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Re: fcntl & cmsg
On Mon, Sep 22, 2014 at 05:26:47AM +0000, David Holland wrote:
> On Fri, Sep 19, 2014 at 03:38:19PM -0400, James K. Lowden wrote:
> > unix(4) contains an "interesting" sentence:
> > "The received descriptor is a duplicate of the sender's
> > descriptor, as if it were created with a call to dup(2). Per-process
> > descriptor flags, set with fcntl(2), are not passed to a receiver."
> > because fcntl(2) doesn't define "per-process" descriptor flags. Does
> > the sentence mean "any flag set with fcntl", or are some flags
> > per-process? If the latter, is the reader supposed to be able to
> > determine which flags are per-process from the context?
> I don't know what the documentation is supposed to mean, but the way
> it's supposed to behave is:
> - fd passing is like calling dup2() except that (once received) the
> new fd appears in another process;
> - this means that the close-on-exec flag, which is per file handle,
> is not shared;
> - everything else is an attribute of the open file object (or in
> some cases the vnode, e.g. with F_SETLK or flock()) and should be
> shared between the two references.
Except that (posix) file locks are per process, not per open file.
This is fubar:
1) you can't lock against another thread in the same process.
2) if another thread opens and closes the same file you lose your locks.
David Laight: david%l8s.co.uk@localhost
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