Provided by: libnbd-dev_1.14.1-1_amd64 
NAME
NBD - OCaml bindings for libnbd.
Module
Module NBD
Documentation
Module NBD
: sig end
OCaml bindings for libnbd.
For full documentation see libnbd-ocaml(3) and libnbd(3).
For examples written in OCaml see the libnbd source code ocaml/examples subdirectory.
exception Error of string * int
Exception thrown when an API call fails.
The string is the error message, and the int is the raw errno (if available).
exception Closed of string
Exception thrown if you call a closed handle.
type cookie = int64
module TLS : sig end
module SIZE : sig end
module CMD_FLAG : sig end
module HANDSHAKE_FLAG : sig end
module STRICT : sig end
module ALLOW_TRANSPORT : sig end
module SHUTDOWN : sig end
val aio_direction_read : int32
val aio_direction_write : int32
val aio_direction_both : int32
val read_data : int32
val read_hole : int32
val read_error : int32
val namespace_base : string
val context_base_allocation : string
val state_hole : int32
val state_zero : int32
module Buffer : sig end
Persistent buffer used in AIO calls.
type t
The handle.
val create : unit -> t
Create a new handle.
val close : t -> unit
Close a handle.
Handles can also be closed by the garbage collector when they become unreachable. This
call is used only if you want to force the handle to close now and reclaim resources
immediately.
val with_handle : (t -> 'a) -> 'a
Wrapper around NBD.create . It calls the function parameter with a newly created handle,
and ensures that NBD.close is always called even if the function throws an exception.
Use this when it is essential that the handle is closed in order to free up external
resources in a timely manner; for example if running the server as a subprocess and you
want to ensure that the subprocess is always killed; or if you need to disconnect from the
server before continuing with another operation.
val set_debug : t -> bool -> unit
set or clear the debug flag
Set or clear the debug flag. When debugging is enabled, debugging messages from the
library are printed to stderr, unless a debugging callback has been defined too (see
nbd_set_debug_callback(3)) in which case they are sent to that function. This flag
defaults to false on newly created handles, except if "LIBNBD_DEBUG=1" is set in the
environment in which case it defaults to true.
val get_debug : t -> bool
return the state of the debug flag
Return the state of the debug flag on this handle.
val set_debug_callback : t -> (string -> string -> int) -> unit
set the debug callback
Set the debug callback. This function is called when the library emits debug messages,
when debugging is enabled on a handle. The callback parameters are "user_data" passed to
this function, the name of the libnbd function emitting the debug message ("context"), and
the message itself ("msg"). If no debug callback is set on a handle then messages are
printed on "stderr".
The callback should not call "nbd_*" APIs on the same handle since it can be called while
holding the handle lock and will cause a deadlock.
val clear_debug_callback : t -> unit
clear the debug callback
Remove the debug callback if one was previously associated with the handle (with
nbd_set_debug_callback(3)). If no callback was associated this does nothing.
val set_handle_name : t -> string -> unit
set the handle name
Handles have a name which is unique within the current process. The handle name is used in
debug output.
Handle names are normally generated automatically and have the form "nbd1", "nbd2", etc.,
but you can optionally use this call to give the handles a name which is meaningful for
your application to make debugging output easier to understand.
val get_handle_name : t -> string
get the handle name
Get the name of the handle. If it was previously set by calling nbd_set_handle_name(3)
then this returns the name that was set. Otherwise it will return a generic name like
"nbd1", "nbd2", etc.
val set_private_data : t -> int -> int
set the per-handle private data
Handles contain a private data field for applications to use for any purpose.
When calling libnbd from C, the type of this field is "uintptr_t" so it can be used to
store an unsigned integer or a pointer.
In non-C bindings it can be used to store an unsigned integer.
This function sets the value of this field and returns the old value (or 0 if it was not
previously set).
val get_private_data : t -> int
get the per-handle private data
Return the value of the private data field set previously by a call to
nbd_set_private_data(3) (or 0 if it was not previously set).
val set_export_name : t -> string -> unit
set the export name
For servers which require an export name or can serve different content on different
exports, set the "export_name" to connect to. The default is the empty string "".
This is only relevant when connecting to servers using the newstyle protocol as the
oldstyle protocol did not support export names. The NBD protocol limits export names to
4096 bytes, but servers may not support the full length. The encoding of export names is
always UTF-8.
When option mode is not in use, the export name must be set before beginning a connection.
However, when nbd_set_opt_mode(3) has enabled option mode, it is possible to change the
export name prior to nbd_opt_go(3). In particular, the use of nbd_opt_list(3) during
negotiation can be used to determine a name the server is likely to accept, and
nbd_opt_info(3) can be used to learn details about an export before connecting.
This call may be skipped if using nbd_connect_uri(3) to connect to a URI that includes an
export name.
val get_export_name : t -> string
get the export name
Get the export name associated with the handle. This is the name that libnbd requests; see
nbd_get_canonical_export_name(3) for determining if the server has a different canonical
name for the given export (most common when requesting the default export name of an empty
string "")
val set_request_block_size : t -> bool -> unit
control whether NBD_OPT_GO requests block size
By default, when connecting to an export, libnbd requests that the server report any block
size restrictions. The NBD protocol states that a server may supply block sizes regardless
of whether the client requests them, and libnbd will report those block sizes (see
nbd_get_block_size(3)); conversely, if a client does not request block sizes, the server
may reject the connection instead of dealing with a client sending unaligned requests.
This function makes it possible to test server behavior by emulating older clients.
Note that even when block size is requested, the server is not obligated to provide any.
Furthermore, if block sizes are provided (whether or not the client requested them),
libnbd enforces alignment to those sizes unless nbd_set_strict_mode(3) is used to bypass
client-side safety checks.
val get_request_block_size : t -> bool
see if NBD_OPT_GO requests block size
Return the state of the block size request flag on this handle.
val set_full_info : t -> bool -> unit
control whether NBD_OPT_GO requests extra details
By default, when connecting to an export, libnbd only requests the details it needs to
service data operations. The NBD protocol says that a server can supply optional
information, such as a canonical name of the export (see nbd_get_canonical_export_name(3))
or a description of the export (see nbd_get_export_description(3)), but that a hint from
the client makes it more likely for this extra information to be provided. This function
controls whether libnbd will provide that hint.
Note that even when full info is requested, the server is not obligated to reply with all
information that libnbd requested. Similarly, libnbd will ignore any optional server
information that libnbd has not yet been taught to recognize. Furthermore, the hint to
request block sizes is independently controlled via nbd_set_request_block_size(3).
val get_full_info : t -> bool
see if NBD_OPT_GO requests extra details
Return the state of the full info request flag on this handle.
val get_canonical_export_name : t -> string
return the canonical export name, if the server has one
The NBD protocol permits a server to report an optional canonical export name, which may
differ from the client's request (as set by nbd_set_export_name(3) or nbd_connect_uri(3)).
This function accesses any name returned by the server; it may be the same as the client
request, but is more likely to differ when the client requested a connection to the
default export name (an empty string "").
Some servers are unlikely to report a canonical name unless the client specifically hinted
about wanting it, via nbd_set_full_info(3).
val get_export_description : t -> string
return the export description, if the server has one
The NBD protocol permits a server to report an optional export description. This function
reports any description returned by the server.
Some servers are unlikely to report a description unless the client specifically hinted
about wanting it, via nbd_set_full_info(3). For qemu-nbd(8), a description is set with
*-D*.
val set_tls : t -> TLS.t -> unit
enable or require TLS (authentication and encryption)
Enable or require TLS (authenticated and encrypted connections) to the NBD server. The
possible settings are:
"LIBNBD_TLS_DISABLE" Disable TLS. (The default setting, unless using nbd_connect_uri(3)
with a URI that requires TLS)
"LIBNBD_TLS_ALLOW" Enable TLS if possible.
This option is insecure (or best effort) in that in some cases it will fall back to an
unencrypted and/or unauthenticated connection if TLS could not be established. Use
"LIBNBD_TLS_REQUIRE" below if the connection must be encrypted.
