The following is a semaphore solution to the readers$-$writers problem:

semaphore mutex $=1$

semaphore wrt $=1$

int variable: readcount $=0$

$0$ Reader: wait $($mutex$)$

readcount $=$ readcount $+1$

if readcount $=1$ then wrt$)$

V$($mutex$)$

$==0V$