nbd_can_block_status_payload(3)

📦
version 1.25.8 · 16 versions
, in centos-stream 10

💅 nbd_can_block_status_payload — libnbd manual, section 3

okay so bestie, you're reading nbd_can_block_status_payload(3) from the libnbd manual, and we're about to make it make sense. no cap. 🫶

✨ NAME ✨

nbd_can_block_status_payload — does the server support the block status payload flag? 👀

🌸 SYNOPSIS 🌸

#include <libnbd.h>

int nbd_can_block_status_payload (
      struct nbd_handle *h
    );

🦋 DESCRIPTION 🦋

So here's the tea: this returns true if the server supports using the "LIBNBD_CMD_FLAG_PAYLOAD_LEN" flag, which is what lets you do filtering of the block status command (peep nbd_block_status_filter(3) for that one). 💅

And it returns false if the server does not support it.

🚨 real talk: this will never return true if nbd_get_extended_headers_negotiated(3) is false. like, ever. that's the rule.

🎀 RETURN VALUE 🎀

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

💀 ERRORS 💀

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). don't be that person who passes NULL, i'm begging 🥺

🔥 NOTES 🔥

🌈 Non-blocking call

This call does not block, because it returns data that is saved in the handle from the NBD protocol handshake. it already knows the answer, it's giving prepared-and-ready. no waiting, no queue, no stress. 👑

👑 HANDLE STATE 👑

nbd_can_block_status_payload 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

like, know where your handle is at before you come at it. vibe check first. 🧚

⭐ VERSION ⭐

This function first appeared in libnbd 1.18. baby is not new but she's not ancient either.

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_BLOCK_STATUS_PAYLOAD 1

🌸 EXAMPLE 🌸

This example is also available as examples/server-flags.c in the libnbd source code. it's a whole main-character file, enjoy 🎀

/* 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 🐛

go read them, iykyk 💖

💜 AUTHORS 💜

  • Eric Blake
  • Richard W.M. Jones

absolute legends, show some respect 🫡

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 🏛️

and that's the page, bestie — courtesy of libnbd-1.25.8, dated 2026-10-13, filed under nbd_can_block_status_payload(3). slay, periodt. 💅✨

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