Character sets and kinds

The IO-Fortran-Library officially supports the standard Fortran character set, based on the US-ASCII character set with default character kind. On most systems, the default kind will consist of precisely one byte per character, equivalent to selected_char_kind("ascii"). Since US-ASCII is a 7-bit character set and most systems use 8-bit characters, it must be noted that only the first 128 characters are system-independent, i.e. achar(0) is portable but achar(255) is not. To maximize portability across systems and compilers, the IO-Fortran-Library does not reference any other character sets or kinds other than the 128-character US-ASCII with default kind.

However, most users should feel free to employ characters outside of the US-ASCII in strings and string expressions (including many Unicode symbols) and these will tend to behave as expected as long as the characters can fit comfortably into one byte and the output unit supports UTF-8 encoding. For instance, inspect the output of the following program:

program main
    use io_fortran_lib
    implicit none (type, external)

    type(String) :: emojis

    emojis = "😂🙈😊🤣" + "😍" - "😂" + "👌"**5
    call emojis%echo("emojis.txt")
    write(*,*) emojis
end program main

The expected result is 🙈😊🤣😍👌👌👌👌👌, which will be displayed properly in any terminal or text file with UTF-8 encoding.