nbd_connect_systemd_socket_activation - Man Page

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).

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).

 ┌─────────┬─────────┐    ┌───────────────┐
 │ 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.

Return Value

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).

Handle State

The handle must be newly created, otherwise this call will return an 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:



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>

 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,
   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

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


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

Referenced By

libnbd(3), libnbd-release-notes-1.2(1), NBD(3), nbd_aio_connect_systemd_socket_activation(3), nbd_connect_command(3), nbdcopy(1), nbdfuse(1), nbd_get_socket_activation_name(3), nbdkit-nbd-plugin(1), nbd_set_socket_activation_name(3), nbdublk(1).

2023-08-30 libnbd-1.17.4