nbd_zero(3)

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

💅 nbd_zero — bestie edition

okay so this is nbd_zero(3) from the libnbd girlypop squad, and it's literally the function that tells your NBD server to go write some zeroes. slay.

✨ NAME ✨

nbd_zero — send write zeroes command to the NBD server. that's it, that's the vibe 🫶

🎀 SYNOPSIS 🎀

#include <libnbd.h>

int nbd_zero (
      struct nbd_handle *h, uint64_t count,
      uint64_t offset, uint32_t flags
    );

like yes, four args, we been knew 💅

🌸 DESCRIPTION 🌸

okay so what this does: it issues a write zeroes command to the NBD server. and if the server supports it, it causes zeroes to be written efficiently, starting at offset and ending at offset + count - 1. no cap, it's giving efficiency icon.

the call returns when the command has been acknowledged by the server, or there is an error. simple as that.

heads up bestie: this will generally return an error if nbd_can_zero(3) is false, or if nbd_is_read_only(3) is true. so like, vibe check your server first 🙈

also — and this is important — not all servers can support a count of 4GiB or larger. nbd_get_extended_headers_negotiated(3) tells you which servers will actually parse a request bigger than 32 bits. and here's the messy part: the NBD protocol does not yet have a way for a client to learn if the server will enforce an even smaller maximum zero size, although a future extension may add a constraint visible in nbd_get_block_size(3). also also, some servers may permit a larger zero request only when the LIBNBD_CMD_FLAG_FAST_ZERO is in use. iykyk 😭

now the flags parameter, queen of options. it may be 0 for no flags, or it may contain:

  • LIBNBD_CMD_FLAG_FUA — meaning the server should not return until the data has been committed to permanent storage. real "don't @ me until it's saved" energy. (if that is supported — some servers cannot do this, see nbd_can_fua(3))
  • LIBNBD_CMD_FLAG_NO_HOLE — meaning the server should favor writing actual allocated zeroes over punching a hole. allocated zeroes >>> hole-punched zeroes, apparently 💀
  • LIBNBD_CMD_FLAG_FAST_ZERO — meaning the server must fail quickly if writing zeroes is no faster than a normal write. like girl, if you can't do it fast, don't do it at all. (if that is supported — some servers cannot do this, see nbd_can_fast_zero(3))

⭐ RETURN VALUE ⭐

if the call is successful, the function returns 0. that's the good ending 🎀

💀 ERRORS 💀

on error -1 is returned. not the good ending.

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 please don't pass NULL, i'm begging 🙏

🦋 NOTES 🦋

Strict call

by default, libnbd will reject attempts to use this function with parameters that are likely to result in server failure — such as requesting an unknown command flag. so it's protective, it's giving older sister energy 💅. the nbd_set_strict_mode(3) function can be used to alter which scenarios should await a server reply rather than failing fast.

🌈 HANDLE STATE 🌈

nbd_zero can be called when the handle is in the following state:

  • Handle created, before connecting — ❌ error
  • Connecting — ❌ error
  • Connecting & handshaking (opt_mode) — ❌ error
  • Connected to the server — ✅ allowed
  • Connection shut down — ❌ error
  • Handle dead — ❌ error

so basically: connect first, then talk to me. periodt. ✨

👑 VERSION 👑

this function first appeared in libnbd 1.0. vintage, but make it iconic.

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

#define LIBNBD_HAVE_NBD_ZERO 1

🔥 SEE ALSO 🔥

nbd_aio_zero(3), nbd_can_fast_zero(3), nbd_can_fua(3), nbd_can_zero(3), nbd_create(3), nbd_get_block_size(3), nbd_get_extended_headers_negotiated(3), nbd_is_read_only(3), nbd_set_strict_mode(3), libnbd(3).

go read them, bestie, they're all connected 🫶

🍓 AUTHORS 🍓

Eric Blake

Richard W.M. Jones

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

okay that's the whole page, mwah 💋 this one's from libnbd-1.25.8, dated 2026-10-13 — and that's the nbd_zero(3) moment. go zero something, responsibly 🌸

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