Resource Interchange File Format Services
The preferred format for multimedia files is resource interchange file format
(RIFF). The RIFF file I/O functions work with the basic buffered and unbuffered
file I/O services. You can open, read, and write RIFF files in the same way as
other file types. For detailed information about RIFF, see
AVIFile Functions and Macros.
RIFF files use four-character codes to identify file elements. These codes are
32-bit quantities representing a sequence of one to four ASCII alphanumeric
characters, padded on the right with space characters. The data type for
four-character codes is
FOURCC. Use the
mmioFOURCC macro to convert four characters into a four-character code. To convert a
null-terminated string into a four-character code, use the
mmioStringToFOURCC function.
The basic building block of a RIFF file is a
chunk. A chunk is a logical unit of multimedia data, such as a single frame in a
video clip. Each chunk contains the following fields:
- A four-character code specifying the chunk identifier
- A doubleword value specifying the size of the data member in the chunk
- A data field
The following illustration shows a "RIFF" chunk that contains two subchunks.
A chunk contained in another chunk is a
subchunk. The only chunks allowed to contain subchunks are those with a chunk
identifier of "RIFF" or "LIST". A chunk that contains another chunk is called a
parent chunk. The first chunk in a RIFF file must be a "RIFF" chunk. All other chunks in
the file are subchunks of the "RIFF" chunk.
"RIFF" chunks include an additional field in the first four bytes of the data
field. This additional field provides the
form type of the field. The form type is a four-character code identifying the format
of the data stored in the file. For example, Microsoft waveform-audio files have
a form type of "WAVE".
"LIST" chunks also include an additional field in the first four bytes of the
data field. This additional field contains the
list type of the field. The list type is a four-character code identifying the contents
of the list. For example, a "LIST" chunk with a list type of "INFO" can
contain "ICOP" and "ICRD" chunks providing copyright and creation date information.
The following illustration shows a "RIFF" chunk that contains a "LIST" chunk and
one other subchunk (the "LIST" chunk contains two subchunks).
Multimedia file I/O services include two functions you can use to navigate
among chunks in a RIFF file:
mmioAscend and
mmioDescend. You can use these functions as high-level seek functions. When you descend
into a chunk, the file position is set to the data field of the chunk (8 bytes
from the beginning of the chunk). For "RIFF" and "LIST" chunks, the file
position is set to the location following the form type or list type (12 bytes from
the beginning of the chunk). When you ascend out of a chunk, the file position is
set to the location following the end of the chunk.
To create a new chunk, use the
mmioCreateChunk function to write a chunk header at the current position in an open file. The
mmioAscend,
mmioDescend, and
mmioCreateChunk functions use the
MMCKINFO structure to specify and retrieve information about "RIFF" chunks.
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