=head1 NAME

perlfilter - Source Filters

=head1 DESCRIPTION

This article is about a little-known feature of Perl called

I<source filters>. Source filters alter the program text of a module

before Perl sees it, much as a C preprocessor alters the source text of

a C program before the compiler sees it. This article tells you more

about what source filters are, how they work, and how to write your

own.

The original purpose of source filters was to let you encrypt your

program source to prevent casual piracy. This isn't all they can do, as

you'll soon learn. But first, the basics.

=head1 CONCEPTS

Before the Perl interpreter can execute a Perl script, it must first

read it from a file into memory for parsing and compilation. If that

script itself includes other scripts with a C<use> or C<require>

statement, then each of those scripts will have to be read from their

respective files as well.

Now think of each logical connection between the Perl parser and an

individual file as a I<source stream>. A source stream is created when

the Perl parser opens a file, it continues to exist as the source code

is read into memory, and it is destroyed when Perl is finished parsing

the file. If the parser encounters a C<require> or C<use> statement in

a source stream, a new and distinct stream is created just for that

file.

The diagram below represents a single source stream, with the flow of

source from a Perl script file on the left into the Perl parser on the

right. This is how Perl normally operates.

    file -------> parser

There are two important points to remember:

=over 5

=item 1.

Although there can be any number of source streams in existence at any

given time, only one will be active.

=item 2.

Every source stream is associated with only one file.

=back

A source filter is a special kind of Perl module that intercepts and

modifies a source stream before it reaches the parser. A source filter

changes our diagram like this:

    file ----> filter ----> parser

If that doesn't make much sense, consider the analogy of a command

pipeline. Say you have a shell script stored in the compressed file

I<trial.gz>. The simple pipeline command below runs the script without

needing to create a temporary file to hold the uncompressed file.

    gunzip -c trial.gz | sh

In this case, the data flow from the pipeline can be represented as follows:

    trial.gz ----> gunzip ----> sh

With source filters, you can store the text of your script compressed and use a source filter to uncompress it for Perl's parser:

     compressed           gunzip

    Perl program ---> source filter ---> parser

=head1 USING FILTERS

So how do you use a source filter in a Perl script? Above, I said that

a source filter is just a special kind of module. Like all Perl

modules, a source filter is invoked with a use statement.

Say you want to pass your Perl source through the C preprocessor before

execution. As it happens, the source filters distribution comes with a C

preprocessor filter module called Filter::cpp.

Below is an example program, C<cpp_test>, which makes use of this filter.

Line numbers have been added to allow specific lines to be referenced

easily.

    1: use Filter::cpp;

    2: #define TRUE 1

    3: $a = TRUE;

    4: print "a = $a\n";

When you execute this script, Perl creates a source stream for the

file. Before the parser processes any of the lines from the file, the

source stream looks like this:

    cpp_test ---------> parser

Line 1, C<use Filter::cpp>, includes and installs the C<cpp> filter

module. All source filters work this way. The use statement is compiled

and executed at compile time, before any more of the file is read, and

it attaches the cpp filter to the source stream behind the scenes. Now

the data flow looks like this:

    cpp_test ----> cpp filter ----> parser

As the parser reads the second and subsequent lines from the source

stream, it feeds those lines through the C<cpp> source filter before

processing them. The C<cpp> filter simply passes each line through the

real C preprocessor. The output from the C preprocessor is then

inserted back into the source stream by the filter.

