nbd_aio_get_direction(3)

📦
version 1.25.8 · 18 versions
, in centos-stream 10

nbd_aio_get_direction(3) — the read/write direction vibe check 💅

okay bestie, welcome to nbd_aio_get_direction(3) from the LIBNBD manual ✨ this one is literally about figuring out which way the connection is facing next: read, write, both, or “none of the above, don’t poll yet.” iconic little state-machine check 🦋

🌸 NAME 🌸

nbd_aio_get_direction — return the read or write direction, bestie. That’s the whole assignment. 💖

✨ 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. short, sweet, and very much the moment ✨

🦋 ERRORS 🦋

This function does not fail. no cap, that’s the whole error section 💀

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

🔥 SEE ALSO 🔥

go give these a little read, iykyk:

💜 AUTHORS 💜

  • Eric Blake
  • Richard W.M. Jones

Copyright Red Hat

🎀 LICENSE 🎀

okay the legal text stays exactly as written, no edits, bestie:

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

catch u later, bestie — this iconic page is from libnbd-1.25.8, dated 2026-10-13 🫶💖

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