---
name: "nbd_can_fua"
section: "3"
platform: "linux"
package: "libnbd"
version: "1.24.4-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/nbd_can_fua"
---

# nbd\_can\_fua(3)

does the server support the FUA flag?

## NAME

nbd\_can\_fua - does the server support the FUA flag?

## SYNOPSIS

```
 #include <libnbd.h>
 int nbd_can_fua (
       struct nbd_handle *h
     );
```

## DESCRIPTION

Returns true if the server supports the FUA flag on certain commands (see [**nbd\_pwrite**(3)](/nbd_pwrite/3.md)).

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)](/libnbd/3.md) 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)](/libnbd/3.md).

## HANDLE STATE

nbd\_can\_fua 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                │
 └─────────────────────────────────────┴─────────────────────────┘
```

## 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_FUA 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 <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);
 }
```

## SEE ALSO

[**nbd\_create**(3)](/nbd_create/3.md), [**nbd\_opt\_info**(3)](/nbd_opt_info/3.md), [**nbd\_pwrite**(3)](/nbd_pwrite/3.md), [**nbd\_trim**(3)](/nbd_trim/3.md), [**nbd\_zero**(3)](/nbd_zero/3.md), "Flag calls" in [**libnbd**(3)](/libnbd/3.md), [**libnbd**(3)](/libnbd/3.md).

## 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