Ncurses - How to handle terminal resizes for an image that is being updated in an infinite loop

Question

I have a program utilizing ncurses to draw a window with a clock timer. It is constantly being refreshed in an infinite loop. It works as intended, however when I resize the terminal window, the entire image gets messed up and all out of sorts.

I've read some items on SIGWINCH for ncurses as well as a few others but they are all relying on a wgetch(win) or getch(). These calls block the loop waiting for user input. This seems like a pretty standard use-case, so trying to understand what is the procedure for detecting the window resizing without blocking the loop.

Note I am on Windows 11 and to compile I run gcc -o test.exe test.c -lncurses -DNCURSES_STATIC

#include <windows.h>
#include <ncurses/ncurses.h>
#include <time.h>

// default window sizes
#define WINDOW_HEIGHT 16
#define WINDOW_WIDTH 45
#define WINDOW_START_Y 5
#define WINDOW_START_X 5

// prototypes
WINDOW *init_window(void);

int main(void)
{
        // global screen init
        initscr();
        WINDOW *win;
        win = init_window();
        refresh();

        // hide cursor
        curs_set(0);

        // draw basic box in the window
        box(win, 0, 0);

        // initialize the timer
        time_t now;
        while (1) {
                
                // update current UTC time
                now = time(0);
                struct tm *tmp = gmtime(&now);

                // modify terminal window
                mvwprintw(win, 1, 1, "           _________           ");
                mvwprintw(win, 2, 1, "          / ======= \\         ");
                mvwprintw(win, 3, 1, "         / __________\\        ");
                mvwprintw(win, 4, 1, "        | ___________ |        ");
                mvwprintw(win, 5, 1, "        | | -       | |        ");
                mvwprintw(win, 6, 1, "        | |         | |              ");
                mvwprintw(win, 7, 1, "        | |_________| |________________  ");
                mvwprintw(win, 8, 1, "        \\=____________/      REM       ) ");
                mvwprintw(win, 9, 1, "        / \"\"\"\"\"\"\"\"\"\"\" \\               /  ");
                mvwprintw(win, 10, 1, "       / ::::::::::::: \\          =D-'   ");
                mvwprintw(win, 11, 1, "      (_________________)                  ");
                mvwprintw(win, 12, 1, "      current time: %02d:%02d:%02d", tmp->tm_hour, tmp->tm_min, tmp->tm_sec);
                wrefresh(win);
        }

        // clean up
        endwin();
        return 0;
}

WINDOW* init_window(void)
{
        // create new window
        WINDOW *win = newwin(WINDOW_HEIGHT, WINDOW_WIDTH, WINDOW_START_Y, WINDOW_START_X);
        if (!win) {
                printf("ERROR: could not create window\n");
                exit(1);
        }
        return win;
}

Answer 1

This should be a duplicate, but the essential point combining SIGWINCH and polled input doesn't appear.

So... modify the program to

• use wgetch with no time delay, to allow it to read KEY_RESIZE
• the window should be resized to allow for the margins, etc
• drawing the window on each call to time is wasteful (check and skip)

Here's a modified program which illustrates those points:

#include <stdlib.h>
#include <ncurses.h>
#include <time.h>

// default window sizes
#define WINDOW_HEIGHT LINES - 2
#define WINDOW_WIDTH COLS - 2
#define WINDOW_START_Y 1
#define WINDOW_START_X 1

// prototypes
WINDOW *init_window(void);

int main(void)
{
        initscr();
        cbreak();
        WINDOW *win = init_window();
        nodelay(win, TRUE);

        // hide cursor
        curs_set(0);

        // draw basic box in the window
        box(win, 0, 0);

        // initialize the timer
        time_t start = time(0);
        while (1) {
                if (wgetch(win) == KEY_RESIZE) {        // process SIGWINCH
                        wresize(win, WINDOW_HEIGHT, WINDOW_WIDTH);
                        clear();
                        wclear(win);
                        box(win, 0, 0);
                        wnoutrefresh(win);
                        start--;        // force a redraw
                }
                
                // update current UTC time
                time_t now = time(0);

                if (now == start)
                        continue;
                start = now;

                struct tm *tmp = gmtime(&now);

                // modify terminal window
                mvwprintw(win, 1, 1, "           _________           ");
                mvwprintw(win, 2, 1, "          / ======= \\         ");
                mvwprintw(win, 3, 1, "         / __________\\        ");
                mvwprintw(win, 4, 1, "        | ___________ |        ");
                mvwprintw(win, 5, 1, "        | | -       | |        ");
                mvwprintw(win, 6, 1, "        | |         | |              ");
                mvwprintw(win, 7, 1, "        | |_________| |________________  ");
                mvwprintw(win, 8, 1, "        \\=____________/      REM       ) ");
                mvwprintw(win, 9, 1, "        / \"\"\"\"\"\"\"\"\"\"\" \\               /  ");
                mvwprintw(win, 10, 1, "       / ::::::::::::: \\          =D-'   ");
                mvwprintw(win, 11, 1, "      (_________________)                  ");
                mvwprintw(win, 12, 1, "      current time: %02d:%02d:%02d", tmp->tm_hour, tmp->tm_min, tmp->tm_sec);
                wnoutrefresh(win);
                doupdate();
        }

        // clean up
        endwin();
        return 0;
}

WINDOW* init_window(void)
{
        // create new window
        WINDOW *win = newwin(WINDOW_HEIGHT, WINDOW_WIDTH, WINDOW_START_Y, WINDOW_START_X);
        if (!win) {
                printf("ERROR: could not create window\n");
                exit(1);
        }
        return win;
}

Now... the answer as posted is ambiguous regarding whether this is (for example) using Cygwin, MinGW or MSYS2. The MinGW configuration doesn't provide a workable KEY_RESIZE; if you are using that, this question should be marked as a duplicate. Otherwise, if the underlying libraries are more-or-less POSIX, the program will work for you.