nbd_aio_get_direction - return the read or write direction


 #include <libnbd.h>

 unsigned nbd_aio_get_direction (
            struct nbd_handle *h


Return the current direction of this connection, which means whether we are next expecting to read data from the server, write data to the server, or both. It returns


We are not expected to interact with the server file descriptor from the current state. It is not worth attempting to use poll(2); if the connection is not dead, then state machine progress must instead come from some other means such as nbd_aio_connect(3).


We are expected next to read from the server. If using poll(2) you would set events = POLLIN. If revents returns POLLIN or POLLHUP you would then call nbd_aio_notify_read(3).

Note that once libnbd reaches nbd_aio_is_ready(3), this direction is returned even when there are no commands in flight (see nbd_aio_in_flight(3)). In a single-threaded use of libnbd, it is not worth polling until after issuing a command, as otherwise the server will never wake up the poll. In a multi-threaded scenario, you can have one thread begin a polling loop prior to any commands, but any other thread that issues a command will need a way to kick the polling thread out of poll in case issuing the command changes the needed polling direction. Possible ways to do this include polling for activity on a pipe-to-self, or using pthread_kill(3) to send a signal that is masked except during ppoll(2).


We are expected next to write to the server. If using poll(2) you would set events = POLLOUT. If revents returns POLLOUT you would then call nbd_aio_notify_write(3).


We are expected next to either read or write to the server. If using poll(2) you would set events = POLLIN|POLLOUT. If only one of POLLIN or POLLOUT is returned, then see above. However, if both are returned, it is better to call only nbd_aio_notify_read(3), as processing the server's reply may change the state of the connection and invalidate the need to write more commands.


This call returns a bitmask.


This function does not fail.

The following parameters must not be NULL: h. For more information see "Non-NULL parameters" in libnbd(3).


This function first appeared in libnbd 1.0.

If you need to test if this function is available at compile time check if the following macro is defined:



nbd_aio_connect(3), nbd_aio_in_flight(3), nbd_aio_is_ready(3), nbd_aio_notify_read(3), nbd_aio_notify_write(3), nbd_create(3), libnbd(3), poll(2), ppoll(2), pthread_kill(3).


Eric Blake

Richard W.M. Jones


Copyright Red Hat


This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

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