Why can't directories be hard-linked on Unix filesystems (e.g. HFS)?
On both my MacBook (OS 10.5.8) and Linux (RHEL4) I've tried to do some directory organiztion using hard links (ln, not ln -s). When I try to hard-link a directory, I get an error message:
{yakut05:81} mkdir tempo
{yakut05:83} ln tempo tempo-link
ln: `tempo': hard link not allowed for directory
{yakut05:84}
The man page for ln doesn't specify this limitation. I wonder if any filesystem gurus out there could explain why it's not allowed? I'm sure it has to do with the inode structure, but I'd like to be clear on the details.
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afaik, can't be done. see
http://developer.apple.com/mac/library/technotes/tn/tn1150.html#HardLinks
http://developer.apple.com/mac/library/technotes/tn/tn1150.html#HardLinks
Just FYI, the stupid new editor won't automatically HTMLify URLs. You have to paste it in, then highlight it, hit Ctrl-L, and paste it again into the box :-(
That aside, thank you much, Sean. It sounds (paraphrasing) like hard links are dealt with almost as though they were low-level symbolic links, and that system treats directory objects differently than file objects.
Now at least I know what's going on!
That aside, thank you much, Sean. It sounds (paraphrasing) like hard links are dealt with almost as though they were low-level symbolic links, and that system treats directory objects differently than file objects.
Now at least I know what's going on!
If I just hit the link button and paste into the little box it works for me.
developer.apple.com/mac/library/technotes/tn/tn1150.html#HardLinks
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developer.apple.com/mac/library/technotes/tn/tn1150.html#HardLinks
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Ah, I didn't know you could open the link box without some text to be used as the anchor. What Sean did above was paste the URL into the main editor, which doesn't auto-convert it.
Yea, the new editor didn't come with instructions (as far as I know). Still knowing some HTML is handier than it used to be.
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I failed to abide by the editor and hit post-comment...got lazy didn't redo it....the shame, the mortal shame ;)
;)
I know. I was too tired to go back and copy past edit the note... The particular plugin they use (or it appears to be the same one) is quite popular, but very clumsy and slow imo.The first time I encountered that editor plugin, I was excited by the possibilities, but equally dissapointed once I used it.. Sorry for the glitch...figured you'd understand. I don't usually do that, btw, just too lazy when I posted.
Total paraphrasing. I'm usually disinclined to commenting by copy-out (clone the results of a net search), but the occasional one doesn't violate values... I have zero experience with macs...well, aside from machine level programming on the first ones (Apple IIE I believe) during engineering course work nearly 30 years ago...but, the question was legit, had merit (imo), was interesting, and seemed like it ought to have an obvious reason, coupled with my foreknowledge that you would already have performed some search...two eyes often better than one in those cases...I rad enough to confirm the logic and figured you'd missed it along the way.I blather. must get some sleep.
I know. I was too tired to go back and copy past edit the note... The particular plugin they use (or it appears to be the same one) is quite popular, but very clumsy and slow imo.The first time I encountered that editor plugin, I was excited by the possibilities, but equally dissapointed once I used it.. Sorry for the glitch...figured you'd understand. I don't usually do that, btw, just too lazy when I posted.
Total paraphrasing. I'm usually disinclined to commenting by copy-out (clone the results of a net search), but the occasional one doesn't violate values... I have zero experience with macs...well, aside from machine level programming on the first ones (Apple IIE I believe) during engineering course work nearly 30 years ago...but, the question was legit, had merit (imo), was interesting, and seemed like it ought to have an obvious reason, coupled with my foreknowledge that you would already have performed some search...two eyes often better than one in those cases...I rad enough to confirm the logic and figured you'd missed it along the way.I blather. must get some sleep.
Thanks! Yeah, I had found it, but didn't read far enough to see the answer. Also, I see the same error on a regular Linux system, so I wasn't sure if it was Mac-specific or not.
Aug 27, 2011. 6:43 PMganglion
says:
I think this is a deliberate design choice, not a bug. If hard linked directories were allowed, then you would have the possibility of creating circular loops in the filesystem structure. This make it much harder for filesystem search algorithms to produce a comprehensive list of files and directories under a certain point.
With a hard link, there is literally no difference between one link name and the other - they are both pointers to inodes - so the search algorithm would have no way of knowing which was the 'real' name and which was the link.
With a hard link, there is literally no difference between one link name and the other - they are both pointers to inodes - so the search algorithm would have no way of knowing which was the 'real' name and which was the link.
Ah, ha! Thank you :-) I see your point about the possibility of completely invisible circular loops. With symlinks, the circularlity is explicit, and can be excluded with careful coding (
find handles it, for example), but that wouldn't be the case with hard links.Aug 27, 2011. 10:32 PMganglion
says:
Yes - that's why you can only soft link directories.
I'm glad this was useful - I found the comment from last year after looking your page, and I wasn't sure whether you would have forgotten about it or found the answer anyway.
I'm glad this was useful - I found the comment from last year after looking your page, and I wasn't sure whether you would have forgotten about it or found the answer anyway.
You won't have overlooked anything obvious or made any basic error. Might the permissions or path be "funny" on the target drive / directory?
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I don't think so. The same ln command, in the same directory, works perfectly well if I apply it to a simple file. That's why I'm confused.
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