                  .-> cpp --.

                  |         |

                  |         |

                  |       <-'

   cpp_test ----> cpp filter ----> parser

The parser then sees the following code:

    use Filter::cpp;

    $a = 1;

    print "a = $a\n";

Let's consider what happens when the filtered code includes another

module with use:

    1: use Filter::cpp;

    2: #define TRUE 1

    3: use Fred;

    4: $a = TRUE;

    5: print "a = $a\n";

The C<cpp> filter does not apply to the text of the Fred module, only

to the text of the file that used it (C<cpp_test>). Although the use

statement on line 3 will pass through the cpp filter, the module that

gets included (C<Fred>) will not. The source streams look like this

after line 3 has been parsed and before line 4 is parsed:

    cpp_test ---> cpp filter ---> parser (INACTIVE)

    Fred.pm ----> parser

As you can see, a new stream has been created for reading the source

from C<Fred.pm>. This stream will remain active until all of C<Fred.pm>

has been parsed. The source stream for C<cpp_test> will still exist,

but is inactive. Once the parser has finished reading Fred.pm, the

source stream associated with it will be destroyed. The source stream

for C<cpp_test> then becomes active again and the parser reads line 4

and subsequent lines from C<cpp_test>.

You can use more than one source filter on a single file. Similarly,

you can reuse the same filter in as many files as you like.

For example, if you have a uuencoded and compressed source file, it is

possible to stack a uudecode filter and an uncompression filter like

this:

    use Filter::uudecode; use Filter::uncompress;

    M'XL(".H<US4''V9I;F%L')Q;>7/;1I;_>_I3=&E=%:F*I"T?22Q/

    M6]9*<IQCO*XFT"0[PL%%'Y+IG?WN^ZYN-$'J.[.JE$,20/?K=_[>

    ...

Once the first line has been processed, the flow will look like this:

    file ---> uudecode ---> uncompress ---> parser

               filter         filter

Data flows through filters in the same order they appear in the source

file. The uudecode filter appeared before the uncompress filter, so the

source file will be uudecoded before it's uncompressed.

=head1 WRITING A SOURCE FILTER

There are three ways to write your own source filter. You can write it

in C, use an external program as a filter, or write the filter in Perl.

I won't cover the first two in any great detail, so I'll get them out

of the way first. Writing the filter in Perl is most convenient, so

I'll devote the most space to it.

=head1 WRITING A SOURCE FILTER IN C

The first of the three available techniques is to write the filter

completely in C. The external module you create interfaces directly

with the source filter hooks provided by Perl.

The advantage of this technique is that you have complete control over

the implementation of your filter. The big disadvantage is the

increased complexity required to write the filter - not only do you

need to understand the source filter hooks, but you also need a

reasonable knowledge of Perl guts. One of the few times it is worth

going to this trouble is when writing a source scrambler. The

C<decrypt> filter (which unscrambles the source before Perl parses it)

included with the source filter distribution is an example of a C

source filter (see Decryption Filters, below).

=over 5

=item B<Decryption Filters>

All decryption filters work on the principle of "security through

obscurity." Regardless of how well you write a decryption filter and

how strong your encryption algorithm is, anyone determined enough can

retrieve the original source code. The reason is quite simple - once

the decryption filter has decrypted the source back to its original

form, fragments of it will be stored in the computer's memory as Perl

parses it. The source might only be in memory for a short period of

time, but anyone possessing a debugger, skill, and lots of patience can

eventually reconstruct your program.

That said, there are a number of steps that can be taken to make life

difficult for the potential cracker. The most important: Write your

decryption filter in C and statically link the decryption module into

the Perl binary. For further tips to make life difficult for the

potential cracker, see the file I<decrypt.pm> in the source filters

distribution.

=back

=head1 CREATING A SOURCE FILTER AS A SEPARATE EXECUTABLE

An alternative to writing the filter in C is to create a separate

executable in the language of your choice. The separate executable

reads from standard input, does whatever processing is necessary, and

writes the filtered data to standard output. C<Filter::cpp> is an

example of a source filter implemented as a separate executable - the

executable is the C preprocessor bundled with your C compiler.

The source filter distribution includes two modules that simplify this

task: C<Filter::exec> and C<Filter::sh>. Both allow you to run any

external executable. Both use a coprocess to control the flow of data

into and out of the external executable. (For details on coprocesses,

see Stephens, W.R., "Advanced Programming in the UNIX Environment."

