putenv is not lock free: https://github.com/illumos/illumos-gate/blob/master/usr/src/...

Ay, yes. Though it could be made lock-less.

I think you meant

    auto ii{};

Not at all. I meant

    std::common_type_t<bool, char> ii{};

Not sure this one is actually valid. You can't infer a type. Maybe in C++40 where types are so 30s?

C++030 is just next year!

Thanks, glad you found it useful :)

Should be fixed, sorry for the inconvenience.

There's a GUI redshift for Windows: https://github.com/maoserr/redshiftgui/downloads