* `m` is an instance of [link beast.ref.boost__beast__file_mode `file_mode`] * `n` is a number of bytes, convertible to `std::size_t` * `o` is a byte offset in the file, convertible to `std::uint64_t` * `b` is any non-const pointer to memory * `c` is any possibly-const pointer to memory * `ec` is a reference of type [link beast.ref.boost__beast__error_code `error_code`]