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Override printing to STDOUT/ERR

by bliako (Chaplain)
on Apr 13, 2018 at 12:53 UTC ( #1212802=perlquestion: print w/replies, xml ) Need Help??
bliako has asked for the wisdom of the Perl Monks concerning the following question:

Hello wise Monks,

I would like to override Perl's printing to STDOUT/STDERR with my own version which simply adds the line number the print function call happened. Ideally, ALL the line numbers/filenames/packagenames in the case of nested calls all the way to the top/main because I want to separate a program's STDOUT depending on which part of the program it originated.

You can assume I have complete control over the source code (of the program which I want to override its print statements) and can change anything I like. HOWEVER, I do not have control of the source-code of the many packages the program may be using and the print statements therein.

Initially I thought to just replace print with my_own_print in the source code and run that. Then I realised that it should only affect print in a function-call context and I used PPI for this purpose. And then I realised that there are other ways of printing to STDOUT/STDERR including the croaks etc. And finally I also realised that print statements in the packages will be out of my control. So this plan collapsed fairly quickly.

Now, I have already read the thread !Overriding Builtin print

and the more recent https://stackoverflow.com/questions/387702/how-can-i-hook-into-perls-print/6281636

However, I wonder if there are any recent developments since then or a definite HOWTO.

thanks, bliako

Replies are listed 'Best First'.
Re: Override printing to STDOUT/ERR
by Corion (Pope) on Apr 13, 2018 at 13:10 UTC

      thanks for the recommendation. Following your suggestion I stumbled upon  Tie::STDOUT (by David Cantrell) which does the trick:

      #!/usr/bin/env perl use strict; use warnings; use Tie::STDOUT print => sub { my @params = @_; my @calhis = (); my $i = 2; # to avoid printing stacktrace for Tie::STDOUT # build a kind of stacktrace: while( my ($package, $filename, $line, $subroutine, $hasargs, $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash) = caller($i++) ){ push(@calhis, "$filename, line $line : $package".'::'."$subrou +tine"); } # print the stacktrace with whatever needs to be printed print join("->", @calhis), " : ", @_; } ; print "from main\n"; sub1("main"); sub2("main"); sub sub1 { my $via = $_[0]; print "from sub1 via $via\n"; } sub sub2 { sub1("sub2"); }
Re: Override printing to STDOUT/ERR
by LanX (Bishop) on Apr 13, 2018 at 13:57 UTC
    I'm still confused about the use case....

    If this is only for debugging purpose to find the line numbers , you can close STDOUT and STDERR and cause warnings. But the string to be printed will be lost.

    DB<6> close STDOUT DB<7> use warnings; print 666 print() on closed filehandle STDOUT at (eval 15)

    Otherwise I concur with Corion to tie a filehandle. caller will tell you where the tied method was called and you can redirect STDOUT and STDERR to these FHs. (also with select )

    Cheers Rolf
    (addicted to the Perl Programming Language and ☆☆☆☆ :)
    Wikisyntax for the Monastery

      What I would like to do eventually is to be able to group program output (to STDOUT) depending on which program block produced it.

      Maybe visualising program output as a tree/graph. Which could also be used to selectively switch on and off program output, highligthing certain sections etc. Lots of possibilities.

      I will give more details in the Meditations section once I have a prototype.

      thanks for the input, bliako

        You may want to consider using on of the logging frameworks, available on CPAN, as opposed to a print based approach

        Log::Log4perl, Log::Any, Log::ger are examples that support directing output of the log statements to different locations based upon a category. A category is generally a package but can be explicitly set as desired.

        Here an example from the Log::ger::Manual::Tutorial::05_Category tutorial

        use Log::ger::Output Composite => ( outputs => { File => [ { conf => {path=>'/path/app.log'}, category_level => { Foo => 'off' }, }, { conf => {path=>'/path/foo.log'}, level => 'off', category_level => { Foo => 'trace' }, }, ], }, );

        lbe

Re: Override printing to STDOUT/ERR
by Tux (Abbot) on Apr 17, 2018 at 10:10 UTC

    Maybe Capture::Tiny serves your purpose enough:

    $ perl -MCapture::Tiny=capture -wE'my($out,$err,$exit)=capture{say"Hel +lo";warn"Die\n";};say"OUT:$out";say"ERR:$err";say"EXIT:$exit";' OUT:Hello ERR:Die EXIT:1

    Enjoy, Have FUN! H.Merijn
Re: Override printing to STDOUT/ERR
by LanX (Bishop) on Apr 13, 2018 at 13:30 UTC
      yeah but what about the print from 3rd party packages?

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