nbd_aio_pread(3)

NAME
nbd_aio_pread - read from the NBD server

SYNOPSIS
 #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
         );

DESCRIPTION
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. Furthermore, if the "error" parameter to "completion_callback" is set or if nbd_aio_command_completed(3) reports failure, and if nbd_get_pread_initialize(3) returns true, then libnbd sanitized "buf", but it is unspecified whether the contents of "buf" will read as zero or as partial results from the server. If nbd_get_pread_initialize(3) returns false, then libnbd did not sanitize "buf", and the contents are undefined on failure.

Other parameters behave as documented in nbd_pread(3).

By default, libnbd will reject attempts to use this function with parameters that are likely to result in server failure, such as requesting an unknown command flag. The nbd_set_strict_mode(3) function can be used to alter which scenarios should await a server reply rather than failing fast.

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

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", "buf". For more information see "Non-NULL parameters" in libnbd(3).

HANDLE STATE
nbd_aio_pread can be called when the handle is in the following state:

 ┌─────────────────────────────────────┬─────────────────────────┐
 │ Handle created, before connecting   │ ❌ error                │
 │ Connecting                          │ ❌ error                │
 │ Connecting & handshaking (opt_mode) │ ❌ error                │
 │ Connected to the server             │ ✅ allowed              │
 │ Connection shut down                │ ❌ error                │
 │ Handle dead                         │ ❌ error                │
 └─────────────────────────────────────┴─────────────────────────┘

VERSION
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

EXAMPLE
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",
           data->offset);
   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];

 int
 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,
                        data[i].offset,
                        (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);
 }

SEE ALSO
nbd_aio_command_completed(3), nbd_aio_pread_structured(3), nbd_create(3), nbd_get_pread_initialize(3), nbd_pread(3), nbd_set_pread_initialize(3), nbd_set_strict_mode(3), "Issuing asynchronous commands" in libnbd(3), libnbd(3).

AUTHORS
Eric Blake

Richard W.M. Jones

COPYRIGHT
Copyright Red Hat

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