nbd\_aio\_get\_direction(3) LIBNBD nbd\_aio\_get\_direction(3)

# [NAME](#NAME)

nbd\_aio\_get\_direction - return the read or write direction

# [SYNOPSIS](#SYNOPSIS)

```
 #include <libnbd.h>
 unsigned nbd_aio_get_direction (
            struct nbd_handle *h
          );
```

# [DESCRIPTION](#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)](/man/os/libnbd/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)](/man/os/libnbd/nbd_aio_connect.3).

"LIBNBD\_AIO\_DIRECTION\_READ" = 1

We are expected next to read from the server. If using [**poll**(2)](/man/os/libnbd/poll.2) you would set "events = POLLIN". If "revents" returns "POLLIN" or "POLLHUP" you would then call [**nbd\_aio\_notify\_read**(3)](/man/os/libnbd/nbd_aio_notify_read.3).

Note that once libnbd reaches [**nbd\_aio\_is\_ready**(3)](/man/os/libnbd/nbd_aio_is_ready.3), this direction is returned even when there are no commands in flight (see [**nbd\_aio\_in\_flight**(3)](/man/os/libnbd/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)](/man/os/libnbd/pthread_kill.3) to send a signal that is masked except during [**ppoll**(2)](/man/os/libnbd/ppoll.2).

"LIBNBD\_AIO\_DIRECTION\_WRITE" = 2

We are expected next to write to the server. If using [**poll**(2)](/man/os/libnbd/poll.2) you would set "events = POLLOUT". If "revents" returns "POLLOUT" you would then call [**nbd\_aio\_notify\_write**(3)](/man/os/libnbd/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)](/man/os/libnbd/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)](/man/os/libnbd/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](#RETURN_VALUE)

This call returns a bitmask.

# [ERRORS](#ERRORS)

This function does not fail.

The following parameters must not be NULL: "h". For more information see "Non-NULL parameters" in [**libnbd**(3)](/man/os/libnbd/libnbd.3).

# [VERSION](#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
```

# [SEE ALSO](#SEE_ALSO)

[**nbd\_aio\_connect**(3)](/man/os/libnbd/nbd_aio_connect.3), [**nbd\_aio\_in\_flight**(3)](/man/os/libnbd/nbd_aio_in_flight.3), [**nbd\_aio\_is\_ready**(3)](/man/os/libnbd/nbd_aio_is_ready.3), [**nbd\_aio\_notify\_read**(3)](/man/os/libnbd/nbd_aio_notify_read.3), [**nbd\_aio\_notify\_write**(3)](/man/os/libnbd/nbd_aio_notify_write.3), [**nbd\_create**(3)](/man/os/libnbd/nbd_create.3), [**libnbd**(3)](/man/os/libnbd/libnbd.3), [**poll**(2)](/man/os/libnbd/poll.2), [**ppoll**(2)](/man/os/libnbd/ppoll.2), [**pthread\_kill**(3)](/man/os/libnbd/pthread_kill.3).

# [AUTHORS](#AUTHORS)

Eric Blake

Richard W.M. Jones

# [COPYRIGHT](#COPYRIGHT)

Copyright Red Hat

# [LICENSE](#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

2026-10-11 libnbd-1.25.7

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Document: nbd_aio_get_direction(3) from libnbd 1.25.7-r3 (Chainguard OS)