Which header file should I include for using _swprintf_s()?

When I used _swprintf(), it said it might be unsafe and I should use _swprintf_s instead.

But when I used _swprintf_s(), it said

error C3861: '_swprintf_s': 开发者_运维问答identifier not found

Which header am I missing? I

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It is swprintf() and swprintf_s(). No leading underscore.