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in reply to Re^2: Introspecting the current Perl file via DATA, even w/o DATA?
in thread Introspecting the current Perl file via DATA, even w/o DATA?

Because the filepointer is in sync with the parsing phases of Perl
Not necessarily, no... Let's see 
BEGIN {
    print "BEGIN\n";
}
UNITCHECK {
    print "UNITCHECK\n";
}
CHECK {
    print "CHECK\n";
}
INIT {
    print "INIT\n";
}
print "MAIN\n";

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Let's strace it (Debian Jessie, Perl 5.22) 
open("t.pl", O_RDONLY)                  = 3
...
read(3, "BEGIN {\n    print \"BEGIN\\n\";\n}\nU"..., 8192) = 146
write(1, "BEGIN\n", 6)                  = 6
read(3, "", 8192)                       = 0
close(3)                                = 0
write(1, "UNITCHECK\n", 10)             = 10
write(1, "CHECK\n", 6)                  = 6
write(1, "INIT\n", 5)                   = 5
write(1, "MAIN\n", 5)                   = 5

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Note that the file is closed right after BEGIN (and before UNITCHECK). So no handle to it can exist. But, if we add 
__DATA__
whatever

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that it changes a bit... 
open("t.pl", O_RDONLY)                  = 3
...
read(3, "BEGIN {\n    print \"BEGIN\\n\";\n}\nU"..., 8192) = 165
write(1, "BEGIN\n", 6)                  = 6
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
write(1, "UNITCHECK\n", 10)             = 10
write(1, "CHECK\n", 6)                  = 6
write(1, "INIT\n", 5)                   = 5
write(1, "MAIN\n", 5)                   = 5
...
close(3)                                = 0
exit_group(0)                           = ?

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Now the file isn't closed until exit. So without __DATA__ there is no handle to the file, and the only handle (that I can find :) is DATA.

Anyway, let us know if you'll find something in perl5db.pl or somewhere else.

Replies are listed 'Best First'.
Re^4: Introspecting the current Perl file via DATA, even w/o DATA? (perl5db.pl)
by LanX (Saint) on Feb 05, 2016 at 15:14 UTC
    > Anyway, let us know if you'll find something in perl5db.pl or somewhere else.

    see http://cpansearch.perl.org/src/SHAY/perl-5.22.1/lib/perl5db.pl

    line 405 

    =head4 C<@dbline>

Local alias to the magical line array, C<@{$main::{'_<'.$filename}}> ,
supplied by the Perl interpreter to the debugger. Contains the source.

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    and line 176 

    =head1 DATA STRUCTURES MAINTAINED BY CORE

There are a number of special data structures provided to the debugger
+ by
the Perl interpreter.

The array C<@{$main::{'_<'.$filename}}> (aliased locally to C<@dbline>
via glob assignment) contains the text from C<$filename>, with each
element corresponding to a single line of C<$filename>.

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    Seems to be magic. Dunno if it depends on debug flags or just the context of the DB package like other magic (e.g. @DB::args )

    Anyway, let us know if you'll find something details about that magic. ;-).

    Cheers Rolf
    (addicted to the Perl Programming Language and ☆☆☆☆ :)
    Je suis Charlie!

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      Anyway, let us know if you'll find something details about that magic. ;-)
      You're so lazy, LanX ;) I'm sure you could do it yourself, but if you insist... 
      # main.pl
BEGIN {
    $^P = 0x400;
}
use test ();

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      and 
      # test.pm
my @source = @{ $main::{ '_<' . __FILE__ } };
print for @source;
1;

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      Now, the thing is, you have to set $^P before the file (test.pm in this case) is even opened. So you can't do it inside test.pm itself, I don't think...

      Is that good enough for you?

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        I found this interesting so I played with it briefly...

        # main.pl
my @source = @{ $main::{ '_<' . __FILE__ } };
shift @source; # get rid of sub DB::DB{};
print for @source;

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        and 
        PERL5DB="sub DB::DB{}" perl -d main.pl

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        Output: 

        my @source = @{ $main::{ '_<' . __FILE__ } };
shift @source; # get rid of sub DB::DB{};
print for @source;

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        Update: I had found @{$main::{'_<'.$filename}} yesterday but didn't have time to play with it.

        Update 2: Added shift.

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        > Is that good enough for you?

        yes, I knew I can count on your collaboration! ;)

        > Now, the thing is, you have to set $^P before the file (test.pm in this case) is even opened.

        I think one could trick that², the question is rather about other penalties included.

        ... let's have a look into perlvar#$^P ...

            x400

    Save source code lines into @{"_<$filename"} .

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        OK this doesn't look to expensive ... benchmark needed.

        (too busy now ;-)

        Cheers Rolf
        (addicted to the Perl Programming Language and ☆☆☆☆ :)
        Je suis Charlie!

        footnotes

        ²) for instance IIRC it's possible to configure global setting for your perl interpreter.

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Re^4: Introsepcting the current Perl file via DATA, even w/o DATA?
by Anonymous Monk on Feb 05, 2016 at 02:28 UTC
    To clarify, the source filter thing should work, and maybe there is a way to make the file to stick around after BEGIN (without __DATA__), but in my opinion that's a bit too obscure and I don't see anything particularly complex or performance-intensive in just opening it again... but YMMV.
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[d/l]
      I think it's the only way, and a quick open if done at the right time, won't penalize the objective.

      Update: What you've shown here is very intriguing. Why are you Anonymonk?

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[d/l]
        Why are you Anonymonk?
        Well, considering that this is the only forum that I post on... I guess if I had to register, I wouldn't bother...
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