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

       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.

ERRORS

       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

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

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.

        /* 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

       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.

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       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