nbd_aio_in_flight - check how many aio commands are still in flight


 #include <libnbd.h>

 int nbd_aio_in_flight (
       struct nbd_handle *h


Return the number of in-flight aio commands that are still awaiting a response from the server before they can be retired. If this returns a non-zero value when requesting a disconnect from the server (see nbd_aio_disconnect(3) and nbd_shutdown(3)), libnbd does not try to wait for those commands to complete gracefully; if the server strands commands while shutting down, nbd_aio_command_completed(3) will report those commands as failed with a status of ENOTCONN.


This call returns an integer ≥ 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. For more information see "Non-NULL parameters" in libnbd(3).


nbd_aio_in_flight can be called when the handle is in the following states:

 │ Handle created, before connecting   │ ❌ error                │
 │ Connecting                          │ ❌ error                │
 │ Connecting & handshaking (opt_mode) │ ❌ error                │
 │ Connected to the server             │ ✅ allowed              │
 │ Connection shut down                │ ✅ allowed              │
 │ Handle dead                         │ ✅ allowed              │


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 <errno.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;

   if (*error) {
     errno = *error;
     perror ("failed to read");
     exit (EXIT_FAILURE);

   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;

 static struct data data[NR_SECTORS];

 main (int argc, char *argv[])
   struct nbd_handle *nbd;
   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_command_completed(3), nbd_aio_disconnect(3), nbd_create(3), nbd_shutdown(3), libnbd(3).


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