Box character doesn't display properly in Linux terminal

Question

I was just writing a C++] program that uses the box characters to display information.  I ran the program on macOS and used the terminal app and it worked fine.

When I switched to Debian Linux using the same code and recompiling it, I only then encountered this problem!  I've searched online for answers, but I have found none related to this problem.  I've tested this with both LXTerminal and URXVT.

Here is my minimal reproducible example of this problem:

#include <ncurses.h>
#include <iostream>

int ch;

int main() {
    setlocale(LC_CTYPE, "");
    initscr();
    noecho();
    keypad(stdscr, true);
    nodelay(stdscr, true);
    curs_set(0);
    start_color();
    use_default_colors();

    init_pair(1, COLOR_BLUE, -1);

    clear();
    refresh();

    while ((ch = getch()) != 'q' && ch != 'Q') {

        attrset(COLOR_PAIR(1));

        mvprintw(0,0,"┏━┓");
        mvprintw(1,0,"┗━┛");

    }

    refresh();
    endwin();

    return 0;
}

Compile this demo with this command below:

g++ -g -c -o main.o test.cpp && g++ -g -o test main.o -std=c++11 -Wall -pedantic -lncurses

or

LDLIBS=-lncurses make test

Instead of displaying

┏━┓
┗━┛

it displays this:

M-b~T~OM-b~T~AM-b~T~S
M-b~T~WM-b~T~AM-b~T~[

Actual screen images

Normal (desired) display on macOS:

Abnormal display on Linux:

Answer 1

The problem is that you're using literal characters ┌, ─, ┐, └, ─, ┘ instead of using the ncurses-independent symbols. You should use the ACS_??CORNER (for Upper/Lower and Left/Right) and ACS_HLINE symbolic names and let ncurses work out how to represent those on your terminal.

Modifying your code from this,

mvprintw(0,0,"┏━┓");
mvprintw(1,0,"┗━┛");

to this,

move(0,0); addch(ACS_ULCORNER); addch(ACS_HLINE); addch(ACS_URCORNER);
move(1,0); addch(ACS_LLCORNER); addch(ACS_HLINE); addch(ACS_LRCORNER);

should ensure that the code works portably. I also changed the end of your main() procedure from a plain return 0 to the following, so that the terminal will be correctly reset when the program ends:

endwin();
exit(0);

Compile test.cpp and execute with,

LDLIBS=-lncurses make test && ./test

Useful references, in no particular order:

• NCURSES Extended Characters Quick Reference Charts and Screen Layout Chart
• NCURSES Programming HOWTO
• add a character (with attributes) to a curses window, then advance the cursor

Answer 2

On MacOS, the -lncurses links to the wide-character configuration of ncurses, which normally is provided by -lncursesw. The setlocale call gives different results in those (normal versus wide characters), and the literal string for the box-characters (which is UTF-8) comes out differently.

Further reading:

• Curses library doesn't support wide char on OS X high sierra, for my explanation
• Alternate Configurations in the ncurses manual
• keyname, which expands on unctrl and addch, noting

• Values above 128 are either meta characters (if the screen has not been initialized, or if meta(3x) has been called with a TRUE parameter), shown in the M-X notation, or are displayed as themselves. In the latter case, the values may not be printable; this follows the X/Open specification.

The underlying issue is that the UTF-8 encoding for the box characters uses byte-codes above 128, and without the locale support used in the wide-character library, you'll get just that—no reassembly of the bytes into a wide-character.

Some other platforms also present -lncurses as an alias for -lncursesw, e.g., (noting a comment), Arch Linux which has this in /usr/lib/ncurses.so:

INPUT(-lncursesw)

Another comment asked about "thick lines". There are a few points here:

• In my answer Double line box_drawing characters in terminal, I mentioned that ncurses provides symbols for convenience in using the Unicode thick and double lines.
• Those have been in ncurses since October 2009:

I only implemented WACS_xxx symbols for thick/double-lines for the usual reasons: it's experimental, and only useful with Unicode.

• The symbols are documented in the wadd_wch manual page.
• The feature is not available for MacOS's bundled ncurses, since that (ncurses 5.7) is about a year older than the change in ncurses.
• It will not work for the Linux console terminal (because that relies upon a font with only 512 glyphs).
• The actual appearance of thick/double lines is unspecified (it depends on the fonts).

Answer 3

The problem was that box character didn't display properly in a Linux terminal. The solution is that you need to compile the code differently.

If you are using printw or some other text displaying method that uses ncurses as a library, then this will apply to you.  This is OS dependent, so if you are on macOS you would use the -lncurses flag when compiling (also according to @terdon this also applies to Arch Linux, but I haven't tested that), and if you are using Linux (I've tested it only for Debian) you need to use the -lncursesw flag.

If you want to automatically make your code cross-compatible for this issue, put this in your makefile.

ifneq ($(OS),Windows_NT)
    UNAME_S := $(shell uname -s)
    ifeq ($(UNAME_S),Linux)
        NCURSES += -lncursesw
    endif
    ifeq ($(UNAME_S),Darwin)
        NCURSES += -lncurses
    endif
endif