nbd_aio_connect_unix - connect to the NBD server over a Unix domain socket


 #include <libnbd.h>

 int nbd_aio_connect_unix (struct nbd_handle *h,
                           const char *unixsocket);


Begin connecting to the NBD server over Unix domain socket (unixsocket). Parameters behave as documented in nbd_connect_unix(3).

You can check if the connection is still connecting by calling nbd_aio_is_connecting(3), or if it has connected to the server and completed the NBD handshake by calling nbd_aio_is_ready(3), on the connection.


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 1.0.

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/aio-connect-read.c in the libnbd source code.

 /* This example shows how to use the AIO (asynchronous) low
  * level API to connect to a server and read the disk.
  * Here are a few ways to try this example:
  * nbdkit -U - linuxdisk . \
  *   --run './aio-connect-read $unixsocket'
  * nbdkit -U - floppy . \
  *   --run './aio-connect-read $unixsocket'
  * nbdkit -U - pattern size=1M \
  *   --run './aio-connect-read $unixsocket'
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>
 #include <inttypes.h>
 #include <assert.h>
 #include <libnbd.h>
 #define NR_SECTORS 32
 #define SECTOR_SIZE 512
 struct data {
   uint64_t offset;
   char sector[SECTOR_SIZE];
 static int
 hexdump (void *user_data, int *error)
   struct data *data = user_data;
   FILE *pp;
   printf ("sector at offset 0x%" PRIx64 ":\n",
   pp = popen ("hexdump -C", "w");
   if (pp == NULL) {
     perror ("popen: hexdump");
     exit (EXIT_FAILURE);
   fwrite (data->sector, SECTOR_SIZE, 1, pp);
   pclose (pp);
   printf ("\n");
   /* Returning 1 from the callback automatically retires
    * the command.
   return 1;
 main (int argc, char *argv[])
   struct nbd_handle *nbd;
   struct data data[NR_SECTORS];
   size_t i;
   if (argc != 2) {
     fprintf (stderr, "%s socket\n", argv[0]);
     exit (EXIT_FAILURE);
   /* Create the libnbd handle. */
   nbd = nbd_create ();
   if (nbd == NULL) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   /* Connect to the NBD server over a Unix domain socket.
    * This only starts the connection.
   if (nbd_aio_connect_unix (nbd, argv[1]) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   /* Wait for the connection to complete.  The use of
    * nbd_poll here is only as an example.  You could also
    * integrate this with poll(2), glib or another main
    * loop.  Read libnbd(3) and the source file lib/poll.c.
   while (!nbd_aio_is_ready (nbd)) {
     if (nbd_poll (nbd, -1) == -1) {
       fprintf (stderr, "%s\n", nbd_get_error ());
       exit (EXIT_FAILURE);
   assert (nbd_get_size (nbd) >= NR_SECTORS * SECTOR_SIZE);
   /* Issue read commands for the first NR sectors. */
   for (i = 0; i < NR_SECTORS; ++i) {
     data[i].offset = i * SECTOR_SIZE;
     /* The callback (hexdump) is called when the command
      * completes.  The buffer must continue to exist while
      * the command is running.
     if (nbd_aio_pread (nbd, data[i].sector, SECTOR_SIZE,
                        (nbd_completion_callback) {
                          .callback = hexdump,
                          .user_data = &data[i],
                        }, 0) == -1) {
       fprintf (stderr, "%s\n", nbd_get_error ());
       exit (EXIT_FAILURE);
   /* Run the main loop until all the commands have
    * completed and retired.  Again the use of nbd_poll
    * here is only as an example.
   while (nbd_aio_in_flight (nbd) > 0) {
     if (nbd_poll (nbd, -1) == -1) {
       fprintf (stderr, "%s\n", nbd_get_error ());
       exit (EXIT_FAILURE);
   /* Close the libnbd handle. */
   nbd_close (nbd);
   exit (EXIT_SUCCESS);


nbd_aio_is_connecting(3), nbd_aio_is_ready(3), nbd_connect_unix(3), nbd_create(3), libnbd(3).


Eric Blake

Richard W.M. Jones


Copyright (C) 2019-2020 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