Provided by: libnbd-dev_1.20.0-1_amd64 bug

NAME

       nbd_opt_list - request the server to list all exports during negotiation

SYNOPSIS

        #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
            );

DESCRIPTION

       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.

RETURN VALUE

       This call returns an integer ≥ 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_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                │
        └─────────────────────────────────────┴─────────────────────────┘

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_LIST 1

EXAMPLE

       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;
          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

       nbd_aio_opt_list(3), nbd_create(3), nbd_opt_go(3), nbd_set_export_name(3),
       nbd_set_opt_mode(3), libnbd(3).

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright Red Hat

LICENSE

       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