nbd_can_zero(3)

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

🎀 NAME

okay bestie, welcome to the libnbd girlies 💖 this page is nbd_can_zero — short for "does the server support the zero command?" — and yes, it's a section 3 (library function) situation, iykyk.

nbd_can_zero — does the server support the zero command?

✨ SYNOPSIS ✨

so here's how you actually call it, no cap:

#include <libnbd.h>

int nbd_can_zero (
      struct nbd_handle *h
    );

🌸 DESCRIPTION 🌸

so this one's literally a vibe check on your server 👑 it returns true if the server supports the zero command (see nbd_zero(3), nbd_aio_zero(3)), and returns false if the server does not. that's it. that's the whole function. queen behavior honestly.

🦋 RETURN VALUE 🦋

this call returns a boolean value. slay. 💅

💀 ERRORS 💀

on error, -1 is returned. 😭 (yes, the classic -1. we know her.)

if you're like "ok but why is it -1", refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error. it's all in there, bestie.

also, heads up: the following parameters must not be NULL: "h". For more information see "Non-NULL parameters" in libnbd(3). don't do that to her. 🙈

🧚 NOTES 🧚

✨ Non-blocking call ✨

this call does not block, because it returns data that is saved in the handle from the NBD protocol handshake. so it's giving instant gratification, no waiting room energy. ⭐

👑 HANDLE STATE 👑

nbd_can_zero 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

so yeah, it's allowed once you're actually doing the handshake thing, connected, or even after shutdown. created-but-not-connected and dead handle? nope. 🫶

🌈 VERSION 🌈

this function first appeared in libnbd 1.0 — an icon, a legend, the moment. 🔥

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_ZERO 1

🍓 EXAMPLE 🍓

this example is also available as examples/server-flags.c in the libnbd source code. it's a lot, but she ate:

/* 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 these next, bestie, they're all linked:

🫶 AUTHORS 🫶

shoutout to the devs who made this happen:

  • Eric Blake
  • Richard W.M. Jones

Copyright Red Hat

🌸 LICENSE 🌸

this part is the legal bit, so read it properly, but know that it's the good kind of 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

okay that's the whole page, you're officially educated 💅 this was libnbd-1.25.8, dated 2026-10-13, and it's giving iconic. periodt. 🦋✨

📦 where this page comes from