Why do I get a segfault in C from declaring a large array on the stack?

Question

I get a segfault from this line of code:

int fatblob[1820][286][5];

Why is that?

Answer 1

You're trying to allocate 1820 * 285 * 5 * sizeof(int) bytes = about 10MB (if sizeof(int) == 4). That's probably more bytes than your OS gives you for stack allocation by default, so you get a stack overflow/segfault.

You can fix this by either asking for extra stack when you create the thread, allocating on the heap, or changing the OS defaults.