Chunks

The basic building block of a RIFF file is called a chunk. Using C syntax, a chunk can be defined as follows:

typedef unsigned long ULONG;
typedef unsigned char BYTE;

typedef ULONG FOURCC;        /* Four-character code                  */

typedef FOURCC CKID;         /* Four-character-code chunk identifier */
typedef ULONG CKSIZE;        /* 32-bit unsigned size value           */

typedef struct {             /* Chunk structure                      */
      CKID           ckID;              /* Chunk type identifier             */
      CKSIZE         ckSize;            /* Chunk size field (size of ckData) */
      BYTE           ckData[ckSize];    /* Chunk data              */
} CK;

A FOURCC is represented as a sequence of one to four ASCII alphanumeric characters, padded on the right with blank characters (ASCII character value 32) as required, with no embedded blanks.

For example, the four-character code 'FOO' is stored as a sequence of four bytes: 'F', 'O', 'O', '' in ascending addresses. For quick comparisons, a four-character code may also be treated as a 32-bit number.

The three parts of the chunk are described in the following table:

Part

Description
ckID
A four-character code that identifies the representation of the chunk data data. A program reading a RIFF file can skip over any chunk whose chunk ID it doesn't recognize; it simply skips the number of bytes specified by ckSize plus the pad byte, if present.
ckSize
A 32-bit unsigned value identifying the size of ckData. This size value does not include the size of the ckID or ckSize fields or the pad byte at the end of ckData.
ckData
Binary data of fixed or variable size. The start of ckData is word-aligned with respect to the start of the RIFF file. If the chunk size is an odd number of bytes, a pad byte with value zero is written after ckData. Word aligning improves access speed (for chunks resident in memory) and maintains compatibility with EA IFF. The ckSize value does not include the pad byte.

We can represent a chunk with the following notation (in this example, the ckSize and pad byte are implicit):

<ckID> ( <ckData> )

Two types of chunks, the 'LIST' and 'RIFF' chunks, may contain nested chunks, or subchunks. These special chunk types are discussed later in this document. All other chunk types store a single element of binary data in <ckData>.

---

[Back: Resource Interchange File Format]
[Next: RIFF Forms]