perlfaq.pod

source of perl for linux application,

First of all, however, you I<can't> take away read permission, because
the source code has to be readable in order to be compiled and
interpreted.  (That doesn't mean that a CGI script's source is
readable by people on the web, though--only by people with access to
the filesystem.)  So you have to leave the permissions at the socially
friendly 0755 level.

Some people regard this as a security problem.  If your program does
insecure things and relies on people not knowing how to exploit those
insecurities, it is not secure.  It is often possible for someone to
determine the insecure things and exploit them without viewing the
source.  Security through obscurity, the name for hiding your bugs
instead of fixing them, is little security indeed.

You can try using encryption via source filters (Filter::* from CPAN),
but any decent programmer will be able to decrypt it.  You can try using
the byte code compiler and interpreter described below, but the curious
might still be able to de-compile it.  You can try using the native-code
compiler described below, but crackers might be able to disassemble it.
These pose varying degrees of difficulty to people wanting to get at
your code, but none can definitively conceal it (true of every
language, not just Perl).

If you're concerned about people profiting from your code, then the
bottom line is that nothing but a restrictive license will give you
legal security.  License your software and pepper it with threatening
statements like ``This is unpublished proprietary software of XYZ Corp.
Your access to it does not give you permission to use it blah blah
blah.''  We are not lawyers, of course, so you should see a lawyer if
you want to be sure your license's wording will stand up in court.

=head2 How can I compile my Perl program into byte code or C?

Malcolm Beattie has written a multifunction backend compiler,
available from CPAN, that can do both these things.  It is included
in the perl5.005 release, but is still considered experimental.
This means it's fun to play with if you're a programmer but not
really for people looking for turn-key solutions.

Merely compiling into C does not in and of itself guarantee that your
code will run very much faster.  That's because except for lucky cases
where a lot of native type inferencing is possible, the normal Perl
run-time system is still present and so your program will take just as
long to run and be just as big.  Most programs save little more than
compilation time, leaving execution no more than 10-30% faster.  A few
rare programs actually benefit significantly (even running several times
faster), but this takes some tweaking of your code.

You'll probably be astonished to learn that the current version of the
compiler generates a compiled form of your script whose executable is
just as big as the original perl executable, and then some.  That's
because as currently written, all programs are prepared for a full
eval() statement.  You can tremendously reduce this cost by building a
shared I<libperl.so> library and linking against that.  See the
F<INSTALL> podfile in the Perl source distribution for details.  If
you link your main perl binary with this, it will make it minuscule.
For example, on one author's system, F</usr/bin/perl> is only 11k in
size!

In general, the compiler will do nothing to make a Perl program smaller,
faster, more portable, or more secure.  In fact, it can make your
situation worse.  The executable will be bigger, your VM system may take
longer to load the whole thing, the binary is fragile and hard to fix,
and compilation never stopped software piracy in the form of crackers,
viruses, or bootleggers.  The real advantage of the compiler is merely
packaging, and once you see the size of what it makes (well, unless
you use a shared I<libperl.so>), you'll probably want a complete
Perl install anyway.

=head2 How can I compile Perl into Java?

You can also integrate Java and Perl with the
Perl Resource Kit from O'Reilly and Associates.  See
http://www.oreilly.com/catalog/prkunix/ .

Perl 5.6 comes with Java Perl Lingo, or JPL.  JPL, still in
development, allows Perl code to be called from Java.  See jpl/README
in the Perl source tree.

=head2 How can I get C<#!perl> to work on [MS-DOS,NT,...]?

For OS/2 just use

    extproc perl -S -your_switches

as the first line in C<*.cmd> file (C<-S> due to a bug in cmd.exe's
`extproc' handling).  For DOS one should first invent a corresponding
batch file and codify it in C<ALTERNATIVE_SHEBANG> (see the
F<INSTALL> file in the source distribution for more information).

The Win95/NT installation, when using the ActiveState port of Perl,
will modify the Registry to associate the C<.pl> extension with the
perl interpreter.  If you install another port, perhaps even building
your own Win95/NT Perl from the standard sources by using a Windows port
of gcc (e.g., with cygwin or mingw32), then you'll have to modify
the Registry yourself.  In addition to associating C<.pl> with the
interpreter, NT people can use: C<SET PATHEXT=%PATHEXT%;.PL> to let them
run the program C<install-linux.pl> merely by typing C<install-linux>.

Macintosh Perl programs will have the appropriate Creator and
Type, so that double-clicking them will invoke the Perl application.

I<IMPORTANT!>: Whatever you do, PLEASE don't get frustrated, and just
throw the perl interpreter into your cgi-bin directory, in order to
get your programs working for a web server.  This is an EXTREMELY big
security risk.  Take the time to figure out how to do it correctly.

=head2 Can I write useful Perl programs on the command line?

