nbd_pread - read from the NBD server


 #include <libnbd.h>

 int nbd_pread (struct nbd_handle *h, void *buf,
                size_t count, uint64_t offset,
                uint32_t flags);


Issue a read command to the NBD server for the range starting at offset and ending at offset + count - 1. NBD can only read all or nothing using this call. The call returns when the data has been read fully into buf or there is an error. See also nbd_pread_structured(3), if finer visibility is required into the server's replies, or if you want to use LIBNBD_CMD_FLAG_DF.

The flags parameter must be 0 for now (it exists for future NBD protocol extensions).

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.


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 connected 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:



This example is also available as examples/fetch-first-sector.c in the libnbd source code.

 /* This example shows how to connect to an NBD server
  * and fetch and print the first sector (usually the
  * boot sector or partition table or filesystem
  * superblock).
  * You can test it with nbdkit like this:
  * nbdkit -U - floppy . \
  *   --run './fetch-first-sector $unixsocket'
  * The nbdkit floppy plugin creates an MBR disk so the
  * first sector is the partition table.
 #include <stdio.h>
 #include <stdlib.h>
 #include <libnbd.h>
 main (int argc, char *argv[])
   struct nbd_handle *nbd;
   char buf[512];
   FILE *pp;
   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.
   if (nbd_connect_unix (nbd, argv[1]) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   /* Read the first sector synchronously. */
   if (nbd_pread (nbd, buf, sizeof buf, 0, 0) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   /* Close the libnbd handle. */
   nbd_close (nbd);
   /* Print the first sector. */
   pp = popen ("hexdump -C", "w");
   if (pp == NULL) {
     perror ("popen: hexdump");
     exit (EXIT_FAILURE);
   fwrite (buf, sizeof buf, 1, pp);
   pclose (pp);
   exit (EXIT_SUCCESS);


nbd_aio_pread(3), nbd_create(3), nbd_get_block_size(3), nbd_pread_structured(3), nbd_set_strict_mode(3), libnbd(3).


Eric Blake

Richard W.M. Jones


Copyright (C) 2019-2021 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.

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