Position cursors, expressed as a byte offset, are maintained on sound and capture buffers indicating where it is safe to write or read data and the data currently being played or captured.
The write cursor is used in sound buffers, moving in advance of the play cursor and indicating the point in the buffer beyond which it is safe to write data.
The buffer area between the play and write cursors is already committed to be played and should not be modified.
The write cursor moves with the play cursor, not with the data being written to the buffer. Thus it is neccessary for your application to maintain its own pointer for the purpose of writing to the buffer.