nbd_opt_go(3)

nbd_opt_go(3) — end negotiation and move on to using an export 🌸

okay so this is the libnbd manual page for nbd_opt_go(3), and yes bestie, we are getting into option-mode business. The vibe: negotiation is wrapping up and you want the server to actually serve the export. Periodt.

✨ NAME ✨

nbd_opt_go — end negotiation and move on to using an export

🌈 SYNOPSIS 🌈

bestie, include the header and call it like this:

        #include <libnbd.h>

        int nbd_opt_go (
              struct nbd_handle *h
            );

💅 DESCRIPTION 💅

So nbd_opt_go is literally you asking the server: “okay bestie, finish negotiation and move on to serving the export I already specified with the most recent nbd_set_export_name(3) or nbd_connect_uri(3).” This can only be used if nbd_set_opt_mode(3) enabled option mode. No option mode, no go. 🙈

By default, libnbd will automatically request all meta contexts registered by nbd_add_meta_context(3) as part of this call; but this can be suppressed with nbd_set_request_meta_context(3), particularly if nbd_opt_set_meta_context(3) was used earlier in the negotiation sequence. So yeah — default behavior is doing the most, but you can turn that off if you already handled it. 💅

If this fails, the server may still be in negotiation, where it is possible to attempt another option such as a different export name; although older servers will instead have killed the connection. So like… sometimes you get another shot, sometimes the server is dramatic and just ends things. 😭

🫶 RETURN VALUE 🫶

If the call is successful the function returns 0.

🚨 ERRORS 🚨

On error -1 is returned.

Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.

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

👑 HANDLE STATE 👑

nbd_opt_go can be called when the handle is in the following state:

• Handle created, before connecting — ❌ error
• Connecting — ❌ error
• Connecting & handshaking (opt_mode) — ✅ allowed
• Connected to the server — ❌ error
• Connection shut down — ❌ error
• Handle dead — ❌ error

🦋 VERSION 🦋

This function first appeared in libnbd 1.4.

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

        #define LIBNBD_HAVE_NBD_OPT_GO 1

🎀 EXAMPLE 🎀

This example is also available as examples/list-exports.c in the libnbd source code.

lowkey it’s a whole program, but here it is, character for character:

        /* This example shows how to list NBD exports.
         *
         * To test this with qemu-nbd:
         *   $ qemu-nbd -x "hello" -t -k /tmp/sock disk.img
         *   $ ./run examples/list-exports /tmp/sock
         *   [0] hello
         *   Which export to connect to (-1 to quit)? 0
         *   Connecting to hello ...
         *   /tmp/sock: hello: size = 2048 bytes
         *
         * To test this with nbdkit (requires 1.22):
         *   $ nbdkit -U /tmp/sock sh - <<\EOF
         *   case $1 in
         *     list_exports) echo NAMES; echo foo; echo foobar ;;
         *     open) echo "$3" ;;
         *     get_size) echo "$2" | wc -c ;;
         *     pread) echo "$2" | dd bs=1 skip=$4 count=$3 ;;
         *     *) exit 2 ;;
         *   esac
         *   EOF
         *   $ ./run examples/list-exports /tmp/sock
         *   [0] foo
         *   [1] foobar
         *   Which export to connect to (-1 to quit)? 1
         *   Connecting to foobar ...
         *   /tmp/sock: foobar: size = 7 bytes
         */

        #include <stdio.h>
        #include <stdlib.h>
        #include <stdbool.h>
        #include <stdint.h>
        #include <string.h>
        #include <inttypes.h>
        #include <errno.h>

        #include <libnbd.h>

        struct export_list {
          int i;
          char **names;
        };

