An Atom is a simple generic data container for holding any type of Plain Old Data (POD). An Atom can contain simple primitive types like integers, floating point numbers, and strings; as well as structured data like lists and dictionary-like Objects. Since Atoms are POD, they can be easily copied (e.g. using memcpy) anywhere and are suitable for use in real-time code.

Every atom starts with an LV2_Atom header, followed by the contents. This allows code to process atoms without requiring special code for every type of data. For example, plugins that mutually understand a type can be used together in a host that does not understand that type, because the host is only required to copy atoms, not interpret their contents. Similarly, plugins (such as routers, delays, or data structures) can meaningfully process atoms of a type unknown to them.

Atoms should be used anywhere values of various types must be stored or transmitted. The port type AtomPort can be used to transmit atoms via ports. An AtomPort that contains an Sequence can be used for sample accurate event communication, such as MIDI, and replaces the earlier event extension.


Each Atom type defines a binary format for use at runtime, but also a serialisation that is natural to express in Turtle format. Thus, this specification defines a powerful real-time appropriate data model, as well as a portable way to serialise any data in that model. This is particularly useful for inter-process communication, saving/restoring state, and describing values in plugin data files.

Custom Atom Types

While it is possible to define new Atom types for any binary format, the standard types defined here are powerful enough to describe almost anything. Implementations SHOULD build structures out of the types provided here, rather than define new binary formats (e.g. use Tuple or Object rather than a new C struct type). Current implementations have support for serialising all standard types, so new binary formats are an implementation burden which harms interoperabilty. In particular, plugins SHOULD NOT expect UI communication or state saving with custom Atom types to work. In general, new Atom types should only be defined where absolutely necessary due to performance reasons and serialisation is not a concern.




Class atom:Atom


Abstract base class for all atoms. An LV2_Atom has a 32-bit size and type followed by a body of size bytes. Atoms MUST be 64-bit aligned.

All concrete Atom types (subclasses of this class) MUST define a precise binary layout for their body.

The type field is the URI of an Atom type mapped to an integer. Implementations SHOULD gracefully pass through, or ignore, atoms with unknown types.

All atoms are POD by definition except references, which as a special case have type = 0. An Atom MUST NOT contain a Reference. It is safe to copy any non-reference Atom with a simple memcpy, even if the implementation does not understand type. Though this extension reserves the type 0 for references, the details of reference handling are currently unspecified. A future revision of this extension, or a different extension, may define how to use non-POD data and references. Implementations MUST NOT send references to another implementation unless the receiver is explicitly known to support references (e.g. by supporting a feature).

The atom with both type and size 0 is null, which is not considered a Reference.

atom:cType LV2_Atom

Class atom:AtomPort

Atom Port

A port which contains an atom:Atom. Ports of this type are connected to an LV2_Atom with a type specified by atom:bufferType.

Output ports with a variably sized type MUST be initialised by the host before every run() to an atom:Chunk with size set to the available space. The plugin reads this size to know how much space is available for writing. In all cases, the plugin MUST write a complete atom (including header) to outputs. However, to be robust, hosts SHOULD initialise output ports to a safe sentinel (e.g. the null Atom) before calling run().

Sub-class oflv2:Port
In domain ofatom:bufferType

Class atom:Blank


This class is deprecated. Use atom:Object with ID 0 instead.

An atom:Object where the LV2_Atom_Object::id is a blank node ID (NOT a URI). The ID of a Blank is valid only within the context the Blank appears in. For ports this is the context of the associated run() call, i.e. all ports share the same context so outputs can contain IDs that correspond to IDs of blanks in the input.

Sub-class ofatom:Object
atom:cType LV2_Atom_Object
owl:deprecated true

Class atom:Bool


An Int where 0 is false and any other value is true.

Sub-class ofatom:Atom
owl:onDatatype xsd:boolean
atom:cType LV2_Atom_Bool

Class atom:Chunk

Chunk of memory

A chunk of memory with undefined contents. This type is used to indicate a certain amount of space is available. For example, output ports with a variably sized type are connected to a Chunk so the plugin knows the size of the buffer available for writing.