Some servers will drop the connection if TLS fails so fallback may not be possible.
"LIBNBD_TLS_REQUIRE" Require an encrypted and authenticated TLS connection. Always fail to
connect if the connection is not encrypted and authenticated.
As well as calling this you may also need to supply the path to the certificates directory
(nbd_set_tls_certificates(3)), the username (nbd_set_tls_username(3)) and/or the
Pre-Shared Keys (PSK) file (nbd_set_tls_psk_file(3)). For now, when using
nbd_connect_uri(3), any URI query parameters related to TLS are not handled automatically.
Setting the level higher than zero will fail if libnbd was not compiled against gnutls;
you can test whether this is the case with nbd_supports_tls(3).
val get_tls : t -> TLS.t
get the TLS request setting
Get the TLS request setting.
Note: If you want to find out if TLS was actually negotiated on a particular connection
use nbd_get_tls_negotiated(3) instead.
val get_tls_negotiated : t -> bool
find out if TLS was negotiated on a connection
After connecting you may call this to find out if the connection is using TLS.
This is only really useful if you set the TLS request mode to "LIBNBD_TLS_ALLOW" (see
nbd_set_tls(3)), because in this mode we try to use TLS but fall back to unencrypted if it
was not available. This function will tell you if TLS was negotiated or not.
In "LIBNBD_TLS_REQUIRE" mode (the most secure) the connection would have failed if TLS
could not be negotiated, and in "LIBNBD_TLS_DISABLE" mode TLS is not tried.
val set_tls_certificates : t -> string -> unit
set the path to the TLS certificates directory
Set the path to the TLS certificates directory. If not set and TLS is used then a compiled
in default is used. For root this is "/etc/pki/libnbd/". For non-root this is
"$HOME/.pki/libnbd" and "$HOME/.config/pki/libnbd". If none of these directories can be
found then the system trusted CAs are used.
This function may be called regardless of whether TLS is supported, but will have no
effect unless nbd_set_tls(3) is also used to request or require TLS.
val set_tls_verify_peer : t -> bool -> unit
set whether we verify the identity of the server
Set this flag to control whether libnbd will verify the identity of the server from the
server's certificate and the certificate authority. This defaults to true when connecting
to TCP servers using TLS certificate authentication, and false otherwise.
This function may be called regardless of whether TLS is supported, but will have no
effect unless nbd_set_tls(3) is also used to request or require TLS.
val get_tls_verify_peer : t -> bool
get whether we verify the identity of the server
Get the verify peer flag.
val set_tls_username : t -> string -> unit
set the TLS username
Set the TLS client username. This is used if authenticating with PSK over TLS is enabled.
If not set then the local username is used.
This function may be called regardless of whether TLS is supported, but will have no
effect unless nbd_set_tls(3) is also used to request or require TLS.
val get_tls_username : t -> string
get the current TLS username
Get the current TLS username.
val set_tls_psk_file : t -> string -> unit
set the TLS Pre-Shared Keys (PSK) filename
Set the TLS Pre-Shared Keys (PSK) filename. This is used if trying to authenticate to the
server using with a pre-shared key. There is no default so if this is not set then PSK
authentication cannot be used to connect to the server.
This function may be called regardless of whether TLS is supported, but will have no
effect unless nbd_set_tls(3) is also used to request or require TLS.
val set_request_structured_replies : t -> bool -> unit
control use of structured replies
By default, libnbd tries to negotiate structured replies with the server, as this protocol
extension must be in use before nbd_can_meta_context(3) or nbd_can_df(3) can return true.
However, for integration testing, it can be useful to clear this flag rather than find a
way to alter the server to fail the negotiation request.
val get_request_structured_replies : t -> bool
see if structured replies are attempted
Return the state of the request structured replies flag on this handle.
Note: If you want to find out if structured replies were actually negotiated on a
particular connection use nbd_get_structured_replies_negotiated(3) instead.
val get_structured_replies_negotiated : t -> bool
see if structured replies are in use
After connecting you may call this to find out if the connection is using structured
replies.
val set_handshake_flags : t -> HANDSHAKE_FLAG.t list -> unit
control use of handshake flags
By default, libnbd tries to negotiate all possible handshake flags that are also supported
by the server, since omitting a handshake flag can prevent the use of other functionality
such as TLS encryption or structured replies. However, for integration testing, it can be
useful to reduce the set of flags supported by the client to test that a particular server
can handle various clients that were compliant to older versions of the NBD specification.
The "flags" argument is a bitmask, including zero or more of the following handshake
flags:
"LIBNBD_HANDSHAKE_FLAG_FIXED_NEWSTYLE" = 1 The server gracefully handles unknown option
requests from the client, rather than disconnecting. Without this flag, a client cannot
safely request to use extensions such as TLS encryption or structured replies, as the
request may cause an older server to drop the connection.
"LIBNBD_HANDSHAKE_FLAG_NO_ZEROES" = 2 If the client is forced to use "NBD_OPT_EXPORT_NAME"
instead of the preferred "NBD_OPT_GO", this flag allows the server to send fewer all-zero
padding bytes over the connection.
For convenience, the constant "LIBNBD_HANDSHAKE_FLAG_MASK" is available to describe all
flags supported by this build of libnbd. Future NBD extensions may add further flags,
which in turn may be enabled by default in newer libnbd. As such, when attempting to
disable only one specific bit, it is wiser to first call nbd_get_handshake_flags(3) and
modify that value, rather than blindly setting a constant value.
val get_handshake_flags : t -> HANDSHAKE_FLAG.t list
see which handshake flags are supported
Return the state of the handshake flags on this handle. When the handle has not yet
completed a connection (see nbd_aio_is_created(3)), this returns the flags that the client
is willing to use, provided the server also advertises those flags. After the connection
is ready (see nbd_aio_is_ready(3)), this returns the flags that were actually agreed on
between the server and client. If the NBD protocol defines new handshake flags, then the
return value from a newer library version may include bits that were undefined at the time
of compilation.
val set_pread_initialize : t -> bool -> unit
control whether libnbd pre-initializes read buffers
By default, libnbd will pre-initialize the contents of a buffer passed to calls such as
nbd_pread(3) to all zeroes prior to checking for any other errors, so that even if a
client application passed in an uninitialized buffer but fails to check for errors, it
will not result in a potential security risk caused by an accidental leak of prior heap
contents (see CVE-2022-0485 in libnbd-security(3) for an example of a security hole in an
application built against an earlier version of libnbd that lacked consistent
pre-initialization). However, for a client application that has audited that an
uninitialized buffer is never dereferenced, or which performs its own pre-initialization,
libnbd's sanitization efforts merely pessimize performance (although the time spent in
pre-initialization may pale in comparison to time spent waiting on network packets).
Calling this function with "request" set to false tells libnbd to skip the buffer
initialization step in read commands.
val get_pread_initialize : t -> bool
see whether libnbd pre-initializes read buffers
Return whether libnbd performs a pre-initialization of a buffer passed to nbd_pread(3) and
similar to all zeroes, as set by nbd_set_pread_initialize(3).
val set_strict_mode : t -> STRICT.t list -> unit
control how strictly to follow NBD protocol
By default, libnbd tries to detect requests that would trigger undefined behavior in the
NBD protocol, and rejects them client side without causing any network traffic, rather
than risking undefined server behavior. However, for integration testing, it can be handy
to relax the strictness of libnbd, to coerce it into sending such requests over the
network for testing the robustness of the server in dealing with such traffic.
The "flags" argument is a bitmask, including zero or more of the following strictness
flags:
"LIBNBD_STRICT_COMMANDS" = 1 If set, this flag rejects client requests that do not comply
with the set of advertised server flags (for example, attempting a write on a read-only
server, or attempting to use "LIBNBD_CMD_FLAG_FUA" when nbd_can_fua(3) returned false). If
clear, this flag relies on the server to reject unexpected commands.
