Bug Hunting Adventures #13: Prime Sums

“Why, yes; and not exactly that either. The fact is, we have all been a good deal puzzled because the affair is so simple, and yet baffles us altogether.”
― Edgar Allan Poe, The Purloined Letter

Below, you find a little C project that doesn’t do what it’s supposed to do, namely print the sum of the first 10 prime numbers. The program builds cleanly with gcc and clang; that is, without any warnings even when using -Wextra -Wall -pedantic -ansi as compiler options. It’s well-formed and doesn’t crash.

What’s the root cause of this bug? What’s the output of the program? Here are the files, you can also find them on GitHub: