nbd\_cache(3) LIBNBD nbd\_cache(3)

# [NAME](#NAME)

nbd\_cache - send cache (prefetch) command to the NBD server

# [SYNOPSIS](#SYNOPSIS)

```
 #include <libnbd.h>
 int nbd_cache (
       struct nbd_handle *h, uint64_t count,
       uint64_t offset, uint32_t flags
     );
```

# [DESCRIPTION](#DESCRIPTION)

Issue the cache (prefetch) command to the NBD server, which if supported by the server causes data to be prefetched into faster storage by the server, speeding up a subsequent [**nbd\_pread**(3)](/man/os/libnbd/nbd_pread.3) call. The server can also silently ignore this command. Note this will generally return an error if [**nbd\_can\_cache**(3)](/man/os/libnbd/nbd_can_cache.3) is false.

Note that not all servers can support a "count" of 4GiB or larger; [**nbd\_get\_extended\_headers\_negotiated**(3)](/man/os/libnbd/nbd_get_extended_headers_negotiated.3) indicates which servers will parse a request larger than 32 bits. The NBD protocol does not yet have a way for a client to learn if the server will enforce an even smaller maximum cache size, although a future extension may add a constraint visible in [**nbd\_get\_block\_size**(3)](/man/os/libnbd/nbd_get_block_size.3).

The "flags" parameter must be 0 for now (it exists for future NBD protocol extensions).

# [RETURN VALUE](#RETURN_VALUE)

If the call is successful the function returns 0.

# [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". For more information see "Non-NULL parameters" in [**libnbd**(3)](/man/os/libnbd/libnbd.3).

# [NOTES](#NOTES)

## [Strict call](#Strict_call)

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.

# [HANDLE STATE](#HANDLE_STATE)

nbd\_cache 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_CACHE 1
```

# [SEE ALSO](#SEE_ALSO)

[**nbd\_aio\_cache**(3)](/man/os/libnbd/nbd_aio_cache.3), [**nbd\_can\_cache**(3)](/man/os/libnbd/nbd_can_cache.3), [**nbd\_create**(3)](/man/os/libnbd/nbd_create.3), [**nbd\_get\_block\_size**(3)](/man/os/libnbd/nbd_get_block_size.3), [**nbd\_get\_extended\_headers\_negotiated**(3)](/man/os/libnbd/nbd_get_extended_headers_negotiated.3), [**nbd\_pread**(3)](/man/os/libnbd/nbd_pread.3), [**nbd\_set\_strict\_mode**(3)](/man/os/libnbd/nbd_set_strict_mode.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

2026-10-13 libnbd-1.25.8

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Document: nbd_cache(3) from libnbd 1.25.8-r0 (Chainguard OS)