nbd_opt_go - Man Page

end negotiation and move on to using an export


 #include <libnbd.h>

 int nbd_opt_go (
       struct nbd_handle *h


Request that the server finish negotiation and move on to serving the export previously specified by 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.

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.

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.

Return Value

If the call is successful the function returns 0.


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

The handle must be negotiating, otherwise this call will return an error.


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:



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

 /* 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 <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;
   return 0;

 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

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).


Eric Blake

Richard W.M. Jones


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

Referenced By

libnbd(3), libnbd-release-notes-1.4(1), NBD(3), nbd_aio_opt_go(3), nbd_aio_opt_set_meta_context(3), nbd_aio_opt_set_meta_context_queries(3), nbd_connect_uri(3), nbd_opt_abort(3), nbd_opt_info(3), nbd_opt_list(3), nbd_opt_list_meta_context(3), nbd_opt_list_meta_context_queries(3), nbd_opt_set_meta_context(3), nbd_opt_set_meta_context_queries(3), nbd_opt_structured_reply(3), nbd_set_export_name(3), nbd_set_opt_mode(3), nbd_set_request_meta_context(3).

2023-08-30 libnbd-1.17.4