Man page 9 : kfifo_to_user

kfifo_to_user - copies data from the fifo into user space

kfifo_to_user( fifo , to , len , copied );

fifo
    address of the fifo to be used

to
    where the data must be copied

len
    the size of the destination buffer

copied
    pointer to output variable to store the number of copied bytes

This macro copies at most len bytes from the fifo into the to buffer and returns -EFAULT/0.

Note that with only one concurrent reader and one concurrent writer, you donAqt need extra locking to use these macro.