Detect OS dark/light color mode setting from within the terminal

detect-color-mode.cpp

// SPDX-License-Identifier: Apache-2.0
// Implements https://contour-terminal.org/vt-extensions/color-palette-update-notifications/

#include <cstdlib>
#include <iostream>
#include <string_view>

#include <termios.h>
#include <unistd.h>

int main()
{
    struct termios oldt {};
    struct termios newt {};
    tcgetattr(STDIN_FILENO, &oldt);
    newt = oldt;
    newt.c_lflag &= ~(ICANON | ECHO);
    tcsetattr(STDIN_FILENO, TCSANOW, &newt);

    // Also send DA1 to detect end of reply, in case the terminal does not support color mode detection
    std::cout << "\033[?996n\033[c";
    std::cout.flush();

    char buf[32];
    size_t n = 0;
    size_t i = 0;
    while (i < sizeof(buf))
    {
        if (read(STDIN_FILENO, buf + i, 1) != 1)
            break;
        else if (buf[i] == 'n')
            n = i + 1;
        else if (buf[i] == 'c')
        {
            ++i;
            break;
        }
        ++i;
    }

    // restore terminal
    tcsetattr(STDIN_FILENO, TCSANOW, &oldt);

    auto const response = std::string_view(buf, n);
    if (response == "\033[?997;1n")
        std::cout << "dark\n";
    else if (response == "\033[?997;2n")
        std::cout << "light\n";
    else
        std::cout << "unknown\n";

    return EXIT_SUCCESS;
}