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 <assert.h>
67 #include <libnbd.h>
69 #define NR_SECTORS 32
71 #define SECTOR_SIZE 512
72 struct data {
74 uint64_t offset;
75 char sector[SECTOR_SIZE];
76 };
77 static int
79 hexdump (void *user_data, int *error)
80 {
81 struct data *data = user_data;
82 FILE *pp;
83 printf ("sector at offset 0x%" PRIx64 ":\n",
85 data->offset);
86 pp = popen ("hexdump -C", "w");
87 if (pp == NULL) {
88 perror ("popen: hexdump");
89 exit (EXIT_FAILURE);
90 }
91 fwrite (data->sector, SECTOR_SIZE, 1, pp);
92 pclose (pp);
93 printf ("\n");
94 /* Returning 1 from the callback automatically retires
96 * the command.
97 */
98 return 1;
99 }
100 static struct data data[NR_SECTORS];
102 int
104 main (int argc, char *argv[])
105 {
106 struct nbd_handle *nbd;
107 size_t i;
108 if (argc != 2) {
110 fprintf (stderr, "%s socket\n", argv[0]);
111 exit (EXIT_FAILURE);
112 }
113 /* Create the libnbd handle. */
115 nbd = nbd_create ();
116 if (nbd == NULL) {
117 fprintf (stderr, "%s\n", nbd_get_error ());
118 exit (EXIT_FAILURE);
119 }
120 /* Connect to the NBD server over a Unix domain socket.
122 * This only starts the connection.
123 */
124 if (nbd_aio_connect_unix (nbd, argv[1]) == -1) {
125 fprintf (stderr, "%s\n", nbd_get_error ());
126 exit (EXIT_FAILURE);
127 }
128 /* Wait for the connection to complete. The use of
130 * nbd_poll here is only as an example. You could also
131 * integrate this with poll(2), glib or another main
132 * loop. Read libnbd(3) and the source file lib/poll.c.
133 */
134 while (!nbd_aio_is_ready (nbd)) {
135 if (nbd_poll (nbd, -1) == -1) {
136 fprintf (stderr, "%s\n", nbd_get_error ());
137 exit (EXIT_FAILURE);
138 }
139 }
140 assert (nbd_get_size (nbd) >= NR_SECTORS * SECTOR_SIZE);
142 /* Issue read commands for the first NR sectors. */
144 for (i = 0; i < NR_SECTORS; ++i) {
145 data[i].offset = i * SECTOR_SIZE;
146 /* The callback (hexdump) is called when the command
148 * completes. The buffer must continue to exist while
149 * the command is running.
150 */
151 if (nbd_aio_pread (nbd, data[i].sector, SECTOR_SIZE,
152 data[i].offset,
153 (nbd_completion_callback) {
154 .callback = hexdump,
155 .user_data = &data[i],
156 }, 0) == -1) {
157 fprintf (stderr, "%s\n", nbd_get_error ());
158 exit (EXIT_FAILURE);
159 }
160 }
161 /* Run the main loop until all the commands have
163 * completed and retired. Again the use of nbd_poll
164 * here is only as an example.
165 */
166 while (nbd_aio_in_flight (nbd) > 0) {
167 if (nbd_poll (nbd, -1) == -1) {
168 fprintf (stderr, "%s\n", nbd_get_error ());
169 exit (EXIT_FAILURE);
170 }
171 }
172 /* Close the libnbd handle. */
174 nbd_close (nbd);
175 exit (EXIT_SUCCESS);
177 }
178
180 nbd_create(3), libnbd(3), poll(2).
181
183 Eric Blake
184 Richard W.M. Jones
186
188 Copyright (C) 2019-2021 Red Hat Inc.
189
191 This library is free software; you can redistribute it and/or modify it
192 under the terms of the GNU Lesser General Public License as published
193 by the Free Software Foundation; either version 2 of the License, or
194 (at your option) any later version.
195 This library is distributed in the hope that it will be useful, but
197 WITHOUT ANY WARRANTY; without even the implied warranty of
198 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
199 Lesser General Public License for more details.
200 You should have received a copy of the GNU Lesser General Public
202 License along with this library; if not, write to the Free Software
203 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
204 02110-1301 USA
205libnbd-1.10.1 2021-10-25 nbd_poll(3)