Even if you limit "side effects" to observable behavior in the abstract machine sense, it's not entirely clear what is meant by a function to be "pure". GCC ha

Even if you limit "side effects" to observable behavior in the abstract machine sense, it's not entirely clear what is meant by a function to be "pure". GCC has two attributes for marking functions, "pure" and "const" (not the language const qualifier). C23 introduced the [[reproducible]] and [[unsequenced]] attributes, that are mostly modeled by the GCC extensions, but with some subtle but important differences in their description. Turns out it's pretty hard to define these concepts if the language is not built around immutability and pure functions from the ground up.