nbd_pread(3)

1nbd_pread(3)                        LIBNBD                        nbd_pread(3)
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NAME

6       nbd_pread - read from the NBD server
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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       Note that libnbd currently enforces a maximum read buffer of 64MiB,
24       even if the server would permit a larger buffer in a single
25       transaction; attempts to exceed this will result in an "ERANGE" error.
26       The server may enforce a smaller limit, which can be learned with
27       nbd_get_block_size(3).
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29       The "flags" parameter must be 0 for now (it exists for future NBD
30       protocol extensions).
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32       Note that if this command fails, and nbd_get_pread_initialize(3)
33       returns true, then libnbd sanitized "buf", but it is unspecified
34       whether the contents of "buf" will read as zero or as partial results
35       from the server.  If nbd_get_pread_initialize(3) returns false, then
36       libnbd did not sanitize "buf", and the contents are undefined on
37       failure.
38
39       By default, libnbd will reject attempts to use this function with
40       parameters that are likely to result in server failure, such as
41       requesting an unknown command flag.  The nbd_set_strict_mode(3)
42       function can be used to alter which scenarios should await a server
43       reply rather than failing fast.
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RETURN VALUE

46       If the call is successful the function returns 0.
47

ERRORS

49       On error "-1" is returned.
50
51       Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
52       of the error.
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HANDLE STATE

55       The handle must be connected with the server, otherwise this call will
56       return an error.
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VERSION

59       This function first appeared in libnbd 1.0.
60
61       If you need to test if this function is available at compile time check
62       if the following macro is defined:
63
64        #define LIBNBD_HAVE_NBD_PREAD 1
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EXAMPLE

67       This example is also available as examples/fetch-first-sector.c in the
68       libnbd source code.
69
70        /* This example shows how to connect to an NBD server
71         * and fetch and print the first sector (usually the
72         * boot sector or partition table or filesystem
73         * superblock).
74         *
75         * You can test it with nbdkit like this:
76         *
77         * nbdkit -U - floppy . \
78         *   --run './fetch-first-sector $unixsocket'
79         *
80         * The nbdkit floppy plugin creates an MBR disk so the
81         * first sector is the partition table.
82         */
83
84        #include <stdio.h>
85        #include <stdlib.h>
86
87        #include <libnbd.h>
88
89        int
90        main (int argc, char *argv[])
91        {
92          struct nbd_handle *nbd;
93          char buf[512];
94          FILE *pp;
95
96          if (argc != 2) {
97            fprintf (stderr, "%s socket\n", argv[0]);
98            exit (EXIT_FAILURE);
99          }
100
101          /* Create the libnbd handle. */
102          nbd = nbd_create ();
103          if (nbd == NULL) {
104            fprintf (stderr, "%s\n", nbd_get_error ());
105            exit (EXIT_FAILURE);
106          }
107
108          /* Connect to the NBD server over a
109           * Unix domain socket.
110           */
111          if (nbd_connect_unix (nbd, argv[1]) == -1) {
112            fprintf (stderr, "%s\n", nbd_get_error ());
113            exit (EXIT_FAILURE);
114          }
115
116          /* Read the first sector synchronously. */
117          if (nbd_pread (nbd, buf, sizeof buf, 0, 0) == -1) {
118            fprintf (stderr, "%s\n", nbd_get_error ());
119            exit (EXIT_FAILURE);
120          }
121
122          /* Close the libnbd handle. */
123          nbd_close (nbd);
124
125          /* Print the first sector. */
126          pp = popen ("hexdump -C", "w");
127          if (pp == NULL) {
128            perror ("popen: hexdump");
129            exit (EXIT_FAILURE);
130          }
131          fwrite (buf, sizeof buf, 1, pp);
132          pclose (pp);
133
134          exit (EXIT_SUCCESS);
135        }
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AUTHORS

143       Eric Blake
144
145       Richard W.M. Jones
146

COPYRIGHT

148       Copyright (C) 2019-2021 Red Hat Inc.
149

LICENSE

151       This library is free software; you can redistribute it and/or modify it
152       under the terms of the GNU Lesser General Public License as published
153       by the Free Software Foundation; either version 2 of the License, or
154       (at your option) any later version.
155
156       This library is distributed in the hope that it will be useful, but
157       WITHOUT ANY WARRANTY; without even the implied warranty of
158       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
159       Lesser General Public License for more details.
160
161       You should have received a copy of the GNU Lesser General Public
162       License along with this library; if not, write to the Free Software
163       Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
164       02110-1301 USA
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168libnbd-1.12.5                     2022-07-10                      nbd_pread(3)