int r = 0;
int w = 0;

read_lock(void)
{
     r++;
     while( w != 0);
}

read_unlock(void)
{
   r—;
}
write_lock(void)
{
 while( (r != 0) &&
            ( w != 0))
  w++;
}

write_unlock(void)
{
    w--;
}

read_lock()
// Critical Section
read_unlock();

write_lock()
// Critical Section
write_unlock();