mmioCreateChunk creates a new chunk; that is, it writes a chunk header starting at the current file position and descends into the chunk. The chunk ID is copied from the ckid field of the provided MMCKINFO structure. Call mmioAscend after the chunk data has been written. The ckSize field is assumed to be a proposed chunk size, if it turns out to be correct; that is, if you write that much data into the chunk before calling mmioAscend to end the chunk, mmioAscend will not have to seek back and correct the chunk header.