1nbd_pread_structured(3)             LIBNBD             nbd_pread_structured(3)
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NAME

6       nbd_pread_structured - read from the NBD server
7

SYNOPSIS

9        #include <libnbd.h>
10
11        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
19        int nbd_pread_structured (struct nbd_handle *h, void *buf,
20                                  size_t count, uint64_t offset,
21                                  nbd_chunk_callback chunk_callback,
22                                  uint32_t flags);
23

DESCRIPTION

25       Issue a read command to the NBD server for the range starting at
26       "offset" and ending at "offset" + "count" - 1.  The server's response
27       may be subdivided into chunks which may arrive out of order before
28       reassembly into the original buffer; the "chunk" callback is used for
29       notification after each chunk arrives, and may perform additional
30       sanity checking on the server's reply. The callback cannot call "nbd_*"
31       APIs on the same handle since it holds the handle lock and will cause a
32       deadlock.  If the callback returns "-1", and no earlier error has been
33       detected, then the overall read command will fail with any non-zero
34       value stored into the callback's "error" parameter (with a default of
35       "EPROTO"); but any further chunks will still invoke the callback.
36
37       The "chunk" function is called once per chunk of data received, with
38       the "user_data" passed to this function.  The "subbuf" and "count"
39       parameters represent the subset of the original buffer which has just
40       been populated by results from the server (in C, "subbuf" always points
41       within the original "buf"; but this guarantee may not extend to other
42       language bindings). The "offset" parameter represents the absolute
43       offset at which "subbuf" begins within the image (note that this is not
44       the relative offset of "subbuf" within the original buffer "buf").
45       Changes to "error" on output are ignored unless the callback fails. The
46       input meaning of the "error" parameter is controlled by the "status"
47       parameter, which is one of
48
49       "LIBNBD_READ_DATA" = 1
50           "subbuf" was populated with "count" bytes of data. On input,
51           "error" contains the errno value of any earlier detected error, or
52           zero.
53
54       "LIBNBD_READ_HOLE" = 2
55           "subbuf" represents a hole, and contains "count" NUL bytes. On
56           input, "error" contains the errno value of any earlier detected
57           error, or zero.
58
59       "LIBNBD_READ_ERROR" = 3
60           "count" is 0, so "subbuf" is unusable. On input, "error" contains
61           the errno value reported by the server as occurring while reading
62           that "offset", regardless if any earlier error has been detected.
63
64       Future NBD extensions may permit other values for "status", but those
65       will not be returned to a client that has not opted in to requesting
66       such extensions. If the server is non-compliant, it is possible for the
67       "chunk" function to be called more times than you expect or with
68       "count" 0 for "LIBNBD_READ_DATA" or "LIBNBD_READ_HOLE". It is also
69       possible that the "chunk" function is not called at all (in particular,
70       "LIBNBD_READ_ERROR" is used only when an error is associated with a
71       particular offset, and not when the server reports a generic error),
72       but you are guaranteed that the callback was called at least once if
73       the overall read succeeds. Libnbd does not validate that the server
74       obeyed the requirement that a read call must not have overlapping
75       chunks and must not succeed without enough chunks to cover the entire
76       request.
77
78       The "flags" parameter may be 0 for no flags, or may contain
79       "LIBNBD_CMD_FLAG_DF" meaning that the server should not reply with more
80       than one fragment (if that is supported - some servers cannot do this,
81       see nbd_can_df(3)). Libnbd does not validate that the server actually
82       obeys the flag.
83
84       By default, libnbd will reject attempts to use this function with
85       parameters that are likely to result in server failure, such as
86       requesting an unknown command flag.  The nbd_set_strict_mode(3)
87       function can be used to alter which scenarios should await a server
88       reply rather than failing fast.
89

RETURN VALUE

91       If the call is successful the function returns 0.
92

ERRORS

94       On error "-1" is returned.
95
96       Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
97       of the error.
98

HANDLE STATE

100       The handle must be connected with the server, otherwise this call will
101       return an error.
102

VERSION

104       This function first appeared in libnbd 1.0.
105
106       If you need to test if this function is available at compile time check
107       if the following macro is defined:
108
109        #define LIBNBD_HAVE_NBD_PREAD_STRUCTURED 1
110

SEE ALSO

112       nbd_aio_pread_structured(3), nbd_can_df(3), nbd_create(3),
113       nbd_get_block_size(3), nbd_pread(3), nbd_set_strict_mode(3), libnbd(3).
114

AUTHORS

116       Eric Blake
117
118       Richard W.M. Jones
119
121       Copyright (C) 2019-2021 Red Hat Inc.
122

LICENSE

124       This library is free software; you can redistribute it and/or modify it
125       under the terms of the GNU Lesser General Public License as published
126       by the Free Software Foundation; either version 2 of the License, or
127       (at your option) any later version.
128
129       This library is distributed in the hope that it will be useful, but
130       WITHOUT ANY WARRANTY; without even the implied warranty of
131       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
132       Lesser General Public License for more details.
133
134       You should have received a copy of the GNU Lesser General Public
135       License along with this library; if not, write to the Free Software
136       Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
137       02110-1301 USA
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141libnbd-1.7.12                     2021-05-29           nbd_pread_structured(3)
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