无头环境下修改FrameBuffer对应内存数据绘图

FrameBuffer,帧缓冲区，显卡或者显示器在文件系统中的映射，里面的数据就是将要显示在界面的内容。

headless,类似于服务器版，只有内核和shell，无X/Wayland等GUI组件的环境，只能通过tty进行交互

首先准备一个空白操作系统，没有X/Wayland桌面服务。

我的环境就是直接安装Ubuntu Server 23.04。

源码

打开系统framebuffer
将framebuffer映射至内存
修改内存数据
同步显示内容

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <sys/mman.h>
#include <sys/ioctl.h>

int main()
{
    int fbfd = 0;
    struct fb_var_screeninfo vinfo;
    struct fb_fix_screeninfo finfo;
    long int screensize = 0;
    char *fbp = 0;
    int x = 0, y = 0;
    long int location = 0;

    // Open the file for reading and writing
    fbfd = open("/dev/fb0", O_RDWR);
    if (fbfd == -1)
    {
        perror("Error: cannot open framebuffer device");
        exit(1);
    }
    printf("The framebuffer device was opened successfully.\n");

    // Get fixed screen information
    if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo) == -1)
    {
        perror("Error reading fixed information");
        exit(2);
    }

    // Get variable screen information
    if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo) == -1)
    {
        perror("Error reading variable information");
        exit(3);
    }

    printf("%dx%d, %dbpp\n", vinfo.xres, vinfo.yres, vinfo.bits_per_pixel);

    // Figure out the size of the screen in bytes
    screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;

    // Map the device to memory
    fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED,
                       fbfd, 0);
    if ((int)fbp == -1)
    {
        perror("Error: failed to map framebuffer device to memory");
        exit(4);
    }
    printf("The framebuffer device was mapped to memory successfully.\n");

    x = 300;
    y = 100; // Where we are going to put the pixel

    // Figure out where in memory to put the pixel
    while (1)
    {
        for (y = 100; y < 300; y++)
        {
            for (x = 100; x < 300; x++)
            {
                location = (x + vinfo.xoffset) * (vinfo.bits_per_pixel / 8) +
                           (y + vinfo.yoffset) * finfo.line_length;

                if (vinfo.bits_per_pixel == 32)
                {
                    *(fbp + location) = 100;                     // Some blue
                    *(fbp + location + 1) = 15 + (x - 100) / 2;  // A little green
                    *(fbp + location + 2) = 200 - (y - 100) / 5; // A lot of red
                    *(fbp + location + 3) = 0;                   // No transparency
                }
                else
                { // assume 16bpp
                    int b = 10;
                    int g = (x - 100) / 6;       // A little green
                    int r = 31 - (y - 100) / 16; // A lot of red
                    unsigned short int t = r << 11 | g << 5 | b;
                    *((unsigned short int *)(fbp + location)) = t;
                }
            }
        }
    }
    munmap(fbp, screensize);
    close(fbfd);
    return 0;
}