1nbd_connect_command(3) LIBNBD nbd_connect_command(3)
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6 nbd_connect_command - connect to NBD server command
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9 #include <libnbd.h>
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11 int nbd_connect_command (struct nbd_handle *h, char **argv);
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14 Run the command as a subprocess and connect to it over stdin/stdout.
15 This is for use with NBD servers which can behave like inetd clients,
16 such as "nbdkit --single".
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18 Subprocess
19 Libnbd will fork the "argv" command and pass the NBD socket to it using
20 file descriptors 0 and 1 (stdin/stdout):
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22 ┌─────────┬─────────┐ ┌────────────────┐
23 │ program │ libnbd │ │ NBD server │
24 │ │ │ │ (argv) │
25 │ │ socket ╍╍╍╍╍╍╍╍▶ stdin/stdout │
26 └─────────┴─────────┘ └────────────────┘
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28 When the NBD handle is closed the server subprocess is killed.
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31 If the call is successful the function returns 0.
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34 On error "-1" is returned.
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36 Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
37 of the error.
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40 The handle must be newly created, otherwise this call will return an
41 error.
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44 This function first appeared in libnbd 1.0.
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46 If you need to test if this function is available at compile time check
47 if the following macro is defined:
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49 #define LIBNBD_HAVE_NBD_CONNECT_COMMAND 1
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52 This example is also available as examples/connect-command.c in the
53 libnbd source code.
54
55 /* This example shows how to run an NBD server
56 * (nbdkit) as a subprocess of libnbd.
57 */
58
59 #include <stdio.h>
60 #include <stdlib.h>
61 #include <string.h>
62
63 #include <libnbd.h>
64
65 int
66 main (int argc, char *argv[])
67 {
68 struct nbd_handle *nbd;
69 char wbuf[512], rbuf[512];
70 size_t i;
71
72 /* Create the libnbd handle. */
73 nbd = nbd_create ();
74 if (nbd == NULL) {
75 fprintf (stderr, "%s\n", nbd_get_error ());
76 exit (EXIT_FAILURE);
77 }
78
79 /* Run nbdkit as a subprocess. */
80 char *args[] = {
81 "nbdkit",
82
83 /* You must use ‘-s’ (which tells nbdkit to serve
84 * a single connection on stdin/stdout).
85 */
86 "-s",
87
88 /* It is recommended to use ‘--exit-with-parent’
89 * to ensure nbdkit is always cleaned up even
90 * if the main program crashes.
91 */
92 "--exit-with-parent",
93
94 /* Use this to enable nbdkit debugging. */
95 "-v",
96
97 /* The nbdkit plugin name - this is a RAM disk. */
98 "memory", "size=1M",
99
100 /* Use NULL to terminate the arg list. */
101 NULL
102 };
103 if (nbd_connect_command (nbd, args) == -1) {
104 fprintf (stderr, "%s\n", nbd_get_error ());
105 exit (EXIT_FAILURE);
106 }
107
108 /* Write some random data to the first sector. */
109 for (i = 0; i < sizeof wbuf; ++i)
110 wbuf[i] = i % 13;
111 if (nbd_pwrite (nbd, wbuf, sizeof wbuf, 0, 0) == -1) {
112 fprintf (stderr, "%s\n", nbd_get_error ());
113 exit (EXIT_FAILURE);
114 }
115
116 /* Read the first sector back. */
117 if (nbd_pread (nbd, rbuf, sizeof rbuf, 0, 0) == -1) {
118 fprintf (stderr, "%s\n", nbd_get_error ());
119 exit (EXIT_FAILURE);
120 }
121
122 /* Close the libnbd handle. */
123 nbd_close (nbd);
124
125 /* What was read must be exactly the same as what
126 * was written.
127 */
128 if (memcmp (rbuf, wbuf, sizeof rbuf) != 0) {
129 fprintf (stderr, "FAILED: "
130 "read data did not match written data\n");
131 exit (EXIT_FAILURE);
132 }
133
134 exit (EXIT_SUCCESS);
135 }
136
138 nbd_create(3), nbd_kill_subprocess(3), libnbd(3).
139
141 Eric Blake
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143 Richard W.M. Jones
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146 Copyright (C) 2019 Red Hat Inc.
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149 This library is free software; you can redistribute it and/or modify it
150 under the terms of the GNU Lesser General Public License as published
151 by the Free Software Foundation; either version 2 of the License, or
152 (at your option) any later version.
153
154 This library is distributed in the hope that it will be useful, but
155 WITHOUT ANY WARRANTY; without even the implied warranty of
156 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
157 Lesser General Public License for more details.
158
159 You should have received a copy of the GNU Lesser General Public
160 License along with this library; if not, write to the Free Software
161 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
162 02110-1301 USA
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166libnbd-1.3.7 2020-04-23 nbd_connect_command(3)