Addison-Wesley, ISBN 0-210-56317-7, pages 441-445.) The difference

between them is that C<Filter::exec> spawns the external command

directly, while C<Filter::sh> spawns a shell to execute the external

command. (Unix uses the Bourne shell; NT uses the cmd shell.) Spawning

a shell allows you to make use of the shell metacharacters and

redirection facilities.

Here is an example script that uses C<Filter::sh>:

    use Filter::sh 'tr XYZ PQR';

    $a = 1;

    print "XYZ a = $a\n";

The output you'll get when the script is executed:

    PQR a = 1

Writing a source filter as a separate executable works fine, but a

small performance penalty is incurred. For example, if you execute the

small example above, a separate subprocess will be created to run the

Unix C command. Each use of the filter requires its own subprocess.

If creating subprocesses is expensive on your system, you might want to

consider one of the other options for creating source filters.

=head1 WRITING A SOURCE FILTER IN PERL

The easiest and most portable option available for creating your own

source filter is to write it completely in Perl. To distinguish this

from the previous two techniques, I'll call it a Perl source filter.

To help understand how to write a Perl source filter we need an example

to study. Here is a complete source filter that performs rot13

decoding. (Rot13 is a very simple encryption scheme used in Usenet

postings to hide the contents of offensive posts. It moves every letter

forward thirteen places, so that A becomes N, B becomes O, and Z

becomes M.)

   package Rot13;

   use Filter::Util::Call;

   sub import {

      my ($type) = @_;

      my ($ref) = [];

      filter_add(bless $ref);

   }

   sub filter {

      my ($self) = @_;

      my ($status);

      tr/n-za-mN-ZA-M/a-zA-Z/

         if ($status = filter_read()) > 0;

      $status;

   }

   1;

All Perl source filters are implemented as Perl classes and have the

same basic structure as the example above.

First, we include the C<Filter::Util::Call> module, which exports a

number of functions into your filter's namespace. The filter shown

above uses two of these functions, C<filter_add()> and

C<filter_read()>.

Next, we create the filter object and associate it with the source

stream by defining the C<import> function. If you know Perl well

enough, you know that C<import> is called automatically every time a

module is included with a use statement. This makes C<import> the ideal

place to both create and install a filter object.

In the example filter, the object (C<$ref>) is blessed just like any

other Perl object. Our example uses an anonymous array, but this isn't

a requirement. Because this example doesn't need to store any context

information, we could have used a scalar or hash reference just as

well. The next section demonstrates context data.

The association between the filter object and the source stream is made

with the C<filter_add()> function. This takes a filter object as a

parameter (C<$ref> in this case) and installs it in the source stream.

Finally, there is the code that actually does the filtering. For this

type of Perl source filter, all the filtering is done in a method

called C<filter()>. (It is also possible to write a Perl source filter

using a closure. See the C<Filter::Util::Call> manual page for more

details.) It's called every time the Perl parser needs another line of

source to process. The C<filter()> method, in turn, reads lines from

the source stream using the C<filter_read()> function.

If a line was available from the source stream, C<filter_read()>

returns a status value greater than zero and appends the line to C<$_>.

A status value of zero indicates end-of-file, less than zero means an

error. The filter function itself is expected to return its status in

the same way, and put the filtered line it wants written to the source

stream in C<$_>. The use of C<$_> accounts for the brevity of most Perl

source filters.

In order to make use of the rot13 filter we need some way of encoding

the source file in rot13 format. The script below, C<mkrot13>, does

just that.

    die "usage mkrot13 filename\n" unless @ARGV;

    my $in = $ARGV[0];

    my $out = "$in.tmp";

    open(IN, "<$in") or die "Cannot open file $in: $!\n";

    open(OUT, ">$out") or die "Cannot open file $out: $!\n";

    print OUT "use Rot13;\n";

    while (<IN>) {

       tr/a-zA-Z/n-za-mN-ZA-M/;

       print OUT;

    }

    close IN;

    close OUT;

    unlink $in;

    rename $out, $in;

If we encrypt this with C<mkrot13>:

    print " hello fred \n";

the result will be this:

    use Rot13;

    cevag "uryyb serq\a";

Running it produces this output:

    hello fred

=head1 USING CONTEXT: THE DEBUG FILTER

The rot13 example was a trivial example. Here's another demonstration

that shows off a few more features.

