Three argument version of open '>-' (updated)

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

In an ancient part of the code at work, we use

open my $output, '>-' or die $!;
[download]

Perl::Critic complains about it and wants us to replace it with a three argument variant. But, what's the 3-arg variant of >-?

I thought

open my $output, '>&', *STDOUT or die $!;
[download]

might be the same, but it isn't, cf.

open my $output, '>-' or die $!;
close *STDOUT;
print {$output} "This fails with Bad file descriptor";
[download]

versus

open my $output, '>&', *STDOUT or die $!;
close *STDOUT;
print {$output} "This prints OK\n";
[download]

Then I tried with

my $output = *STDOUT;
[download]

but it's different, too:

close STDOUT;
print {$output}
    'Besides Bad file descriptor, this will also warn "print() on clos
+ed filehandle STDOUT"';
[download]

Is there a three-argument version of open that behaves the same as '>-'?

Update: Here's a SSCCE. Closing STDOUT is a global state, so call this with an argument (1, 2, 3, 4) to test a particular implementation.

#!/usr/bin/perl
use strict;
use warnings;

sub original {
    open my $output, '>-' or die $!;
    $output
}

sub assign {
    my $output = *STDOUT;
    $output
}

sub dup {
    open my $output, '>&', *STDOUT or die $!;
    $output
}

sub fn {
    open my $output, '>>&=', *STDOUT->fileno or die $!;
    $output
}

use Test::More;

sub test {
    my ($open) = @_;
    my @W;
    local $SIG{__WARN__} = sub { push @W, @_ };

    my $output = $open->();
    close *STDOUT;
    my $v = print {$output} "abc\n";

    isnt $v, 1, 'fails';
    like $!, qr/Bad file descriptor/, 'Exception';
    is scalar @W, 0, 'no warnings';
    chomp, diag "($_)" for @W;
    done_testing();
}

my %dispatch = (1 => \&original,
                2 => \&assign,
                3 => \&dup,
                4 => \&fn);

my $what = shift;
my $code = $dispatch{$what} or die 'Not associated';
test($code);
[download]

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

Comment on Three argument version of open '>-' (updated) Select or Download Code

Replies are listed 'Best First'.
Re: Three argument version of open '>-' (de-critic) by LanX (Saint) on Mar 02, 2021 at 02:16 UTC
> Perl::Critic complains about it and wants us to replace it with a three argument variant. But, what's the 3-arg variant of >-? From a pragmatic point of view, I'd say: Adjust the Perl::Critic rule! I mean it's an interesting question, but probably it's not possible to duplicate all these exact implementation details. This particular 2 args syntax OTOH is documented and legit. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery} update and by all communist gods, why would you like to have a duplicate of STDOUT which fails globally without warning? Probably this feature is simply broken.	[reply]
Re^2: Three argument version of open '>-' (updated) by jcb (Parson) on Mar 02, 2021 at 02:41 UTC
Seconding this: `Perl::Critic` is simply wrong here. The primary risk of two-argument open is the possibility of a "strange" filename improperly triggering the magic (How about a "file" named '`rm -rf / \|`'?), but in this case the filename is a literal constant and that is not possible. This is a bug in Perl::Critic — your particular use of the two-argument syntax is fine.	[reply] [d/l]
Re: Three argument version of open '>-' by Fletch (Bishop) on Mar 01, 2021 at 22:23 UTC
If the OS provides it perhaps explicitly opening `/dev/stdout` as the filename? Or maybe no because that's going to behave like your "prints OK" block because it'd return a new descriptor on the device. Slightly ebil but maybe `open( my $output, ">>&=", STDOUT->fileno )` would get it? (I want to say that should create a new handle instance pointing to the same underlying descriptor which seems to be what >- was doing). The cake is a lie. The cake is a lie. The cake is a lie.	[reply] [d/l] [select]
Re: Three argument version of open '>-' by LanX (Saint) on Mar 01, 2021 at 22:21 UTC
Maybe try `open $fh, '>&', STDOUT;` behavior looks consistent when autoflush is activated. Otherwise please use a similar code-template for a SSCCE. `use v5.12; use warnings; use Data::Dump qw/pp dd/; sub tst { my $vers = shift; open STDOUT, ">-"; # restore STDOUT $\|=1; say STDERR "********* testing $vers\n"; my $fh; open $fh, ">-" if $vers eq "open2"; $fh = \STDOUT if $vers eq "copyref"; open $fh, '>&', STDOUT if $vers eq "redirect"; $fh->autoflush; say "$vers 1"; close STDOUT; say $fh "$vers 2"; say "$vers 3"; } tst("redirect"); tst("open2"); tst("copyref");` [download] -- mode: compilation; default-directory: "d:/tmp/pm/" -- Compilation started at Mon Mar 1 23:21:00 C:/Strawberry/perl/bin\perl.exe -w d:/tmp/pm/redirect_STDOUT.pl ****** testing redirect redirect 1 redirect 2 say() on closed filehandle STDOUT at d:/tmp/pm/redirect_STDOUT.pl line + 32. ***** testing open2 open2 1 open2 2 say() on closed filehandle STDOUT at d:/tmp/pm/redirect_STDOUT.pl line + 32. ******* testing copyref copyref 1 say() on closed filehandle STDOUT at d:/tmp/pm/redirect_STDOUT.pl line + 30. say() on closed filehandle STDOUT at d:/tmp/pm/redirect_STDOUT.pl line + 32. Compilation finished at Mon Mar 1 23:21:00 [download] Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply] [d/l] [select]
Re^2: Three argument version of open '>-' by choroba (Cardinal) on Mar 01, 2021 at 23:06 UTC
OP updated witn a SSCCE. `map{substr$_->[0],$_->[1]\|\|0,1}[\\|\|{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^ARGV,3]`	[reply] [d/l]
Re^3: Three argument version of open '>-' by LanX (Saint) on Mar 01, 2021 at 23:24 UTC
Thanks, it's clearer now, you want that the "copy FH" fails if STDOUT is closed, but without emitting a warning. Sorry, no clue yet... update BTW: you can automatize your test-runs with an `exec $0 #Test` if @ARGV is empty :) and maybe you should also consider testing different states of autoflush. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply] [d/l]
Re: Three argument version of open '>-' by LanX (Saint) on Mar 01, 2021 at 21:55 UTC
same behavior for me `DB<47> open $fh,">-" DB<48> say $fh "bla" bla DB<49> say "bla" bla DB<50> close STDOUT DB<51> say $fh "bla" DB<52>` [download] copy solution `DB<53> say "bla" bla DB<54> $fh=\*STDOUT # copy DB<55> say $fh "bla" bla DB<56> close STDOUT DB<57> say $fh "bla" DB<58>` [download] what am I missing? EDIT: Both approaches warn under warnings if STDOUT was closed. UPDATE I just remembered that the debugger does scary redirections under the hood, so these tests may not be reliable. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply] [d/l] [select]

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