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 int
102 main (int argc, char *argv[])
103 {
104 struct nbd_handle *nbd;
105 struct data data[NR_SECTORS];
106 size_t i;
107 if (argc != 2) {
109 fprintf (stderr, "%s socket\n", argv[0]);
110 exit (EXIT_FAILURE);
111 }
112 /* Create the libnbd handle. */
114 nbd = nbd_create ();
115 if (nbd == NULL) {
116 fprintf (stderr, "%s\n", nbd_get_error ());
117 exit (EXIT_FAILURE);
118 }
119 /* Connect to the NBD server over a Unix domain socket.
121 * This only starts the connection.
122 */
123 if (nbd_aio_connect_unix (nbd, argv[1]) == -1) {
124 fprintf (stderr, "%s\n", nbd_get_error ());
125 exit (EXIT_FAILURE);
126 }
127 /* Wait for the connection to complete. The use of
129 * nbd_poll here is only as an example. You could also
130 * integrate this with poll(2), glib or another main
131 * loop. Read libnbd(3) and the source file lib/poll.c.
132 */
133 while (!nbd_aio_is_ready (nbd)) {
134 if (nbd_poll (nbd, -1) == -1) {
135 fprintf (stderr, "%s\n", nbd_get_error ());
136 exit (EXIT_FAILURE);
137 }
138 }
139 assert (nbd_get_size (nbd) >= NR_SECTORS * SECTOR_SIZE);
141 /* Issue read commands for the first NR sectors. */
143 for (i = 0; i < NR_SECTORS; ++i) {
144 data[i].offset = i * SECTOR_SIZE;
145 /* The callback (hexdump) is called when the command
147 * completes. The buffer must continue to exist while
148 * the command is running.
149 */
150 if (nbd_aio_pread (nbd, data[i].sector, SECTOR_SIZE,
151 data[i].offset,
152 (nbd_completion_callback) {
153 .callback = hexdump,
154 .user_data = &data[i],
155 }, 0) == -1) {
156 fprintf (stderr, "%s\n", nbd_get_error ());
157 exit (EXIT_FAILURE);
158 }
159 }
160 /* Run the main loop until all the commands have
162 * completed and retired. Again the use of nbd_poll
163 * here is only as an example.
164 */
165 while (nbd_aio_in_flight (nbd) > 0) {
166 if (nbd_poll (nbd, -1) == -1) {
167 fprintf (stderr, "%s\n", nbd_get_error ());
168 exit (EXIT_FAILURE);
169 }
170 }
171 /* Close the libnbd handle. */
173 nbd_close (nbd);
174 exit (EXIT_SUCCESS);
176 }
177
179 nbd_create(3), libnbd(3), poll(2).
180
182 Eric Blake
183 Richard W.M. Jones
185
187 Copyright (C) 2019-2020 Red Hat Inc.
188
190 This library is free software; you can redistribute it and/or modify it
191 under the terms of the GNU Lesser General Public License as published
192 by the Free Software Foundation; either version 2 of the License, or
193 (at your option) any later version.
194 This library is distributed in the hope that it will be useful, but
196 WITHOUT ANY WARRANTY; without even the implied warranty of
197 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
198 Lesser General Public License for more details.
199 You should have received a copy of the GNU Lesser General Public
201 License along with this library; if not, write to the Free Software
202 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
203 02110-1301 USA
204libnbd-1.6.2 2021-03-02 nbd_poll(3)