Say you wanted to include a lot of debugging code in your Perl script

during development, but you didn't want it available in the released

product. Source filters offer a solution. In order to keep the example

simple, let's say you wanted the debugging output to be controlled by

an environment variable, C<DEBUG>. Debugging code is enabled if the

variable exists, otherwise it is disabled.

Two special marker lines will bracket debugging code, like this:

    ## DEBUG_BEGIN

    if ($year > 1999) {

       warn "Debug: millennium bug in year $year\n";

    }

    ## DEBUG_END

The filter ensures that Perl parses the code between the <DEBUG_BEGIN>

and C<DEBUG_END> markers only when the C<DEBUG> environment variable

exists. That means that when C<DEBUG> does exist, the code above

should be passed through the filter unchanged. The marker lines can

also be passed through as-is, because the Perl parser will see them as

comment lines. When C<DEBUG> isn't set, we need a way to disable the

debug code. A simple way to achieve that is to convert the lines

between the two markers into comments:

    ## DEBUG_BEGIN

    #if ($year > 1999) {

    #     warn "Debug: millennium bug in year $year\n";

    #}

    ## DEBUG_END

Here is the complete Debug filter:

    package Debug;

    use strict;

    use warnings;

    use Filter::Util::Call;

    use constant TRUE => 1;

    use constant FALSE => 0;

    sub import {

       my ($type) = @_;

       my (%context) = (

         Enabled => defined $ENV{DEBUG},

         InTraceBlock => FALSE,

         Filename => (caller)[1],

         LineNo => 0,

         LastBegin => 0,

       );

       filter_add(bless \%context);

    }

    sub Die {

       my ($self) = shift;

       my ($message) = shift;

       my ($line_no) = shift || $self->{LastBegin};

       die "$message at $self->{Filename} line $line_no.\n"

    }

    sub filter {

       my ($self) = @_;

       my ($status);

       $status = filter_read();

       ++ $self->{LineNo};

       # deal with EOF/error first

       if ($status <= 0) {

           $self->Die("DEBUG_BEGIN has no DEBUG_END")

               if $self->{InTraceBlock};

           return $status;

       }

       if ($self->{InTraceBlock}) {

          if (/^\s*##\s*DEBUG_BEGIN/ ) {

              $self->Die("Nested DEBUG_BEGIN", $self->{LineNo})

          } elsif (/^\s*##\s*DEBUG_END/) {

              $self->{InTraceBlock} = FALSE;

          }

          # comment out the debug lines when the filter is disabled

          s/^/#/ if ! $self->{Enabled};

       } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {

          $self->{InTraceBlock} = TRUE;

          $self->{LastBegin} = $self->{LineNo};

       } elsif ( /^\s*##\s*DEBUG_END/ ) {

          $self->Die("DEBUG_END has no DEBUG_BEGIN", $self->{LineNo});

       }

       return $status;

    }

    1;

The big difference between this filter and the previous example is the

use of context data in the filter object. The filter object is based on

a hash reference, and is used to keep various pieces of context

information between calls to the filter function. All but two of the

hash fields are used for error reporting. The first of those two,

Enabled, is used by the filter to determine whether the debugging code

should be given to the Perl parser. The second, InTraceBlock, is true

when the filter has encountered a C<DEBUG_BEGIN> line, but has not yet

encountered the following C<DEBUG_END> line.

