perlfilter - Source Filters
-
=head1 DESCRIPTION
This article is about a little-known feature of Perl called
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 C<-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.
+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 ;
+ 1: use Filter::cpp;
2: #define TRUE 1
- 3: $a = TRUE ;
- 4: print "a = $a\n" ;
+ 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
The parser then sees the following code:
- use Filter::cpp ;
- $a = 1 ;
- print "a = $a\n" ;
+ 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 ;
+ 1: use Filter::cpp;
2: #define TRUE 1
- 3: use Fred ;
- 4: $a = TRUE ;
- 5: print "a = $a\n" ;
+ 3: use Fred;
+ 4: $a = TRUE;
+ 5: print "a = $a\n";
The C<cpp> filter does not apply to the text of the Fred module, only
to the text of the file that used it (C<cpp_test>). Although the use
possible to stack a uudecode filter and an uncompression filter like
this:
- use Filter::uudecode ; use Filter::uncompress ;
+ use Filter::uudecode; use Filter::uncompress;
M'XL(".H<US4''V9I;F%L')Q;>7/;1I;_>_I3=&E=%:F*I"T?22Q/
M6]9*<IQCO*XFT"0[PL%%'Y+IG?WN^ZYN-$'J.[.JE$,20/?K=_[>
...
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
+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
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
+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.
Here is an example script that uses C<Filter::sh>:
- use Filter::sh 'tr XYZ PQR' ;
- $a = 1 ;
- print "XYZ a = $a\n" ;
+ use Filter::sh 'tr XYZ PQR';
+ $a = 1;
+ print "XYZ a = $a\n";
The output you'll get when the script is executed:
becomes M.)
- package Rot13 ;
+ package Rot13;
- use Filter::Util::Call ;
+ use Filter::Util::Call;
sub import {
- my ($type) = @_ ;
- my ($ref) = [] ;
- filter_add(bless $ref) ;
+ my ($type) = @_;
+ my ($ref) = [];
+ filter_add(bless $ref);
}
sub filter {
- my ($self) = @_ ;
- my ($status) ;
+ my ($self) = @_;
+ my ($status);
tr/n-za-mN-ZA-M/a-zA-Z/
- if ($status = filter_read()) > 0 ;
- $status ;
+ if ($status = filter_read()) > 0;
+ $status;
}
1;
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" ;
+ 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: $!\n";
- print OUT "use Rot13;\n" ;
+ print OUT "use Rot13;\n";
while (<IN>) {
- tr/a-zA-Z/n-za-mN-ZA-M/ ;
- print OUT ;
+ tr/a-zA-Z/n-za-mN-ZA-M/;
+ print OUT;
}
close IN;
If we encrypt this with C<mkrot13>:
- print " hello fred \n" ;
+ print " hello fred \n";
the result will be this:
use Rot13;
- cevag "uryyb serq\a" ;
+ cevag "uryyb serq\a";
Running it produces this output:
## DEBUG_BEGIN
if ($year > 1999) {
- warn "Debug: millennium bug in year $year\n" ;
+ warn "Debug: millennium bug in year $year\n";
}
## DEBUG_END
## DEBUG_BEGIN
#if ($year > 1999) {
- # warn "Debug: millennium bug in year $year\n" ;
+ # warn "Debug: millennium bug in year $year\n";
#}
## DEBUG_END
package Debug;
use strict;
- use Filter::Util::Call ;
+ use warnings;
+ use Filter::Util::Call;
- use constant TRUE => 1 ;
- use constant FALSE => 0 ;
+ use constant TRUE => 1;
+ use constant FALSE => 0;
sub import {
- my ($type) = @_ ;
+ my ($type) = @_;
my (%context) = (
Enabled => defined $ENV{DEBUG},
InTraceBlock => FALSE,
Filename => (caller)[1],
LineNo => 0,
LastBegin => 0,
- ) ;
- filter_add(bless \%context) ;
+ );
+ filter_add(bless \%context);
}
sub Die {
- my ($self) = shift ;
- my ($message) = shift ;
- my ($line_no) = shift || $self->{LastBegin} ;
+ my ($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() ;
- ++ $self->{LineNo} ;
+ my ($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};
+ 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 ;
+ $self->{InTraceBlock} = FALSE;
}
# comment out the debug lines when the filter is disabled
- s/^/#/ if ! $self->{Enabled} ;
+ s/^/#/ if ! $self->{Enabled};
} elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
- $self->{InTraceBlock} = TRUE ;
- $self->{LastBegin} = $self->{LineNo} ;
+ $self->{InTraceBlock} = TRUE;
+ $self->{LastBegin} = $self->{LineNo};
} elsif ( /^\s*##\s*DEBUG_END/ ) {
$self->Die("DEBUG_END has no DEBUG_BEGIN", $self->{LineNo});
}
- return $status ;
+ return $status;
}
- 1 ;
+ 1;
The big difference between this filter and the previous example is the
use of context data in the filter object. The filter object is based on
essence of the filter is as follows:
sub filter {
- my ($self) = @_ ;
- my ($status) ;
- $status = filter_read() ;
+ my ($self) = @_;
+ my ($status);
+ $status = filter_read();
# deal with EOF/error first
- return $status if $status <= 0 ;
+ 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} ;
+ s/^/#/ if ! $self->{Enabled};
} elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
- $self->{InTraceBlock} = TRUE ;
+ $self->{InTraceBlock} = TRUE;
}
- return $status ;
+ return $status;
}
Be warned: just as the C-preprocessor doesn't know C, the Debug filter
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.
+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
into this:
sub MySub($$@) {
- my ($first) = shift ;
- my ($second) = shift ;
- my (@rest) = @_ ;
+ my ($first) = shift;
+ my ($second) = shift;
+ my (@rest) = @_;
...
}
you know. The tricky bit will be choosing how much knowledge of Perl's
syntax you want your filter to have.
+=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.
+
+=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>
https://perl5.git.perl.org / perl5.git / blobdiff