nbd_is_read_only(3)

💅 nbd_is_read_only — is the NBD export read-only?

okay so bestie, welcome to nbd_is_read_only(3) from the libnbd manual, section 3 🎀 this one's a lil baby function but she's lowkey iconic, let's get into it ✨

---

🌸 NAME

nbd_is_read_only — is the NBD export read-only?

🦋 SYNOPSIS

#include <libnbd.h>

int nbd_is_read_only (
      struct nbd_handle *h
    );

💖 DESCRIPTION

So like. This one returns true if the NBD export is read-only. That's the whole vibe. And if it is read-only? Then writes and write-like operations will fail — like, they're just not gonna happen, no cap 😭 periodt.

✨ RETURN VALUE

This call returns a boolean value. simple as that 🫶

🔥 ERRORS

On error -1 is returned. yep, the classic.

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). like pls don't pass NULL, she will NOT be okay with that 💀

🧚 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 she already knows the tea from earlier, she doesn't need to go ask again 💅

🌈 HANDLE STATE

nbd_is_read_only 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

bestie the pattern is: if the handshake already happened (or is happening), you're so good. if the handle is brand new or straight-up deceased, that's a no 🙈

🎀 VERSION

This function first appeared in libnbd 1.0. she's an OG, a founding member of the group chat 👑

If you need to test if this function is available at compile time check if the following macro is defined:

#define LIBNBD_HAVE_NBD_IS_READ_ONLY 1

🍓 EXAMPLE

This example is also available as examples/server-flags.c in the libnbd source code. yes bestie, it's a real file, go look at it 💜

/* 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);
}

ngl that PRINT_FLAG macro is kind of a serve. she ate 💅

👑 SEE ALSO

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

💜 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

---

and that's the whole page bestie!! this lil gem is libnbd-1.25.8, dated 2026-10-13, and it says nbd_is_read_only(3) right there in the corner 🫶 go check your export flags, ily, bye 💖🎀🦋