On Wed, 20 Nov 2002, jayanth wrote:
> Some developers here have encountered a scenario where the file
> descriptor flags and the socket flags seem to be out of sync.
>
> if an application does:
>
> listen(listenfd)
> while (!done) {
> select()
> <-------------------- new connection arrives before fcntl()
> fcntl(listenfd,O_NONBLOCK)
> newfd = accept(listenfd,...)
> fnctl(listenfd,0) /* make socket blocking */
> if (newfd & O_NONBLOCK)
Better delete the last line. newfd doesn't contain any file flags.
> /* fd is O_NONBLOCK, but socket is blocking */
> }
>
> At this point socket is blocking because the state
> of the new socket = state of the listen socket only during the connection
> setup phase, not during the accept phase. However, the filedescriptor
> flags are copied during the accept phase. So at this point
> the filedescriptor flags are nonblocking but the socket is actually blocking.
This is related to my pet peeve of keeping the O_NONBLOCK flag in several
places (one of them inadequate) and using the wrong copies. For sockets,
the per-socket flag is adequate since open() doesn't work on sockets so
there only needs to be one flag per socket, and using the per-socket flag
mostly works right. For devices that can be opened more than once, a
per-device flag is inadequate since file flags are supposed to be per-open.
Non-broken device drivers use the copy of the file flags passed to their
read/write/ioctl/close/etc functions, but making a copy causes synchronization
problems. However, accessing the per-file flag from the device layer or
the socket layer would be a layering violation.
> Agreed, that the solution is to have the application set NONBLOCK before
> the listen() call, but it seems incorrect to have the newfd's flags and socket
> state be out of sync.
You could also use "fnctl(newfd, 0)" to sync things for newfd.
> Copying the state of the socket during the accept might lead to a slightly
> different behaviour, but will solve this particular problem.
I think this is the correct fix. The states are supposed to be consistent.
This is enforced by fcntl() if the file flag state using it. I think you
can still make a mess by changing the state using ioctl().
Bruce
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