nbd_opt_list - request the server to list all exports during negotiation


 #include <libnbd.h>

 typedef struct {
   int (*callback) (void *user_data, const char *name,
                    const char *description);
   void *user_data;
   void (*free) (void *user_data);
 } nbd_list_callback;

 int nbd_opt_list (
       struct nbd_handle *h,
       nbd_list_callback list_callback


Request that the server list all exports that it supports. This can only be used if nbd_set_opt_mode(3) enabled option mode.

The list function is called once per advertised export, with any user_data passed to this function, and with name and description supplied by the server. Many servers omit descriptions, in which case description will be an empty string. Remember that it is not safe to call nbd_set_export_name(3) from within the context of the callback function; rather, your code must copy any name needed for later use after this function completes. At present, the return value of the callback is ignored, although a return of -1 should be avoided.

For convenience, when this function succeeds, it returns the number of exports that were advertised by the server.

Not all servers understand this request, and even when it is understood, the server might intentionally send an empty list to avoid being an information leak, may encounter a failure after delivering partial results, or may refuse to answer more than one query per connection in the interest of avoiding negotiation that does not resolve. Thus, this function may succeed even when no exports are reported, or may fail but have a non-empty list. Likewise, the NBD protocol does not specify an upper bound for the number of exports that might be advertised, so client code should be aware that a server may send a lengthy list.

For nbd-server(1) you will need to allow clients to make list requests by adding allowlist=true to the [generic] section of /etc/nbd-server/config. For qemu-nbd(8), a description is set with -D.


This call returns an integer ≥ 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_opt_list 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                │


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_opt_list(3), nbd_create(3), nbd_opt_go(3), nbd_set_export_name(3), nbd_set_opt_mode(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.

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