"LIBNBD_STRICT_FLAGS" = 2 If set, this flag rejects client requests that attempt to set a
command flag not recognized by libnbd (those outside of "LIBNBD_CMD_FLAG_MASK"), or a flag
not normally associated with a command (such as using "LIBNBD_CMD_FLAG_FUA" on a read
command). If clear, all flags are sent on to the server, even if sending such a flag may
cause the server to change its reply in a manner that confuses libnbd, perhaps causing
deadlock or ending the connection.
Flags that are known by libnbd as associated with a given command (such as
"LIBNBD_CMD_FLAG_DF" for nbd_pread_structured(3) gated by nbd_can_df(3)) are controlled by
"LIBNBD_STRICT_COMMANDS" instead.
Note that the NBD protocol only supports 16 bits of command flags, even though the libnbd
API uses "uint32_t"; bits outside of the range permitted by the protocol are always a
client-side error.
"LIBNBD_STRICT_BOUNDS" = 3 If set, this flag rejects client requests that would exceed the
export bounds without sending any traffic to the server. If clear, this flag relies on the
server to detect out-of-bounds requests.
"LIBNBD_STRICT_ZERO_SIZE" = 4 If set, this flag rejects client requests with length 0. If
clear, this permits zero-length requests to the server, which may produce undefined
results.
"LIBNBD_STRICT_ALIGN" = 5 If set, and the server provided minimum block sizes (see
nbd_get_block_size(3), this flag rejects client requests that do not have length and
offset aligned to the server's minimum requirements. If clear, unaligned requests are sent
to the server, where it is up to the server whether to honor or reject the request.
For convenience, the constant "LIBNBD_STRICT_MASK" is available to describe all strictness
flags supported by this build of libnbd. Future versions of libnbd may add further flags,
which are likely to be enabled by default for additional client-side filtering. As such,
when attempting to relax only one specific bit while keeping remaining checks at the
client side, it is wiser to first call nbd_get_strict_mode(3) and modify that value,
rather than blindly setting a constant value.
val get_strict_mode : t -> STRICT.t list
see which strictness flags are in effect
Return flags indicating which protocol strictness items are being enforced locally by
libnbd rather than the server. The return value from a newer library version may include
bits that were undefined at the time of compilation.
val set_opt_mode : t -> bool -> unit
control option mode, for pausing during option negotiation
Set this flag to true in order to request that a connection command "nbd_connect_*" will
pause for negotiation options rather than proceeding all the way to the ready state, when
communicating with a newstyle server. This setting has no effect when connecting to an
oldstyle server.
When option mode is enabled, you have fine-grained control over which options are
negotiated, compared to the default of the server negotiating everything on your behalf
using settings made before starting the connection. To leave the mode and proceed on to
the ready state, you must use nbd_opt_go(3) successfully; a failed nbd_opt_go(3) returns
to the negotiating state to allow a change of export name before trying again. You may
also use nbd_opt_abort(3) to end the connection without finishing negotiation.
val get_opt_mode : t -> bool
return whether option mode was enabled
Return true if option negotiation mode was enabled on this handle.
val opt_go : t -> unit
end negotiation and move on to using an export
Request that the server finish negotiation and move on to serving the export previously
specified by the most recent nbd_set_export_name(3) or nbd_connect_uri(3). This can only
be used if nbd_set_opt_mode(3) enabled option mode.
If this fails, the server may still be in negotiation, where it is possible to attempt
another option such as a different export name; although older servers will instead have
killed the connection.
val opt_abort : t -> unit
end negotiation and close the connection
Request that the server finish negotiation, gracefully if possible, then close the
connection. This can only be used if nbd_set_opt_mode(3) enabled option mode.
val opt_list : t -> (string -> string -> int) -> int
request the server to list all exports during negotiation
Request that the server list all exports that it supports. This can only be used if
nbd_set_opt_mode(3) enabled option mode.
The "list" function is called once per advertised export, with any "user_data" passed to
this function, and with "name" and "description" supplied by the server. Many servers omit
descriptions, in which case "description" will be an empty string. Remember that it is not
safe to call nbd_set_export_name(3) from within the context of the callback function;
rather, your code must copy any "name" needed for later use after this function completes.
At present, the return value of the callback is ignored, although a return of -1 should be
avoided.
For convenience, when this function succeeds, it returns the number of exports that were
advertised by the server.
Not all servers understand this request, and even when it is understood, the server might
intentionally send an empty list to avoid being an information leak, may encounter a
failure after delivering partial results, or may refuse to answer more than one query per
connection in the interest of avoiding negotiation that does not resolve. Thus, this
function may succeed even when no exports are reported, or may fail but have a non-empty
list. Likewise, the NBD protocol does not specify an upper bound for the number of exports
that might be advertised, so client code should be aware that a server may send a lengthy
list.
For nbd-server(1) you will need to allow clients to make list requests by adding
"allowlist=true" to the " generic " section of /etc/nbd-server/config. For qemu-nbd(8), a
description is set with *-D*.
val opt_info : t -> unit
request the server for information about an export
Request that the server supply information about the export name previously specified by
the most recent nbd_set_export_name(3) or nbd_connect_uri(3). This can only be used if
nbd_set_opt_mode(3) enabled option mode.
If successful, functions like nbd_is_read_only(3) and nbd_get_size(3) will report details
about that export. In general, if nbd_opt_go(3) is called next, that call will likely
succeed with the details remaining the same, although this is not guaranteed by all
servers.
Not all servers understand this request, and even when it is understood, the server might
fail the request even when a corresponding nbd_opt_go(3) would succeed.
val opt_list_meta_context : t -> (string -> int) -> int
request the server to list available meta contexts
Request that the server list available meta contexts associated with the export previously
specified by the most recent nbd_set_export_name(3) or nbd_connect_uri(3). This can only
be used if nbd_set_opt_mode(3) enabled option mode.
The NBD protocol allows a client to decide how many queries to ask the server. Rather than
taking that list of queries as a parameter to this function, libnbd reuses the current
list of requested meta contexts as set by nbd_add_meta_context(3); you can use
nbd_clear_meta_contexts(3) to set up a different list of queries. When the list is empty,
a server will typically reply with all contexts that it supports; when the list is
non-empty, the server will reply only with supported contexts that match the client's
request. Note that a reply by the server might be encoded to represent several feasible
contexts within one string, rather than multiple strings per actual context name that
would actually succeed during nbd_opt_go(3); so it is still necessary to use
nbd_can_meta_context(3) after connecting to see which contexts are actually supported.
The "context" function is called once per server reply, with any "user_data" passed to
this function, and with "name" supplied by the server. Remember that it is not safe to
call nbd_add_meta_context(3) from within the context of the callback function; rather,
your code must copy any "name" needed for later use after this function completes. At
present, the return value of the callback is ignored, although a return of -1 should be
avoided.
For convenience, when this function succeeds, it returns the number of replies returned by
the server.
Not all servers understand this request, and even when it is understood, the server might
intentionally send an empty list because it does not support the requested context, or may
encounter a failure after delivering partial results. Thus, this function may succeed even
when no contexts are reported, or may fail but have a non-empty list. Likewise, the NBD
protocol does not specify an upper bound for the number of replies that might be
advertised, so client code should be aware that a server may send a lengthy list.
val add_meta_context : t -> string -> unit
ask server to negotiate metadata context
During connection libnbd can negotiate zero or more metadata contexts with the server.
Metadata contexts are features (such as "base:allocation") which describe information
returned by the nbd_block_status(3) command (for "base:allocation" this is whether blocks
of data are allocated, zero or sparse).
This call adds one metadata context to the list to be negotiated. You can call it as many
times as needed. The list is initially empty when the handle is created; you can check the
contents of the list with nbd_get_nr_meta_contexts(3) and nbd_get_meta_context(3), or
clear it with nbd_clear_meta_contexts(3).
The NBD protocol limits meta context names to 4096 bytes, but servers may not support the
full length. The encoding of meta context names is always UTF-8.
Not all servers support all metadata contexts. To learn if a context was actually
negotiated, call nbd_can_meta_context(3) after connecting.
