PERLFILTER(1) Perl Programmers Reference Guide PERLFILTER(1)

PERLFILTER(1) Perl Programmers Reference Guide PERLFILTER(1) #

PERLFILTER(1) Perl Programmers Reference Guide PERLFILTER(1)

NNAAMMEE #

 perlfilter - Source Filters

DDEESSCCRRIIPPTTIIOONN #

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

CCOONNCCEEPPTTSS #

 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 "use" or "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 _s_o_u_r_c_e _s_t_r_e_a_m. 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 "require" or "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:

 1.   Although there can be any number of source streams in existence at
      any given time, only one will be active.

 2.   Every source stream is associated with only one file.

 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
 _t_r_i_a_l_._g_z. 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

UUSSIINNGG FFIILLTTEERRSS #

 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, "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, "use Filter::cpp", includes and installs the "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 "cpp" source filter before
 processing them. The "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 "cpp" filter does not apply to the text of the Fred module, only to
 the text of the file that used it ("cpp_test"). Although the use
 statement on line 3 will pass through the cpp filter, the module that
 gets included ("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
 "Fred.pm". This stream will remain active until all of "Fred.pm" has been
 parsed. The source stream for "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 "cpp_test"
 then becomes active again and the parser reads line 4 and subsequent
 lines from "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.

WWRRIITTIINNGG AA SSOOUURRCCEE FFIILLTTEERR #

 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.

WWRRIITTIINNGG AA SSOOUURRCCEE FFIILLTTEERR IINN CC #

 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 "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).

 DDeeccrryyppttiioonn FFiilltteerrss
      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 _d_e_c_r_y_p_t_._p_m in the source
      filters distribution.

CCRREEAATTIINNGG AA SSOOUURRCCEE FFIILLTTEERR AASS AA SSEEPPAARRAATTEE EEXXEECCUUTTAABBLLEE #

 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. "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: "Filter::exec" and "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
 "Filter::exec" spawns the external command directly, while "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 "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 "tr"
 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.

WWRRIITTIINNGG AA SSOOUURRCCEE FFIILLTTEERR IINN PPEERRLL #

 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 "Filter::Util::Call" module, which exports a number
 of functions into your filter's namespace. The filter shown above uses
 two of these functions, "filter_add()" and "filter_read()".

 Next, we create the filter object and associate it with the source stream
 by defining the "import" function. If you know Perl well enough, you know
 that "import" is called automatically every time a module is included
 with a use statement. This makes "import" the ideal place to both create
 and install a filter object.

 In the example filter, the object ($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 "filter_add()" function. This takes a filter object as a
 parameter ($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
 "filter()". (It is also possible to write a Perl source filter using a
 closure. See the "Filter::Util::Call" manual page for more details.) It's
 called every time the Perl parser needs another line of source to
 process. The "filter()" method, in turn, reads lines from the source
 stream using the "filter_read()" function.

 If a line was available from the source stream, "filter_read()" returns a
 status value greater than zero and appends the line to $_.  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 $_.
 The use of $_ 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, "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 "mkrot13":

     print " hello fred \n";

 the result will be this:

     use Rot13;
     cevag "uryyb serq\a";

 Running it produces this output:

     hello fred

UUSSIINNGG CCOONNTTEEXXTT:: TTHHEE DDEEBBUUGG FFIILLTTEERR #

 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, "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 "DEBUG_END" markers only when the "DEBUG" environment variable
 exists. That means that when "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 "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 v5.36;
     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 "DEBUG_BEGIN" line, but has not yet encountered the
 following "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 <<EOM;

##DEBUG_BEGIN #

EOM #

 Such things aside, you can see that a lot can be achieved with a modest
 amount of code.

CCOONNCCLLUUSSIIOONN #

 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
 "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 @_ 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.

LLIIMMIITTAATTIIOONNSS #

 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 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 "/.../" delimiters and
 don't have a modifier "//x" are indistinguishable from code chunks
 beginning with the division operator "/". As a workaround you must use
 "m/.../" or "m?...?" for such patterns. Also, the presence of regexes
 specified with raw "?...?" delimiters may cause mysterious errors. The
 workaround is to use "m?...?" instead.  See
 <https://metacpan.org/pod/Switch#LIMITATIONS>.

 Currently the content of the "__DATA__" block is not filtered.

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

TTHHIINNGGSS TTOO LLOOOOKK OOUUTT FFOORR #

 Some Filters Clobber the "DATA" Handle
      Some source filters use the "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 "DATA" filehandle, but on the other
      hand totally ignore the text after "__DATA__".

RREEQQUUIIRREEMMEENNTTSS #

 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.

AAUUTTHHOORR #

 Paul Marquess <Paul.Marquess@btinternet.com>

 Reini Urban <rurban@cpan.org>

CCooppyyrriigghhttss 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.

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