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


       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).


       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 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_PREAD 1


       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_pread_structured(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