Man Pages

perlfilter(1) - phpMan perlfilter(1) - phpMan

Command: man perldoc info search(apropos)  

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

       perlfilter - Source Filters

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

       Before the Perl interpreter can execute a Perl script, it must first read it from a file into memory for pars-
       ing 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 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 "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

       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 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

       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. You could use the existing
       "-P" command line option to do this, but as it happens, the source filters distribution comes with a C prepro-
       cessor filter module called Filter::cpp. Let's use that instead.

       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 prepro-
       cessor. 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

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

  ----> parser

       As you can see, a new stream has been created for reading the source from "". This stream will remain
       active until all of "" has been parsed. The source stream for "cpp_test" will still exist, but is inac-
       tive. Once the parser has finished reading, 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

       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;

       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.

       There are three ways to write your own source filter. You can write it in C, use an external program as a fil-
       ter, 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.

       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).

       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, 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 in
            the source filters module.

       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 fil-
       tered data to standard output. "Filter:cpp" is an example of a source filter implemented as a separate exe-
       cutable - 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 "Fil-
       ter::exec" spawns the external command directly, while "Filter::sh" spawns a shell to execute the external com-
       mand. (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.

       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);

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


       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 anony-
       mous 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 fil-
       tering is done in a method called "filter()". (It is also possible to write a Perl source filter using a clo-
       sure. 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 fil-
       ter function itself is expected to return its status in the same way, and put the filtered line it wants writ-
       ten 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>) {
              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

       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

       When the "DEBUG" environment variable exists, the filter ensures that Perl parses only the code between the
       "DEBUG_BEGIN" and "DEBUG_END" markers. 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 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;


       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 informa-
       tion 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;

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

       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 lim-
       ited 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.

       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.

       The Source Filters distribution is available on CPAN, in


       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.

       Paul Marquess <>

       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

perl v5.8.8                       2006-01-07                     PERLFILTER(1)