nbd_connect_sy...cket_activation(3)

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

✨ nbd_connect_systemd_socket_activation — connect using systemd socket activation ✨

okay bestie, this one lives in the libnbd manual (section 3, the "functions" section, iykyk) and it is literally the glow-up of spawning a server and vibing with it 💅

🌸 NAME 🌸

nbd_connect_systemd_socket_activation — connect using systemd socket activation

🎀 SYNOPSIS 🎀

#include <libnbd.h>

int nbd_connect_systemd_socket_activation (
      struct nbd_handle *h, char **argv
    );

💖 DESCRIPTION 💖

okay so the whole move here: you give it a command, it runs that command as a subprocess, and then it connects to it using systemd socket activation. that's it, that's the vibe 🫶

it's especially useful for running qemu-nbd(1) as a subprocess of libnbd — for example to use it to open qcow2 files. iconic behavior honestly.

🔌 Socket activation 🔌

libnbd will fork the argv command and pass an NBD socket to it using special LISTEN_* environment variables (as defined by the systemd socket activation protocol). very official. very protocol. we love to see it 🎀

┌─────────┬─────────┐    ┌───────────────┐
│ program │ libnbd  │    │  qemu-nbd or  │
│         │         │    │  other server │
│         │ socket ╍╍╍╍╍╍╍╍▶             │
└─────────┴─────────┘    └───────────────┘
real talk: when the NBD handle is closed, the server subprocess is killed. like, bye. 💀

🏷️ Socket name 🏷️

the socket activation protocol lets you optionally give the socket a name — a little nickname for your socket, cute 🥺 if you use it, the name is passed to the NBD server using the LISTEN_FDNAMES environment variable.

to provide a socket name, call nbd_set_socket_activation_name(3) before calling the connect function. order matters, bestie, don't do it after 🌸

⭐ RETURN VALUE ⭐

if the call is successful, the function returns 0. slay ✨

💅 ERRORS 💅

on error, -1 is returned. not slay 😭

refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error — that's where the actual tea is 🍓

the following parameters must not be NULL: h, argv. for more information see "Non-NULL parameters" in libnbd(3). like, don't hand it nothing and expect magic, okay? 🙈

🦋 NOTES 🦋

🧚 Blocking connect call 🧚

this call returns when the connection has been made — it blocks, it waits, it's committed. by default, this proceeds all the way to transmission phase, but nbd_set_opt_mode(3) can be used for manual control over option negotiation performed before transmission phase. main character energy: you get to drive 🎀

👑 HANDLE STATE 👑

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

  • Handle created, before connecting — ✅ allowed
  • Connecting — ❌ error
  • Connecting & handshaking (opt_mode) — ❌ error
  • Connected to the server — ❌ error
  • Connection shut down — ❌ error
  • Handle dead — ❌ error
read the room: it's an ✅ before you connect situation and literally nothing else 🌈

🔥 VERSION 🔥

this function first appeared in libnbd 1.2.

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

#define LIBNBD_HAVE_NBD_CONNECT_SYSTEMD_SOCKET_ACTIVATION 1

🌟 EXAMPLE 🌟

this example is also available as examples/open-qcow2.c in the libnbd source code. yes, it's real, go look 👀

/* This example shows how to use qemu-nbd
 * to open a local qcow2 file.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <libnbd.h>

int
main (int argc, const char *argv[])
{
  const char *filename;
  struct nbd_handle *nbd;
  char buf[512];
  FILE *fp;

  if (argc != 2) {
    fprintf (stderr, "open-qcow2 file.qcow2\n");
    exit (EXIT_FAILURE);
  }
  filename = argv[1];

  /* Create the libnbd handle. */
  nbd = nbd_create ();
  if (nbd == NULL) {
    fprintf (stderr, "%s\n", nbd_get_error ());
    exit (EXIT_FAILURE);
  }

  /* Run qemu-nbd as a subprocess using
   * systemd socket activation.
   */
  char *args[] = {
    "qemu-nbd", "-f", "qcow2",
    (char *)filename,
    NULL
  };
  if (nbd_connect_systemd_socket_activation (nbd,
                                             args) == -1) {
    fprintf (stderr, "%s\n", nbd_get_error ());
    exit (EXIT_FAILURE);
  }

  /* Read the first sector and print it. */
  if (nbd_pread (nbd, buf, sizeof buf, 0, 0) == -1) {
    fprintf (stderr, "%s\n", nbd_get_error ());
    exit (EXIT_FAILURE);
  }

  fp = popen ("hexdump -C", "w");
  if (fp == NULL) {
    perror ("popen: hexdump");
    exit (EXIT_FAILURE);
  }
  fwrite (buf, sizeof buf, 1, fp);
  pclose (fp);

  /* Close the libnbd handle. */
  nbd_close (nbd);

  exit (EXIT_SUCCESS);
}

🌈 SEE ALSO 🌈

💜 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

okurrr that's the whole page 💅 this absolute queen of a manpage came to you courtesy of libnbd-1.25.8, dated 2026-10-1 — periodt, go connect something 🦋✨

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