LV2 Atom

Abstract base class for all atoms. An LV2_Atom has a 32-bit type and size 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 ignore, or pass through, 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.

A port which contains an lv2:Atom. Ports of this type will be connected to a 64-bit aligned LV2_Atom immediately followed by size bytes of data.

This is an abstract port type with incomplete semantics which can not be used directly as a port type. Atom ports should be either a atom:ValuePort or a atom:MessagePort.

Before calling a method on a plugin that writes to an AtomPort output, the host MUST set the size of the Atom in that output to the amount of available memory immediately following the Atom header. The plugin MUST write a valid Atom to that port; leaving it untouched is illegal. If there is no reasonable value to write to the port, the plugin MUST write null (the Atom with both type and size 0).

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.

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

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

This type is compatible with rdf: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 <http://lexvo.org/id/term/LANG> where LANG is an ISO 693-2 or ISO 693-3 language code.

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->datatype  = 0;
     lit->lang      = map("http://lexvo.org/id/term/en");
     memcpy(LV2_ATOM_CONTENTS(LV2_Atom_Literal, lit),
            "Hello",
            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->datatype  = map("http://www.w3.org/2008/turtle#turtle");
     lit->lang      = 0;
     memcpy(LV2_ATOM_CONTENTS(LV2_Atom_Literal, lit),
            ttl,
            strlen(ttl) + 1);  // Assumes enough space
}

An AtomPort that contains transient data which is consumed or sent. The Atom contained in a MessagePort is time-dependent and only valid for a single run invocation. Unlike a ValuePort, a MessagePort may be used to manipulate internal plugin state.

Intuitively, a MessagePort contains a message or event which is reacted to once (not a value which is computed with any number of times).

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 has a uint32_t id and uint32_t type, followed by a series of atom:Property bodies (without headers, i.e. LV2_Atom_Property_Body). The LV2_Atom_Object::type field is semantically equivalent to a property with key rdf:type, but is included in the structure to allow for fast dispatch.

This is an abstract Atom type, an Object is always either a atom:Resource or a atom:Blank.

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.

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

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

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 can be used for free-form strings, but in most cases it is better to use atom:Literal since this supports a language tag or datatype. Implementations SHOULD NOT use atom:String unless translating the string does not make sense and the string has no meaningful datatype.

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.

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

An AtomPort that contains a persistent value. A value is time-independent and may be used numerous times. A ValuePort is pure in the sense that it may affect output but MUST NOT affect persistent plugin state in any externally visible way.

• If a plugin has fixed values for all inputs, all ValuePort outputs are also fixed regardless of the number of times the plugin is run.
• If a plugin has fixed input values for all ports except a ValuePort, each value of that port corresponds to a single set of values for all ValuePort outputs.
• If the plugin saves state other than port values (e.g. using the LV2 State extension), changing only the value of a ValuePort input MUST NOT change that state. In other words, value port changes MUST NOT trigger a state change that requires a save.

Value ports are essentially purely functional ports: if a plugin has only value ports, that plugin is purely functional. Hosts may elect to cache output and avoid calling run() if the output is already known according to these rules.

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

An LV2_Atom_Vector is a 32-bit elem_count and elem_type followed by elem_count atom bodies of type elem_type. The element type must be a fixed size atom:Atom type, i.e. the size of each element is the vector's size / elem_count.

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

struct VectorOf42Floats {
    uint32_t type;        // map(expand("atom:Vector"))
    uint32_t size;        // sizeof(LV2_Atom_Vector) + (42 * sizeof(float);
    uint32_t elem_count;  // 42
    uint32_t elem_type;   // map(expand("atom:Float"))
    float    elems[32];
};

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

<plugin>
    lv2:port [
        a lv2:InputPort , atom:ValuePort ;
        atom:bufferType atom:Double ;
    ] .

Note this property only indicates the atom types a port may be directly connected to, it is not recursive. If a port can be connected to a collection, use atom:supports to indicate which element types are understood. If a port supports heterogeneous collections (collections that can contain several types of elements at once), implementations MUST gracefully handle any types that are present in the collection, even if those types are not explicitly supported.

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 supported by a port. For example, a port that receives MIDI events is described like so:

<plugin>
    lv2:port [
        a lv2:InputPort , atom:MessagePort ;
        atom:bufferType atom:Sequence ;
        atom:supports midi:MidiEvent ;
    ] .