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.

       Note that not all servers can support a "count" of 4GiB or larger.  The NBD protocol does
       not yet have a way for a client to learn if the server will enforce an even smaller
       maximum block status size, although a future extension may add a constraint visible in
       nbd_get_block_size(3).

       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_get_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