Creating a new file descriptor from stdout with fcntl fails in files

Question

I have a simple test file that looks like this:

use v6.c;
use NativeCall;

sub fcntl(int32, int32 --> int32) is native { * }
sub close(int32 --> int32) is native { * }

my $fd := fcntl($*OUT.native-descriptor, 0);
say $fd;
close($fd);

The file descriptor gets returned is -1, which is not what I want. But when I run the same code in a REPL, I get what I'm looking for:

> use NativeCall
Nil
> sub fcntl(int32, int32 --> int32) is native { * }
sub fcntl (int32 $, int32 $ --> int32) { #`(Sub+{Callable[int32]}+{NativeCall::Native[Sub+{Callable[int32]},Str]}|17126514527616) ... }
> sub close(int32 --> int32) is native { * }
sub close (int32 $ --> int32) { #`(Sub+{Callable[int32]}+{NativeCall::Native[Sub+{Callable[int32]},Str]}|17126514527904) ... }
> my $fd := fcntl($*OUT.native-descriptor, 0)
15
> say $fd
15
> close($fd)
0

Why doesn't fcntl create a new file descriptor in the file like it does in the REPL?

Edit: I'm running OpenBSD 6.2, Rakudo 2018.02

Works here. I am using Ubuntu 17.10. Rakudo version 2018.02.1 — Håkon Hægland, Mar 30 '18 at 13:17
That script outputs `13` on two distinct local CentOS 7 clusters running rakudo-star-2018.01. — Christopher Bottoms, Mar 30 '18 at 15:12
This ended up being a difference in how OpenBSD handles fcntl compared to Linux. I'll explain in the answer — Kaiepi, Mar 30 '18 at 15:44

score 8 · Accepted Answer · edited Mar 30 '18 at 16:54

8

When using the F_DUPFD flag with fcntl (which was 0), OpenBSD requires that the status flags the original file descriptor had be passed on to the new one as well. So this will work:

use v6.c;
use NativeCall;

constant F_DUPFD = 0;
constant F_GETFD = 1;

sub fcntl(int32, int32, int32 --> int32) is native { * }
sub close(int32 --> int32) is native { * }

sub MAIN() {
    my $out-fd := $*OUT.native-descriptor;
    my $flags  := fcntl($out-fd, F_GETFD, 0);
    my $fd     := fcntl($out-fd, F_DUPFD, $flags);
    say $fd; # 15
    close($fd);
}

edited Mar 30 '18 at 16:54

Christopher Bottoms

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answered Mar 30 '18 at 15:55

Kaiepi

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Nice! Would you please post a little info on how you found the answer? – Christopher Bottoms Mar 30 '18 at 16:11

Creating a new file descriptor from stdout with fcntl fails in files

1 Answers1