On Monday 01 October 2007 00:11, Davide Libenzi wrote: > 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.
My use case is: I want to do a nonblocking read on descriptor 0 (stdin). It may be a pipe or a socket. There may be other processes which share this descriptor with me, I simply cannot know that. And they, too, may want to do reads on it. I want to do nonblocking read in such a way that neither those other processes will ever see fd switching to O_NONBLOCK and back, and I also want to be safe from other processes doing the same. I don't see how this can be done using standard unix primitives. -- vda - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
