nbd_get_size - return the export size

       Eric Blake

       Richard W.M. Jones

       Copyright Red Hat

       Returns the size in bytes of the NBD export.

       Note that this call fails with "EOVERFLOW" for an unlikely server that advertises a size which cannot fit
       in a 64-bit signed integer.

       nbdinfo(1) --size option is a way to access this API from shell scripts.

       This call does not block, because it returns data that is saved in the handle from the NBD protocol
       handshake.

       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).

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

        /* This example shows how to connect to an NBD
         * server and read the size of the disk.
         *
         * You can test it with nbdkit like this:
         *
         * nbdkit -U - memory 1M \
         *   --run './get-size $unixsocket'
         */

        #include <stdio.h>
        #include <stdlib.h>
        #include <stdint.h>
        #include <inttypes.h>

        #include <libnbd.h>

        int
        main (int argc, char *argv[])
        {
          struct nbd_handle *nbd;
          int64_t size;

          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);
          }

          /* Connect to the NBD server over a
           * Unix domain socket.
           */
          if (nbd_connect_unix (nbd, argv[1]) == -1) {
            fprintf (stderr, "%s\n", nbd_get_error ());
            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: size = %" PRIi64 " bytes\n",
                  argv[1], size);

          /* Close the libnbd handle. */
          nbd_close (nbd);

          exit (EXIT_SUCCESS);
        }

       nbd_get_size can be called when the handle is in the following states:

        ┌─────────────────────────────────────┬─────────────────────────┐
        │ Handle created, before connecting   │ ❌ error                │
        │ Connecting                          │ ❌ error                │
        │ Connecting & handshaking (opt_mode) │ ✅ allowed              │
        │ Connected to the server             │ ✅ allowed              │
        │ Connection shut down                │ ✅ allowed              │
        │ Handle dead                         │ ❌ error                │
        └─────────────────────────────────────┴─────────────────────────┘

       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

libnbd-1.22.2                                      2025-06-16                                    nbd_get_size(3)

       nbd_get_size - return the export size

       This call returns a 64 bit signed integer ≥ 0.

nbd_create(3), nbd_opt_info(3), "Size of the export" in libnbd(3), libnbd(3), nbdinfo(1).

        #include <libnbd.h>

        int64_t nbd_get_size (
                  struct nbd_handle *h
                );

       This function first appeared in libnbd 1.0.

       If you need to test if this function is available at compile time check if the following macro is
       defined:

        #define LIBNBD_HAVE_NBD_GET_SIZE 1