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perlfilter ()
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         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


         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
         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
         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
             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 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
         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 preprocessor
         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 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)
    ----> 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 inactive. 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 `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 ;
         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 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.


         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
         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
              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 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


         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 ;
               $status ;
         All Perl source filters are implemented as Perl classes and
         have the same basic structure as the example above.
         First, we include the `Filter::Util::Call' module, which
         exports a number of functions into your filter's namespace.
         The filter shown above uses two of these functions,
         `filter_add()' and `filter_read()'.
         Next, we create the filter object and associate it with the
         source stream by defining the `import' function. If you know
         Perl well enough, you know that `import' is called
         automatically every time a module is included with a use
         statement. This makes `import' the ideal place to both
         create and install a filter object.
         In the example filter, the object (`$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 (`$ref' in this case) and
         installs it in the source stream.
         Finally, there is the code that actually does the filtering.
         For this type of Perl source filter, all the filtering is
         done in a method called `filter()'. (It is also possible to
         write a Perl source filter using a closure. See the
         `Filter::Util::Call' manual page for more details.) It's
         called every time the Perl parser needs another line of
         source to process. The `filter()' method, in turn, reads
         lines from the source stream using the `filter_read()'
         If a line was available from the source stream,
         `filter_read()' returns a status value greater than zero and
         appends the line to `$_'.  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


         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
             ## 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 $status ;
             1 ;
         The big difference between this filter and the previous
         example is the use of context data in the filter object. The
         filter object is based on a hash reference, and is used to
         keep various pieces of context information between calls to
         the filter function. All but two of the hash fields are used
         for error reporting. The first of those two, Enabled, is
         used by the filter to determine whether the debugging code
         should be given to the Perl parser. The second,
         InTraceBlock, is true when the filter has encountered a
         `DEBUG_BEGIN' line, but has not yet encountered the
         following `DEBUG_END' line.
         If you ignore all the error checking that most of the code
         does, the essence of the filter is as follows:
             sub filter {
                my ($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
             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 a 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.


         The Source Filters distribution is available on CPAN, in


         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 itself.

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