[prev in list] [next in list] [prev in thread] [next in thread] 

List:       dng
Subject:    [DNG] the openat suite of functions look useful.
From:       Hendrik Boom <hendrik () topoi ! pooq ! com>
Date:       2020-08-28 14:47:34
Message-ID: 20200828144734.pbchnzhe2svm473j () topoi ! pooq ! com
[Download RAW message or body]

On Fri, Aug 28, 2020 at 03:39:54PM +0200, Adam Borowski wrote:
> On Fri, Aug 28, 2020 at 08:37:07AM -0400, Hendrik Boom wrote:
> > > >>>> Is there an establiched word in the Linux/Unix xommunity
> > > >>>> for something which might be a file or a directory?  
> 
> > > Therefore it looks to me more like an implementation
> > > detail than a sensible concept.
> > 
> > I do miss the ability to identify a directory independent of its name.  
> > It can be important if a directory is renamed and you have active data in it.
> > 
> > There's one directory that works for -- the current directory.  If you 
> > rename that directory you're still in it.  
> > 
> > In the 80's or 90's (I forget) there was a proposal to the ISO from 
> > Japan to establish a standard OS-independent OS interface for 
> > programmers to use.  I lost against POSIX.
> > 
> > But it had a machanism whereby opening a file could be done in two 
> > phases.
> > 
> > First you get a "lock" on a file.  That gives you a connection to that 
> > file.  It's not an exclusive kind of lock (unless you ask it to be); 
> > it's just a way of unambiguously identifying the file as long as it 
> > exists and you keep the lock.  It hangs on to the file even if someone 
> > renames it.  (not sure what happens if the file is deleted, though)
> 
> _Any_ file descriptor does that.  And a deleted file stays there, merely
> with a 0 link count -- if there's an open descriptor, you can link the file
> back into the filesystem.  That's even the recommended way to create a new
> file atomically -- instead of the old write+fsync+rename trick that leaves
> junk upon a crash.
> 
> And if you don't need to do any I/O on the file, O_PATH won't waste time.
> 
> > Then afterward you can open it, close it, set exclusive 
> > read/write access, release exclusive access, etc, using the lock instead 
> > of the name.
> > 
> > I've often regretted there's no such similar feature in Linux.  The 
> > assurance that several uses of a file or directory are indeed to the 
> > same file or directory.  You can do some of this by keeping a file open, 
> > but I know of nothing like that for directories. 
> 
> man name_to_handle_at, open_by_handle_at.  But this interface sucks and is
> superfluous with regular openat() -- which is also portable.
> 
> So _always_ use openat() if you suspect something might happen to the
> underlying directories.

Very useful information!  Thank you.

-- hendrik
_______________________________________________
Dng mailing list
Dng@lists.dyne.org
https://mailinglists.dyne.org/cgi-bin/mailman/listinfo/dng

[prev in list] [next in list] [prev in thread] [next in thread]