The single parameter is the name of the metadata context, for example
"LIBNBD_CONTEXT_BASE_ALLOCATION". <libnbd.h> includes defined constants beginning with
"LIBNBD_CONTEXT_" for some well-known contexts, but you are free to pass in other
contexts.
Other metadata contexts are server-specific, but include "qemu:dirty-bitmap:..." and
"qemu:allocation-depth" for qemu-nbd (see qemu-nbd *-B* and *-A* options).
val get_nr_meta_contexts : t -> int
return the current number of requested meta contexts
During connection libnbd can negotiate zero or more metadata contexts with the server.
Metadata contexts are features (such as "base:allocation") which describe information
returned by the nbd_block_status(3) command (for "base:allocation" this is whether blocks
of data are allocated, zero or sparse).
This command returns how many meta contexts have been added to the list to request from
the server via nbd_add_meta_context(3). The server is not obligated to honor all of the
requests; to see what it actually supports, see nbd_can_meta_context(3).
val get_meta_context : t -> int -> string
return the i'th meta context request
During connection libnbd can negotiate zero or more metadata contexts with the server.
Metadata contexts are features (such as "base:allocation") which describe information
returned by the nbd_block_status(3) command (for "base:allocation" this is whether blocks
of data are allocated, zero or sparse).
This command returns the i'th meta context request, as added by nbd_add_meta_context(3),
and bounded by nbd_get_nr_meta_contexts(3).
val clear_meta_contexts : t -> unit
reset the list of requested meta contexts
During connection libnbd can negotiate zero or more metadata contexts with the server.
Metadata contexts are features (such as "base:allocation") which describe information
returned by the nbd_block_status(3) command (for "base:allocation" this is whether blocks
of data are allocated, zero or sparse).
This command resets the list of meta contexts to request back to an empty list, for
re-population by further use of nbd_add_meta_context(3). It is primarily useful when
option negotiation mode is selected (see nbd_set_opt_mode(3)), for altering the list of
attempted contexts between subsequent export queries.
val set_uri_allow_transports : t -> ALLOW_TRANSPORT.t list -> unit
set the allowed transports in NBD URIs
Set which transports are allowed to appear in NBD URIs. The default is to allow any
transport.
The "mask" parameter may contain any of the following flags ORed together:
"LIBNBD_ALLOW_TRANSPORT_TCP" "LIBNBD_ALLOW_TRANSPORT_UNIX" "LIBNBD_ALLOW_TRANSPORT_VSOCK"
For convenience, the constant "LIBNBD_ALLOW_TRANSPORT_MASK" is available to describe all
transports recognized by this build of libnbd. A future version of the library may add new
flags.
val set_uri_allow_tls : t -> TLS.t -> unit
set the allowed TLS settings in NBD URIs
Set which TLS settings are allowed to appear in NBD URIs. The default is to allow either
non-TLS or TLS URIs.
The "tls" parameter can be:
"LIBNBD_TLS_DISABLE" TLS URIs are not permitted, ie. a URI such as "nbds://..." will be
rejected.
"LIBNBD_TLS_ALLOW" This is the default. TLS may be used or not, depending on whether the
URI uses "nbds" or "nbd".
"LIBNBD_TLS_REQUIRE" TLS URIs are required. All URIs must use "nbds".
val set_uri_allow_local_file : t -> bool -> unit
set the allowed transports in NBD URIs
Allow NBD URIs to reference local files. This is *disabled* by default.
Currently this setting only controls whether the "tls-psk-file" parameter in NBD URIs is
allowed.
val connect_uri : t -> string -> unit
connect to NBD URI
Connect (synchronously) to an NBD server and export by specifying the NBD URI. This call
parses the URI and calls nbd_set_export_name(3) and nbd_set_tls(3) and other calls as
needed, followed by nbd_connect_tcp(3) or nbd_connect_unix(3).
This call returns when the connection has been made.
Example URIs supported "nbd://example.com" Connect over TCP, unencrypted, to "example.com"
port 10809.
"nbds://example.com" Connect over TCP with TLS, to "example.com" port 10809. If the server
does not support TLS then this will fail.
"nbd+unix:///foo?socket=/tmp/nbd.sock" Connect over the Unix domain socket /tmp/nbd.sock
to an NBD server running locally. The export name is set to "foo" (note without any
leading "/" character).
"nbds+unix://alice@/?socket=/tmp/nbd.sock&tls-certificat es=certs" Connect over a Unix
domain socket, enabling TLS and setting the path to a directory containing certificates
and keys.
"nbd+vsock:///" In this scenario libnbd is running in a virtual machine. Connect over
"AF_VSOCK" to an NBD server running on the hypervisor.
Supported URI formats The following schemes are supported in the current version of
libnbd:
"nbd:" Connect over TCP without using TLS.
"nbds:" Connect over TCP. TLS is required and the connection will fail if the server does
not support TLS.
"nbd+unix:" "nbds+unix:" Connect over a Unix domain socket, without or with TLS
respectively. The "socket" parameter is required.
"nbd+vsock:" "nbds+vsock:" Connect over the "AF_VSOCK" transport, without or with TLS
respectively.
The authority part of the URI (" username@ servername :port ") is parsed depending on the
transport. For TCP it specifies the server to connect to and optional port number. For
"+unix" it should not be present. For "+vsock" the server name is the numeric CID (eg. 2
to connect to the host), and the optional port number may be present. If the "username" is
present it is used for TLS authentication.
For all transports, an export name may be present, parsed in accordance with the NBD URI
specification.
Finally the query part of the URI can contain:
socket=SOCKET Specifies the Unix domain socket to connect on. Must be present for the
"+unix" transport and must not be present for the other transports.
tls-certificates=DIR Set the certificates directory. See nbd_set_tls_certificates(3). Note
this is not allowed by default - see next section.
tls-psk-file=PSKFILE Set the PSK file. See nbd_set_tls_psk_file(3). Note this is not
allowed by default - see next section.
Disable URI features For security reasons you might want to disable certain URI features.
Pre-filtering URIs is error-prone and should not be attempted. Instead use the libnbd APIs
below to control what can appear in URIs. Note you must call these functions on the same
handle before calling nbd_connect_uri(3) or nbd_aio_connect_uri(3).
TCP, Unix domain socket or "AF_VSOCK" transports Default: all allowed
To select which transports are allowed call nbd_set_uri_allow_transports(3).
TLS Default: both non-TLS and TLS connections allowed
To force TLS off or on in URIs call nbd_set_uri_allow_tls(3).
Connect to Unix domain socket in the local filesystem Default: allowed
To prevent this you must disable the "+unix" transport using
nbd_set_uri_allow_transports(3).
Read from local files Default: denied
To allow URIs to contain references to local files (eg. for parameters like
"tls-psk-file") call nbd_set_uri_allow_local_file(3).
Overriding the export name It is possible to override the export name portion of a URI by
using nbd_set_opt_mode(3) to enable option mode, then using nbd_set_export_name(3) and
nbd_opt_go(3) as part of subsequent negotiation.
Optional features This call will fail if libnbd was not compiled with libxml2; you can
test whether this is the case with nbd_supports_uri(3).
Support for URIs that require TLS will fail if libnbd was not compiled with gnutls; you
can test whether this is the case with nbd_supports_tls(3).
Constructing a URI from an existing connection See nbd_get_uri(3).
val connect_unix : t -> string -> unit
connect to NBD server over a Unix domain socket
Connect (synchronously) over the named Unix domain socket ("unixsocket") to an NBD server
running on the same machine. This call returns when the connection has been made.
val connect_vsock : t -> int64 -> int64 -> unit
connect to NBD server over AF_VSOCK protocol
Connect (synchronously) over the "AF_VSOCK" protocol from a virtual machine to an NBD
server, usually running on the host. The "cid" and "port" parameters specify the server
address. Usually "cid" should be 2 (to connect to the host), and "port" might be 10809 or
another port number assigned to you by the host administrator. This call returns when the
connection has been made.
val connect_tcp : t -> string -> string -> unit
connect to NBD server over a TCP port
Connect (synchronously) to the NBD server listening on "hostname:port". The "port" may be
a port name such as "nbd", or it may be a port number as a string such as "10809". This
call returns when the connection has been made.
val connect_socket : t -> Unix.file_descr -> unit
connect directly to a connected socket
Pass a connected socket "sock" through which libnbd will talk to the NBD server.
