(Or "primitive") A built-in function or operator is one provided by the lowest level of a language implementation. This usually means it is not possible (or efficient) to express it in the language itself. Typical examples are the basic arithmetic and Boolean operators (in C syntax: +, -, *, /, %, !, &&, ||), bit manipulation operators (~, &, |, ^) and I/O primitives. Other common functions may be provided in libraries but are not built-in if they are written in the language being implemented.
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