Beefy Boxes and Bandwidth Generously Provided by pair Networks
Don't ask to ask, just ask
 
PerlMonks  

Comment on

( #3333=superdoc: print w/ replies, xml ) Need Help??

A few days ago, someone posted a question about reading directories. To discard the current and parent directories that are returned by readdir, someone suggested to:

opendir(DIR, "/users/foo/"); @bar = readdir(DIR); closedir(DIR); for (1..2) {shift @bar;} #get rid of '.' and '..'

Notwithstanding the lack of error checking, and a dubious array hack better handled by splice, merlyn subsequently pointed out that "There is no promise that the first two elements returned from a readdir are dot and dotdot," and I nodded in agreement.

Then I reflected on the fact that I have been using Perl for nearly ten years; I have use Perl on a half a dozen Unix variants, on VMS, on MS-DOS and Win32. In all that time, grovelling through filesystems has always been a large component of my work and yet I cannot recall a single time when . and .. were returned in any but the first two positions. Maybe it happens once in a while for any script in production -- I'm talking about when I'm stepping through some development code I'm testing.

I heed the warning and know that this behaviour is not guaranteed, so I dutifully code:

while( defined( my $file = readdir(DIR) )) { next if $file eq '.' or $file eq '..'; munge($file); }

but maybe it would be more elegant to be able to write code that throws away the first two results returned by readdir and then have a while block that doesn't contain first next if ... check. What I mean is that it's a nearly-invariant test that could be hoisted out of the loop if readdir was a little more deterministic.

My questions are

  1. Does there exist a platform where readdir returns . and .. in other than the first two return values?
  2. Is readdir guaranteed to return . and .. as the first two return values on any platform (specifically *BSD, Linux, Solaris and Win32)? (or more specifically the filesystem in use, such as ext2, ffs, ntfs...)

I realise my assumption is based on the notion that I consider a directory stream to be a linear list. Walking down it is akin to accessing array elements. A hash-based directory stream would produce unordered results, but I'm not sure I've ever encountered one, at least as far as the visible behaviour from userspace is concerned.

--
g r i n d e r

In reply to Is readdir ever deterministic? by grinder

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":



  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?
    Username:
    Password:

    What's my password?
    Create A New User
    Chatterbox?
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others browsing the Monastery: (3)
    As of 2015-07-05 18:26 GMT
    Sections?
    Information?
    Find Nodes?
    Leftovers?
      Voting Booth?

      The top three priorities of my open tasks are (in descending order of likelihood to be worked on) ...









      Results (67 votes), past polls