1nbd_pread_structured(3) LIBNBD nbd_pread_structured(3)
2
6 nbd_pread_structured - read from the NBD server
7
9 #include <libnbd.h>
10 typedef struct {
12 int (*callback) (void *user_data, const void *subbuf,
13 size_t count, uint64_t offset,
14 unsigned status, int *error);
15 void *user_data;
16 void (*free) (void *user_data);
17 } nbd_chunk_callback;
18 int nbd_pread_structured (
20 struct nbd_handle *h, void *buf, size_t count,
21 uint64_t offset, nbd_chunk_callback chunk_callback,
22 uint32_t flags
23 );
24
26 Issue a read command to the NBD server for the range starting at
27 "offset" and ending at "offset" + "count" - 1. The server's response
28 may be subdivided into chunks which may arrive out of order before
29 reassembly into the original buffer; the "chunk" callback is used for
30 notification after each chunk arrives, and may perform additional
31 sanity checking on the server's reply. The callback cannot call "nbd_*"
32 APIs on the same handle since it holds the handle lock and will cause a
33 deadlock. If the callback returns -1, and no earlier error has been
34 detected, then the overall read command will fail with any non-zero
35 value stored into the callback's "error" parameter (with a default of
36 "EPROTO"); but any further chunks will still invoke the callback.
37 The "chunk" function is called once per chunk of data received, with
39 the "user_data" passed to this function. The "subbuf" and "count"
40 parameters represent the subset of the original buffer which has just
41 been populated by results from the server (in C, "subbuf" always points
42 within the original "buf"; but this guarantee may not extend to other
43 language bindings). The "offset" parameter represents the absolute
44 offset at which "subbuf" begins within the image (note that this is not
45 the relative offset of "subbuf" within the original buffer "buf").
46 Changes to "error" on output are ignored unless the callback fails. The
47 input meaning of the "error" parameter is controlled by the "status"
48 parameter, which is one of
49 "LIBNBD_READ_DATA" = 1
51 "subbuf" was populated with "count" bytes of data. On input,
52 "error" contains the errno value of any earlier detected error, or
53 zero.
54 "LIBNBD_READ_HOLE" = 2
56 "subbuf" represents a hole, and contains "count" NUL bytes. On
57 input, "error" contains the errno value of any earlier detected
58 error, or zero.
59 "LIBNBD_READ_ERROR" = 3
61 "count" is 0, so "subbuf" is unusable. On input, "error" contains
62 the errno value reported by the server as occurring while reading
63 that "offset", regardless if any earlier error has been detected.
64 Future NBD extensions may permit other values for "status", but those
66 will not be returned to a client that has not opted in to requesting
67 such extensions. If the server is non-compliant, it is possible for the
68 "chunk" function to be called more times than you expect or with
69 "count" 0 for "LIBNBD_READ_DATA" or "LIBNBD_READ_HOLE". It is also
70 possible that the "chunk" function is not called at all (in particular,
71 "LIBNBD_READ_ERROR" is used only when an error is associated with a
72 particular offset, and not when the server reports a generic error),
73 but you are guaranteed that the callback was called at least once if
74 the overall read succeeds. Libnbd does not validate that the server
75 obeyed the requirement that a read call must not have overlapping
76 chunks and must not succeed without enough chunks to cover the entire
77 request.
78 Note that libnbd currently enforces a maximum read buffer of 64MiB,
80 even if the server would permit a larger buffer in a single
81 transaction; attempts to exceed this will result in an "ERANGE" error.
82 The server may enforce a smaller limit, which can be learned with
83 nbd_get_block_size(3).
84 The "flags" parameter may be 0 for no flags, or may contain
86 "LIBNBD_CMD_FLAG_DF" meaning that the server should not reply with more
87 than one fragment (if that is supported - some servers cannot do this,
88 see nbd_can_df(3)). Libnbd does not validate that the server actually
89 obeys the flag.
90 Note that if this command fails, and nbd_get_pread_initialize(3)
92 returns true, then libnbd sanitized "buf", but it is unspecified
93 whether the contents of "buf" will read as zero or as partial results
94 from the server. If nbd_get_pread_initialize(3) returns false, then
95 libnbd did not sanitize "buf", and the contents are undefined on
96 failure.
97 By default, libnbd will reject attempts to use this function with
99 parameters that are likely to result in server failure, such as
100 requesting an unknown command flag. The nbd_set_strict_mode(3)
101 function can be used to alter which scenarios should await a server
102 reply rather than failing fast.
103
105 If the call is successful the function returns 0.
106
108 On error -1 is returned.
109 Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
111 of the error.
112 The following parameters must not be NULL: "h", "buf". For more
114 information see "Non-NULL parameters" in libnbd(3).
115
117 The handle must be connected with the server, otherwise this call will
118 return an error.
119
121 This function first appeared in libnbd 1.0.
122 If you need to test if this function is available at compile time check
124 if the following macro is defined:
125 #define LIBNBD_HAVE_NBD_PREAD_STRUCTURED 1
127
129 nbd_aio_pread_structured(3), nbd_can_df(3), nbd_create(3),
130 nbd_get_block_size(3), nbd_get_pread_initialize(3), nbd_pread(3),
131 nbd_set_pread_initialize(3), nbd_set_request_block_size(3),
132 nbd_set_strict_mode(3), libnbd(3).
133
135 Eric Blake
136 Richard W.M. Jones
138
140 Copyright Red Hat
141
143 This library is free software; you can redistribute it and/or modify it
144 under the terms of the GNU Lesser General Public License as published
145 by the Free Software Foundation; either version 2 of the License, or
146 (at your option) any later version.
147 This library is distributed in the hope that it will be useful, but
149 WITHOUT ANY WARRANTY; without even the implied warranty of
150 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
151 Lesser General Public License for more details.
152 You should have received a copy of the GNU Lesser General Public
154 License along with this library; if not, write to the Free Software
155 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
156 02110-1301 USA
157libnbd-1.18.1 2023-10-31 nbd_pread_structured(3)