The caller is responsible for creating and connecting this socket by some method, before
passing it to libnbd.
If this call returns without error then socket ownership is passed to libnbd. Libnbd will
close the socket when the handle is closed. The caller must not use the socket in any way.
val connect_command : t -> string list -> unit
connect to NBD server command
Run the command as a subprocess and connect to it over stdin/stdout. This is for use with
NBD servers which can behave like inetd clients, such as nbdkit(1) using the
*-s*/*--single* flag, and nbd-server(1) with port number set to 0.
To run qemu-nbd(1), use nbd_connect_systemd_socket_activation(3) instead.
Subprocess Libnbd will fork the "argv" command and pass the NBD socket to it using file
descriptors 0 and 1 (stdin/stdout):
┌─────────┬─────────┐ ┌────────────────┐ │ program │ libnbd │ │ NBD server │ │
│ │ │ (argv) │ │ │ socket ╍╍╍╍╍╍╍╍▶ stdin/stdout │
└─────────┴─────────┘ └────────────────┘
When the NBD handle is closed the server subprocess is killed.
val connect_systemd_socket_activation : t -> string list -> unit
connect using systemd socket activation
Run the command as a subprocess and connect to it using systemd socket activation.
This is especially useful for running qemu-nbd(1) as a subprocess of libnbd, for example
to use it to open qcow2 files.
To run nbdkit as a subprocess, this function can be used, or nbd_connect_command(3).
To run nbd-server(1) as a subprocess, this function cannot be used, you must use
nbd_connect_command(3).
Socket activation Libnbd will fork the "argv" command and pass an NBD socket to it using
special "LISTEN_*" environment variables (as defined by the systemd socket activation
protocol).
┌─────────┬─────────┐ ┌───────────────┐ │ program │ libnbd │ │ qemu-nbd or │ │
│ │ │ other server │ │ │ socket ╍╍╍╍╍╍╍╍▶ │
└─────────┴─────────┘ └───────────────┘
When the NBD handle is closed the server subprocess is killed.
val is_read_only : t -> bool
is the NBD export read-only?
Returns true if the NBD export is read-only; writes and write-like operations will fail.
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val can_flush : t -> bool
does the server support the flush command?
Returns true if the server supports the flush command (see nbd_flush(3),
nbd_aio_flush(3)). Returns false if the server does not.
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val can_fua : t -> bool
does the server support the FUA flag?
Returns true if the server supports the FUA flag on certain commands (see nbd_pwrite(3)).
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val is_rotational : t -> bool
is the NBD disk rotational (like a disk)?
Returns true if the disk exposed over NBD is rotational (like a traditional floppy or hard
disk). Returns false if the disk has no penalty for random access (like an SSD or RAM
disk).
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val can_trim : t -> bool
does the server support the trim command?
Returns true if the server supports the trim command (see nbd_trim(3), nbd_aio_trim(3)).
Returns false if the server does not.
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val can_zero : t -> bool
does the server support the zero command?
Returns true if the server supports the zero command (see nbd_zero(3), nbd_aio_zero(3)).
Returns false if the server does not.
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val can_fast_zero : t -> bool
does the server support the fast zero flag?
Returns true if the server supports the use of the "LIBNBD_CMD_FLAG_FAST_ZERO" flag to the
zero command (see nbd_zero(3), nbd_aio_zero(3)). Returns false if the server does not.
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val can_df : t -> bool
does the server support the don't fragment flag to pread?
Returns true if the server supports structured reads with an ability to request a
non-fragmented read (see nbd_pread_structured(3), nbd_aio_pread_structured(3)). Returns
false if the server either lacks structured reads or if it does not support a
non-fragmented read request.
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val can_multi_conn : t -> bool
does the server support multi-conn?
Returns true if the server supports multi-conn. Returns false if the server does not.
It is not safe to open multiple handles connecting to the same server if you will write to
the server and the server does not advertise multi-conn support. The safe way to check for
this is to open one connection, check this flag is true, then open further connections as
required.
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val can_cache : t -> bool
does the server support the cache command?
Returns true if the server supports the cache command (see nbd_cache(3),
nbd_aio_cache(3)). Returns false if the server does not.
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val can_meta_context : t -> string -> bool
does the server support a specific meta context?
Returns true if the server supports the given meta context (see nbd_add_meta_context(3)).
Returns false if the server does not.
The single parameter is the name of the metadata context, for example
"LIBNBD_CONTEXT_BASE_ALLOCATION". <libnbd.h> includes defined constants for well-known
namespace contexts beginning with "LIBNBD_CONTEXT_", but you are free to pass in other
contexts.
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val get_protocol : t -> string
return the NBD protocol variant
Return the NBD protocol variant in use on the connection. At the moment this returns one
of the strings "oldstyle", "newstyle" or "newstyle-fixed". Other strings might be
returned in the future. Most modern NBD servers use "newstyle-fixed".
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val get_size : t -> int64
return the export size
Returns the size in bytes of the NBD export.
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val get_block_size : t -> SIZE.t -> int64
return a specific server block size constraint
Returns a specific size constraint advertised by the server, if any. If the return is
zero, the server did not advertise a constraint. "size_type" must be one of the following
constraints:
"LIBNBD_SIZE_MINIMUM" = 0 If non-zero, this will be a power of 2 between 1 and 64k; any
client request that is not aligned in length or offset to this size is likely to fail with
"EINVAL". The image size will generally also be a multiple of this value (if not, the
final few bytes are inaccessible while obeying alignment constraints). If zero, it is
safest to assume a minimum block size of 512, although many servers support a minimum
block size of 1. If the server provides a constraint, then libnbd defaults to honoring
that constraint client-side unless "LIBNBD_STRICT_ALIGN" is cleared in
nbd_set_strict_mode(3).
"LIBNBD_SIZE_PREFERRED" = 1 If non-zero, this is a power of 2 representing the preferred
size for efficient I/O. Smaller requests may incur overhead such as read-modify-write
cycles that will not be present when using I/O that is a multiple of this value. This
value may be larger than the size of the export. If zero, using 4k as a preferred block
size tends to give decent performance.
"LIBNBD_SIZE_MAXIMUM" = 2 If non-zero, this represents the maximum length that the server
is willing to handle during nbd_pread(3) or nbd_pwrite(3). Other functions like
nbd_zero(3) may still be able to use larger sizes. Note that this function returns what
the server advertised, but libnbd itself imposes a maximum of 64M. If zero, some NBD
servers will abruptly disconnect if a transaction involves more than 32M.
Future NBD extensions may result in additional "size_type" values. Note that by default,
libnbd requests all available block sizes, but that a server may differ in what sizes it
chooses to report if nbd_set_request_block_size(3) alters whether the client requests
sizes.
This call does not block, because it returns data that is saved in the handle from the NBD
protocol handshake.
val pread : ?flags:CMD_FLAG.t list -> t -> bytes -> int64 -> unit
read from the NBD server
Issue a read command to the NBD server for the range starting at "offset" and ending at
"offset" + "count" - 1. NBD can only read all or nothing using this call. The call returns
when the data has been read fully into "buf" or there is an error. See also
nbd_pread_structured(3), if finer visibility is required into the server's replies, or if
you want to use "LIBNBD_CMD_FLAG_DF".
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 must be 0 for now (it exists for future NBD protocol extensions).
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.
val pread_structured : ?flags:CMD_FLAG.t list -> t -> bytes -> int64 -> (bytes -> int64 ->
int -> int Stdlib.ref -> int) -> unit
read from the NBD server
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.
val pwrite : ?flags:CMD_FLAG.t list -> t -> bytes -> int64 -> unit
write to the NBD server
Issue a write command to the NBD server, writing the data in "buf" to the range starting
at "offset" and ending at "offset" + "count" - 1. NBD can only write all or nothing using
this call. The call returns when the command has been acknowledged by the server, or there
is an error. Note this will generally return an error if nbd_is_read_only(3) is true.
