In memory filehandles

polettix has asked for the wisdom of the Perl Monks concerning the following question:

Esteemed Monks,

I'm having a bit of troubles with in-memory filehandles, which seem to change shape behind the scenes, as the following example code seems to demonstrate.

#!/usr/bin/perl
use strict;
use warnings;
 
use IO::Handle;
 
my $data = 'some sample data here';
 
open my $fh, '<', \$data or die "open(): $!";
print {*STDERR} 'ref $fh yields ', ref $fh, "\n";
print_prop('$fh->can("opened")', $fh->can('opened'));
print_prop('$fh->can("seek")', $fh->can('opened'));
print_prop('$fh->can("seekable")', $fh->can('opened'));
print_prop('$fh is IO::Handle', $fh->isa('IO::Handle'));
print_prop('$fh->can("opened")', $fh->can("opened"));
print_prop('$fh->opened()', $fh->opened());
print_prop('$fh->can("seek")', $fh->can("seek"));
eval {print_prop('$fh->seek(0, 0)', $fh->seek(0, 0));} or print "$@";
print_prop('$fh is tied', tied $fh);
close $fh;
 
sub print_prop {
   my ($msg, $flag) = @_;
   print {*STDERR} $msg, '? ', ($flag ? 'yes' : 'no'), "\n";
   return;
}
 
__END__
ref $fh yields GLOB
$fh->can("opened")? no
$fh->can("seek")? no
$fh->can("seekable")? no
$fh is IO::Handle? no
$fh->can("opened")? no
$fh->opened()? yes
$fh->can("seek")? no
Can't locate object method "seek" via package "IO::Handle" at memhandl
+e.pl line 19.
$fh is tied? no
[download]

Good monk dada suggested that there is a fallback to IO::Handle somewhere, and this makes sense from what I see. Now I would like to understand:

is this theory correct?
is IO::Handle the only possible fallback?
why the fallback class doesn't support seek() and tell()? I'm able to use seek() and tell() functions on them! (e.g. seek($fh, 0, SEEK_SET); works)

Any light is highly appreciated :)

Flavio
perl -ple'$_=reverse' <<<ti.xittelop@oivalf

Don't fool yourself.

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Re: In memory filehandles by ysth (Canon) on Jan 26, 2006 at 11:56 UTC
The method-calls-on-filehandles work because perl makes any filehandle an IO::Handle (or a FileHandle if you've loaded FileHandle, but that's just a wrapper around IO::Handle now I think). But IO::Handle doesn't "handle" everything; in particular, it's not an IO::Seekable. Because it perl does this across the board for every filehandle, it can't assume that it is seekable.	[reply]
Re^2: In memory filehandles by polettix (Vicar) on Jan 26, 2006 at 13:15 UTC
So, this basically means that there aren't any hooks to plug IO::File instead of IO::Handle where needed. As a workaround, I'm using something like this: `sub open_memory { my ($mode, $scalar_ref) = @_; open my $fh, $mode, $scalar_ref or die "open(): $!"; my $retval = IO::File->new_from_fd($fh, $mode) or die "IO::File->new_from_fd() failed"; return $retval; } ## end sub open_memory` [download] This makes clear that in-memory filehandles aren't a real substitute for IO::String-s as I used to believe. Flavio perl -ple'$_=reverse' <<<ti.xittelop@oivalf Don't fool yourself.	[reply] [d/l]
Re^3: In memory filehandles by ysth (Canon) on Jan 26, 2006 at 15:39 UTC
Or you could just `use FileHandle;`. Or you could `push @IO::Handle::ISA, "IO::File"`. Or call `$fh->IO::File::seek(...)`. Except for giving easy access to the per-filehandle punctuation variables, I think the whole "let's pretend filehandles are objects" thing is pretty silly.	[reply] [d/l] [select]
Re^4: In memory filehandles by polettix (Vicar) on Jan 27, 2006 at 09:38 UTC
Re^5: In memory filehandles by ysth (Canon) on Jan 27, 2006 at 10:31 UTC
Some notes below your chosen depth have not been shown here
Re: In memory filehandles by crouchingpenguin (Priest) on Jan 27, 2006 at 01:45 UTC
Use IO::Scalar for this. #!/usr/bin/perl use strict; use warnings; use IO::Scalar; my $data = 'some sample data here'; my $fh = IO::Scalar->new(\$data); print {STDERR} 'ref $fh yields ', ref $fh, "\n"; print_prop('$fh->can("opened")', $fh->can('opened')); print_prop('$fh->can("seek")', $fh->can('opened')); print_prop('$fh->can("seekable")', $fh->can('opened')); print_prop('$fh is IO::Scalar', $fh->isa('IO::Scalar')); print_prop('$fh->can("opened")', $fh->can("opened")); print_prop('$fh->opened()', $fh->opened()); print_prop('$fh->can("seek")', $fh->can("seek")); eval {print_prop('$fh->seek(0, 0)', $fh->seek(0, 0));} or print "$@"; print_prop('$fh is tied', tied $fh); close $fh; sub print_prop { my ($msg, $flag) = @_; print {STDERR} $msg, '? ', ($flag ? 'yes' : 'no'), "\n"; return; } [download] which yields `ref $fh yields IO::Scalar $fh->can("opened")? yes $fh->can("seek")? yes $fh->can("seekable")? yes $fh is IO::Scalar? yes $fh->can("opened")? yes $fh->opened()? yes $fh->can("seek")? yes $fh->seek(0, 0)? yes $fh is tied? no` [download] cp ---- "Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic."	[reply] [d/l] [select]
Re^2: In memory filehandles by polettix (Vicar) on Jan 27, 2006 at 09:26 UTC
crouchingpenguin, thanks for the suggestion. I don't like the IO::Scalar solution very much, for the same reasons I don't like using IO::String in perl 5.8.x: in-memory filehandles should work out of the box (at least this is what I thought). I tried to code a quick solution here just to work around the problem without the need to install yet another module. There is also another reason why I don't like IO::Scalar very much: it does not work with Archive::Zip. This is where all really started: you can't pass an in-memory filehandle to Archive::Zip, simply because it contains a seekability test that will fail. And, sadly enough, it will explicitly fail for IO::Scalar as well. Flavio perl -ple'$_=reverse' <<<ti.xittelop@oivalf Don't fool yourself.	[reply]