The decc$validate_wchar function provides a convenient way to
    verify whether a specified argument of wchar_t type is a valid
    wide character in the current program's locale.

    One reason to call decc$validate_wchar is that the isw* wide-
    character classification functions and macros do not validate
    their argument before dereferencing the classmask array
    describing character properties. Passing an isw* function a value
    that exceeds the maximum wide-character value for the current
    program's locale can result in an attempt to access memory beyond
    the allocated classmask array.

    A standard way to validate a wide character is to call the wctomb
    function, but this way is less convenient because it requires
    declaring a multibyte character array of sufficient size and
    passing it to wctomb.