Simple example of writing directly to `/dev/fb0` in C on Linux

main.c

#include <stdint.h>

#include <sys/ioctl.h>
#include <linux/fb.h>

typedef union {
    uint32_t rgba;
    struct {
        uint8_t b, g, r, a;
    };
} Color;

#define RGBA(val) ((Color) { .rgba = val })

typedef struct {
    size_t width;
    size_t height;
    size_t stride;
    Color *pixels;
} Canvas;

#define CANVAS_AT(c, x, y) (c).pixels[(y) * (c).stride + (x)]

Canvas canvas_alloc(size_t width, size_t height) {
    return (Canvas) {
        .width = width,
        .height = height,
        .stride = width,
        .pixels = malloc(sizeof(Color)*width*height),
    };
}

void canvas_fill(Canvas c, Color color) {
    for (size_t y = 0; y < c.height; y++) {
        for (size_t x = 0; x < c.width; x++) {
            CANVAS_AT(c, x, y) = color;
        }
    }
}

void canvas_fill_rect(Canvas c, int x0, int y0, int w, int h, Color color) {
    for (int x = x0; x <= x0 + w; x++) {
        if (0 <= x && x < c.width) {
            for (int y = y0; y <= y0 + h; y++) {
                if (0 <= y && y < c.height) {
                    CANVAS_AT(c, x, y) = color;
                }
            }
        }
    }
}

void canvas_write_to_fb0(Canvas c, int fd) {
    lseek(fd, 0, SEEK_SET);

    if (c.width != c.stride) {
        for (size_t y = 0; y < c.height; y++) {
            for (size_t x = 0; x < c.width; x++) {
                write(fd, &CANVAS_AT(c, x, y), sizeof(Color));
            }
        }
    } else {
        write(fd, c.pixels, sizeof(Color)*c.width*c.height);
    }
}

int main(void) {
    if (setuid(0) < 0) {
        fprintf(stderr, "ERROR: Could not setuid(0): %s\n", strerror(errno));
        if (errno == EPERM) {
            fprintf(stderr, "Maybe try running with sudo?\n");
        }
        return 1;
    }

    const char *file_path = "/dev/fb0";
    int fd = open(file_path, O_RDWR);
    if (fd < 0) {
        fprintf(stderr, "ERROR: Could not open file %s: %s\n", file_path, strerror(errno));
        return 1;
    }

    printf("File descriptor of %s is %d\n", file_path, fd);

    struct fb_var_screeninfo screeninfo;
    if (ioctl(fd, FBIOGET_VSCREENINFO, &screeninfo) < 0) {
        fprintf(stderr, "ERROR: Could not use ioctl(): %s\n", strerror(errno));
        return 1;
    }

    if (screeninfo.bits_per_pixel != 32) {
        fprintf(stderr, "ERROR: color depths other than RGBA32? fuck that shii\n");
        return 1;
    }

    Canvas c = canvas_alloc(screeninfo.xres, screeninfo.yres);

    int x = 0;
    int y = 0;
    int w = 50;

    while (1) {
        canvas_fill(c, RGBA(0xFF181818));
        canvas_fill_rect(c, x, y, w, w, RGBA(0xFFFF2020));

        x += 1;
        y += 1;

        canvas_write_to_fb0(c, fd);

        usleep(16);
    }

    return 0;
}