The use of a Chunk should be constrained to a local scope, since interpreting it is impossible without context. However, if serialised to RDF, a Chunk may be represented directly as an xsd:base64Binary string, e.g.:

[] eg:someChunk "vu/erQ=="^^xsd:base64Binary .
Sub-class ofatom:Atom
owl:onDatatype xsd:base64Binary

Class atom:Double

64-bit floating point number
Sub-class ofatom:Number
owl:onDatatype xsd:double
atom:cType LV2_Atom_Double

Class atom:Event


An atom with a time stamp prefix, typically an element of an atom:Sequence. Note this is not an Atom type.

atom:cType LV2_Atom_Event

Class atom:Float

32-bit floating point number
Sub-class ofatom:Number
owl:onDatatype xsd:float
atom:cType LV2_Atom_Float

Class atom:Int

Signed 32-bit integer
Sub-class ofatom:Number
owl:onDatatype xsd:int
atom:cType LV2_Atom_Int

Class atom:Literal

String Literal

A UTF-8 encoded string literal, with an optional datatype or language.

This type is compatible with rdfs:Literal and is capable of expressing a string in any language or a value of any type. A Literal has a datatype and lang followed by string data in UTF-8 encoding. The length of the string data in bytes is size - sizeof(LV2_Atom_Literal), including the terminating NULL character. The lang field SHOULD be a URI of the form <> or <> where LANG is a 3-character ISO 693-3 language code, or a 2-character ISO 693-1 language code, respectively.

A Literal may have a datatype OR a lang, but never both.

For example, a Literal can be "Hello" in English:

void set_to_hello_in_english(LV2_Atom_Literal* lit) {
     lit->atom.type     = map(expand("atom:Literal"));
     lit->atom.size     = 14;
     lit->body.datatype = 0;
     lit->body.lang     = map("");
     memcpy(LV2_ATOM_CONTENTS(LV2_Atom_Literal, lit),
            sizeof("Hello"));  // Assumes enough space

or a Turtle string:

void set_to_turtle_string(LV2_Atom_Literal* lit, const char* ttl) {
     lit->atom.type     = map(expand("atom:Literal"));
     lit->atom.size     = 64;
     lit->body.datatype = map("");
     lit->body.lang     = 0;
     memcpy(LV2_ATOM_CONTENTS(LV2_Atom_Literal, lit),
            strlen(ttl) + 1);  // Assumes enough space
Sub-class ofatom:Atom
atom:cType LV2_Atom_Literal

Class atom:Long

Signed 64-bit integer
Sub-class ofatom:Number
atom:cType LV2_Atom_Long
owl:onDatatype xsd:long

Class atom:Number

Sub-class ofatom:Atom

Class atom:Object


An Object is an atom with a set of properties. This corresponds to an RDF Resource, and can be thought of as a dictionary with URID keys.

An LV2_Atom_Object body has a uint32_t id and type, followed by a series of atom:Property bodies (LV2_Atom_Property_Body). The LV2_Atom_Object_Body::otype field is equivalent to a property with key rdf:type, but is included in the structure to allow for fast dispatching.

Code SHOULD check for objects using lv2_atom_forge_is_object() or lv2_atom_forge_is_blank() if a forge is available, rather than checking the atom type directly. This will correctly handle the deprecated atom:Resource and atom:Blank types.

When serialised to RDF, an Object is represented as a resource, e.g.:

    eg:firstPropertyKey "first property value" ;
    eg:secondPropertyKey "first loser" ;
    eg:andSoOn "and so on" .
Sub-class ofatom:Atom
atom:cType LV2_Atom_Object

Class atom:Path

File path string

A local file path.

A Path is a URI reference with only a path component: no scheme, authority, query, or fragment. In particular, paths to files in the same bundle may be cleanly written in Turtle files as a relative URI. However, implementations may assume any binary Path (e.g. in an event payload) is a valid file path which can passed to system functions like fopen() directly, without any character encoding or escape expansion required.

