nbd_set_opt_mode - control option mode, for pausing during option negotiation


 #include <libnbd.h>

 int nbd_set_opt_mode (
       struct nbd_handle *h, bool enable


Set this flag to true in order to request that a connection command nbd_connect_* will pause for negotiation options rather than proceeding all the way to the ready state, when communicating with a newstyle server. This setting has no effect when connecting to an oldstyle server.

Note that libnbd defaults to attempting NBD_OPT_STARTTLS, NBD_OPT_EXTENDED_HEADERS, and NBD_OPT_STRUCTURED_REPLY before letting you control remaining negotiation steps; if you need control over these steps as well, first set nbd_set_tls(3) to LIBNBD_TLS_DISABLE, and nbd_set_request_extended_headers(3) or nbd_set_request_structured_replies(3) to false, before starting the connection attempt.

When option mode is enabled, you have fine-grained control over which options are negotiated, compared to the default of the server negotiating everything on your behalf using settings made before starting the connection. To leave the mode and proceed on to the ready state, you must use nbd_opt_go(3) successfully; a failed nbd_opt_go(3) returns to the negotiating state to allow a change of export name before trying again. You may also use nbd_opt_abort(3) or nbd_shutdown(3) to end the connection without finishing negotiation.


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).


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

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


nbd_aio_connect(3), nbd_aio_is_negotiating(3), nbd_create(3), nbd_get_opt_mode(3), nbd_opt_abort(3), nbd_opt_go(3), nbd_opt_info(3), nbd_opt_list(3), nbd_opt_list_meta_context(3), nbd_opt_set_meta_context(3), nbd_opt_starttls(3), nbd_opt_structured_reply(3), nbd_set_request_extended_headers(3), nbd_set_request_structured_replies(3), nbd_set_tls(3), nbd_shutdown(3), libnbd(3).


Eric Blake

Richard W.M. Jones


Copyright Red Hat


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.

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