nbd_connect_command - connect to NBD server command


 #include <libnbd.h>

 int nbd_connect_command (
       struct nbd_handle *h, char **argv


Run the command as a subprocess and connect to it over stdin/stdout. This is for use with NBD servers which can behave like inetd clients, such as nbdkit(1) using the -s/--single flag, and nbd-server(1) with port number set to 0.

To run qemu-nbd(1), use nbd_connect_systemd_socket_activation(3) instead.


Libnbd will fork the argv command and pass the NBD socket to it using file descriptors 0 and 1 (stdin/stdout):

 ┌─────────┬─────────┐    ┌────────────────┐
 │ program │ libnbd  │    │   NBD server   │
 │         │         │    │       (argv)   │
 │         │ socket ╍╍╍╍╍╍╍╍▶ stdin/stdout │
 └─────────┴─────────┘    └────────────────┘

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

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_command 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 1.0.

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/connect-command.c in the libnbd source code.

 /* This example shows how to run an NBD server
  * (nbdkit) as a subprocess of libnbd.

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

 #include <libnbd.h>

 main (int argc, char *argv[])
   struct nbd_handle *nbd;
   char wbuf[512], rbuf[512];
   size_t i;

   /* Create the libnbd handle. */
   nbd = nbd_create ();
   if (nbd == NULL) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);

   /* Run nbdkit as a subprocess. */
   char *args[] = {

     /* You must use ‘-s’ (which tells nbdkit to serve
      * a single connection on stdin/stdout).

     /* It is recommended to use ‘--exit-with-parent’
      * to ensure nbdkit is always cleaned up even
      * if the main program crashes.

     /* Use this to enable nbdkit debugging. */

     /* The nbdkit plugin name - this is a RAM disk. */
     "memory", "size=1M",

     /* Use NULL to terminate the arg list. */
   if (nbd_connect_command (nbd, args) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);

   /* Write some random data to the first sector. */
   for (i = 0; i < sizeof wbuf; ++i)
     wbuf[i] = i % 13;
   if (nbd_pwrite (nbd, wbuf, sizeof wbuf, 0, 0) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);

   /* Read the first sector back. */
   if (nbd_pread (nbd, rbuf, sizeof rbuf, 0, 0) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);

   /* Close the libnbd handle. */
   nbd_close (nbd);

   /* What was read must be exactly the same as what
    * was written.
   if (memcmp (rbuf, wbuf, sizeof rbuf) != 0) {
     fprintf (stderr, "FAILED: "
              "read data did not match written data\n");
     exit (EXIT_FAILURE);

   exit (EXIT_SUCCESS);


nbd_aio_connect_command(3), nbd_connect_systemd_socket_activation(3), nbd_create(3), nbd_kill_subprocess(3), nbd_set_opt_mode(3), libnbd(3), nbdkit(1).


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