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perlfilter(3)	      User Contributed Perl Documentation	 perlfilter(3)

       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 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 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 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

	   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. 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) ----> 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

	   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	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 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
	    in the source filters distribution.

       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.

       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	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

	   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:

	   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:

	   #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
       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;

       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 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.

       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

       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

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

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

       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__".

       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

       Paul Marquess <>

       Reini Urban <>

       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.

perl v5.32.1			  2020-06-17			 perlfilter(3)


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