Any implemenation that creates a Path atom to transmit to another is responsible for ensuring it is valid. A Path SHOULD always be absolute, unless there is some mechanism in place that defines a base path. Since this is not the case for plugin instances, effectively any Path sent to or received from a plugin instance MUST be absolute.

Sub-class ofatom:URI
owl:onDatatype atom:URI

Class atom:Property


A property of an atom:Object. An LV2_Atom_Property has a URID key and context, and an Atom value. This corresponds to an RDF Property, where the key is the predicate and the value is the object.

The context field can be used to specify a different context for each property, where this is useful. Otherwise, it may be 0.

Properties generally only exist as part of an atom:Object. Accordingly, they will typically be represented directly as properties in RDF (see atom:Object). If this is not possible, they may be expressed as partial reified statements, e.g.:

    rdf:predicate eg:theKey ;
    rdf:object eg:theValue .
Sub-class ofatom:Atom
atom:cType LV2_Atom_Property

Class atom:Resource


This class is deprecated. Use atom:Object instead.

An atom:Object where the id field is a URID, i.e. an Object with a URI.

Sub-class ofatom:Object
owl:deprecated true
atom:cType LV2_Atom_Object

Class atom:Sequence


A sequence of atom:Event, i.e. a series of time-stamped Atoms.

LV2_Atom_Sequence_Body.unit describes the time unit for the contained atoms. If the unit is known from context (e.g. run() stamps are always audio frames), this field may be zero. Otherwise, it SHOULD be either units:frame or units:beat, in which case ev.time.frames or is valid, respectively.

If serialised to RDF, a Sequence has a similar form to atom:Vector, but for brevity the elements may be assumed to be atom:Event, e.g.:

    a atom:Sequence ;
    rdf:value (
            atom:frameTime 1 ;
            rdf:value "901A01"^^midi:MidiEvent
        ] [
            atom:frameTime 3 ;
            rdf:value "902B02"^^midi:MidiEvent
    ) .
Sub-class ofatom:Atom
atom:cType LV2_Atom_Sequence

Class atom:Sound


An atom:Vector of atom:Float which represents an audio waveform. The format is the same as the buffer format for lv2:AudioPort (except the size may be arbitrary). An atom:Sound inherently depends on the sample rate, which is assumed to be known from context. Because of this, directly serialising an atom:Sound is probably a bad idea, use a standard format like WAV instead.

Sub-class ofatom:Vector
atom:cType LV2_Atom_Sound

Class atom:String


A UTF-8 encoded string.

The body of an LV2_Atom_String is a C string in UTF-8 encoding, i.e. an array of bytes (uint8_t) terminated with a NULL byte ('\0').

This type is for free-form strings, but SHOULD NOT be used for typed data or text in any language. Use atom:Literal unless translating the string does not make sense and the string has no meaningful datatype.

Sub-class ofatom:Atom
owl:onDatatype xsd:string
atom:cType LV2_Atom_String

Class atom:Tuple


A series of Atoms with varying type and size.

The body of a Tuple is simply a series of complete atoms, each aligned to 64 bits.

If serialised to RDF, a Tuple SHOULD have the form:

     a atom:Tuple ;
     rdf:value (
     ) .
Sub-class ofatom:Atom

Class atom:URI

URI string

A URI string. This is useful when a URI is needed but mapping is inappropriate, for example with temporary or relative URIs. Since the ability to distinguish URIs from plain strings is often necessary, URIs MUST NOT be transmitted as atom:String.

This is not strictly a URI, since UTF-8 is allowed. Escaping and related issues are the host's responsibility.

Sub-class ofatom:String
owl:onDatatype xsd:anyURI

Class atom:URID

Integer URID

An unsigned 32-bit integer mapped from a URI (e.g. with LV2_URID_Map).

Sub-class ofatom:Atom
atom:cType LV2_Atom_URID

Class atom:Vector


A homogeneous series of atom bodies with equivalent type and size.

