How could I guarantee a terminal has Unicode/wide character support with NCURSES?

Question

I am developing an NCURSES application for a little TUI (text user interface) exercise. Unfortunately, I do not have the option of using the ever-so-wonderful-and-faithful ASCII. My program uses a LOT of Unicode box drawing characters.

My program can already detect if the terminal is color-capable. I need to do something like:

if(!supportsUnicode()) //I prefer camel-case, it's just the way I am.
{
    fprintf(stderr, "This program requires a Unicode-capable terminal.\n\r");
    exit(1);
}
else
{
    //Yay, we have Unicode! some random UI-related code goes here.
}

This isn't just a matter of simply including ncursesw and just setting the locale. I need to get specific terminal info and actually throw an error if it's not gonna happen. I need to, for example, throw an error when the user tries to run the program in the lovely XTerm rather than the Unicode-capable UXTerm.

Answer 1

You can't. ncurses(w) uses termcap to determine what capabilities a terminal has, and that looks at the $TERM environment variable to determine what terminal is being used. There is no special value of that variable that indicates that a terminal supports Unicode; both XTerm and UXTerm set TERM=xterm. Many other terminal applications use that value of $TERM as well, including both ones that support Unicode and ones that don't. (Indeed, in many terminal emulators, it's possible to enable and disable Unicode support at runtime.)

If you want to start outputting Unicode text to the terminal, you will just have to take it on faith that the user's terminal will support that.

If all you want to do is output box drawing characters, though, you may not need Unicode at all — those characters are available as part of the VT100 graphical character set. You can output these characters in a ncurses application using the ACS_* constants (e.g, ACS_ULCORNER for ?), or use a function like box() to draw a larger figure for you.

Answer 2

The nl_langinfo() function shall return a pointer to a string containing information relevant to the particular language or cultural area defined in the current locale.

#include <langinfo.h>
#include <locale.h>
#include <stdbool.h>
#include <string.h>

bool supportsUnicode()
{
        setlocale(LC_CTYPE, "");
        return strcmp(nl_langinfo(CODESET), "UTF-8") ? false : true;
}

Refer to htop source code which can draw lines with/without Unicode.