There are three fields in the MMIOINFO structure used for reading and writing the file I/O buffer: pchNext, pchEndRead, and pchEndWrite. The pchNext field points to the next location in the buffer to read or write. You must increment pchNext as you read and write the buffer. The pchEndRead field identifies the last valid character you can read from the buffer. Likewise, pchEndWrite identifies the last location in the buffer you can write. To be precise, both pchEndRead and pchEndWrite point to the memory location following the last valid data in the buffer.