Readers-Writers with Semaphores

shared int readcount=0; sem mutex=1, wrt=1;
Reader: wait(mutex); readcount++; if(readcount==1) wait(wrt); signal(mutex); read; wait(mutex); readcount--; if(readcount==0) signal(wrt); signal(mutex);
Writer: wait(wrt); write; signal(wrt);
Readers preference variant, writer may starve; for writer preference add extra sem.