The contents of the header must represent the structure that is expected by the I/O procedure. It does not represent the manner in which the data will be saved by the I/O procedure, because the I/O procedure might translate the data in some manner. The plBytesWritten parameter value might differ from the actual number of bytes written to the file in the case of translations.

This function can be used in conjunction with mmioSet to set (write) a specific track header into a multiple track movie file.