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

I'm tearing my hair out over what should be a Perl 101 issue, and just can't tell what's going on. I have a script that processes some stuff, and then sends a directory over to a separate module for further processing. The module is unable to read the directory, and I don't know why. The code (so far!) is incredibly simple: 
sub process_directory {
    my ($self, $dir) = @_;

    print "TESTING: dir is [$dir]\n";

    open my $dh, $dir or die "Could not open dir [$dir] for reading: $
+!\n";
    my @files = readdir $dh;

    print "FILES: @files\n";

    closedir $dh;
}

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When I run this, I get: 
TESTING: dir is [/tmp/testdirectory]
readdir() attempted on invalid dirhandle $dh at /path/to/MyModel.pm li
+ne 14.
FILES: 
closedir() attempted on invalid dirhandle $dh at /path/to/MyModel.pm l
+ine 18.

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/tmp/testdirectory exists, it's a directory, and the caller has permissions to read it.

I'm using strict and warnings. I don't get this--the open didn't fail; what's the problem here? I've searched for this, and the usual error is that people try reading a file as a directory or something like that; that's not the case here.

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Re: "readdir() attempted on invalid dirhandle" error?
by Paladin (Vicar) on Jul 10, 2024 at 20:05 UTC
    opendir, not open. You are doing the opposite, trying to read a directory as a file.
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Re: "readdir() attempted on invalid dirhandle" error?
by NERDVANA (Priest) on Jul 10, 2024 at 20:14 UTC
    Well, in building my reply, I just learned something. Linux lets you open a directory as a file without an error, but then you can't do anything with it.

    Anyway, the answer you're looking for is perldoc -f opendir. This corresponds to the C library opendir/readdir/seekdir/telldir/rewinddir/scandir/closedir family of functions which operate on a DIR* struct, which is very different from a file handle. I think the error message tries to convey that with "dirhandle" language instead of "filehandle".

    Meanwhile, I recommend Path::Tiny or Path::Class is you're doing any significant amount of work with browsing file trees. If you can't use modules, there's also the handy 'glob' function with the convenient built-in syntax of my @files= <$dir/*>; Note that that one returns full relative paths instead of just the file names, which is sometimes more useful (and sometimes not).

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      > Linux lets you open a directory as a file without an error, but then you can't do anything with it.

      You can do something.

      open my $d, '<', '..' or die $!;  # Doesn't die.
say -e $d;                        # 1.
say -d $d;                        # 1.
say -f $d;                        # Nothing.
chdir $d or die $!;               # Switches to the parent directory (
+read "perldoc -f chdir").

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      map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]
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        From chdir function ... 
        ...
chdir FILEHANDLE
chdir DIRHANDLE
...
  On systems that support fchdir(2), you may pass a filehandle or dire
+ctory handle as the argument. On systems that don't support fchdir(2)
+, passing handles raises an exception.

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        ... fchdir(2) ought to be present on POSIX-conforming systems, from "fchdir(2)" manual page of FreeBSD 14 ... 

        ...
STANDARDS
     The chdir() system call is expected to conform to IEEE Std 1003.1
+-1990 (“POSIX.1”).

HISTORY
  The chdir() system call appeared in Version 1 AT&T UNIX. The fchdir(
+) system call appeared in 4.2BSD.

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      Interesting. Also, WTAF!

      Same call -- open my $dh, q[<], q[./] or die $! -- also succeeds on FreeBSD 14.

      Thinking bit more, does not make sense: Should Perl call open(2) with O_DIRECTORY flag -- where available -- so that call will fail if path is not a directory? Apparently it already does not (feel free to correct).

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      (OP here) God, I knew it was something stupid and obvious like that! But why on earth would Perl not return an error when opening (not opendiring) a directory? Aargh. Thank you (both) for this. This task doesn't require any fancy tree-walking, the directory will just be a flat folder with files in it, so this is pretty much all I need for this purpose.
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        But why on earth would Perl not return an error when opening (not opendiring) a directory?

        As others have shown, there's nothing intrinsically invalid about open on a directory. However I think Perl is definitely capable of giving a more informative error message when you try to use a filehandle in a context that requires a directory handle. I'd recommend opening a github issue tagged as 'Wishlist' suggesting that there's an opportunity for improvement here.

        Update: the issue now exists as #22394.

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