read closed directory handle?

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

Why does this work?

#!/usr/bin/perl -w
use strict;

opendir (TOPDIR,"$ENV{HOME}")
   or die "Thus shall ye likewise perish: $!";
close TOPDIR;
while ($_=readdir(TOPDIR)){
  print "$_\n";
}
[download]

This will read the _closed_ directory handle, without an error, if invoked as % perl test_file (where test_file) contains the above code.

But, I just discovered that if I chmod 0755 test_file, the code fails without a warning when invoked directly. So, I'm more confused than ever. What is happening here?

mkmcconn

Comment on read closed directory handle? Download Code

Replies are listed 'Best First'.
Re: read closed directory handle? by KM (Priest) on Jan 27, 2001 at 02:41 UTC
You did a close() on a filehandle which is a directory. You need closedir(). Cheers, KM	[reply]
Re: read closed directory handle? by lemming (Priest) on Jan 27, 2001 at 02:42 UTC
You're closing the FILEHANDLE with close. I don't think a warning is generated when trying to close a directory handle. If you use closedir, you'll wind up with nothing printed. Update: It doesn't hurt to check if you successfully close file or directory handles. That would show the error with the close, instead of closedir. My guess on why warnings and strict didn't show anything is that you used the variables involved, but that's only a guess. readline will bring up a warning on a closed filehandle, but readdir doesn't seem to care that's it tries reading a closed directory handle.	[reply]
Re: read closed directory handle? by mkmcconn (Chaplain) on Jan 27, 2001 at 03:01 UTC
Thanks to KM and lemming for the speedy reply. I didn't realize that I couldn't resubmit the post, or I would have cleaned up the errors I made in testing the code as posted, and I would have rephrased the question: "Why didn't this do what I expect". It will, of course, run when invoked directly. The point my two helpers made, is the lesson to take away: that close(FILEHANDLE) and closedir(DIRHANDLE) are not the same thing, FILEHANDLE and DIRHANDLE have separate namespaces (and thou shalt thoroughly test all operations, with warnings) Thanks again, brethren. mkmcconn	[reply]


Think about Loose Coupling
	PerlMonks