nbd_is_read_only - Man Page

​ #include <libnbd.h>
​
​ int nbd_is_read_only (
​       struct nbd_handle *h
​     );

Returns true if the NBD export is read-only; writes and write-like operations will fail.

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_is_read_only 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_IS_READ_ONLY 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_create(3), ​nbd_opt_info(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