[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Old Index]
Re: fcntl & cmsg
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.
> McKusick distinguishes between a "file entry" describing an open file,
I think by "file entry" this means "open file object" or in NetBSD,
"struct file". The per-process open file table is an array of
references to these.
They aren't the same as vnodes; vnodes are one layer deeper.
Some people use the words "file descriptor" to refer to the open file
object as well as to the integer that consitutes a user-level
reference to it. This can occasionally be confusing...
> (Can anyone explain why close-on-exec isn't just another option for
> F_SETFL? I see that dup(2) preserves the F_SETFL flags but not
> close-on-exec. Interesting choice....)
Because it's per-handle instead of per-object. That in turn is to make
close-on-exec and I/O redirection interact productively.
David A. Holland
Main Index |
Thread Index |