nbd_can_trim(3)

💅 nbd_can_trim(3) — aka the libnbd glow-up page

okay so real talk, bestie: this is a page from the libnbd manual, section 3, and it's about one (1) tiny function that answers one (1) tiny question. let's get into it 🎀

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✨ NAME ✨

nbd_can_trim — does the server support the trim command?

like. does it or does it not. we're about to find out 💅

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🦋 SYNOPSIS 🦋

#include <libnbd.h>

int nbd_can_trim (
      struct nbd_handle *h
    );

one handle in, one int out. minimal. iconic. no notes ✨

---

🌸 DESCRIPTION 🌸

so here's the tea 🍵 — this function returns true if the server supports the trim command (peep nbd_trim(3) and nbd_aio_trim(3), they're the whole point of this vibe check). and it returns false if the server does not. that's it. that's the function. iykyk 😌

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💖 RETURN VALUE 💖

it returns a boolean value. true or false. yes or no. slay or don't slay. 🔥

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🙈 ERRORS 🙈

on error, -1 is returned. 😭

for how to get further details of the error, go read "ERROR HANDLING" in libnbd(3) — it's all spelled out there, promise.

also heads up: these parameters must not be NULL: "h". like, genuinely not. for more info see "Non-NULL parameters" in libnbd(3).

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🧚 NOTES 🧚

💤 Non-blocking call

this call does not block — and the reason is lowkey cute: it just returns data that's already saved in the handle from the NBD protocol handshake. no waiting, no queueing, no stress. she already knows. 👑

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👑 HANDLE STATE 👑

so nbd_can_trim 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

periodt. three green lights, three red flags. 🚦

---

⭐ VERSION ⭐

this function first appeared in libnbd 1.0. vintage. classic. ate and left no crumbs.

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_TRIM 1

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🔥 EXAMPLE 🔥

this example is also available as examples/server-flags.c in the libnbd source code. (yes, it also does a bunch of other flag calls, not just trim — she's an overachiever, we love her for 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);
}

---

🌈 SEE ALSO 🌈

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

that's the whole friend group right there, go say hi 🫶

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🍓 AUTHORS 🍓

• Eric Blake
• Richard W.M. Jones

legends. both of them. 🙌

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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

ok bestie for real for real: that license paragraph is NOT a vibe, it's legally binding. the "WITHOUT ANY WARRANTY" bit is in all caps for a reason. no cap. 💀

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so yeah, that's the whole page 🦋 this one's from libnbd-1.25.8, dated 2026-10-13, and it was served to you fresh as nbd_can_trim(3). go check your trim support, main character. mwah 💋✨