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desire to pass file handles by value

by mandog (Curate)
on Nov 07, 2003 at 02:22 UTC ( #305217=perlquestion: print w/ replies, xml ) Need Help??
mandog has asked for the wisdom of the Perl Monks concerning the following question:

I'd like to pass file handles to subs by value. The FAQ gave me a recipe but the position of the file gets changed.

#!/usr/bin/perl -w use strict; use 5.6.1; sub find { local *FH = shift; my $find_me = shift; my $count = 0; my $tell = tell(FH); while (<FH>) { $count++ if (/$find_me/); } return "count: $count tell: $tell\n"; } open OUT, '>', '/tmp/tmp.txt' or die "$!\n"; print OUT while (<DATA>); close OUT or die "$!\n"; open IN, '/tmp/tmp.txt' or die "$!\n"; print find( *IN, 'a' ); print find( *IN, 'a' ); print find( *IN, 'd' ); print find( *IN, 'e' ); #count: 3 tell: 0 #count: 0 tell: 13 #count: 0 tell: 13 #count: 0 tell: 13 __DATA__ a a a c d e

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Re: desire to pass file handles by value
by Roger (Parson) on Nov 07, 2003 at 02:42 UTC
    I think you *need* to rewind the file by calling seek FH, 0, 0; in the subroutine find.

    The file operation in Perl is ultimately linked to the underlying unix file handles. When you read from a Perl file handle, Perl uses and updates the underlying unix file handle, thus giving the side effect.

    I think ideally you are looking for the Perl's equivalent to the C stdio's dup function, to duplicate/copy an existing file handle into a new file handle (not just an alias to the existing file handle, but a new independent file handle), and operate on the second file handle in the subroutine. I am not aware of such facility in Perl 5 (my lack of research perhaps), but I think Perl 6 has implimented the dup function for the File Object.

    Update:
    Ok, I just learned that you can duplicate a file handle in Perl with open NEW, "<&OLD";. I quickly came up with the following code -

    #!/usr/bin/perl -w use strict; sub find { local *IN = shift; my $find_me = shift; my $count = 0; # duplicate existing file handle open F2, '<&', *IN or die "Can not duplicate file handle"; my $tell = tell(F2); while (<F2>) { $count++ if (/$find_me/); } return "count: $count tell: $tell\n"; close F2; } open OUT, '>', 'tmp.txt' or die "$!\n"; print OUT while (<DATA>); close OUT or die "$!\n"; open IN, 'tmp.txt' or die "$!\n"; print find( *IN, 'a' ); print find( *IN, 'a' ); print find( *IN, 'd' ); print find( *IN, 'e' ); __DATA__ a a a c d e
    The output is still -
    #count: 3 tell: 0 #count: 0 tell: 13 #count: 0 tell: 13 #count: 0 tell: 13
    That didn't work either! Ok, that taught me a lession - my assumption on the duplicated file handle could be wrong. I need some re-education.

    Fellow monks, could you please tell me what is wrong with the duplicated file handle? Is perl actually creating a second independent file handle? Am I doing the right thing at all?

    Thanks!

      Thanks for the help. For now, I'm going to record the orignal file position on entry to the sub & restore it on exit.

      #... my $pos=tell(FH); #... seek(FH,$pos,0); return

      The file handle duplicating thing looks interesting though...

      update: fixed dumb semantic error

        Now if only you could localize a variable that represents the file position, you wouldn't have to manually save/restore its value. Hmmm....
        package Tie::Scalar::FHPos; use FileHandle; sub TIESCALAR { my( $pkg, $fh ) = @_; bless [$fh], $pkg; } sub FETCH { my( $self ) = @_; $self->[0]->tell } sub STORE { my( $self, $pos ) = @_; $self->[0]->seek($pos,0); }
        Test:
        open F, "< foo.dat"; our $pos; tie $pos, 'Tie::Scalar::FHPos', \*F; { local $pos = 20; print "$pos: ", scalar(<F>); } print "$pos: ", scalar(<F>);
        Cool!

        jdporter
        The 6th Rule of Perl Club is -- There is no Rule #6.

      The file operation in Perl is ultimately linked to the underlying unix file handles. When you read from a Perl file handle, Perl uses and updates the underlying unix file handle, thus giving the side effect.
      Well not on windows, or other non-unix operating systems ....
      Update: Ok, I just learned that you can duplicate a file handle in Perl with open NEW, "<&OLD";. I quickly came up with the following code -

      From a Linux man page...

      After successful return of dup or dup2, the old and new descriptors may be used interchangeably. They share locks, file position pointers and flags; for example, if the file position is modified by using lseek on one of the descriptors, the position is also changed for the other.
      --
      James Antill
Re: desire to pass file handles by value
by pg (Canon) on Nov 07, 2003 at 03:12 UTC

    Besides what Roger said, if this is all what you want to do, you can:

    use Data::Dumper; use strict; use warnings; my @lines = <DATA>; my $count = {}; map {chomp;$count->{$_} ++} @lines; print Dumper($count); __DATA__ a a a c d e

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