1nbd_pread(3)                        LIBNBD                        nbd_pread(3)
2
3
4

NAME

6       nbd_pread - read from the NBD server
7

SYNOPSIS

9        #include <libnbd.h>
10
11        int nbd_pread (struct nbd_handle *h, void *buf,
12                       size_t count, uint64_t offset,
13                       uint32_t flags);
14

DESCRIPTION

16       Issue a read command to the NBD server for the range starting at
17       "offset" and ending at "offset" + "count" - 1.  NBD can only read all
18       or nothing using this call.  The call returns when the data has been
19       read fully into "buf" or there is an error.  See also
20       nbd_pread_structured(3), if finer visibility is required into the
21       server's replies, or if you want to use "LIBNBD_CMD_FLAG_DF".
22
23       The "flags" parameter must be 0 for now (it exists for future NBD
24       protocol extensions).
25

RETURN VALUE

27       If the call is successful the function returns 0.
28

ERRORS

30       On error "-1" is returned.
31
32       Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
33       of the error.
34

HANDLE STATE

36       The handle must be connected and finished handshaking with the server,
37       otherwise this call will return an error.
38

VERSION

40       This function first appeared in libnbd 1.0.
41
42       If you need to test if this function is available at compile time check
43       if the following macro is defined:
44
45        #define LIBNBD_HAVE_NBD_PREAD 1
46

EXAMPLE

48       This example is also available as examples/fetch-first-sector.c in the
49       libnbd source code.
50
51        /* This example shows how to connect to an NBD server
52         * and fetch and print the first sector (usually the
53         * boot sector or partition table or filesystem
54         * superblock).
55         *
56         * You can test it with nbdkit like this:
57         *
58         * nbdkit -U - floppy . \
59         *   --run './fetch-first-sector $unixsocket'
60         *
61         * The nbdkit floppy plugin creates an MBR disk so the
62         * first sector is the partition table.
63         */
64
65        #include <stdio.h>
66        #include <stdlib.h>
67
68        #include <libnbd.h>
69
70        int
71        main (int argc, char *argv[])
72        {
73          struct nbd_handle *nbd;
74          char buf[512];
75          FILE *pp;
76
77          if (argc != 2) {
78            fprintf (stderr, "%s socket\n", argv[0]);
79            exit (EXIT_FAILURE);
80          }
81
82          /* Create the libnbd handle. */
83          nbd = nbd_create ();
84          if (nbd == NULL) {
85            fprintf (stderr, "%s\n", nbd_get_error ());
86            exit (EXIT_FAILURE);
87          }
88
89          /* Connect to the NBD server over a
90           * Unix domain socket.
91           */
92          if (nbd_connect_unix (nbd, argv[1]) == -1) {
93            fprintf (stderr, "%s\n", nbd_get_error ());
94            exit (EXIT_FAILURE);
95          }
96
97          /* Read the first sector synchronously. */
98          if (nbd_pread (nbd, buf, sizeof buf, 0, 0) == -1) {
99            fprintf (stderr, "%s\n", nbd_get_error ());
100            exit (EXIT_FAILURE);
101          }
102
103          /* Close the libnbd handle. */
104          nbd_close (nbd);
105
106          /* Print the first sector. */
107          pp = popen ("hexdump -C", "w");
108          if (pp == NULL) {
109            perror ("popen: hexdump");
110            exit (EXIT_FAILURE);
111          }
112          fwrite (buf, sizeof buf, 1, pp);
113          pclose (pp);
114
115          exit (EXIT_SUCCESS);
116        }
117

SEE ALSO

119       nbd_aio_pread(3), nbd_pread_structured(3), nbd_create(3), libnbd(3).
120

AUTHORS

122       Eric Blake
123
124       Richard W.M. Jones
125
127       Copyright (C) 2019 Red Hat Inc.
128

LICENSE

130       This library is free software; you can redistribute it and/or modify it
131       under the terms of the GNU Lesser General Public License as published
132       by the Free Software Foundation; either version 2 of the License, or
133       (at your option) any later version.
134
135       This library is distributed in the hope that it will be useful, but
136       WITHOUT ANY WARRANTY; without even the implied warranty of
137       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
138       Lesser General Public License for more details.
139
140       You should have received a copy of the GNU Lesser General Public
141       License along with this library; if not, write to the Free Software
142       Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
143       02110-1301 USA
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147libnbd-1.2.1                      2019-11-14                      nbd_pread(3)
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