Date: Wed, 08 Jan 2020 21:30:00 -0800 From: "Ronald F. Guilmette" <rfg@tristatelogic.com> To: freebsd-questions@freebsd.org Subject: Re: Independence of file descriptor flags across forks (or lack thereof) Message-ID: <8433.1578547800@segfault.tristatelogic.com> In-Reply-To: <20200108161544.76f64dd6eb0d1edb134a5b0f@sohara.org>
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In message <20200108161544.76f64dd6eb0d1edb134a5b0f@sohara.org>,
Steve O'Hara-Smith <steve@sohara.org> wrote:
> OK I can explain this:
>
> fd1 = open ("/some/file", O_RDONLY);
> fd2 = open ("/some/file", O_RDWR);
>
> Now I have two file descriptors referring to the same file with
>different file access modes. I can set flags on fd1 and fd2 independently.
>That is what the precisely phrased paragraph above is referring to. Each
>file descriptor (with values like 3) refers to a table which identifies the
>kernel data structure representing the open file. It is that kernel data
>structure that holds such things as the file position, mode, flags etc.
>
> Now if I fork this process then the table of open file descriptors
>gets copied so now two processes are sharing the same kernel data
>structures.
All I can say is that the kernel implementors of both FreeBSD and also
Linux seem to agree with your viewpoint. I myself am not persuaded
that the two file descriptors that exist after a fork should behave any
differently from the two in your example above, but they do, so I guess
I just have to deal with it.
Regards,
rfg
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