nbd_opt_list - Man Page

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.

Handle State

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

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

Referenced By

libnbd(3), libnbd-release-notes-1.4(1), NBD(3), nbd_aio_opt_list(3), nbd_set_export_name(3), nbd_set_opt_mode(3).

2020-10-05 libnbd-1.5.4