On Sun, 30 Sep 2007, Denys Vlasenko 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?
I'm not sure I understood correctly your use case. But, if you have two
processes/threads randomly switching O_NONBLOCK on/off, your problems
arise not only the F_SETFL time.
If one of the tasks is not expecting an fd to be O_NONBLOCK, that will
likely end up not handling correctly read/write-miss situations.
In that case it'd be better to keep the fd as O_NONBLOCK, and manually
create blocking behaviour (when needed) with poll+read/write.
- Davide
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