that doesn't work, try seeking to a different part of
    the file, reading something, and then seeking back.  If that doesn't
    work, give up on your stdio package and use sysread.  You can't call
    stdio's clearerr() from Perl, so if you get EINTR from a signal
    handler, you're out of luck.  Best to just use sysread() from the
    start for the tty.


5.18) How can I translate tildes in a filename?

    Perl doesn't expand tildes -- the shell (ok, some shells) do.
    The classic request is to be able to do something like:

        open(FILE, "~/dir1/file1");
        open(FILE, "~tchrist/dir1/file1");

    which doesn't work.  (And you don't know it, because you
    did a system call without an "|| die" clause! :-)

    If you *know* you're on a system with the csh, and you *know*
    that Larry hasn't internalized file globbing, then you could
    get away with

        $filename = <~tchrist/dir1/file1>;

    but that's pretty iffy.

    A better way is to do the translation yourself, as in:

        $filename =~ s#^~(\w+)(/.*)?$#(getpwnam($1))[7].$2#e;

    More robust and efficient versions that checked for error conditions,
    handed simple ~/blah notation, and cached lookups are all reasonable
    enhancements.


5.19) How can I convert my shell script to Perl?

    Larry's standard answer is to send it through the shell to perl filter,
    otherwise known at [e-mail].  Contrary to popular belief, Tom
    Christiansen isn't a real person.  He is actually a highly advanced
    artificial intelligence experiment written by a graduate student at the
    University of Colorado.  Some of the earlier tasks he was programmed to
    perform included:

        * monitor comp.lang.perl.misc and collect statistics on which
      questions were asked with which frequency and to respond to them
      with stock answers.  Tom's programming has since outgrown this
      paltry task, and it was assigned to an undergraduate student from
      the University of Florida.  After all, we all know that students
      from UF aren't able to do much more than documentation anyway.
      Against all odds, that undergraduate student has become a
      professional system administrator, perl programmer, and now
      author of the second edition of "Programming Perl".
        * convert shell programs to perl programs

    (This *IS* a joke... please quit calling me and asking about it!)

    Actually, there is no automatic machine translator.  Even if there
    were, you wouldn't gain a lot, as most of the external programs would
    still get called.  It's the same problem as blind translation into C:
    you're still apt to be bogged down by exec()s.  You have to analyze
    the dataflow and algorithm and rethink it for optimal speedup.  It's
    not uncommon to see one, two, or even three orders of magnitude of
    speed difference between the brute-force and the recoded approaches.


5.20) Can I use Perl to run a telnet or ftp session?

    Sure, you can connect directly to them using sockets, or you can run a
    session on a pty.  In either case, Randal's chat2 package, which is
    distributed with the perl source, will come in handly.  It address
    much the same problem space as Don Libes's expect package does.  Two
    examples of using managing an ftp session using chat2 can be found on
    convex.com in /pub/perl/scripts/ftp-chat2.shar .

    Caveat lector: chat2 is documented only by example, may not run on
    System V systems, and is subtly machine dependent both in its ideas
    of networking and in pseudottys.  See also question 4.21, "Why doesn't
    my sockets program work under System V (Solaris)?"

    Randal also has code showing an example socket session for handling the
    telnet protocol to get a weather report.  This can be found at
    ftp.cis.ufl.edu:/pub/perl/scripts/getduatweather.pl.

    Gene Spafford* has a nice ftp library package that will help with ftp.


5.21) Why do I sometimes get an "Arguments too long" error when I use <*>?

    As of perl4.036, there is a certain amount of globbing that is passed
    out to the shell and not handled internally.  The following code (which
    will, roughly, emulate "chmod 0644 *")

        while (<*>) {
            chmod 0644, $_;
        }

    is the equivalent of

        open(FOO, "echo * | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
        while (<FOO>) {
            chop;
            chmod 0644, $_;
        }

    Until globbing is built into Perl, you will need to use some form of
    non-globbing work around.

    Something like the following will work:

        opendir(DIR,'.');
        chmod 0644, grep(/\.c$/, readdir(DIR));
        closedir(DIR);

    This example is taken directly from "Programming Perl" page 78.

    If you've installed tcsh as /bin/csh, you'll never have this problem.

5.22) How do I do a "tail -f" in Perl?

    Larry says that the solution is to put a call to seek in yourself.
    First try

            seek(GWFILE, 0, 1);

    The statement seek(GWFILE, 0, 1); doesn't change the current position,
    but it does clear the end-of-file condition on the handle, so that the
    next <GWFILE> makes Perl try again to read something.

    If that doesn't work (depends on your stdio implementation), then
    you need something more like this:


    for (;;) {
        for ($curpos = tell(GWFILE); $_ = <GWFILE>; $curpos = tell(GWFILE)) {
            # search for some stuff and put it into files
        }
        sleep for a while
        seek(GWFILE, $curpos, 0);
    }


5.23) Is there a way to hide perl's command line from programs such as "ps"?

    Generally speaking, if you need to do this you're either using poor
    programming practices or are far too paranoid for your own good.  If you
    need to do this to hide a password being entered on the command line,
    recode the program to read the password from a file or to prompt for
    it. (see question 4.24)  Typing a password on the command line is
    inherently insecure as anyone can look over your shoulder to see it.

