Re: [9fans] what a mess the Unix world has become

["Douglas A. Gwyn" <DAGwyn@null.net>]

Aharon Robbins wrote:
>>I think that one really should be able to do "test -r foo && cat foo"
>>without cat failing because of permissions.

Yes, that was the intention.  But the question comes into
even sharper focus if you try to test for writability.
What OS facility is there for determining whether a file
would be writable without actually modifying the file?
The answer: access(2).  Not a mere examination of the
mode bits; not a trial write to the file.  When something
like NFS is bolted onto Unix, access(2) is one of the
things that needs to be made to work right; if NFS has
complicated access rules then that is what access() needs
to tap into.  (Better in the filesystem support than in
user-mode utilities.)
VAX/VMS had a "probe" call that, as I recall, checked
whether an operation would succeed but didn't do the
operation.
Of course, all such facilities are somewhat limited in
that the condition of the file might change between a
probe and actually doing the operation.  There are
transaction-oriented filesystems (MUMPS comes to mind)
that cluster requests until a "commit" and implement a
well-defined rollback scheme when the commit is
unsuccessful.