A generic value container and several data types.
This extension defines a generic container for data, called an Atom, and several basic Atom types which can be used to express structured data. Atoms are (with one exception) Plain Old Data (POD), which means they can be easily copied generically (e.g. using memcpy), and are suitable for use in high-performance and real-time code. Since Atom communication can be implemented generically, plugins that understand some type can be used together in a host that does not understand that type. Similarly, plugins (such as routers, delays, or data stores) can meaningfully process atoms of a type unknown to them. Atoms can and should be used anywhere values of various types must be stored or transmitted. This extension defines a port type, atom:AtomPort, for transmitting atoms via ports. The atom:Sequence type in an atom:AtomPort replaces the LV2 event extension. The types defined in this extension should be powerful enough to express almost any structure. Implementations SHOULD build structures out of the types provided here, rather than define new binary formats (e.g. use atom:Object rather than a new C struct type). New binary formats are an implementation burden which harms interoperabilty, and should only be defined where absolutely necessary. Implementing this extension requires a facility for mapping URIs to integers, such as the LV2 URID extension.
The identifier for a C type describing the in-memory representation of an instance of this class.
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 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.
An Int where 0 is false and any other value is true.
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 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/iso639-3/LANG> or <http://lexvo.org/id/iso639-1/LANG> 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->datatype = 0; lit->lang = map("http://lexvo.org/id/iso639-1/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 }
A local file path string. This is identical in format to atom:String, except the string is a path. Since the ability to distinguish paths from plain strings is often necessary, paths MUST NOT be transmitted as atom:String.
A URI string. This is identical in format to atom:String, except the string is a URI. 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 issues are the host's responsibility.
An unsigned 32-bit integer mapped from a URI (e.g. with LV2_URID_Map).
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.
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.
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 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.
An atom:Object where the id field is a URID, i.e. an Object with a URI.
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: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.
A unit for atom:Event time stamps.
Time stamp in audio frames. Typically used for events.
Time stamp in beats. Typically used for events.
Time in audio frames. Converting this to absolute time depends on the sample rate. When this is the stamp unit for an atom:Sequence, its events have int64_t time stamps (event.time.frames)
Time in beats. Converting this to absolute time depends on the tempo. When this is the stamp unit for an atom:Sequence, the events in that sequence have a double stamp (event.time.beats).
An atom with a time stamp header prepended, typically an element of an atom:Sequence. Note this is not an Atom type.
A sequence of atom:Event, i.e. a series of time-stamped Atoms.
A port which contains an lv2: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().
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: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.
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 or understood by a port. For example, a port that receives MIDI events is described like so: <plugin> lv2:port [ a lv2:InputPort , atom:AtomPort ; atom:bufferType atom:Sequence ; atom:supports midi:MidiEvent ; ] .
Transfer of individual events. Useful as the format for a LV2_UI_Write_Function, or the port_protocol for LV2_PUI_Host_Descriptor::write_port(). 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.