1nbd_aio_pread(3) LIBNBD nbd_aio_pread(3)
2
6 nbd_aio_pread - read from the NBD server
7
9 #include <libnbd.h>
10 typedef struct {
12 int (*callback) (void *user_data, int *error);
13 void *user_data;
14 void (*free) (void *user_data);
15 } nbd_completion_callback;
16 int64_t nbd_aio_pread (struct nbd_handle *h, void *buf,
18 size_t count, uint64_t offset,
19 nbd_completion_callback completion_callback,
20 uint32_t flags);
21
23 Issue a read command to the NBD server.
24 To check if the command completed, call nbd_aio_command_completed(3).
26 Or supply the optional "completion_callback" which will be invoked as
27 described in "Completion callbacks" in libnbd(3).
28 Note that you must ensure "buf" is valid until the command has
30 completed. Other parameters behave as documented in nbd_pread(3).
31
33 This call returns the 64 bit cookie of the command. The cookie is ≥ 1.
34 Cookies are unique (per libnbd handle, not globally).
35
37 On error "-1" is returned.
38 Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
40 of the error.
41
43 The handle must be connected and finished handshaking with the server,
44 otherwise this call will return an error.
45
47 This function first appeared in libnbd 1.0.
48 If you need to test if this function is available at compile time check
50 if the following macro is defined:
51 #define LIBNBD_HAVE_NBD_AIO_PREAD 1
53
55 This example is also available as examples/aio-connect-read.c in the
56 libnbd source code.
57 /* This example shows how to use the AIO (asynchronous) low
59 * level API to connect to a server and read the disk.
60 *
61 * Here are a few ways to try this example:
62 *
63 * nbdkit -U - linuxdisk . \
64 * --run './aio-connect-read $unixsocket'
65 *
66 * nbdkit -U - floppy . \
67 * --run './aio-connect-read $unixsocket'
68 *
69 * nbdkit -U - pattern size=1M \
70 * --run './aio-connect-read $unixsocket'
71 */
72 #include <stdio.h>
74 #include <stdlib.h>
75 #include <stdint.h>
76 #include <inttypes.h>
77 #include <assert.h>
78 #include <libnbd.h>
80 #define NR_SECTORS 32
82 #define SECTOR_SIZE 512
83 struct data {
85 uint64_t offset;
86 char sector[SECTOR_SIZE];
87 };
88 static int
90 hexdump (void *user_data, int *error)
91 {
92 struct data *data = user_data;
93 FILE *pp;
94 printf ("sector at offset 0x%" PRIx64 ":\n",
96 data->offset);
97 pp = popen ("hexdump -C", "w");
98 if (pp == NULL) {
99 perror ("popen: hexdump");
100 exit (EXIT_FAILURE);
101 }
102 fwrite (data->sector, SECTOR_SIZE, 1, pp);
103 pclose (pp);
104 printf ("\n");
105 /* Returning 1 from the callback automatically retires
107 * the command.
108 */
109 return 1;
110 }
111 int
113 main (int argc, char *argv[])
114 {
115 struct nbd_handle *nbd;
116 struct data data[NR_SECTORS];
117 size_t i;
118 if (argc != 2) {
120 fprintf (stderr, "%s socket\n", argv[0]);
121 exit (EXIT_FAILURE);
122 }
123 /* Create the libnbd handle. */
125 nbd = nbd_create ();
126 if (nbd == NULL) {
127 fprintf (stderr, "%s\n", nbd_get_error ());
128 exit (EXIT_FAILURE);
129 }
130 /* Connect to the NBD server over a Unix domain socket.
132 * This only starts the connection.
133 */
134 if (nbd_aio_connect_unix (nbd, argv[1]) == -1) {
135 fprintf (stderr, "%s\n", nbd_get_error ());
136 exit (EXIT_FAILURE);
137 }
138 /* Wait for the connection to complete. The use of
140 * nbd_poll here is only as an example. You could also
141 * integrate this with poll(2), glib or another main
142 * loop. Read libnbd(3) and the source file lib/poll.c.
143 */
144 while (!nbd_aio_is_ready (nbd)) {
145 if (nbd_poll (nbd, -1) == -1) {
146 fprintf (stderr, "%s\n", nbd_get_error ());
147 exit (EXIT_FAILURE);
148 }
149 }
150 assert (nbd_get_size (nbd) >= NR_SECTORS * SECTOR_SIZE);
152 /* Issue read commands for the first NR sectors. */
154 for (i = 0; i < NR_SECTORS; ++i) {
155 data[i].offset = i * SECTOR_SIZE;
156 /* The callback (hexdump) is called when the command
158 * completes. The buffer must continue to exist while
159 * the command is running.
160 */
161 if (nbd_aio_pread (nbd, data[i].sector, SECTOR_SIZE,
162 data[i].offset,
163 (nbd_completion_callback) {
164 .callback = hexdump,
165 .user_data = &data[i],
166 }, 0) == -1) {
167 fprintf (stderr, "%s\n", nbd_get_error ());
168 exit (EXIT_FAILURE);
169 }
170 }
171 /* Run the main loop until all the commands have
173 * completed and retired. Again the use of nbd_poll
174 * here is only as an example.
175 */
176 while (nbd_aio_in_flight (nbd) > 0) {
177 if (nbd_poll (nbd, -1) == -1) {
178 fprintf (stderr, "%s\n", nbd_get_error ());
179 exit (EXIT_FAILURE);
180 }
181 }
182 /* Close the libnbd handle. */
184 nbd_close (nbd);
185 exit (EXIT_SUCCESS);
187 }
188
190 "Issuing asynchronous commands" in libnbd(3),
191 nbd_aio_pread_structured(3), nbd_pread(3), nbd_create(3), libnbd(3).
192
194 Eric Blake
195 Richard W.M. Jones
197
199 Copyright (C) 2019 Red Hat Inc.
200
202 This library is free software; you can redistribute it and/or modify it
203 under the terms of the GNU Lesser General Public License as published
204 by the Free Software Foundation; either version 2 of the License, or
205 (at your option) any later version.
206 This library is distributed in the hope that it will be useful, but
208 WITHOUT ANY WARRANTY; without even the implied warranty of
209 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
210 Lesser General Public License for more details.
211 You should have received a copy of the GNU Lesser General Public
213 License along with this library; if not, write to the Free Software
214 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
215 02110-1301 USA
216libnbd-1.2.1 2019-11-14 nbd_aio_pread(3)