hdlr

hdlr, short for Handler Reference Box, is a metadata box used in the ISO Base Media File Format (ISOBMF), including MP4 and related containers. It is identified by the four-character code hdlr and is located within the Media Box (mdia) of a track. The purpose of the hdlr box is to declare the media handler type for the track and to provide a human-readable name for the component that processes the track’s media data.

The hdlr box contains several fields: version and flags, a 4-byte pre_defined field, a 4-byte handler_type, a 12-byte reserved field, and a name string that is typically null-terminated. The handler_type specifies the kind of data the track carries, using common four-character codes such as vide for video tracks, soun for audio tracks, meta for metadata, and hint for hint tracks. The name field offers a descriptive label, such as "Video Media Handler" or "Sound Media Handler," which can aid in debugging or editing but is not required for playback.

Role and usage: the hdlr box helps media players and authoring tools route and process track data correctly by indicating the intended processing component. While some files may omit certain fields or use defaults, a valid handler_type is generally expected for proper interpretation of tracks.

See also: MP4 file format, ISO Base Media File Format, QuickTime File Format.