    If you feel you really must overwrite the command line and hide it, you
    can assign to the variable "$0".  For example:

        #!/usr/local/bin/perl
        $0 = "Hidden from prying eyes";

        open(PS, "ps |") || die "Can't PS: $!";
        while (<PS>) {
            next unless m/$$/;
            print
        }

    It should be noted that some OSes, like Solaris 2.X, read directly from
    the kernel information, instead of from the program's stack, and hence
    don't allow you to change the command line.


5.24) I {changed directory, modified my environment} in a perl script.  How
      come the change disappeared when I exited the script?  How do I get
      my changes to be visible?

    In the strictest sense, it "can't" be done.  However, there is special
    shell magic which may allow you to do it.  I suggest checking out
    comp.unix.shell and reading the comp.unix.questions FAQ.

    When perl is started, you are creating a child process.  Due to the way
    the Unix system is designed, children cannot permanently affect their
    parent shells.

    When a child process is created, it inherits a copy of its parents
    environment (variables, current directory, etc).  When the child
    changes this environment, it is changing the copy and not the original,
    so the parent isn't affected.

    If you must change the parent from within a perl script, you could try
    having it write out a shell script or a C-shell script and then using
    ". script" or "source script" (sh, Csh, respectively)


5.25) How can I use pass a filehandle to a function, or make a list of
      filehandles?

    If you've ever tried to use a variable for a filehandle, you may well
    have had some problems.  This is just revealing one of the icky places
    in perl: filehandles aren't first-class citizens the way everything
    else is, and it really gets in the way sometimes.

    Of course, it's just fine to say

        $fh = "/some/path";
        open($fh, "< $fh");
        print $fh "string\n";

    But you'll still get into trouble for trying:

        $fharray[$i] = "/some/path";
        open($fharray[$i], "< $fharray[$i]");
        print $fharray[$i] "stuff\n";

    You can also do this:

        $tmp_fh = $fharray[$i];
        print $tmp_fh "stuff\n";

    But this is ok:

        print { $fharray[$i] } "stuff\n";

    There are about four ways of passing in a filehandle.  The way
    everyone tries is just to pass it as a string.

        printit(Some_Handle);

    Unfortunately, that doesn't work so well, because the package that
    the printit() function is executing in may in fact not be the
    one that the handle was opened it:

    A simple fix would be to pass it in fully qualified;

        printit(main::Some_Handle);

    because if you don't, the function is going to have to do
    some crazy thing like this:

    CODE 1:

        sub printit {
            my $fh = shift;
            my $package = (caller)[0];
            $fh =~ s/^[^':]+$/$package::$&/;
            while (<$fh>) {
                print;
            }
        }

    A better solution is to pass a typeglob instead:

    CODE 2:

        printit(*Some_Handle);
        sub printit {
            local *FH = shift;
            while (<FH>) {
                print;
            }
        }

    However, it turns out that you don't have to use a typeglob inside
    the function.  This also works:

    CODE 3:

        printit(*Some_Handle);
        sub printit {
            my $fh = shift;
            while (<$fh>) {
                print;
            }
        }

    As does this even, in case you want to make an object by blessing
    your reference:

    CODE 4:

        printit(\*Some_Handle);
        sub printit {
            my $fh = shift;
            while (<$fh>) {
                print;
            }
        }

    I used to think that you had to use 1 or preferably 2, but apparently
    you can get away with number 3 and 4 as well.  This is nice because it
    avoids ever assigning to a typeglob as we do it 2, which is a bit
    risky.

    Some other problems with #1: if you're using strict subs, then you
    aren't going to be able to do that: the strict subs will gets you.
    Instead, you'll have to pass in 'main::Some_Handle', but then down in
    your function, you'll get blown away by strict refs, because you'll be
    using a string a symbol.  So really, the best way is to pass the
    typeglob (or occasionally a reference to the same).

5.26) How can open a file with a leading ">" or trailing blanks?

    Normally perl ignores trailing blanks in filenames, and interprets
    certain leading characters (or a trailing "|") to mean something
    special.  To avoid this, you might want to use a routine like this.
    It makes non-fullpathnames into explicit relative ones, and tacks
    a trailing null byte on the name to make perl leave it alone:

        sub safe_filename {
          local($_) = shift;
          m#^/# ? "$_\0" : "./$_\0";
        }
        $fn = &safe_filename("<<<something really wicked   ");
        open(FH, "> $fn") || "couldn't open $fn: $!";


5.27) How can I tell if a variable is tainted?

    This function works reasonably well to figure out whether a variable
    will be disliked by the taint checks automatically enabled by setuid
    execution:

        sub tainted {
            ! eval { join('',@_), kill 0; 1; };
        }

    and in particular, never does any system calls.
--
Stephen P Potter        Pencom Systems Administration              Beaching It
[e-mail]    Pager: 1-800-759-8888, 547-9561     Work: 703-860-2222
  Cthulhu for President in '96: When You're Tired of the Lesser of Two Evils
--
Stephen P Potter        Pencom Systems Administration              Beaching It
[e-mail]    Pager: 1-800-759-8888, 547-9561     Work: 703-860-2222
"I don't care whether people actually like Perl, just so long as they *think*
    they like it...  ;-)"    -Larry Wall