nbd\_aio\_pread\_structured(3) LIBNBD nbd\_aio\_pread\_structured(3)

# [NAME](#NAME)

nbd\_aio\_pread\_structured - read from the NBD server

# [SYNOPSIS](#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;
 typedef struct {
   int (*callback) (void *user_data, int *error);
   void *user_data;
   void (*free) (void *user_data);
 } nbd_completion_callback;
 int64_t nbd_aio_pread_structured (
           struct nbd_handle *h, void *buf, size_t count,
           uint64_t offset,
           nbd_chunk_callback chunk_callback,
           nbd_completion_callback completion_callback,
           uint32_t flags
         );
```

# [DESCRIPTION](#DESCRIPTION)

Issue a read command to the NBD server.

To check if the command completed, call [**nbd\_aio\_command\_completed**(3)](/man/os/libnbd/nbd_aio_command_completed.3). Or supply the optional "completion\_callback" which will be invoked as described in "Completion callbacks" in [**libnbd**(3)](/man/os/libnbd/libnbd.3).

Note that you must ensure "buf" is valid until the command has completed. Furthermore, if the "error" parameter to "completion\_callback" is set or if [**nbd\_aio\_command\_completed**(3)](/man/os/libnbd/nbd_aio_command_completed.3) reports failure, and if [**nbd\_get\_pread\_initialize**(3)](/man/os/libnbd/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)](/man/os/libnbd/nbd_get_pread_initialize.3) returns false, then libnbd did not sanitize "buf", and the contents are undefined on failure.

Other parameters behave as documented in [**nbd\_pread\_structured**(3)](/man/os/libnbd/nbd_pread_structured.3).

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)](/man/os/libnbd/nbd_set_strict_mode.3) function can be used to alter which scenarios should await a server reply rather than failing fast.

# [RETURN VALUE](#RETURN_VALUE)

This call returns the 64 bit cookie of the command. The cookie is ≥ 1. Cookies are unique (per libnbd handle, not globally).

# [ERRORS](#ERRORS)

On error -1 is returned.

Refer to "ERROR HANDLING" in [**libnbd**(3)](/man/os/libnbd/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)](/man/os/libnbd/libnbd.3).

# [HANDLE STATE](#HANDLE_STATE)

nbd\_aio\_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](#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_AIO_PREAD_STRUCTURED 1
```

# [SEE ALSO](#SEE_ALSO)

[**nbd\_aio\_command\_completed**(3)](/man/os/libnbd/nbd_aio_command_completed.3), [**nbd\_aio\_pread**(3)](/man/os/libnbd/nbd_aio_pread.3), [**nbd\_create**(3)](/man/os/libnbd/nbd_create.3), [**nbd\_get\_pread\_initialize**(3)](/man/os/libnbd/nbd_get_pread_initialize.3), [**nbd\_pread\_structured**(3)](/man/os/libnbd/nbd_pread_structured.3), [**nbd\_set\_pread\_initialize**(3)](/man/os/libnbd/nbd_set_pread_initialize.3), [**nbd\_set\_strict\_mode**(3)](/man/os/libnbd/nbd_set_strict_mode.3), "Issuing asynchronous commands" in [**libnbd**(3)](/man/os/libnbd/libnbd.3), [**libnbd**(3)](/man/os/libnbd/libnbd.3).

# [AUTHORS](#AUTHORS)

Eric Blake

Richard W.M. Jones

# [COPYRIGHT](#COPYRIGHT)

Copyright Red Hat

# [LICENSE](#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

2026-10-11 libnbd-1.25.7

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Document: nbd_aio_pread_structured(3) from libnbd 1.25.7-r3 (Chainguard OS)