        /* Callback function for nbd_opt_list */
        static int
        list_one (void *opaque, const char *name,
                  const char *description)
        {
          struct export_list *l = opaque;
          char **names;

          printf ("[%d] %s\n", l->i, name);
          if (*description)
            printf ("  (%s)\n", description);
          names = realloc (l->names,
                           (l->i + 1) * sizeof *names);
          if (!names) {
            perror ("realloc");
            exit (EXIT_FAILURE);
          }
          names[l->i] = strdup (name);
          if (!names[l->i]) {
            perror ("strdup");
            exit (EXIT_FAILURE);
          }
          l->names = names;
          l->i++;
          return 0;
        }

        int
        main (int argc, char *argv[])
        {
          struct nbd_handle *nbd;
          int i;
          const char *name;
          int64_t size;
          struct export_list list = { 0 };

          if (argc != 2) {
            fprintf (stderr, "%s socket\n", argv[0]);
            exit (EXIT_FAILURE);
          }

          /* Create the libnbd handle. */
          nbd = nbd_create ();
          if (nbd == NULL) {
            fprintf (stderr, "%s\n", nbd_get_error ());
            exit (EXIT_FAILURE);
          }

          /* Set opt mode. */
          nbd_set_opt_mode (nbd, true);

          /* Connect to the NBD server over a
           * Unix domain socket.  If we did not
           * end up in option mode, then a
           * listing is not possible.
           */
          if (nbd_connect_unix (nbd, argv[1]) == -1) {
            fprintf (stderr, "%s\n", nbd_get_error ());
            exit (EXIT_FAILURE);
          }
          if (!nbd_aio_is_negotiating (nbd)) {
            fprintf (stderr, "Server does not support "
                     "listing exports.\n");
            exit (EXIT_FAILURE);
          }

          /* Print the export list. */
          if (nbd_opt_list (nbd,
                            (nbd_list_callback) {
                              .callback = list_one,
                              .user_data = &list, }) == -1) {
            fprintf (stderr, "%s\n", nbd_get_error ());
            exit (EXIT_FAILURE);
          }

          /* Display the list of exports. */
          printf ("Which export to connect to? ");
          if (scanf ("%d", &i) != 1) exit (EXIT_FAILURE);
          if (i == -1) {
            if (nbd_opt_abort (nbd) == -1) {
              fprintf (stderr, "%s\n", nbd_get_error ());
              exit (EXIT_FAILURE);
            }
            nbd_close (nbd);
            exit (EXIT_SUCCESS);
          }
          if (i < 0 || i >= list.i) {
            fprintf (stderr, "index %d out of range", i);
            exit (EXIT_FAILURE);
          }
          name = list.names[i];
          printf ("Connecting to %s ...\n", name);

          /* Resume connecting to the chosen export. */
          if (nbd_set_export_name (nbd, name) == -1 ||
              nbd_opt_go (nbd) == -1) {
            fprintf (stderr, "%s\n", nbd_get_error ());
            exit (EXIT_FAILURE);
          }
          if (!nbd_aio_is_ready (nbd)) {
            fprintf (stderr, "server closed early\n");
            exit (EXIT_FAILURE);
          }

          /* Read the size in bytes and print it. */
          size = nbd_get_size (nbd);
          if (size == -1) {
            fprintf (stderr, "%s\n", nbd_get_error ());
            exit (EXIT_FAILURE);
          }
          printf ("%s: %s: size = %" PRIi64 " bytes\n",
                  argv[1], name, size);

          /* Close the libnbd handle. */
          nbd_close (nbd);

          for (i = 0; i < list.i; i++)
            free (list.names[i]);
          free (list.names);

          exit (EXIT_SUCCESS);
        }

🔥 SEE ALSO 🔥

go read: nbd_add_meta_context(3), nbd_aio_opt_go(3), nbd_connect_uri(3), nbd_create(3), nbd_opt_abort(3), nbd_opt_info(3), nbd_opt_set_meta_context(3), nbd_set_export_name(3), nbd_set_opt_mode(3), nbd_set_request_meta_context(3), libnbd(3).

🌟 AUTHORS 🌟

• Eric Blake
• Richard W.M. Jones

🍓 COPYRIGHT 🍓

Copyright Red Hat

🧚 LICENSE 🧚

legal time, no skips:

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

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mwah 💋 — this little love letter is from the libnbd-1.25.8 manual, dated 2026-10-13. bestie, go forth and negotiate responsibly. 🫶