Hi Jens,

On 08/25/2017 10:55 PM, Jens Axboe wrote:

On 08/25/2017 02:51 PM, Michael Kerrisk (man-pages) wrote:

quoted

quoted

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quoted

Do you mean here "file descriptor" or "file description (i.e., the
open file handle)? Maybe you mean the former, but I want to confirm.

I do mean file descriptor.

So, what are the semantics if a file descriptor is duplicated using
dup(2) or similar? If I understand correctly, then the write lifetime
hint has no effect for the new file descriptor, right?

If it's dup(2)'ed, then the new file descriptor will refer to the same
hints as the previous. See attached test file.

But then isn't this exactly the point I asked about: are the hints
private to a file descriptor or are they associated with the open file
description (open file table entry, "struct file")? You said "I do
mean file descriptor", but actually I understand what you just said
now as "hints are associated with the open file description, which may
be referred to by multiple duplicated file descriptors". Can you
clarify?

You are right, I misunderstood your original question. They do follow
the file description. So the dup'ed one will return the same as the
original, even if the hints on the original fd get modified. That is the
expected behavior.

So, I am still confused. I was wondering whether the hints are 
associated with the open file description (OFD), rather than the 
file descriptor. You said yes, then say that the dup'ed file 
descriptor will have the same hints even if the hints on the
original file descriptor are modified. To me that sounds like:
the hints are associated with the file descriptor, and not the
OFD, and during dup(2) the hints are *copied* to the the new
file descriptor, with the result that after the dup(2) the hints
can be modified independently for the two file descriptors.

Can you clarify please?

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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