Note that libnbd currently enforces a maximum write 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_FUA" meaning
that the server should not return until the data has been committed to permanent storage
(if that is supported - some servers cannot do this, see nbd_can_fua(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) function can be used to alter which scenarios should await a server
reply rather than failing fast.
val shutdown : ?flags:SHUTDOWN.t list -> t -> unit
disconnect from the NBD server
Issue the disconnect command to the NBD server. This is a nice way to tell the server we
are going away, but from the client's point of view has no advantage over abruptly closing
the connection (see nbd_close(3)).
This function works whether or not the handle is ready for transmission of commands. If
more fine-grained control is needed, see nbd_aio_disconnect(3).
The "flags" argument is a bitmask, including zero or more of the following shutdown flags:
"LIBNBD_SHUTDOWN_ABANDON_PENDING" = 0x10000 If there are any pending requests which have
not yet been sent to the server (see nbd_aio_in_flight(3)), abandon them without sending
them to the server, rather than the usual practice of issuing those commands before
informing the server of the intent to disconnect.
For convenience, the constant "LIBNBD_SHUTDOWN_MASK" is available to describe all shutdown
flags recognized by this build of libnbd. A future version of the library may add new
flags.
val flush : ?flags:CMD_FLAG.t list -> t -> unit
send flush command to the NBD server
Issue the flush command to the NBD server. The function should return when all write
commands which have completed have been committed to permanent storage on the server. Note
this will generally return an error if nbd_can_flush(3) is false.
The "flags" parameter must be 0 for now (it exists for future NBD protocol extensions).
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.
val trim : ?flags:CMD_FLAG.t list -> t -> int64 -> int64 -> unit
send trim command to the NBD server
Issue a trim command to the NBD server, which if supported by the server causes a hole to
be punched in the backing store starting at "offset" and ending at "offset" + "count" - 1.
The call returns when the command has been acknowledged by the server, or there is an
error. Note this will generally return an error if nbd_can_trim(3) is false or
nbd_is_read_only(3) is true.
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 trim size, although a future extension may add a constraint visible in
nbd_get_block_size(3).
The "flags" parameter may be 0 for no flags, or may contain "LIBNBD_CMD_FLAG_FUA" meaning
that the server should not return until the data has been committed to permanent storage
(if that is supported - some servers cannot do this, see nbd_can_fua(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) function can be used to alter which scenarios should await a server
reply rather than failing fast.
val cache : ?flags:CMD_FLAG.t list -> t -> int64 -> int64 -> unit
send cache (prefetch) command to the NBD server
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) call. The server can also silently ignore this command. Note this will
generally return an error if nbd_can_cache(3) is false.
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 cache size, although a future extension may add a constraint visible in
nbd_get_block_size(3).
The "flags" parameter must be 0 for now (it exists for future NBD protocol extensions).
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.
val zero : ?flags:CMD_FLAG.t list -> t -> int64 -> int64 -> unit
send write zeroes command to the NBD server
Issue a write zeroes command to the NBD server, which if supported by the server causes a
zeroes to be written efficiently starting at "offset" and ending at "offset"
-"count" - 1. The call returns when the command has been acknowledged by the server, or
there is an error. Note this will generally return an error if nbd_can_zero(3) is false
or nbd_is_read_only(3) is true.
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 zero size, although a future extension may add a constraint visible in
nbd_get_block_size(3). Also, some servers may permit a larger zero request only when the
"LIBNBD_CMD_FLAG_FAST_ZERO" is in use.
The "flags" parameter may be 0 for no flags, or may contain "LIBNBD_CMD_FLAG_FUA" meaning
that the server should not return until the data has been committed to permanent storage
(if that is supported - some servers cannot do this, see nbd_can_fua(3)),
"LIBNBD_CMD_FLAG_NO_HOLE" meaning that the server should favor writing actual allocated
zeroes over punching a hole, and/or "LIBNBD_CMD_FLAG_FAST_ZERO" meaning that the server
must fail quickly if writing zeroes is no faster than a normal write (if that is supported
- some servers cannot do this, see nbd_can_fast_zero(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) function can be used to alter which scenarios should await a server
reply rather than failing fast.
val block_status : ?flags:CMD_FLAG.t list -> t -> int64 -> int64 -> (string -> int64 ->
int64 array -> int Stdlib.ref -> int) -> unit
send block status command to the NBD server
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.
val poll : t -> int -> int
poll the handle once
This is a simple implementation of poll(2) which is used internally by synchronous API
calls. On success, it returns 0 if the "timeout" (in milliseconds) occurs, or 1 if the
poll completed and the state machine progressed. Set "timeout" to -1 to block indefinitely
(but be careful that eventual action is actually expected - for example, if the connection
is established but there are no commands in flight, using an infinite timeout will
permanently block).
This function is mainly useful as an example of how you might integrate libnbd with your
own main loop, rather than being intended as something you would use.
val aio_connect : t -> string -> unit
connect to the NBD server
Begin connecting to the NBD server. The "addr" and "addrlen" parameters specify the
address of the socket to connect to.
You can check if the connection is still connecting by calling nbd_aio_is_connecting(3),
or if it has connected to the server and completed the NBD handshake by calling
nbd_aio_is_ready(3), on the connection.
val aio_connect_uri : t -> string -> unit
connect to an NBD URI
Begin connecting to the NBD URI "uri". Parameters behave as documented in
nbd_connect_uri(3).
You can check if the connection is still connecting by calling nbd_aio_is_connecting(3),
or if it has connected to the server and completed the NBD handshake by calling
nbd_aio_is_ready(3), on the connection.
val aio_connect_unix : t -> string -> unit
connect to the NBD server over a Unix domain socket
Begin connecting to the NBD server over Unix domain socket ("unixsocket"). Parameters
behave as documented in nbd_connect_unix(3).
You can check if the connection is still connecting by calling nbd_aio_is_connecting(3),
or if it has connected to the server and completed the NBD handshake by calling
nbd_aio_is_ready(3), on the connection.
val aio_connect_vsock : t -> int64 -> int64 -> unit
connect to the NBD server over AF_VSOCK socket
Begin connecting to the NBD server over the "AF_VSOCK" protocol to the server "cid:port".
Parameters behave as documented in nbd_connect_vsock(3).
You can check if the connection is still connecting by calling nbd_aio_is_connecting(3),
or if it has connected to the server and completed the NBD handshake by calling
nbd_aio_is_ready(3), on the connection.
val aio_connect_tcp : t -> string -> string -> unit
connect to the NBD server over a TCP port
Begin connecting to the NBD server listening on "hostname:port". Parameters behave as
documented in nbd_connect_tcp(3).
You can check if the connection is still connecting by calling nbd_aio_is_connecting(3),
or if it has connected to the server and completed the NBD handshake by calling
nbd_aio_is_ready(3), on the connection.
val aio_connect_socket : t -> Unix.file_descr -> unit
connect directly to a connected socket
Begin connecting to the connected socket "fd". Parameters behave as documented in
nbd_connect_socket(3).
You can check if the connection is still connecting by calling nbd_aio_is_connecting(3),
or if it has connected to the server and completed the NBD handshake by calling
nbd_aio_is_ready(3), on the connection.
val aio_connect_command : t -> string list -> unit
connect to the NBD server
Run the command as a subprocess and begin connecting to it over stdin/stdout. Parameters
behave as documented in nbd_connect_command(3).
You can check if the connection is still connecting by calling nbd_aio_is_connecting(3),
or if it has connected to the server and completed the NBD handshake by calling
nbd_aio_is_ready(3), on the connection.
val aio_connect_systemd_socket_activation : t -> string list -> unit
connect using systemd socket activation
Run the command as a subprocess and begin connecting to it using systemd socket
activation. Parameters behave as documented in nbd_connect_systemd_socket_activation(3).
