Ubuntu Manpage: nbd_aio_get_direction - return the read or write direction

NAME

       nbd_aio_get_direction - return the read or write direction

SYNOPSIS

        #include <libnbd.h>

        unsigned nbd_aio_get_direction (
                   struct nbd_handle *h
                 );

DESCRIPTION

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

0 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).

"LIBNBD_AIO_DIRECTION_READ" = 1
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).

"LIBNBD_AIO_DIRECTION_WRITE" = 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).

"LIBNBD_AIO_DIRECTION_BOTH" = 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.

RETURN VALUE

       This call returns a bitmask.

ERRORS

       This function does not fail.

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

VERSION

       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:

        #define LIBNBD_HAVE_NBD_AIO_GET_DIRECTION 1

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright Red Hat

LICENSE

       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.

       You should have received a copy of the GNU Lesser General Public License along with this
       library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth
       Floor, Boston, MA 02110-1301 USA