nbd_can_df(3)

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

SYNOPSIS
 #include <libnbd.h>

 int nbd_can_df (
       struct nbd_handle *h
     );

DESCRIPTION
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.

RETURN VALUE
This call returns a boolean value.

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
The handle must be negotiating, or connected with the server, or shut down, otherwise this call will return an error.

VERSION
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

EXAMPLE
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>

 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);
 }

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