You can check if the connection is still connecting by calling nbd_aio_is_connecting(3),
or if it has connected to the server and completed the NBD handshake by calling
nbd_aio_is_ready(3), on the connection.
val aio_opt_go : ?completion:(int Stdlib.ref -> int) -> t -> unit
end negotiation and move on to using an export
Request that the server finish negotiation and move on to serving the export previously
specified by the most recent nbd_set_export_name(3) or nbd_connect_uri(3). This can only
be used if nbd_set_opt_mode(3) enabled option mode.
To determine when the request completes, wait for nbd_aio_is_connecting(3) to return
false. Or supply the optional "completion_callback" which will be invoked as described in
"Completion callbacks" in libnbd(3), except that it is automatically retired regardless of
return value. Note that directly detecting whether the server returns an error (as is done
by the return value of the synchronous counterpart) is only possible with a completion
callback; however it is also possible to indirectly detect an error when
nbd_aio_is_negotiating(3) returns true.
val aio_opt_abort : t -> unit
end negotiation and close the connection
Request that the server finish negotiation, gracefully if possible, then close the
connection. This can only be used if nbd_set_opt_mode(3) enabled option mode.
To determine when the request completes, wait for nbd_aio_is_connecting(3) to return
false.
val aio_opt_list : ?completion:(int Stdlib.ref -> int) -> t -> (string -> string -> int)
-> unit
request the server to list all exports during negotiation
Request that the server list all exports that it supports. This can only be used if
nbd_set_opt_mode(3) enabled option mode.
To determine when the request completes, wait for nbd_aio_is_connecting(3) to return
false. Or supply the optional "completion_callback" which will be invoked as described in
"Completion callbacks" in libnbd(3), except that it is automatically retired regardless of
return value. Note that detecting whether the server returns an error (as is done by the
return value of the synchronous counterpart) is only possible with a completion callback.
val aio_opt_info : ?completion:(int Stdlib.ref -> int) -> t -> unit
request the server for information about an export
Request that the server supply information about the export name previously specified by
the most recent nbd_set_export_name(3) or nbd_connect_uri(3). This can only be used if
nbd_set_opt_mode(3) enabled option mode.
To determine when the request completes, wait for nbd_aio_is_connecting(3) to return
false. Or supply the optional "completion_callback" which will be invoked as described in
"Completion callbacks" in libnbd(3), except that it is automatically retired regardless of
return value. Note that detecting whether the server returns an error (as is done by the
return value of the synchronous counterpart) is only possible with a completion callback.
val aio_opt_list_meta_context : ?completion:(int Stdlib.ref -> int) -> t -> (string ->
int) -> int
request the server to list available meta contexts
Request that the server list available meta contexts associated with the export previously
specified by the most recent nbd_set_export_name(3) or nbd_connect_uri(3). This can only
be used if nbd_set_opt_mode(3) enabled option mode.
To determine when the request completes, wait for nbd_aio_is_connecting(3) to return
false. Or supply the optional "completion_callback" which will be invoked as described in
"Completion callbacks" in libnbd(3), except that it is automatically retired regardless of
return value. Note that detecting whether the server returns an error (as is done by the
return value of the synchronous counterpart) is only possible with a completion callback.
val aio_pread : ?completion:(int Stdlib.ref -> int) -> ?flags:CMD_FLAG.t list -> t ->
Buffer.t -> int64 -> cookie
read from the NBD server
Issue a read command to the NBD server.
To check if the command completed, call nbd_aio_command_completed(3). Or supply the
optional "completion_callback" which will be invoked as described in "Completion
callbacks" in 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)
reports failure, and if 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.
Other parameters behave as documented in nbd_pread(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) function can be used to alter which scenarios should await a server
reply rather than failing fast.
val aio_pread_structured : ?completion:(int Stdlib.ref -> int) -> ?flags:CMD_FLAG.t list
-> t -> Buffer.t -> int64 -> (bytes -> int64 -> int -> int Stdlib.ref -> int) -> cookie
read from the NBD server
Issue a read command to the NBD server.
To check if the command completed, call nbd_aio_command_completed(3). Or supply the
optional "completion_callback" which will be invoked as described in "Completion
callbacks" in 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)
reports failure, and if 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.
Other parameters behave as documented in 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) function can be used to alter which scenarios should await a server
reply rather than failing fast.
val aio_pwrite : ?completion:(int Stdlib.ref -> int) -> ?flags:CMD_FLAG.t list -> t ->
Buffer.t -> int64 -> cookie
write to the NBD server
Issue a write command to the NBD server.
To check if the command completed, call nbd_aio_command_completed(3). Or supply the
optional "completion_callback" which will be invoked as described in "Completion
callbacks" in libnbd(3).
Note that you must ensure "buf" is valid until the command has completed. Other parameters
behave as documented in nbd_pwrite(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) function can be used to alter which scenarios should await a server
reply rather than failing fast.
val aio_disconnect : ?flags:CMD_FLAG.t list -> t -> unit
disconnect from the NBD server
Issue the disconnect command to the NBD server. This is not a normal command because NBD
servers are not obliged to send a reply. Instead you should wait for nbd_aio_is_closed(3)
to become true on the connection. Once this command is issued, you cannot issue any
further commands.
Although libnbd does not prevent you from issuing this command while still waiting on the
replies to previous commands, the NBD protocol recommends that you wait until there are no
other commands in flight (see nbd_aio_in_flight(3)), to give the server a better chance at
a clean shutdown.
The "flags" parameter must be 0 for now (it exists for future NBD protocol extensions).
There is no direct synchronous counterpart; however, nbd_shutdown(3) will call this
function if appropriate.
val aio_flush : ?completion:(int Stdlib.ref -> int) -> ?flags:CMD_FLAG.t list -> t ->
cookie
send flush command to the NBD server
Issue the flush command to the NBD server.
To check if the command completed, call nbd_aio_command_completed(3). Or supply the
optional "completion_callback" which will be invoked as described in "Completion
callbacks" in libnbd(3).
Other parameters behave as documented in nbd_flush(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) function can be used to alter which scenarios should await a server
reply rather than failing fast.
val aio_trim : ?completion:(int Stdlib.ref -> int) -> ?flags:CMD_FLAG.t list -> t -> int64
-> int64 -> cookie
send trim command to the NBD server
Issue a trim command to the NBD server.
To check if the command completed, call nbd_aio_command_completed(3). Or supply the
optional "completion_callback" which will be invoked as described in "Completion
callbacks" in libnbd(3).
Other parameters behave as documented in nbd_trim(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) function can be used to alter which scenarios should await a server
reply rather than failing fast.
val aio_cache : ?completion:(int Stdlib.ref -> int) -> ?flags:CMD_FLAG.t list -> t ->
int64 -> int64 -> cookie
send cache (prefetch) command to the NBD server
Issue the cache (prefetch) command to the NBD server.
To check if the command completed, call nbd_aio_command_completed(3). Or supply the
optional "completion_callback" which will be invoked as described in "Completion
callbacks" in libnbd(3).
Other parameters behave as documented in nbd_cache(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) function can be used to alter which scenarios should await a server
reply rather than failing fast.
val aio_zero : ?completion:(int Stdlib.ref -> int) -> ?flags:CMD_FLAG.t list -> t -> int64
-> int64 -> cookie
send write zeroes command to the NBD server
Issue a write zeroes command to the NBD server.
To check if the command completed, call nbd_aio_command_completed(3). Or supply the
optional "completion_callback" which will be invoked as described in "Completion
callbacks" in libnbd(3).
Other parameters behave as documented in nbd_zero(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) function can be used to alter which scenarios should await a server
reply rather than failing fast.
val aio_block_status : ?completion:(int Stdlib.ref -> int) -> ?flags:CMD_FLAG.t list -> t
-> int64 -> int64 -> (string -> int64 -> int64 array -> int Stdlib.ref -> int) -> cookie
send block status command to the NBD server
Send the block status command to the NBD server.
To check if the command completed, call nbd_aio_command_completed(3). Or supply the
optional "completion_callback" which will be invoked as described in "Completion
callbacks" in libnbd(3).
