The following figure shows how the file I/O buffer is advanced as a file is read from.

• The application opens the file for buffered I/O. The buffer is initially empty, so mmioOpen sets pchNext and pchEndRead to point to the beginning of the file I/O buffer.

• The application calls mmioAdvance to fill the I/O buffer. The mmioAdvance function fills the buffer and sets pchNext to point to the beginning of the buffer.

• The application reads from the I/O buffer and increments pchNext.

• The application continues to read the buffer and call mmioAdvance to refill the buffer when it is empty. When mmioAdvance reaches the end of the file, there is not enough information to fill the buffer. The mmioAdvance function sets pchEndRead to point to the end of the valid data in the buffer.
  

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