Provided by: libnbd-dev_1.2.2-1ubuntu2_amd64 bug


       nbd_can_df - does the server support the don't fragment flag to pread?


        #include <libnbd.h>

        int nbd_can_df (struct nbd_handle *h);


       Returns true if the server supports structured reads with an ability to request a non-
       fragmented read (see nbd_pread_structured(3), nbd_aio_pread_structured(3)).  Returns false
       if the server either lacks structured reads or if it does not support a non-fragmented
       read request.

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


       This call returns a boolean value.


       On error "-1" is returned.

       Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.


       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_CAN_DF 1


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

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

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

        #include <libnbd.h>

        main (int argc, char *argv[])
          struct nbd_handle *nbd;
          int flag;

          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 and print the flags. */
        #define PRINT_FLAG(flag_fn)                     \
          flag = flag_fn (nbd);                         \
          if (flag == -1) {                             \
            fprintf (stderr, "%s\n", nbd_get_error ()); \
            exit (EXIT_FAILURE);                        \
          }                                             \
          printf (#flag_fn " = %s\n",                   \
                  flag ? "true" : "false");

          PRINT_FLAG (nbd_can_cache);
          PRINT_FLAG (nbd_can_df);
          PRINT_FLAG (nbd_can_flush);
          PRINT_FLAG (nbd_can_fua);
          PRINT_FLAG (nbd_can_multi_conn);
          PRINT_FLAG (nbd_can_trim);
          PRINT_FLAG (nbd_can_zero);
        #if LIBNBD_HAVE_NBD_CAN_FAST_ZERO /* Added in 1.2 */
          PRINT_FLAG (nbd_can_fast_zero);
          PRINT_FLAG (nbd_is_read_only);
          PRINT_FLAG (nbd_is_rotational);

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

          exit (EXIT_SUCCESS);


       "Flag calls" in libnbd(3), nbd_pread_structured(3), nbd_aio_pread_structured(3),
       nbd_create(3), libnbd(3).


       Eric Blake

       Richard W.M. Jones


       Copyright (C) 2019 Red Hat Inc.


       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