As far as I understand copyright is automatic, at least under the Berne Convention. There is no automatic license, so for anything "large" or "worthy" enough to fall under the Berne Convention, you'll need to get an explicit license from the author, if you involve a legal counselor. If you don't involve a legal counselor, I assume that code posted, for example, in reply to a question would be licensed implicitly for reuse, or even be in the public domain as far as such a construct exists in your jurisdiction.
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