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.

Note that libnbd currently enforces a maximum read buffer of 64MiB, even if the server would permit a larger buffer in a single transaction; attempts to exceed this will result in an ERANGE error. The server may enforce a smaller limit, which can be learned with nbd_get_block_size(3).

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

Note that if this command fails, and 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.

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 following parameters must not be NULL: h, buf. For more information see "Non-NULL parameters" in libnbd(3).


nbd_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                │


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_get_pread_initialize(3), nbd_pread_structured(3), nbd_set_pread_initialize(3), nbd_set_strict_mode(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