NAME

       nbd_block_status - send block status command to the NBD server

SYNOPSIS

        #include <libnbd.h>

        typedef struct {
          int (*callback) (void *user_data,
                           const char *metacontext,
                           uint64_t offset, uint32_t *entries,
                           size_t nr_entries, int *error);
          void *user_data;
          void (*free) (void *user_data);
        } nbd_extent_callback;

        int nbd_block_status (struct nbd_handle *h, uint64_t count,
                              uint64_t offset,
                              nbd_extent_callback extent_callback,
                              uint32_t flags);

DESCRIPTION

       Issue the block status command to the NBD server.  If supported by the server, this causes metadata
       context information about blocks beginning from the specified offset to be returned. The "count"
       parameter is a hint: the server may choose to return less status, or the final block may extend beyond
       the requested range. If multiple contexts are supported, the number of blocks and cumulative length of
       those blocks need not be identical between contexts.

       Depending on which metadata contexts were enabled before connecting (see nbd_add_meta_context(3)) and
       which are supported by the server (see nbd_can_meta_context(3)) this call returns information about
       extents by calling back to the "extent" function.  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 block status command will fail with any non-zero value
       stored into the callback's "error" parameter (with a default of "EPROTO"); but any further contexts will
       still invoke the callback.

       The "extent" function is called once per type of metadata available, with the "user_data" passed to this
       function.  The "metacontext" parameter is a string such as "base:allocation".  The "entries" array is an
       array of pairs of integers with the first entry in each pair being the length (in bytes) of the block and
       the second entry being a status/flags field which is specific to the metadata context.  (The number of
       pairs passed to the function is "nr_entries/2".)  The NBD protocol document in the section about
       "NBD_REPLY_TYPE_BLOCK_STATUS" describes the meaning of this array; for contexts known to libnbd,
       <libnbd.h> contains constants beginning with "LIBNBD_STATE_" that may help decipher the values.  On entry
       to the callback, the "error" parameter contains the errno value of any previously detected error, but
       even if an earlier error was detected, the current "metacontext" and "entries" are valid.

       It is possible for the extent function to be called more times than you expect (if the server is buggy),
       so always check the "metacontext" field to ensure you are receiving the data you expect.  It is also
       possible that the extent function is not called at all, even for metadata contexts that you requested.
       This indicates either that the server doesn't support the context or for some other reason cannot return
       the data.

       The "flags" parameter may be 0 for no flags, or may contain "LIBNBD_CMD_FLAG_REQ_ONE" meaning that the
       server should return only one extent per metadata context where that extent does not exceed "count"
       bytes; however, libnbd does not validate that the server obeyed the flag.

       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_BLOCK_STATUS 1

SEE ALSO

       nbd_add_meta_context(3), nbd_aio_block_status(3), nbd_can_meta_context(3), nbd_create(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