1nbd_poll(3) LIBNBD nbd_poll(3)
2
6 nbd_poll - poll the handle once
7
9 #include <libnbd.h>
10 int nbd_poll (struct nbd_handle *h, int timeout);
12
14 This is a simple implementation of poll(2) which is used internally by
15 synchronous API calls. On success, it returns 0 if the "timeout" (in
16 milliseconds) occurs, or 1 if the poll completed and the state machine
17 progressed. Set "timeout" to "-1" to block indefinitely (but be careful
18 that eventual action is actually expected - for example, if the
19 connection is established but there are no commands in flight, using an
20 infinite timeout will permanently block).
21 This function is mainly useful as an example of how you might integrate
23 libnbd with your own main loop, rather than being intended as something
24 you would use.
25
27 This call returns an integer ≥ 0.
28
30 On error "-1" is returned.
31 Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
33 of the error.
34
36 This function first appeared in libnbd 1.0.
37 If you need to test if this function is available at compile time check
39 if the following macro is defined:
40 #define LIBNBD_HAVE_NBD_POLL 1
42
44 This example is also available as examples/aio-connect-read.c in the
45 libnbd source code.
46 /* This example shows how to use the AIO (asynchronous) low
48 * level API to connect to a server and read the disk.
49 *
50 * Here are a few ways to try this example:
51 *
52 * nbdkit -U - linuxdisk . \
53 * --run './aio-connect-read $unixsocket'
54 *
55 * nbdkit -U - floppy . \
56 * --run './aio-connect-read $unixsocket'
57 *
58 * nbdkit -U - pattern size=1M \
59 * --run './aio-connect-read $unixsocket'
60 */
61 #include <stdio.h>
63 #include <stdlib.h>
64 #include <stdint.h>
65 #include <inttypes.h>
66 #include <errno.h>
67 #include <assert.h>
68 #include <libnbd.h>
70 #define NR_SECTORS 32
72 #define SECTOR_SIZE 512
73 struct data {
75 uint64_t offset;
76 char sector[SECTOR_SIZE];
77 };
78 static int
80 hexdump (void *user_data, int *error)
81 {
82 struct data *data = user_data;
83 FILE *pp;
84 if (*error) {
86 errno = *error;
87 perror ("failed to read");
88 exit (EXIT_FAILURE);
89 }
90 printf ("sector at offset 0x%" PRIx64 ":\n",
92 data->offset);
93 pp = popen ("hexdump -C", "w");
94 if (pp == NULL) {
95 perror ("popen: hexdump");
96 exit (EXIT_FAILURE);
97 }
98 fwrite (data->sector, SECTOR_SIZE, 1, pp);
99 pclose (pp);
100 printf ("\n");
101 /* Returning 1 from the callback automatically retires
103 * the command.
104 */
105 return 1;
106 }
107 static struct data data[NR_SECTORS];
109 int
111 main (int argc, char *argv[])
112 {
113 struct nbd_handle *nbd;
114 size_t i;
115 if (argc != 2) {
117 fprintf (stderr, "%s socket\n", argv[0]);
118 exit (EXIT_FAILURE);
119 }
120 /* Create the libnbd handle. */
122 nbd = nbd_create ();
123 if (nbd == NULL) {
124 fprintf (stderr, "%s\n", nbd_get_error ());
125 exit (EXIT_FAILURE);
126 }
127 /* Connect to the NBD server over a Unix domain socket.
129 * This only starts the connection.
130 */
131 if (nbd_aio_connect_unix (nbd, argv[1]) == -1) {
132 fprintf (stderr, "%s\n", nbd_get_error ());
133 exit (EXIT_FAILURE);
134 }
135 /* Wait for the connection to complete. The use of
137 * nbd_poll here is only as an example. You could also
138 * integrate this with poll(2), glib or another main
139 * loop. Read libnbd(3) and the source file lib/poll.c.
140 */
141 while (!nbd_aio_is_ready (nbd)) {
142 if (nbd_poll (nbd, -1) == -1) {
143 fprintf (stderr, "%s\n", nbd_get_error ());
144 exit (EXIT_FAILURE);
145 }
146 }
147 assert (nbd_get_size (nbd) >= NR_SECTORS * SECTOR_SIZE);
149 /* Issue read commands for the first NR sectors. */
151 for (i = 0; i < NR_SECTORS; ++i) {
152 data[i].offset = i * SECTOR_SIZE;
153 /* The callback (hexdump) is called when the command
155 * completes. The buffer must continue to exist while
156 * the command is running.
157 */
158 if (nbd_aio_pread (nbd, data[i].sector, SECTOR_SIZE,
159 data[i].offset,
160 (nbd_completion_callback) {
161 .callback = hexdump,
162 .user_data = &data[i],
163 }, 0) == -1) {
164 fprintf (stderr, "%s\n", nbd_get_error ());
165 exit (EXIT_FAILURE);
166 }
167 }
168 /* Run the main loop until all the commands have
170 * completed and retired. Again the use of nbd_poll
171 * here is only as an example.
172 */
173 while (nbd_aio_in_flight (nbd) > 0) {
174 if (nbd_poll (nbd, -1) == -1) {
175 fprintf (stderr, "%s\n", nbd_get_error ());
176 exit (EXIT_FAILURE);
177 }
178 }
179 /* Close the libnbd handle. */
181 nbd_close (nbd);
182 exit (EXIT_SUCCESS);
184 }
185
187 nbd_create(3), libnbd(3), poll(2).
188
190 Eric Blake
191 Richard W.M. Jones
193
195 Copyright (C) 2019-2021 Red Hat Inc.
196
198 This library is free software; you can redistribute it and/or modify it
199 under the terms of the GNU Lesser General Public License as published
200 by the Free Software Foundation; either version 2 of the License, or
201 (at your option) any later version.
202 This library is distributed in the hope that it will be useful, but
204 WITHOUT ANY WARRANTY; without even the implied warranty of
205 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
206 Lesser General Public License for more details.
207 You should have received a copy of the GNU Lesser General Public
209 License along with this library; if not, write to the Free Software
210 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
211 02110-1301 USA
212libnbd-1.12.5 2022-07-10 nbd_poll(3)