Other parameters behave as documented in nbd_block_status(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) function can be used to alter which scenarios should await a server
reply rather than failing fast.
val aio_get_fd : t -> Unix.file_descr
return file descriptor associated with this connection
Return the underlying file descriptor associated with this connection. You can use this to
check if the file descriptor is ready for reading or writing and call
nbd_aio_notify_read(3) or nbd_aio_notify_write(3). See also nbd_aio_get_direction(3). Do
not do anything else with the file descriptor.
val aio_get_direction : t -> int
return the read or write direction
Return the current direction of this connection, which means whether we are next expecting
to read data from the server, write data to the server, or both. It returns
0 We are not expected to interact with the server file descriptor from the current
state. It is not worth attempting to use poll(2); if the connection is not dead, then
state machine progress must instead come from some other means such as nbd_aio_connect(3).
"LIBNBD_AIO_DIRECTION_READ" = 1 We are expected next to read from the server. If using
poll(2) you would set "events = POLLIN". If "revents" returns "POLLIN" or "POLLHUP" you
would then call nbd_aio_notify_read(3).
Note that once libnbd reaches nbd_aio_is_ready(3), this direction is returned even when
there are no commands in flight (see nbd_aio_in_flight(3)). In a single-threaded use of
libnbd, it is not worth polling until after issuing a command, as otherwise the server
will never wake up the poll. In a multi-threaded scenario, you can have one thread begin a
polling loop prior to any commands, but any other thread that issues a command will need a
way to kick the polling thread out of poll in case issuing the command changes the needed
polling direction. Possible ways to do this include polling for activity on a
pipe-to-self, or using pthread_kill(3) to send a signal that is masked except during
ppoll(2).
"LIBNBD_AIO_DIRECTION_WRITE" = 2 We are expected next to write to the server. If using
poll(2) you would set "events = POLLOUT". If "revents" returns "POLLOUT" you would then
call nbd_aio_notify_write(3).
"LIBNBD_AIO_DIRECTION_BOTH" = 3 We are expected next to either read or write to the
server. If using poll(2) you would set "events = POLLIN|POLLOUT". If only one of "POLLIN"
or "POLLOUT" is returned, then see above. However, if both are returned, it is better to
call only nbd_aio_notify_read(3), as processing the server's reply may change the state of
the connection and invalidate the need to write more commands.
val aio_notify_read : t -> unit
notify that the connection is readable
Send notification to the state machine that the connection is readable. Typically this is
called after your main loop has detected that the file descriptor associated with this
connection is readable.
val aio_notify_write : t -> unit
notify that the connection is writable
Send notification to the state machine that the connection is writable. Typically this is
called after your main loop has detected that the file descriptor associated with this
connection is writable.
val aio_is_created : t -> bool
check if the connection has just been created
Return true if this connection has just been created. This is the state before the handle
has started connecting to a server. In this state the handle can start to be connected by
calling functions such as nbd_aio_connect(3).
val aio_is_connecting : t -> bool
check if the connection is connecting or handshaking
Return true if this connection is connecting to the server or in the process of
handshaking and negotiating options which happens before the handle becomes ready to issue
commands (see nbd_aio_is_ready(3)).
val aio_is_negotiating : t -> bool
check if connection is ready to send handshake option
Return true if this connection is ready to start another option negotiation command while
handshaking with the server. An option command will move back to the connecting state (see
nbd_aio_is_connecting(3)). Note that this state cannot be reached unless requested by
nbd_set_opt_mode(3), and even then it only works with newstyle servers; an oldstyle server
will skip straight to nbd_aio_is_ready(3).
val aio_is_ready : t -> bool
check if the connection is in the ready state
Return true if this connection is connected to the NBD server, the handshake has
completed, and the connection is idle or waiting for a reply. In this state the handle is
ready to issue commands.
val aio_is_processing : t -> bool
check if the connection is processing a command
Return true if this connection is connected to the NBD server, the handshake has
completed, and the connection is processing commands (either writing out a request or
reading a reply).
Note the ready state (nbd_aio_is_ready(3)) is not included. In the ready state commands
may be *in flight* (the *server* is processing them), but libnbd is not processing them.
val aio_is_dead : t -> bool
check if the connection is dead
Return true if the connection has encountered a fatal error and is dead. In this state the
handle may only be closed. There is no way to recover a handle from the dead state.
val aio_is_closed : t -> bool
check if the connection is closed
Return true if the connection has closed. There is no way to reconnect a closed
connection. Instead you must close the whole handle.
val aio_command_completed : t -> int64 -> bool
check if the command completed
Return true if the command completed. If this function returns true then the command was
successful and it has been retired. Return false if the command is still in flight. This
can also fail with an error in case the command failed (in this case the command is also
retired). A command is retired either via this command, or by using a completion callback
which returns 1.
The "cookie" parameter is the positive unique 64 bit cookie for the command, as returned
by a call such as nbd_aio_pread(3).
val aio_peek_command_completed : t -> int64
check if any command has completed
Return the unique positive 64 bit cookie of the first non-retired but completed command, 0
if there are in-flight commands but none of them are awaiting retirement, or -1 on error
including when there are no in-flight commands. Any cookie returned by this function must
still be passed to nbd_aio_command_completed(3) to actually retire the command and learn
whether the command was successful.
val aio_in_flight : t -> int
check how many aio commands are still in flight
Return the number of in-flight aio commands that are still awaiting a response from the
server before they can be retired. If this returns a non-zero value when requesting a
disconnect from the server (see nbd_aio_disconnect(3) and nbd_shutdown(3)), libnbd does
not try to wait for those commands to complete gracefully; if the server strands commands
while shutting down, nbd_aio_command_completed(3) will report those commands as failed
with a status of "ENOTCONN".
val connection_state : t -> string
return string describing the state of the connection
Returns a descriptive string for the state of the connection. This can be used for
debugging or troubleshooting, but you should not rely on the state of connections since it
may change in future versions.
val get_package_name : t -> string
return the name of the library
Returns the name of the library, always "libnbd" unless the library was modified with
another name at compile time.
val get_version : t -> string
return the version of the library
Return the version of libnbd. This is returned as a string in the form
"major.minor.release" where each of major, minor and release is a small positive integer.
For example:
minor ↓ "1.0.3" ↑ ↑ major release
major = 0 The major number was 0 for the early experimental versions of libnbd where we
still had an unstable API.
major = 1 The major number is 1 for the versions of libnbd with a long-term stable API and
ABI. It is not anticipated that major will be any number other than 1.
minor = 0, 2, ... (even) The minor number is even for stable releases.
minor = 1, 3, ... (odd) The minor number is odd for development versions. Note that new
APIs added in a development version remain experimental and subject to change in that
branch until they appear in a stable release.
release The release number is incremented for each release along a particular branch.
val kill_subprocess : t -> int -> unit
kill server running as a subprocess
This call may be used to kill the server running as a subprocess that was previously
created using nbd_connect_command(3). You do not need to use this call. It is only needed
if the server does not exit when the socket is closed.
The "signum" parameter is the optional signal number to send (see signal(7)). If "signum"
is 0 then "SIGTERM" is sent.
val supports_tls : t -> bool
true if libnbd was compiled with support for TLS
Returns true if libnbd was compiled with gnutls which is required to support TLS
encryption, or false if not.
val supports_uri : t -> bool
true if libnbd was compiled with support for NBD URIs
Returns true if libnbd was compiled with libxml2 which is required to support NBD URIs, or
false if not.
val get_uri : t -> string
construct an NBD URI for a connection
This makes a best effort attempt to construct an NBD URI which could be used to connect
back to the same server (using nbd_connect_uri(3)).
In some cases there is not enough information in the handle to successfully create a URI
(eg. if you connected with nbd_connect_socket(3)). In such cases the call returns "NULL"
and further diagnostic information is available via nbd_get_errno(3) and nbd_get_error(3)
as usual.
Even if a URI is returned it is not guaranteed to work, and it may not be optimal.