1nbd_block_status(3) LIBNBD nbd_block_status(3)
2
6 nbd_block_status - send block status command to the NBD server
7
9 #include <libnbd.h>
10 typedef struct {
12 int (*callback) (void *user_data,
13 const char *metacontext,
14 uint64_t offset, uint32_t *entries,
15 size_t nr_entries, int *error);
16 void *user_data;
17 void (*free) (void *user_data);
18 } nbd_extent_callback;
19 int nbd_block_status (struct nbd_handle *h, uint64_t count,
21 uint64_t offset,
22 nbd_extent_callback extent_callback,
23 uint32_t flags);
24
26 Issue the block status command to the NBD server. If supported by the
27 server, this causes metadata context information about blocks beginning
28 from the specified offset to be returned. The "count" parameter is a
29 hint: the server may choose to return less status, or the final block
30 may extend beyond the requested range. If multiple contexts are
31 supported, the number of blocks and cumulative length of those blocks
32 need not be identical between contexts.
33 Depending on which metadata contexts were enabled before connecting
35 (see nbd_add_meta_context(3)) and which are supported by the server
36 (see nbd_can_meta_context(3)) this call returns information about
37 extents by calling back to the "extent" function. The callback cannot
38 call "nbd_*" APIs on the same handle since it holds the handle lock and
39 will cause a deadlock. If the callback returns "-1", and no earlier
40 error has been detected, then the overall block status command will
41 fail with any non-zero value stored into the callback's "error"
42 parameter (with a default of "EPROTO"); but any further contexts will
43 still invoke the callback.
44 The "extent" function is called once per type of metadata available,
46 with the "user_data" passed to this function. The "metacontext"
47 parameter is a string such as "base:allocation". The "entries" array
48 is an array of pairs of integers with the first entry in each pair
49 being the length (in bytes) of the block and the second entry being a
50 status/flags field which is specific to the metadata context. (The
51 number of pairs passed to the function is "nr_entries/2".) The NBD
52 protocol document in the section about "NBD_REPLY_TYPE_BLOCK_STATUS"
53 describes the meaning of this array; for contexts known to libnbd,
54 <libnbd.h> contains constants beginning with "LIBNBD_STATE_" that may
55 help decipher the values. On entry to the callback, the "error"
56 parameter contains the errno value of any previously detected error.
57 It is possible for the extent function to be called more times than you
59 expect (if the server is buggy), so always check the "metacontext"
60 field to ensure you are receiving the data you expect. It is also
61 possible that the extent function is not called at all, even for
62 metadata contexts that you requested. This indicates either that the
63 server doesn't support the context or for some other reason cannot
64 return the data.
65 The "flags" parameter may be 0 for no flags, or may contain
67 "LIBNBD_CMD_FLAG_REQ_ONE" meaning that the server should return only
68 one extent per metadata context where that extent does not exceed
69 "count" bytes; however, libnbd does not validate that the server obeyed
70 the flag.
71 By default, libnbd will reject attempts to use this function with
73 parameters that are likely to result in server failure, such as
74 requesting an unknown command flag. The nbd_set_strict_mode(3)
75 function can be used to alter which scenarios should await a server
76 reply rather than failing fast.
77
79 If the call is successful the function returns 0.
80
82 On error "-1" is returned.
83 Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
85 of the error.
86
88 The handle must be connected with the server, otherwise this call will
89 return an error.
90
92 This function first appeared in libnbd 1.0.
93 If you need to test if this function is available at compile time check
95 if the following macro is defined:
96 #define LIBNBD_HAVE_NBD_BLOCK_STATUS 1
98
100 nbd_add_meta_context(3), nbd_aio_block_status(3),
101 nbd_can_meta_context(3), nbd_create(3), nbd_set_strict_mode(3),
102 libnbd(3).
103
105 Eric Blake
106 Richard W.M. Jones
108
110 Copyright (C) 2019-2020 Red Hat Inc.
111
113 This library is free software; you can redistribute it and/or modify it
114 under the terms of the GNU Lesser General Public License as published
115 by the Free Software Foundation; either version 2 of the License, or
116 (at your option) any later version.
117 This library is distributed in the hope that it will be useful, but
119 WITHOUT ANY WARRANTY; without even the implied warranty of
120 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
121 Lesser General Public License for more details.
122 You should have received a copy of the GNU Lesser General Public
124 License along with this library; if not, write to the Free Software
125 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
126 02110-1301 USA
127libnbd-1.6.2 2021-03-02 nbd_block_status(3)