nbd_connect_systemd_socket_activation - Man Page

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);
 }

Authors

Eric Blake

Richard W.M. Jones

Copyright

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

Info

2026-10-03 libnbd-1.25.7