On Fri, Sep 14, 2012 at 6:01 AM, Roland Mainz <[email protected]> wrote:
> Attached (as "astksh_chroot_cd_devfd_cd_f_20120911_001.diff.txt") is a
> patch which fixes two issues with "cd":
> 1. $ cd /dev/fd/$fd # doesn't work in chroot'ed environments when
> /dev/fd is not mounted (thanks to CERN staff for reporting this).
> 2. POSIX does not mandate _any_ paths. Since doing a "cd" relative to
> a directory fd has become very popular the request was made to add an
> alternative to using /dev/fd which is more or less acceptable for the
> POSIX people. Based on that and a few discussions I added the option
> -f to cd that a directory descriptor can be passed and the path given
> is relative to that file descriptor.
>
> Notes:
> - The code introduces a new function called |pathdevfd2relpathfd()| in
> libast which can extract the fd number from a /dev/fd/$fd path (even
> nested)
> - If files or directories are opened relative to /dev/fd/$fd/$path
> |sfopen()|/|sfopenat()| will now bypass the /dev/fd filesystem
> completely (which gives a nice performance boost). The only exception
> is that this can *NOT* be done for a plain /dev/fd/$fd, e.g.
> /dev/fd/15. The problem is that there is AFAIK no way to open a file
> from a file descriptor without using the /dev/fd filesystem or using
> |dup()| ... but |dup()| rules itself out because the resulting
> "cloned" fd still shares attributes like the current seek position
> with the original fd...
>
> Glenn: Is it acceptable for |sfopen()| to |dup()| the incoming file fd
> (see "Notes" above) ?
More details on the problem with |dup()| sharing attributes like the
seek position between both old and new fd:
- I tested /dev/fd on Linux (SuSE 12.1) ... it seems that |dup(15)| is
not identical to |open("/dev/fd/15", ...)| ... as expected the seek
position is shared for |dup()| but the fd returned by |open()| has
it's own seek position
- AFAIK the issue is irrelevant for pipes/fifos/sockets... right ?
- What about using |pread()|/|pwrite()| instead of |read()|/|write()|
on normal, plain files ? This would bypass the issue with the seek
position being shared...
----
Bye,
Roland
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