Provided by: libnbd-dev_1.14.1-1_amd64 
NAME
nbd_pread_structured - read from the NBD server
SYNOPSIS
#include <libnbd.h>
typedef struct {
int (*callback) (void *user_data, const void *subbuf,
size_t count, uint64_t offset,
unsigned status, int *error);
void *user_data;
void (*free) (void *user_data);
} nbd_chunk_callback;
int nbd_pread_structured (struct nbd_handle *h, void *buf,
size_t count, uint64_t offset,
nbd_chunk_callback chunk_callback,
uint32_t flags);
DESCRIPTION
Issue a read command to the NBD server for the range starting at "offset" and ending at
"offset" + "count" - 1. The server's response may be subdivided into chunks which may
arrive out of order before reassembly into the original buffer; the "chunk" callback is
used for notification after each chunk arrives, and may perform additional sanity checking
on the server's reply. The callback cannot call "nbd_*" APIs on the same handle since it
holds the handle lock and will cause a deadlock. If the callback returns "-1", and no
earlier error has been detected, then the overall read command will fail with any non-zero
value stored into the callback's "error" parameter (with a default of "EPROTO"); but any
further chunks will still invoke the callback.
The "chunk" function is called once per chunk of data received, with the "user_data"
passed to this function. The "subbuf" and "count" parameters represent the subset of the
original buffer which has just been populated by results from the server (in C, "subbuf"
always points within the original "buf"; but this guarantee may not extend to other
language bindings). The "offset" parameter represents the absolute offset at which
"subbuf" begins within the image (note that this is not the relative offset of "subbuf"
within the original buffer "buf"). Changes to "error" on output are ignored unless the
callback fails. The input meaning of the "error" parameter is controlled by the "status"
parameter, which is one of
"LIBNBD_READ_DATA" = 1
"subbuf" was populated with "count" bytes of data. On input, "error" contains the
errno value of any earlier detected error, or zero.
"LIBNBD_READ_HOLE" = 2
"subbuf" represents a hole, and contains "count" NUL bytes. On input, "error" contains
the errno value of any earlier detected error, or zero.
"LIBNBD_READ_ERROR" = 3
"count" is 0, so "subbuf" is unusable. On input, "error" contains the errno value
reported by the server as occurring while reading that "offset", regardless if any
earlier error has been detected.
Future NBD extensions may permit other values for "status", but those will not be returned
to a client that has not opted in to requesting such extensions. If the server is non-
compliant, it is possible for the "chunk" function to be called more times than you expect
or with "count" 0 for "LIBNBD_READ_DATA" or "LIBNBD_READ_HOLE". It is also possible that
the "chunk" function is not called at all (in particular, "LIBNBD_READ_ERROR" is used only
when an error is associated with a particular offset, and not when the server reports a
generic error), but you are guaranteed that the callback was called at least once if the
overall read succeeds. Libnbd does not validate that the server obeyed the requirement
that a read call must not have overlapping chunks and must not succeed without enough
chunks to cover the entire request.
Note that libnbd currently enforces a maximum read buffer of 64MiB, even if the server
would permit a larger buffer in a single transaction; attempts to exceed this will result
in an "ERANGE" error. The server may enforce a smaller limit, which can be learned with
nbd_get_block_size(3).
The "flags" parameter may be 0 for no flags, or may contain "LIBNBD_CMD_FLAG_DF" meaning
that the server should not reply with more than one fragment (if that is supported - some
servers cannot do this, see nbd_can_df(3)). Libnbd does not validate that the server
actually obeys the flag.
Note that if this command fails, and nbd_get_pread_initialize(3) returns true, then libnbd
sanitized "buf", but it is unspecified whether the contents of "buf" will read as zero or
as partial results from the server. If nbd_get_pread_initialize(3) returns false, then
libnbd did not sanitize "buf", and the contents are undefined on failure.
By default, libnbd will reject attempts to use this function with parameters that are
likely to result in server failure, such as requesting an unknown command flag. The
nbd_set_strict_mode(3) function can be used to alter which scenarios should await a server
reply rather than failing fast.
RETURN VALUE
If the call is successful the function returns 0.
ERRORS
On error "-1" is returned.
Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.
HANDLE STATE
The handle must be connected with the server, otherwise this call will return an error.
VERSION
This function first appeared in libnbd 1.0.
If you need to test if this function is available at compile time check if the following
macro is defined:
#define LIBNBD_HAVE_NBD_PREAD_STRUCTURED 1
SEE ALSO
nbd_aio_pread_structured(3), nbd_can_df(3), nbd_create(3), nbd_get_block_size(3),
nbd_get_pread_initialize(3), nbd_pread(3), nbd_set_pread_initialize(3),
nbd_set_request_block_size(3), nbd_set_strict_mode(3), libnbd(3).
AUTHORS
Eric Blake
Richard W.M. Jones
COPYRIGHT
Copyright (C) 2019-2021 Red Hat Inc.
LICENSE
This library is free software; you can redistribute it and/or modify it under the terms of
the GNU Lesser General Public License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License along with this
library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth
Floor, Boston, MA 02110-1301 USA