http://elm-chan.org/fsw/ff/pf/read.html

pf_read
The pf_read function reads data from the file.
FRESULT pf_read (
  void* Buffer,       /* Pointer to the read buffer */
  WORD ByteToRead,    /* Number of bytes to read */
  WORD* BytesRead     /* Pointer to the variable to return number of bytes read */
);
Parameters
Buffer
Pointer to the buffer to store the read data. A NULL specifies the destination is an outgoing stream.
ByteToRead
Number of bytes to read.
BytesRead
Pointer to the WORD variable to return number of bytes read.


请问大家
A NULL specifies the destination is an outgoing stream.
是什么意思，谢谢