An LV2_Atom_Vector is a 32-bit child_size and child_type followed by size / child_size atom bodies.

For example, an atom:Vector containing 42 elements of type atom:Float:

struct VectorOf42Floats {
    uint32_t size;        // sizeof(LV2_Atom_Vector_Body) + (42 * sizeof(float);
    uint32_t type;        // map(expand("atom:Vector"))
    uint32_t child_size;  // sizeof(float)
    uint32_t child_type;  // map(expand("atom:Float"))
    float    elems[42];

Note that it is possible to construct a valid Atom for each element of the vector, even by an implementation which does not understand child_type.

If serialised to RDF, a Vector SHOULD have the form:

     a atom:Vector ;
     atom:childType atom:Int ;
     rdf:value (
     ) .
Sub-class ofatom:Atom
atom:cType LV2_Atom_Vector

Property atom:beatTime

beat time

Time stamp in beats. Typically used for events.

OWL TypeDatatype Property

Property atom:bufferType

buffer type

Indicates that an AtomPort may be connected to a certain Atom type. A port MAY support several buffer types. The host MUST NOT connect a port to an Atom with a type not explicitly listed with this property. The value of this property MUST be a sub-class of atom:Atom. For example, an input port that is connected directly to an LV2_Atom_Double value is described like so:

    lv2:port [
        a lv2:InputPort , atom:AtomPort ;
        atom:bufferType atom:Double ;
    ] .

This property only describes the types a port may be directly connected to. It says nothing about the expected contents of containers. For that, use atom:supports.

OWL TypeObject Property

Property atom:cType

C type

The identifier for a C type describing the binary representation of an Atom of this type.

OWL TypeDatatype Property

Property atom:childType

child type

The type of a container's children.

OWL TypeObject Property

Property atom:frameTime

frame time

Time stamp in audio frames. Typically used for events.

OWL TypeDatatype Property

Property atom:supports


Indicates that a particular Atom type is supported.

This property is defined loosely, it may be used to indicate that anything supports an Atom type, wherever that may be useful. It applies recursively where collections are involved.

In particular, this property can be used to describe which event types are expected by a port. For example, a port that receives MIDI events is described like so:

    lv2:port [
        a lv2:InputPort , atom:AtomPort ;
        atom:bufferType atom:Sequence ;
        atom:supports midi:MidiEvent ;
    ] .

Instance atom:atomTransfer

atom transfer

Transfer of the complete atom in a port buffer. Useful as the format for a LV2UI_Write_Function.

This protocol applies to atom ports. The host must transfer the complete atom contained in the port, including header.


Instance atom:eventTransfer

event transfer

Transfer of individual events in a port buffer. Useful as the format for a LV2UI_Write_Function.

This protocol applies to ports which contain events, usually in an atom:Sequence. The host must transfer each individual event to the recipient. The format of the received data is an LV2_Atom, there is no timestamp header.



Version 2.0 (2014-08-08)
  • Add lv2_atom_sequence_clear() and lv2_atom_sequence_append_event() helper functions.
  • Add lv2_atom_forge_key() for terser object writing.
  • Deprecate Blank and Resource in favour of just Object.
  • Add lv2_atom_forge_is_object_type() and lv2_atom_forge_is_blank() to ease backwards compatibility.
Version 1.8 (2014-01-04)
  • Make lv2_atom_*_is_end() arguments const.
Version 1.6 (2013-05-26)
  • Fix crash in forge.h when pushing atoms to a full buffer.
Version 1.4 (2013-01-27)
  • Remove atom:stringType in favour of owl:onDatatype so generic tools can understand and validate atom literals.
  • Improve atom documentation.
  • Fix lv2_atom_sequence_end().
Version 1.2 (2012-10-14)
  • Fix lv2_atom_object_next() on 32-bit platforms.
  • Fix outdated documentation in forge.h.
  • Fix implicit conversions in forge.h that are invalid in C++11.
  • Use consistent label style.
  • Add lv2_atom_object_body_get().
Version 1.0 (2012-04-17)
  • Initial release.