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Re: Distinguishing a filehandle for an in-memory string

by roboticus (Chancellor)
on May 18, 2018 at 01:26 UTC ( #1214806=note: print w/replies, xml ) Need Help??


in reply to Distinguishing a filehandle for an in-memory string

jrw:

You could do something like this:

sub is_FH_a_wrapped_scalar { my $FH = shift; return grep { $_ eq "scalar} } PerlIO::get_layers($FH); }

This uses PerlIO to find the layers of I/O handlers on your filehandle. If one of them is named "scalar", then the filehandle is ultimately wrapping a scalar variable.

Unfortunately, this is the only way I've found to do it, and PerlIO might not be compiled in all versions you use.

$ perl pm_1214805.pl FH is a wrapped scalar, do something else! <read><5></etc/> <sysread><5><SYSTE> <read><5><SYSTE> $ cat pm_1214805.pl #!/usr/bin/perl use strict; use warnings; sub is_fh_a_wrapped_scalar { my $fh = shift; my @layers = PerlIO::get_layers($fh); return grep { $_ eq "scalar" } @layers; } sub dbg { my ($op, $fh, $rc, $scalar) = @_; $rc = "UNDEF" unless defined $rc; print "<$op><$rc><$scalar>\n"; close $fh or die; } sub doit_read { my ($fh) = @_; my $rc = read $fh, my $scalar, 5; dbg "read", $fh, $rc, $scalar; } sub doit_sysread { my ($fh) = @_; if (is_fh_a_wrapped_scalar($fh)) { print "FH is a wrapped scalar, do something else!\n"; } else { my $rc = sysread $fh, my $scalar, 5; dbg "sysread", $fh, $rc, $scalar; } } my $fh; open $fh, "<", \"/etc/passwd" or die; doit_sysread $fh; open $fh, "<", \"/etc/passwd" or die; doit_read $fh; open $fh, "<", "/etc/passwd" or die; doit_sysread $fh; open $fh, "<", "/etc/passwd" or die; doit_read $fh;

...roboticus

When your only tool is a hammer, all problems look like your thumb.

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Re^2: Distinguishing a filehandle for an in-memory string
by jrw (Monk) on May 18, 2018 at 03:21 UTC
    Thanks, I'll check out how get_layers() is implemented. If I can figure out how to unwrap it myself (in Pure Perl), then I will use that. I tried looking at it today for a bit, but couldn't figure out how to unpeel what the $fh typeglob contained.

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