linux-kernel - Re: F_DUPFD_CLOEXEC implementation

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Message-ID: <47004662$0$241$e4fe514c@news.xs4all.nl>
Date:	01 Oct 2007 00:59:14 GMT
From:	"Miquel van Smoorenburg" <miquels@...tron.nl>
To:	linux-kernel@...r.kernel.org
Subject: Re: F_DUPFD_CLOEXEC implementation

In article <200709300131.49320.vda.linux@...glemail.com>,
Denys Vlasenko  <vda.linux@...glemail.com> wrote:
>Hi Ulrich,
>
>On Friday 28 September 2007 18:34, Ulrich Drepper wrote:
>> One more small change to extend the availability of creation of
>> file descriptors with FD_CLOEXEC set.  Adding a new command to
>> fcntl() requires no new system call and the overall impact on
>> code size if minimal.
>
>Tangential question: do you have any idea how userspace can
>safely do nonblocking read or write on a potentially-shared fd?
>
>IIUC, currently it cannot be done without races:
>
>old_flags = fcntl(fd, F_GETFL);
>...other process may change flags!...
>fcntl(fd, F_SETFL, old_flags | O_NONBLOCK);
>read(fd, ...)
>...other process may see flags changed under its feet!...
>fcntl(fd, F_SETFL, old_flags);
>
>Can this be fixed?

This is for sockets, right ? Just use revc() instead of read().

	n = recv(filedesc, buffer, buflen, MSG_DONTWAIT);

.. is equivalent to setting O_NONBLOCK. See "man recv".

Mike.
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