Thread demo

• slowprimes.c implements a naive method of calculating the number of primes less than N by checking if each number is divisible by some other number. This problem is easily parallelizable as checking a number is independent from checking other numbers. slowprimes.py implements the same algorithm in Python.
• primes.c implements the sieve of Eratosthenes, which is asymptotically faster than the naive method. We can see orders of magnitude of performance improvement over the parallelized version of the above. However, this algorithm is not so cleanly divisible into subproblems, and we should see some but not much time improvement when using multithreading.
• badcnt.c shows what could happen when two threads compete for the same memory resource.
• goodcnt.c shows how to use semaphores to synchronize access to shared resources.
• Finally, bomb.c shows how to communicate between two threads.