nbd_can_cache - Man Page

does the server support the cache command?

Synopsis

 #include <libnbd.h>

 int nbd_can_cache (struct nbd_handle *h);

Description

Returns true if the server supports the cache command (see nbd_cache(3), nbd_aio_cache(3)).  Returns false if the server does not.

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.

Handle State

The handle must be connected and finished handshaking 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_CACHE 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;
   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);
 #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_cache(3), nbd_cache(3), nbd_create(3), “Flag calls” in libnbd(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), nbd_aio_cache(3), nbd_cache(3).

2020-03-31 libnbd-1.3.6 LIBNBD