It's not quite a "pair". It's a "tagged union" in C-speak, "enum" in Rust-speak and "sum type" in CS-speak. In this case, numerical result and errno will be l

It's not quite a "pair". It's a "tagged union" in C-speak, "enum" in Rust-speak and "sum type" in CS-speak. In this case, numerical result and errno will be located in the same memory, but have different tag.