Yes.  Read L<perlrun> for more information.  Some examples follow.
(These assume standard Unix shell quoting rules.)

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

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

    # remove (most) comments from C program
    perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c

    # make file a month younger than today, defeating reaper daemons
    perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' *

    # find first unused uid
    perl -le '$i++ while getpwuid($i); print $i'

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

OK, the last one was actually an Obfuscated Perl Contest entry. :-)

=head2 Why don't Perl one-liners work on my DOS/Mac/VMS system?

The problem is usually that the command interpreters on those systems
have rather different ideas about quoting than the Unix shells under
which the one-liners were created.  On some systems, you may have to
change single-quotes to double ones, which you must I<NOT> do on Unix
or Plan9 systems.  You might also have to change a single % to a %%.

For example:

    # Unix
    perl -e 'print "Hello world\n"'

    # DOS, etc.
    perl -e "print \"Hello world\n\""

    # Mac
    print "Hello world\n"
     (then Run "Myscript" or Shift-Command-R)

    # VMS
    perl -e "print ""Hello world\n"""

The problem is that none of these examples are reliable: they depend on the
command interpreter.  Under Unix, the first two often work. Under DOS,
it's entirely possible that neither works.  If 4DOS was the command shell,
you'd probably have better luck like this:

  perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""

Under the Mac, it depends which environment you are using.  The MacPerl
shell, or MPW, is much like Unix shells in its support for several
quoting variants, except that it makes free use of the Mac's non-ASCII
characters as control characters.

Using qq(), q(), and qx(), instead of "double quotes", 'single
quotes', and `backticks`, may make one-liners easier to write.

There is no general solution to all of this.  It is a mess, pure and
simple.  Sucks to be away from Unix, huh? :-)

[Some of this answer was contributed by Kenneth Albanowski.]

=head2 Where can I learn about CGI or Web programming in Perl?

For modules, get the CGI or LWP modules from CPAN.  For textbooks,
see the two especially dedicated to web stuff in the question on
books.  For problems and questions related to the web, like ``Why
do I get 500 Errors'' or ``Why doesn't it run from the browser right
when it runs fine on the command line'', see these sources:

    WWW Security FAQ
        http://www.w3.org/Security/Faq/

    Web FAQ
        http://www.boutell.com/faq/

    CGI FAQ
        http://www.webthing.com/tutorials/cgifaq.html

    HTTP Spec
        http://www.w3.org/pub/WWW/Protocols/HTTP/

    HTML Spec
        http://www.w3.org/TR/REC-html40/
        http://www.w3.org/pub/WWW/MarkUp/

    CGI Spec
        http://www.w3.org/CGI/

    CGI Security FAQ
        http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt

=head2 Where can I learn about object-oriented Perl programming?

A good place to start is L<perltoot>, and you can use L<perlobj>,
L<perlboot>, and L<perlbot> for reference.  Perltoot didn't come out
until the 5.004 release; you can get a copy (in pod, html, or
postscript) from http://www.perl.com/CPAN/doc/FMTEYEWTK/ .

=head2 Where can I learn about linking C with Perl? [h2xs, xsubpp]

If you want to call C from Perl, start with L<perlxstut>,
moving on to L<perlxs>, L<xsubpp>, and L<perlguts>.  If you want to
call Perl from C, then read L<perlembed>, L<perlcall>, and
L<perlguts>.  Don't forget that you can learn a lot from looking at
how the authors of existing extension modules wrote their code and
solved their problems.

=head2 I've read perlembed, perlguts, etc., but I can't embed perl in
my C program; what am I doing wrong?

Download the ExtUtils::Embed kit from CPAN and run `make test'.  If
the tests pass, read the pods again and again and again.  If they
fail, see L<perlbug> and send a bug report with the output of
C<make test TEST_VERBOSE=1> along with C<perl -V>.

=head2 When I tried to run my script, I got this message. What does it
mean?

A complete list of Perl's error messages and warnings with explanatory
text can be found in L<perldiag>. You can also use the splain program
(distributed with Perl) to explain the error messages:

    perl program 2>diag.out
    splain [-v] [-p] diag.out

or change your program to explain the messages for you:

    use diagnostics;

or

    use diagnostics -verbose;

=head2 What's MakeMaker?

This module (part of the standard Perl distribution) is designed to
write a Makefile for an extension module from a Makefile.PL.  For more
information, see L<ExtUtils::MakeMaker>.

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
All rights reserved.

When included as an integrated part of the Standard Distribution
of Perl or of its documentation (printed or otherwise), this works is
covered under Perl's Artistic License.  For separate distributions of
all or part of this FAQ outside of that, see L<perlfaq>.

Irrespective of its distribution, all code examples here are in the public
domain.  You are permitted and encouraged to use this code and any
derivatives thereof in your own programs for fun or for profit as you
see fit.  A simple comment in the code giving credit to the FAQ would
be courteous but is not required.