Indication of extended attributes availability.

[Václav Haisman]

Robert Watson wrote, On 22.3.2008 21:00:
 >[...]
> mount flags are normally used for places where there is a desire to 
> report an administrative setting, and on the whole, extended attributes 
> are a property of the file system type, and not a per-mount setting.  
> UFS1 extended attributes are intended to be the exception rather than 
> the rule.  The way fpathconf() works inside the kernel is that the 
> request is delivered directly to the file system that implements the 
> target of the path provided, so it can return information on whatever 
> granularity it wants -- be it per-mount, per-volume, etc.  I think the 
> Solaris model sounds pretty sensible, although one thing worth 
> considering is that Solari's extended attributes may, in fact, be file 
> forks or streams, and called extended attributes due to NFSv4 using that 
> terminology (an unfortunate overloading inconsistent with the use in 
> many OS's).  In which case we might want to use a different name.  It 
> would be worth checking Linux and Mac OS X as well.
> 
I have done some digging using the Man pages mirrors on www.freebsd.org.

1. Only Solaris has the _PC_XATTR_*. According to 
<http://www.freebsd.org/cgi/man.cgi?query=fsattr&apropos=0&sektion=0&manpath=SunOS+5.10&format=html> 
  _PC_XATTR_* are really about extended attributes. Citation from the page:

"Not all implementations are able to, or want to, support the full model. For 
example, the implementation for the UFS file system allows only regular files 
as attributes (for example, no sub-directories) and rejects attempts to place 
attributes on attributes."

This sounds close enough to what FreeBSD has.

2. Neither Linux nor Darwin seem to support querying availability of extended 
attributes even though they support their use using getxattr() etc.


--
VH

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