Ubuntu Manpage: nbd_pread_structured - read from the NBD server

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.

       The following parameters must not be NULL: "h", "buf".  For more information see "Non-NULL parameters" in
       libnbd(3).

HANDLE STATE

       nbd_pread_structured can be called when the handle is in the following state:

        ┌─────────────────────────────────────┬─────────────────────────┐
        │ Handle created, before connecting   │ ❌ error                │
        │ Connecting                          │ ❌ error                │
        │ Connecting & handshaking (opt_mode) │ ❌ error                │
        │ Connected to the server             │ ✅ allowed              │
        │ Connection shut down                │ ❌ error                │
        │ Handle dead                         │ ❌ 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

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright Red Hat

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

libnbd-1.20.0                                      2024-04-05                            nbd_pread_structured(3)

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

HANDLE STATE

VERSION

SEE ALSO

AUTHORS

COPYRIGHT

LICENSE