Provided by: libnbd-dev_1.2.2-1ubuntu2_amd64 bug


       nbd_aio_pread - read from the NBD server


        #include <libnbd.h>

        typedef struct {
          int (*callback) (void *user_data, int *error);
          void *user_data;
          void (*free) (void *user_data);
        } nbd_completion_callback;

        int64_t nbd_aio_pread (struct nbd_handle *h, void *buf,
                               size_t count, uint64_t offset,
                               nbd_completion_callback completion_callback,
                               uint32_t flags);


       Issue a read command to the NBD server.

       To check if the command completed, call nbd_aio_command_completed(3).  Or supply the
       optional "completion_callback" which will be invoked as described in "Completion
       callbacks" in libnbd(3).

       Note that you must ensure "buf" is valid until the command has completed.  Other
       parameters behave as documented in nbd_pread(3).


       This call returns the 64 bit cookie of the command.  The cookie is ≥ 1.  Cookies are
       unique (per libnbd handle, not globally).


       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 connected and finished handshaking with the server, 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:

        #define LIBNBD_HAVE_NBD_AIO_PREAD 1


       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);


       "Issuing asynchronous commands" in libnbd(3), nbd_aio_pread_structured(3), nbd_pread(3),
       nbd_create(3), libnbd(3).


       Eric Blake

       Richard W.M. Jones


       Copyright (C) 2019 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