nbd_can_df - Man Page

 #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.

The following parameters must not be NULL: h. For more information see "Non-NULL parameters" in libnbd(3).

nbd_can_df 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 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 <stdbool.h>

 #include <libnbd.h>

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

   /* Request full information. */
 #if LIBNBD_HAVE_NBD_SET_FULL_INFO /* Added in 1.4 */
   if (nbd_set_full_info (nbd, true) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
 #endif

   /* 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);
   }

   /* See if the server provided extra details,
    * using functions added in 1.4
    */
 #if LIBNBD_HAVE_NBD_GET_EXPORT_DESCRIPTION
   str = nbd_get_canonical_export_name (nbd);
   if (str)
     printf ("canonical_name = %s\n", str);
   free (str);
   str = nbd_get_export_description (nbd);
   if (str)
     printf ("description = %s\n", str);
   free (str);
 #endif

   /* 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);
 #endif
 #if LIBNBD_HAVE_NBD_CAN_BLOCK_STATUS_PAYLOAD
   /* Added in 1.18 */
   PRINT_FLAG (nbd_can_block_status_payload);
 #endif
   PRINT_FLAG (nbd_is_read_only);
   PRINT_FLAG (nbd_is_rotational);

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

   exit (EXIT_SUCCESS);
 }

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

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(3), NBD(3), nbdinfo(1), nbd_pread_structured(3), nbd_set_request_structured_replies(3), nbd_set_strict_mode(3).