nbd_can_df(3)

📦
version 1.25.8 · 18 versions
, in centos-stream 10

🦋 nbd_can_df — your libnbd bestie asks: does the server do the don't-fragment thing?

okay so this is a page from the libnbd(3) manual, and it's literally all about one tiny little yes/no question you can ask your NBD handle. sit down, grab a strawberry 🍓, let's go.

✨ 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 💖

So basically this returns true if the server supports structured reads with the ability to request a non-fragmented read — and like, the functions that care about this are nbd_pread_structured(3) and nbd_aio_pread_structured(3), iykyk.

It returns false if the server either:

  • lacks structured reads entirely, or
  • has them but does not support a non-fragmented read request.

Either way you get your answer. no cap. 💅

🎀 RETURN VALUE 🎀

This call returns a boolean value. that's it, that's the whole vibe.

🔥 ERRORS 🔥

On error -1 is returned. 😭

For how to get further details of the error, go read "ERROR HANDLING" in libnbd(3).

Also heads up: the following parameters must not be NULL: "h". For more info see "Non-NULL parameters" in libnbd(3).

👑 NOTES 👑

⭐ Non-blocking call

This call does not block — and that's the slay part — because it just returns data that's already saved in the handle from the NBD protocol handshake. it's giving "already knew that" energy.

🧚 HANDLE STATE 🧚

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
v important bestie: dead handle = error, pre-connect handle = error. don't come for this function before the handshake, it's not the moment. 🙈

🌈 VERSION 🌈

This function first appeared in libnbd 1.0. iconic behavior, honestly.

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 <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);
}

like, look at PRINT_FLAG (nbd_can_df); in there — that's our girl in her natural habitat. 💜

🌻 SEE ALSO 🌻

💫 AUTHORS 💫

  • Eric Blake
  • Richard W.M. Jones

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

k that's the whole page bestie, mwah 💋 — this little love letter came to you from libnbd version libnbd-1.25.8, dated 2026-10-13. go forth and check your flags, periodt. 🎀✨

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