nbd_can_cache(3)

💅 nbd_can_cache(3) 💅

okay bestie, welcome to LIBNBD — yes that's the manual we're in, no cap — and this page is all about nbd_can_cache. Let's get into it ✨

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🌸 NAME 🌸

nbd_can_cache — does the server support the cache command? 👀

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🎀 SYNOPSIS 🎀

#include <libnbd.h>

int nbd_can_cache (
      struct nbd_handle *h
    );

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🦋 DESCRIPTION 🦋

So like, this one returns true if the server supports the cache command (peep nbd_cache(3) and nbd_aio_cache(3), iykyk). And it returns false if the server does not. That's the whole vibe check fr fr. 💖

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⭐ RETURN VALUE ⭐

This call returns a boolean value. Simple as that, slay. 💅

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💀 ERRORS 💀

On error -1 is returned. (yeah, the classic 😭)

Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error — that's where the real tea is.

Also heads up: the following parameters must not be NULL: "h". For more info, see "Non-NULL parameters" in libnbd(3). Don't come for me, that's just how it is 🫶

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👑 NOTES 👑

Non-blocking call

This call does not block, because it returns data that is saved in the handle from the NBD protocol handshake. Like, it already knows, bestie — the info is literally just sitting there. ⚡

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🌈 HANDLE STATE 🌈

You can call nbd_can_cache when the handle is in these 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

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🧚 VERSION 🧚

This function first appeared in libnbd 1.0. OG behavior. ✨

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

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🔥 EXAMPLE 🔥

This example is also available as examples/server-flags.c in the libnbd source code. Go look, it's iconic.

/* 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);
}

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🍓 SEE ALSO 🍓

nbd_aio_cache(3), nbd_cache(3), nbd_create(3), nbd_opt_info(3), "Flag calls" in libnbd(3), libnbd(3).

Go read them, they're all linked and everything, bestie 🫶

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💜 AUTHORS 💜

Eric Blake

Richard W.M. Jones

(absolute legends, ty for your service 🫡)

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🌸 COPYRIGHT 🌸

Copyright Red Hat

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🎀 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

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okay that's the whole page, bestie, you ate that 💅✨ this edition was libnbd-1.25.8, dated 2026-10-13 — so like, it's giving fresh, it's giving current, periodt 🎀🦋💖