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

This is related to my earlier post, Teeing STDOUT and STDERR to files using filehandle references, from about a month ago.
I'm redirecting STDOUT and STDERR to files with code similar (but with more function calls in the way) to this: 
$this->{'stdout_log'} = \*FH_STDOUT; # These files have already been o
+pened.
$this->{'stderr_log'} = \*FH_STDERR;

# Save the previous filehandles for STDOUT and STDERR so we
# can restore them when we're done.
$this->{'_stdout'} = \*STDOUT;
$this->{'_stderr'} = \*STDERR;

# Set the new filehandles for STDOUT and STDERR
*STDOUT = $this->{'stdout_log'};
*STDERR = $this->{'stderr_log'};

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Later, I try to restore STDOUT and STDERR before I close the logfiles: 
if(defined($this->{'_stdout'}))
{
    *STDOUT = $this->{'_stdout'};
    print STDOUT "STDOUT restored\n"; # Filehandle explicitly specifie
+d for debugging only...
    undef($this->{'_stdout'});
}
if(defined($this->{'_stderr'}))
{
    *STDERR = $this->{'_stderr'};
    print STDERR "STDERR restored\n";
    undef($this->{'_stderr'});
}

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I successfully log STDOUT/ERR to a file, but when I attempt to restore them later, the if blocks evaluate, but the print statements still go to the logfiles, instead of the TTY. Following that, of course, when I close the files and attempt to print to STDOUT and STDERR, I get: 
print() on closed filehandle main::STDOUT at FSLAMTest.pm line 312.

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It would seem that my attempt to preserve the original STDOUT and STDERR is not successful. Any suggestions? TIA.

--isotope
http://www.skylab.org/~isotope/

Replies are listed 'Best First'.
Re: Restoring STDOUT and STDERR after having redirected them to files
by isotope (Deacon) on Jan 13, 2001 at 00:19 UTC
    chipmunk accurately identified the source of my problem. I had thought I was copying the filehandles, but I was actually dup'ing globs, so I was stomping the filehandle value anyway. Unfortunately, the fileno approach is only useful if I use it everywhere in my code, which I didn't, for other reasons. I did, however, manage to put together a solution that works exactly as desired, massaging things into the approach suggested in perlfunc:open: 
    # Save the previous filehandles for STDOUT and STDERR so we
# can restore them when we're done.
local *REAL_STDOUT;
local *REAL_STDERR;
open(REAL_STDOUT, ">&STDOUT");
open(REAL_STDERR, ">&STDERR");
$this->{'_stdout'} = *REAL_STDOUT;
$this->{'_stderr'} = *REAL_STDERR;

# Set the new filehandles for STDOUT and STDERR
local *STDOUT_LOG;
local *STDERR_LOG;
*STDOUT_LOG = $this->{'stdout_log'};
*STDERR_LOG = $this->{'stderr_log'};
open(STDOUT, ">&STDOUT_LOG");
open(STDERR, ">&STDERR_LOG");

    [download]
    I can restore STDOUT and STDERR later with this code: 
    if(defined($this->{'_stdout'}))
{
    local *REAL_STDOUT;
    *REAL_STDOUT = $this->{'_stdout'};
    open(STDOUT, ">&REAL_STDOUT") or warn "Couldn't restore STDOUT: $!
+";
    print STDOUT "STDOUT restored\n";
    undef($this->{'_stdout'});
}
if(defined($this->{'_stderr'}))
{
    local *REAL_STDERR;
    *REAL_STDERR = $this->{'_stderr'};
    open(STDERR, ">&REAL_STDERR") or warn "Couldn't restore STDERR: $!
+";
    print STDOUT "(STDERR restored)\n";
    print STDERR "STDERR restored\n";
    undef($this->{'_stderr'});
}

    [download]
    Everything's happy now.

    --isotope
    http://www.skylab.org/~isotope/
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Re: Restoring STDOUT and STDERR after having redirected them to files
by chipmunk (Parson) on Jan 12, 2001 at 22:46 UTC
    Have you reviewed the examples in the documentation for open on saving and restoring STDOUT and STDERR? I suspect that your glob assignments are overwriting each other, rather than saving the old values.
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[reply]
        Ah... But, reading a little further down in the documentation for open... 
            If you specify '<&=N', where N is a number, then Perl will do an
    equivalent of C's fdopen() of that file descriptor; this is more
    parsimonious of file descriptors.  For example:
 
       open(FILEHANDLE, "<&=$fd")

        [download]
        So, using fileno() on your globs, you can dup your filehandles like in the open example, except with the numbers instead of the names! An updated snippet from your code: 
        # restore STDOUT
my $fd = fileno($this->{'_stdout'});
open(STDOUT, ">&=$fd");
print STDOUT "STDOUT restored\n";

        [download]
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