(Recent) StackOverflow contributions are licensed under CC BY-SA 4.0 by default (though the author can of course release it under any additional licenses they choose): https://stackoverflow.com/help/licensing

If the code is really sufficiently trivial (and I’d guess that most code samples you’ll find on StackOverflow are) you may have a fair use argument in the US. Generally speaking though (and especially for anything nontrivial) you need to respect the license. CC BY-SA 4.0 is one-way compatible with GPLv3, though, so that helps if you’re including it in a GPLv3 codebase: https://creativecommons.org/2015/10/08/cc-by-sa-4-0-now-one-...