in reply to Re: STDERR going to string
in thread STDERR going to string

I had something like this too but it doesn't work on Windows. Here is a stripped down example using Inline::C. On non-Windows (cygwin included), it outputs correctly: |ERROR| On Windows, it gives: || ERROR which shows you that the STDERR isn't being captured. 
use File::Temp qw(tempfile);
use Inline C;

open my $olderr, '>&STDERR'; # save STDERR
my ($fh, $fn) = tempfile();
open STDERR, '>&', $fh;

test_err();

open STDERR, '>&', $olderr; # reset STDERR
close $fh;

open my $e, '<', $fn;
my $err = <$e>;
print "|$err|\n";

__END__
__C__
void test_err()
{
(void)fprintf (stderr, "ERROR");
}

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Re^3: STDERR going to string
by salva (Canon) on Oct 11, 2011 at 14:20 UTC
    Are you using ActiveState or Strawberry Perl? and which version?

    It seems that there is a missmatch between what Perl sees as its file descriptor 2 and the C lib file descriptor 2. 

    use File::Temp qw(tempfile);
use Inline 'C';

open my $olderr, '>&STDERR'; # save STDERR
my ($fh, $fn) = tempfile();
open STDERR, '>&', $fh;
printf "fileno STDERR: %d\n", fileno(STDERR);
print STDERR "foo!";
test_err();
open STDERR, '>&', $olderr; # reset STDERR
close $fh;
open my $e, '<', $fn;
my $err = <$e>;
print "|$err|\n";

__END__
__C__
void test_err() {
    FILE *err = fdopen(2, "a+");
    (void)fprintf (err, "ERROR");
}

# here it says:
# fileno STDERR: 2
# |foo!|
# ERROR

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    Besides that, don't trust what you get from Inline::C, the perlio.h header is included before your C code and most stdio functions are replaced by macros calling into perl own implementations (that not-too-unsurprisingly, do not work as expected either):

    Read more... (723 Bytes)

    update: the funny thing is that when a subprocess is launched, perl file descriptors are the ones inherited:

    Read more... (792 Bytes)
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      Hmm. On Windows I'm currently using a self-compiled 5.14.1 using the strawberry environment but I see the same with "real" strawberry 5.12.2. It seems that re-opening STDERR on windows decouples perl STDERR and the C library STDERR which I can't quite understand since I assumed that they were both fd=2 and I can redirect them both with 2>somewhere on the command line. It's just that perl doesn't see the C library STDERR as STDERR any more. The actual library is using really basic stdio ( fprint(stderr ... ) which is most certainly fd=2. Can perl and C have "different" STDERRs on windows?
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        Can perl and C have "different" STDERRs on windows?

        No. There is only one STDERR on windows and it is always fileno 2. Perl wraps things so that stderr is a variable, but fileno 2 is always the original stderr.

        AFAIK, despite claims to the contrary, there is no way to get it to work on win32.

        If it was possible it would work already :)

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