nbd_connect_systemd_socket_activation(3) LIBNBD nbd_connect_systemd_socket_activation(3)

nbd_connect_systemd_socket_activation - connect using systemd socket activation

#include <libnbd.h>
int nbd_connect_systemd_socket_activation (
      struct nbd_handle *h, char **argv
    );

Run the command as a subprocess and connect to it using systemd socket activation.

This is especially useful for running qemu-nbd(1) as a subprocess of libnbd, for example to use it to open qcow2 files.

To run nbdkit as a subprocess, this function can be used, or nbd_connect_command(3).

To run nbd-server(1) as a subprocess, this function cannot be used, you must use nbd_connect_command(3).

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).

┌─────────┬─────────┐    ┌───────────────┐
│ program │ libnbd  │    │  qemu-nbd or  │
│         │         │    │  other server │
│         │ socket ╍╍╍╍╍╍╍╍▶             │
└─────────┴─────────┘    └───────────────┘

When the NBD handle is closed the server subprocess is killed.

Socket name

The socket activation protocol lets you optionally give the socket a name. If used, 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.

This call returns when the connection has been made. 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.

If the call is successful the function returns 0.

On error -1 is returned.

Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.

The following parameters must not be NULL: "h", "argv". For more information see "Non-NULL parameters" in libnbd(3).

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                │
└─────────────────────────────────────┴─────────────────────────┘

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

This example is also available as examples/open-qcow2.c in the libnbd source code.

/* 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);
}

nbd_aio_connect_systemd_socket_activation(3), nbd_connect_command(3), nbd_create(3), nbd_get_socket_activation_name(3), nbd_kill_subprocess(3), nbd_set_opt_mode(3), nbd_set_socket_activation_name(3), libnbd(3), qemu-nbd(1), http://0pointer.de/blog/projects/socket-activation.html.

Eric Blake

Richard W.M. Jones

Copyright Red Hat

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

2024-05-31 libnbd-1.20.0