If you ignore all the error checking that most of the code does, the

essence of the filter is as follows:

    sub filter {

       my ($self) = @_;

       my ($status);

       $status = filter_read();

       # deal with EOF/error first

       return $status if $status <= 0;

       if ($self->{InTraceBlock}) {

          if (/^\s*##\s*DEBUG_END/) {

             $self->{InTraceBlock} = FALSE

          }

          # comment out debug lines when the filter is disabled

          s/^/#/ if ! $self->{Enabled};

       } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {

          $self->{InTraceBlock} = TRUE;

       }

       return $status;

    }

Be warned: just as the C-preprocessor doesn't know C, the Debug filter

doesn't know Perl. It can be fooled quite easily:

    print <

    ##DEBUG_BEGIN

    EOM

Such things aside, you can see that a lot can be achieved with a modest

amount of code.

=head1 CONCLUSION

You now have better understanding of what a source filter is, and you

might even have a possible use for them. If you feel like playing with

source filters but need a bit of inspiration, here are some extra

features you could add to the Debug filter.

First, an easy one. Rather than having debugging code that is

all-or-nothing, it would be much more useful to be able to control

which specific blocks of debugging code get included. Try extending the

syntax for debug blocks to allow each to be identified. The contents of

the C<DEBUG> environment variable can then be used to control which

blocks get included.

Once you can identify individual blocks, try allowing them to be

nested. That isn't difficult either.

Here is an interesting idea that doesn't involve the Debug filter.

Currently Perl subroutines have fairly limited support for formal

parameter lists. You can specify the number of parameters and their

type, but you still have to manually take them out of the C<@_> array

yourself. Write a source filter that allows you to have a named

parameter list. Such a filter would turn this:

    sub MySub ($first, $second, @rest) { ... }

into this:

    sub MySub($$@) {

       my ($first) = shift;

       my ($second) = shift;

       my (@rest) = @_;

       ...

    }

Finally, if you feel like a real challenge, have a go at writing a

full-blown Perl macro preprocessor as a source filter. Borrow the

useful features from the C preprocessor and any other macro processors

you know. The tricky bit will be choosing how much knowledge of Perl's

syntax you want your filter to have.

=head1 LIMITATIONS

Source filters only work on the string level, thus are highly limited

in its ability to change source code on the fly. It cannot detect

comments, quoted strings, heredocs, it is no replacement for a real

parser.

The only stable usage for source filters are encryption, compression,

or the byteloader, to translate binary code back to source code.

See for example the limitations in L<Switch>, which uses source filters,

and thus is does not work inside a string eval, the presence of

regexes with embedded newlines that are specified with raw C</.../>

delimiters and don't have a modifier C<//x> are indistinguishable from

code chunks beginning with the division operator C. As a workaround

you must use C<m/.../> or C<m?...?> for such patterns. Also, the presence of

regexes specified with raw C delimiters may cause mysterious

errors. The workaround is to use C<m?...?> instead.  See

L<http://search.cpan.org/perldoc?Switch#LIMITATIONS>

Currently the content of the C<__DATA__> block is not filtered.

Currently internal buffer lengths are limited to 32-bit only.

=head1 THINGS TO LOOK OUT FOR

=over 5

=item Some Filters Clobber the C<DATA> Handle

Some source filters use the C<DATA> handle to read the calling program.

When using these source filters you cannot rely on this handle, nor expect

any particular kind of behavior when operating on it.  Filters based on

Filter::Util::Call (and therefore Filter::Simple) do not alter the C<DATA>

filehandle, but on the other hand totally ignore the text after C<__DATA__>.

=back

=head1 REQUIREMENTS

The Source Filters distribution is available on CPAN, in 

    CPAN/modules/by-module/Filter

Starting from Perl 5.8 Filter::Util::Call (the core part of the

Source Filters distribution) is part of the standard Perl distribution.

Also included is a friendlier interface called Filter::Simple, by

Damian Conway.

=head1 AUTHOR

Paul Marquess E<lt>Paul.Marquess@btinternet.comE<gt>

Reini Urban E<lt>rurban@cpan.orgE<gt>

=head1 Copyrights

The first version of this article originally appeared in The Perl

Journal #11, and is copyright 1998 The Perl Journal. It appears

courtesy of Jon Orwant and The Perl Journal.  This document may be

distributed under the same terms as Perl itself.