framebuffer 应用层函数

显示屏里面有一些自带的驱动寄存器，我们要操作显示器的亮灭显示及颜色等，其实就是对该显示器的寄存器进行操作，有一些寄存器是只读的，里面包含了该显示屏的一些基本且不可更改的信息，比如说该显示器实际的长和宽，还有一些信息是可以改变的，比如说屏幕的分辨率。

我们首先需要读取显示器找中的一些特定的信息，在程序的实现中是通过
ret = ioctl(fd, FBIOGET_FSCREENINFO, &finfo);
来实现读取硬件不可变的信息。
而通过
ret = ioctl(fd, FBIOGET_VSCREENINFO, &vinfo);
则可以读取硬件的可变信息，然后再对这些读取到的信息进行调整，然后再写进去到屏幕的寄存器中即可改变显示器的设置

        vinfo.xres = 1024;        vinfo.yres = 600;        vinfo.xres_virtual = 1024;        vinfo.yres_virtual = 1200;        ret = ioctl(fd, FBIOPUT_VSCREENINFO, &vinfo); 

而显示背景和显示字体颜色其实就是对某个像素点的地址进行写值，地址的不同决定了像素点的坐标不同，值的不同决定了该坐标像素点的颜色和亮度不同。首先需要获得该显示屏地址的基址，然后随着偏移地址累加会逐渐扫描各个像素点。我们只需要在里面赋相应的值即可。

 pfb = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);   *(pfb + y * WIDTH + x) = color;

完整代码如下：
本程序参考朱友鹏老师视频程序

#include <stdio.h>#include <sys/types.h>#include <sys/stat.h>#include <fcntl.h>#include <linux/fb.h>#include <sys/ioctl.h>#include <sys/mman.h>#define FBDEVICE        "/dev/fb0"//#define WIDTH         800     //#define HEIGHT        480#define WIDTH           1024    #define HEIGHT          600#define WHITE           0xffffffff      // test ok#define BLACK           0x00000000#define RED             0xffff0000#define GREEN           0xff00ff00      // test ok#define BLUE            0xff0000ff                      #define GREENP          0x0000ff00   void draw_back(unsigned int width, unsigned int height, unsigned int color);void draw_line(unsigned int color);unsigned int *pfb = NULL;int main(void){        int fd = -1, ret = -1;        struct fb_fix_screeninfo finfo = {0};        struct fb_var_screeninfo vinfo = {0};        fd = open(FBDEVICE, O_RDWR);        if (fd < 0)        {                perror("open");                return -1;        }        printf("open %s success.\n", FBDEVICE);        ret = ioctl(fd, FBIOGET_FSCREENINFO, &finfo);  //        if (ret < 0)        {                perror("ioctl");                return -1;        }        printf("smem_start = 0x%x, smem_len = %u.\n", finfo.smem_start, finfo.smem_len);        ret = ioctl(fd, FBIOGET_VSCREENINFO, &vinfo);    //        if (ret < 0)        {                perror("ioctl");                return -1;        }        printf("xres = %u, yres = %u.\n", vinfo.xres, vinfo.yres);        printf("xres_virtual = %u, yres_virtual = %u.\n", vinfo.xres_virtual, vinfo.yres_virtual);        printf("bpp = %u.\n", vinfo.bits_per_pixel);        vinfo.xres = 1024;        vinfo.yres = 600;        vinfo.xres_virtual = 1024;        vinfo.yres_virtual = 1200;        ret = ioctl(fd, FBIOPUT_VSCREENINFO, &vinfo);          if (ret < 0)         {                perror("ioctl");                return -1;        }        ret = ioctl(fd, FBIOGET_VSCREENINFO, &vinfo);        if (ret < 0)        {                perror("ioctl");                return -1;        }        printf("xres = %u, yres = %u.\n", vinfo.xres, vinfo.yres);        printf("xres_virtual = %u, yres_virtual = %u.\n", vinfo.xres_virtual, vinfo.yres_virtual);        printf("bpp = %u.\n", vinfo.bits_per_pixel);        unsigned long len = vinfo.xres_virtual * vinfo.yres_virtual * vinfo.bits_per_pixel / 8;        printf("len = %ld\n", len);        pfb = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);  //get address for back color        if (NULL == pfb)        {                perror("mmap");                return -1;        }        printf("pfb = %p.\n", pfb);        draw_back(WIDTH, HEIGHT, WHITE);  //          draw_line(RED);        close(fd);        return 0;}void draw_back(unsigned int width, unsigned int height, unsigned int color){    unsigned int x, y;    for (y=0; y<height; y++)    {        for (x=0; x<width; x++)        {            *(pfb + y * WIDTH + x) = color;        }    }}void draw_line(unsigned int color){    unsigned int x, y;    for (x=50; x<600; x++)    {        *(pfb + 200 * WIDTH + x) = color;    }}