In article <[email protected]>,
Denys Vlasenko <[email protected]> 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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