Provided by: libnbd-dev_1.2.2-1ubuntu2_amd64 bug


       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 it is usually better to use

   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.


       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 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, 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);


       nbd_connect_command(3), nbd_kill_subprocess(3), qemu-nbd(1),, nbd_create(3), libnbd(3).


       Eric Blake

       Richard W.M. Jones


       Copyright (C) 2019 Red Hat Inc.


       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