1nbd_pwrite(3) LIBNBD nbd_pwrite(3)
2
6 nbd_pwrite - write to the NBD server
7
9 #include <libnbd.h>
10 int nbd_pwrite (struct nbd_handle *h, const void *buf,
12 size_t count, uint64_t offset,
13 uint32_t flags);
14
16 Issue a write command to the NBD server, writing the data in "buf" to
17 the range starting at "offset" and ending at "offset" + "count" - 1.
18 NBD can only write all or nothing using this call. The call returns
19 when the command has been acknowledged by the server, or there is an
20 error. Note this will generally return an error if nbd_is_read_only(3)
21 is true.
22 The "flags" parameter may be 0 for no flags, or may contain
24 "LIBNBD_CMD_FLAG_FUA" meaning that the server should not return until
25 the data has been committed to permanent storage (if that is supported
26 - some servers cannot do this, see nbd_can_fua(3)).
27 By default, libnbd will reject attempts to use this function with
29 parameters that are likely to result in server failure, such as
30 requesting an unknown command flag. The nbd_set_strict_mode(3)
31 function can be used to alter which scenarios should await a server
32 reply rather than failing fast.
33
35 If the call is successful the function returns 0.
36
38 On error "-1" is returned.
39 Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
41 of the error.
42
44 The handle must be connected with the server, otherwise this call will
45 return an error.
46
48 This function first appeared in libnbd 1.0.
49 If you need to test if this function is available at compile time check
51 if the following macro is defined:
52 #define LIBNBD_HAVE_NBD_PWRITE 1
54
56 This example is also available as examples/reads-and-writes.c in the
57 libnbd source code.
58 /* This example shows how to do synchronous reads
60 * and writes randomly over the first megabyte of an
61 * NBD server. Note this will destroy any existing
62 * content on the NBD server.
63 *
64 * To test it with nbdkit and a RAM disk:
65 *
66 * nbdkit -U - memory 1M \
67 * --run './simple-reads-and-writes $unixsocket'
68 */
69 #include <stdio.h>
71 #include <stdlib.h>
72 #include <inttypes.h>
73 #include <assert.h>
74 #include <time.h>
75 #include <libnbd.h>
77 int
79 main (int argc, char *argv[])
80 {
81 struct nbd_handle *nbd;
82 char buf[512];
83 size_t i;
84 int64_t exportsize;
85 uint64_t offset;
86 srand (time (NULL));
88 if (argc != 2) {
90 fprintf (stderr, "%s socket\n", argv[0]);
91 exit (EXIT_FAILURE);
92 }
93 /* Create the libnbd handle. */
95 nbd = nbd_create ();
96 if (nbd == NULL) {
97 fprintf (stderr, "%s\n", nbd_get_error ());
98 exit (EXIT_FAILURE);
99 }
100 /* Connect to the NBD server over a
102 * Unix domain socket.
103 */
104 if (nbd_connect_unix (nbd, argv[1]) == -1) {
105 fprintf (stderr, "%s\n", nbd_get_error ());
106 exit (EXIT_FAILURE);
107 }
108 /* Get the size of the disk and check
110 * it's large enough.
111 */
112 exportsize = nbd_get_size (nbd);
113 if (exportsize == -1) {
114 fprintf (stderr, "%s\n", nbd_get_error ());
115 exit (EXIT_FAILURE);
116 }
117 assert (exportsize >= sizeof buf);
118 /* Check that the server is writable. */
120 if (nbd_is_read_only (nbd) == 1) {
121 fprintf (stderr, "%s: "
122 "error: this NBD export is read-only\n",
123 argv[0]);
124 exit (EXIT_FAILURE);
125 }
126 for (i = 0; i < sizeof buf; ++i)
128 buf[i] = rand ();
129 /* 1000 writes. */
131 for (i = 0; i < 1000; ++i) {
132 offset = rand () % (exportsize - sizeof buf);
133 if (nbd_pwrite (nbd, buf, sizeof buf,
135 offset, 0) == -1) {
136 fprintf (stderr, "%s\n", nbd_get_error ());
137 exit (EXIT_FAILURE);
138 }
139 }
140 /* 1000 reads and writes. */
142 for (i = 0; i < 1000; ++i) {
143 offset = rand () % (exportsize - sizeof buf);
144 if (nbd_pread (nbd, buf, sizeof buf,
145 offset, 0) == -1) {
146 fprintf (stderr, "%s\n", nbd_get_error ());
147 exit (EXIT_FAILURE);
148 }
149 offset = rand () % (exportsize - sizeof buf);
151 if (nbd_pwrite (nbd, buf, sizeof buf,
152 offset, 0) == -1) {
153 fprintf (stderr, "%s\n", nbd_get_error ());
154 exit (EXIT_FAILURE);
155 }
156 }
157 /* Sends a graceful shutdown to the server. */
159 if (nbd_shutdown (nbd, 0) == -1) {
160 fprintf (stderr, "%s\n", nbd_get_error ());
161 exit (EXIT_FAILURE);
162 }
163 nbd_close (nbd);
165 exit (EXIT_SUCCESS);
167 }
168
170 nbd_aio_pwrite(3), nbd_can_fua(3), nbd_create(3),
171 nbd_get_block_size(3), nbd_is_read_only(3), nbd_set_strict_mode(3),
172 libnbd(3).
173
175 Eric Blake
176 Richard W.M. Jones
178
180 Copyright (C) 2019-2020 Red Hat Inc.
181
183 This library is free software; you can redistribute it and/or modify it
184 under the terms of the GNU Lesser General Public License as published
185 by the Free Software Foundation; either version 2 of the License, or
186 (at your option) any later version.
187 This library is distributed in the hope that it will be useful, but
189 WITHOUT ANY WARRANTY; without even the implied warranty of
190 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
191 Lesser General Public License for more details.
192 You should have received a copy of the GNU Lesser General Public
194 License along with this library; if not, write to the Free Software
195 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
196 02110-1301 USA
197libnbd-1.6.2 2021-03-02 nbd_pwrite(3)