nbd_pread(3)                        LIBNBD                        nbd_pread(3)


NAME
       nbd_pread - read from the NBD server

SYNOPSIS
        #include <libnbd.h>

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

DESCRIPTION
       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.

RETURN VALUE
       If the call is successful the function returns 0.

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_pread can be called when the handle is in the following state:

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        | Handle created, before connecting   | <?> error                |
        | Connecting                          | <?> error                |
        | Connecting & handshaking (opt_mode) | <?> error                |
        | Connected to the server             | <?> allowed              |
        | Connection shut down                | <?> error                |
        | Handle dead                         | <?> error                |
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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_PREAD 1

EXAMPLE
       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>

        int
        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);
        }

SEE ALSO
       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).

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

libnbd-1.20.0                     2024-05-31                      nbd_pread(3)