This Stack Overflow is a fairly decent answer to the question "What is a segmentation fault?": https://stackoverflow.com/questions/2346806/what-is-a-segmentation-fault

But, in short, a segmentation fault happens when your program tries to access memory that hasn't been allocated to it. There are several ways that can happen, but common ones include trying to use an invalid or null pointer, or going outside the bounds of an array. Those are probably your most likely suspects, especially with the kind of code you are writing at the moment.