(Update: Wrapped the redirecting code into a
convenient, reusable function.)
The following code shows one way of doing what you want in 5.6.1, all
wrapped into a tidy function. It does use a temporary file, but the
file is cleaned up automatically, and File::Temp uses a
random file name to protect against certain kinds of file-based
attacks.
use File::Temp qw( tempfile );
sub capture_output {
my $target_fh = shift;
my $temp_fh = tempfile();
my $temp_fd = fileno $temp_fh;
local *SAVED;
local *TARGET = $target_fh;
open SAVED, ">&TARGET" or die "can't remember target";
open TARGET, ">&=$temp_fd" or die "can't redirect target";
my $saved_fh = *SAVED;
return sub {
seek $temp_fh, 0, 0 or die "can't seek"; # rewind
my $captured_output = do { local $/; <$temp_fh> };
close $temp_fh or die "can't close temp file handle";
local (*SAVED, *TARGET) = ($saved_fh, $target_fh);
open TARGET, ">&SAVED" or die "can't restore target";
close SAVED or die "can't close SAVED";
return $captured_output;
}
}
With this code in place, capturing output from any filehandle
becomes easy:
print STDERR "before redirection\n";
my $recorder = capture_output(*STDERR); # start
print STDERR "during redirection\n";
my $saved_output = $recorder->(); # stop
print STDERR "after redirection\n";
print "Saved output = $saved_output";
The output is what you would expect (tested on Perl v5.6.1 built for
i386-linux):
before redirection
after redirection
Saved output = during redirection
Sidebar: I don't know why you think a file-based
solution is a hack. Unless you have a really good reason not to use a
temp file, you'll probably find it to be the most practical,
general-purpose solution.
Because you want to capture what is written
to STDERR, you must preserve typical file-descriptor
semantics. Using a temp file gives you this behavior for free.
Yes, you could try to use a tied filehandle, but the 5.6.1
perltie docs say that tied-filehandle support is only "partially
implemented." More importantly, you'll be hosed if what you're
testing ends up writing to the stderr file descriptor directly,
bypassing your filehandle. This can easily happen if your code uses
external libraries, which just about all code does implicitly
via the standard C libraries that Perl links against.
Given that the temp-file solution will work in all situations
and has the advantage of being drop-dead simple, my recommendation
is to use it. Only if it proves inadequate in actual
practice should you worry about more complicated solutions.
Cheers, Tom
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