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in reply to Re^3: Looking for suitable spells to get open to return a filehandle from a module
in thread SOLVED: Looking for suitable spells to get open to return a filehandle from a module

What is the effect of the invocation of Symbol::gensym?

In this case, just a lazily copied expression. Quoting Symbol:

Symbol::gensym creates an anonymous glob and returns a reference to it. Such a glob reference can be used as a file or directory handle.

And that's it. It was needed in ancient perls. Perl 5.6.1 introduced autovivication for file and directory handles (see perl561delta), so Symbol::gensym is technically no longer needed. open my $fh, ... or open local $fh, ... is sufficient.

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

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Re^5: Looking for suitable spells to get open to return a filehandle from a module
by pryrt (Abbot) on Dec 05, 2016 at 21:40 UTC
    ... so Symbol::gensym is technically no longer needed

    But there are times when it's still useful... like when you want open3() to keep STDOUT and STDERR separate (by default, if you pass it undef for both STDOUT and STDERR, STDERR will be directed to the same filehandle that's created for STDOUT)

    #!/usr/bin/env perl
use warnings;
use strict;
use IPC::Open3;
use Symbol qw(gensym);
use POSIX qw(:sys_wait_h);

# I'm sometimes using perl 5.8.5, so //'<undef>' unavailable
# verified later on 5.14.2
sub defor($) { defined($_[0]) ? $_[0] : '<undef>' }

foreach my $run (1..3) {
    my ($out, $err) = ('','');
    my ($fhi, $fho, $fhe) =
        ($run == 1) ? (undef, undef, undef) :           # when fhe==un
+def goes to open3, STDERR will be redirected to STDOUT
        ($run == 2) ? (undef, undef, gensym) :          # this keeps S
+TDERR and STDOUT separate
                      (gensym, gensym, gensym) ;        # as does this
+, the first two gensym are unecessary, but the third is needed

    my $pid = open3( $fhi, $fho, $fhe, qq[cat - ; perl -le 'print STDE
+RR "this should be STDERR (run#$run)...\nand so should this (run#$run
+)\n"'] );
    print { $fhi } "[run#$run] This should be STDOUT\n";
    close($fhi);

    $out .= join '', map { s/^/STDOUT> / if $_ ne $/; $_ } readline($f
+ho)     if defined $fho;
    $err .= join '', map { s/^/STDOUT> / if $_ ne $/; $_ } readline($f
+he)     if defined $fhe;
    print "Read STDOUT:\n$out\n" if length($out);
    print "Read STDERR:\n$err\n" if length($err);
    printf "run#%d --- fhi='%s' fho='%s' fhe='%s'\n", $run, defor $fhi
+, defor $fho, defor $fhe;
    print "="x75, "\n\n";
}

__END__
Read STDOUT:
STDOUT> [run#1] This should be STDOUT
STDOUT> this should be STDERR (run#1)...
STDOUT> and so should this (run#1)


run#1 --- fhi='GLOB(0x92fdba4)' fho='GLOB(0x92fdbcc)' fhe='<undef>'
======================================================================
+=====

Read STDOUT:
STDOUT> [run#2] This should be STDOUT

Read STDERR:
STDOUT> this should be STDERR (run#2)...
STDOUT> and so should this (run#2)


run#2 --- fhi='GLOB(0x93811c0)' fho='GLOB(0x932bc60)' fhe='GLOB(0x92fd
+ba4)'
======================================================================
+=====

Read STDOUT:
STDOUT> [run#3] This should be STDOUT

Read STDERR:
STDOUT> this should be STDERR (run#3)...
STDOUT> and so should this (run#3)


run#3 --- fhi='GLOB(0x93811c0)' fho='GLOB(0x932bc60)' fhe='GLOB(0x92fd
+ba4)'
======================================================================
+=====

    [download]

    Yes, I know the OP didn't include open3, but since I was recently learning the ins and outs of open3 filehandles, I thought I'd mention this exception to your general rule, for posterity's sake.

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