This is an implementation of the Sieve of Eratosthenes in C. I originally wrote something similar in PHP for an interview question and found it to be a fun little exercise. PHP was going painfully slow for values greater than 10000 or so, so I decided to get closer to the metal.
Usage is fairly straightforward. Invoke the executable from the command line with an argument indicating the number to calculate up to.
./primes 1000
will calculate all the primes up to 1000.