How can I hook into Perl's print?

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Here\'s a scenario. You have a large amount of legacy scripts, all using a common library. Said scripts use the \'print\' statement for diagnostic output. No changes are

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闹比i

2020-11-29 04:06

There are a number of built-ins that you can override (see perlsub). However, print is one of the built-ins that doesn't work this way. The difficulties of overriding print are detailed at this perlmonk's thread.

However, you can

Create a package
Tie a handle
Select this handle.

Now, a couple of people have given the basic framework, but it works out kind of like this:

package IO::Override;
use base qw<Tie::Handle>;
use Symbol qw<geniosym>;

sub TIEHANDLE { return bless geniosym, __PACKAGE__ }

sub PRINT { 
    shift;
    # You can do pretty much anything you want here. 
    # And it's printing to what was STDOUT at the start.
    # 
    print $OLD_STDOUT join( '', 'NOTICE: ', @_ );
}

tie *PRINTOUT, 'IO::Override';
our $OLD_STDOUT = select( *PRINTOUT );

You can override printf in the same manner:

sub PRINTF { 
    shift;
    # You can do pretty much anything you want here. 
    # And it's printing to what was STDOUT at the start.
    # 
    my $format = shift;
    print $OLD_STDOUT join( '', 'NOTICE: ', sprintf( $format, @_ ));
}

See Tie::Handle for what all you can override of STDOUT's behavior.

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旧巷少年郎

2020-11-29 04:10

You could run the script from a wrapper script that captures the original script's stdout and writes the output somewhere sensible.

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萌比男神i

2020-11-29 04:14
You can use Perl's select to redirect STDOUT.
```
open my $fh, ">log.txt";
print "test1\n";
my $current_fh = select $fh;
print "test2\n";
select $current_fh;
print "test3\n";
```
The file handle could be anything, even a pipe to another process that post processes your log messages.

PerlIO::tee in the PerlIO::Util module seems to allows you to 'tee' the output of a file handle to multiple destinations (e.g. log processor and STDOUT).
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南旧

2020-11-29 04:26

This isn't the answer to your issue but you should be able to adopt the logic for your own use. If not, maybe someone else will find it useful.

Catching malformed headers before they happen...

package PsychicSTDOUT;
use strict;

my $c = 0;
my $malformed_header = 0;
open(TRUE_STDOUT, '>', '/dev/stdout');
tie *STDOUT, __PACKAGE__, (*STDOUT);

sub TIEHANDLE {
    my $class = shift;
    my $handles = [@_];
    bless $handles, $class;
    return $handles;
}

sub PRINT {
    my $class = shift;
    if (!$c++ && @_[0] !~ /^content-type/i) {
        my (undef, $file, $line) = caller;
        print STDERR "Missing content-type in $file at line $line!!\n";
        $malformed_header = 1;
    }
    return 0 if ($malformed_header);
    return print TRUE_STDOUT @_;
}
1;

usage:

use PsychicSTDOUT;
print "content-type: text/html\n\n"; #try commenting out this line
print "<html>\n";
print "</html>\n";

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孤街浪徒

2020-11-29 04:27

Lots of choices. Use select() to change the filehandle that print defaults to. Or tie STDOUT. Or reopen it. Or apply an IO layer to it.

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