Date: Wed, 8 Jan 2020 16:15:44 +0000 From: Steve O'Hara-Smith <steve@sohara.org> To: "Ronald F. Guilmette" <rfg@tristatelogic.com> Cc: freebsd-questions@freebsd.org Subject: Re: Independence of file descriptor flags across forks (or lack thereof) Message-ID: <20200108161544.76f64dd6eb0d1edb134a5b0f@sohara.org> In-Reply-To: <99921.1578497412@segfault.tristatelogic.com> References: <20200108114244.b431a9ae0170ec947e6fb7d8@sohara.org> <99921.1578497412@segfault.tristatelogic.com>
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On Wed, 08 Jan 2020 07:30:12 -0800
"Ronald F. Guilmette" <rfg@tristatelogic.com> wrote:
> File status flags and file access modes are associated with the
> file description and do not affect other file descriptors that
> refer to the same file with different open file descriptions.
>
> Why else would the authors of this standard have included such stilted
> and, admittedly, imprecise language as that if they were NOT attempting
> to say exactly what I have said?
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.
--
Steve O'Hara-Smith <steve@sohara.org>
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