if(ioctl(fb_fd,FBIOGET_VSCREENINFO,&fb_varinfo)==-1){
  printf("Error:ioctl:FBIOGET_VSCREENINFO/n");
  goto fail;
 }
 printf("ioctl FBIOGET_VSCREENINFO ok/n"); 
 printf("xres=%d/n",fb_varinfo.xres);
 printf("yres=%d/n",fb_varinfo.yres);
 printf("xres_virtual=%d/n",fb_varinfo.xres_virtual);
 printf("yres_virtual=%d/n",fb_varinfo.yres_virtual);
 printf("xoffset=%d/n",fb_varinfo.xoffset);
 printf("yoffset=%d/n",fb_varinfo.yoffset);
 printf("bits_per_pixel=%d/n",fb_varinfo.bits_per_pixel);
 printf("grayscale=%d/n",fb_varinfo.grayscale);
 printf("red=%#x/n",fb_varinfo.red);
 printf("green=%#x/n",fb_varinfo.green);
 printf("blue=%#x/n",fb_varinfo.blue);
 printf("transp=%d/n",fb_varinfo.transp);
 printf("nonstd=%d/n",fb_varinfo.nonstd);
 printf("activate=%d/n",fb_varinfo.activate);
 printf("height=%d/n",fb_varinfo.height);
 printf("width=%d/n",fb_varinfo.width);
 printf("accel_flags=%d/n",fb_varinfo.accel_flags);
 printf("pixclock=%d/n",fb_varinfo.pixclock);
 printf("left_margin=%d/n",fb_varinfo.left_margin);
 printf("right_margin=%d/n",fb_varinfo.right_margin);
 printf("upper_margin=%d/n",fb_varinfo.upper_margin);
 printf("lower_margin=%d/n",fb_varinfo.lower_margin);
 printf("hsync_len=%d/n",fb_varinfo.hsync_len);
 printf("vsync_len=%d/n",fb_varinfo.vsync_len);
 printf("sync=%d/n",fb_varinfo.sync);
 printf("vmode=%d/n",fb_varinfo.vmode);
 printf("rotate=%d/n",fb_varinfo.rotate);
 printf("reserved[0]=%d/n",fb_varinfo.reserved[0]);
 printf("reserved[1]=%d/n",fb_varinfo.reserved[1]);
 printf("reserved[2]=%d/n",fb_varinfo.reserved[2]);
 printf("reserved[3]=%d/n",fb_varinfo.reserved[3]);
 printf("reserved[4]=%d/n",fb_varinfo.reserved[4]);
 
 close(fb_fd);
 return 0;
fail:
 close(fb_fd);
 return -1;
}

从输出结果来看xres=800,yres=600,从这里可以看出这个FrameBuffer的最大值为800x600
得到framebuffer的信息后，我们开辟一段内存，然后映射framebuffer的内存,
这样当向这段内存写入信息时，framebuffer中就会写入相同的数据，也就会有数据表示的图像显示在屏幕上。

例：在屏幕上显示一个渐变窗体
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include

int flush_screen(char *buf,char *framebuf,int width,int height);

int main(int argc, char *argv[])
{
        int fb_fd=0;
        struct fb_fix_screeninfo fb_fixinfo;
        struct fb_var_screeninfo fb_varinfo;
    char *fb_addr;
    unsigned long fb_size;
    char *buffer;
    int i=0;

       if((fb_fd=open("/dev/fb0", O_RDWR))<0){
         printf("open framebuffer device failed./n");
         return -1;
       }
    printf("open framebuffer device ok/n");
     
        if(ioctl(fb_fd, FBIOGET_FSCREENINFO, &fb_fixinfo) == -1)
        {
                printf("ioctl FBIOGET_FSCREENINFO failed./n");
                return -1;
        }
    printf("ioctl FBIOGET_FSCREENINFO ok/n");
        if(ioctl(fb_fd, FBIOGET_VSCREENINFO, &fb_varinfo) == -1)
        {
                printf("ioctl FBIOGET_VSCREENINFO failed./n");
                return -1;
        }
    printf("ioctl FBIOGET_VSCREENINFO ok/n");
        fb_size = fb_varinfo.yres * fb_fixinfo.line_length;   //整个framebuffer的大小
        fb_addr = (char *)mmap(NULL, fb_size, PROT_READ|PROT_WRITE,MAP_SHARED, fb_fd, 0); //将framebuffer的物理地址映射到进程地址空间
        buffer=(char*)malloc(fb_varinfo.yres * fb_fixinfo.line_length); //申请一个缓冲区，采用双缓冲方式
    for(i=0;i<=0xff;i+=0x0f){
          memset(buffer,i,fb_varinfo.yres * fb_fixinfo.line_length);
          flush_screen(buffer,fb_addr,fb_varinfo.xres,fb_varinfo.yres);
      sleep(1);
    }
        if (!fb_addr)
                exit(1);
        free(buffer);
        return 0;
}

int flush_screen(char *buf,char *framebuf,int width,int height)
{
                char *t_data = buf;
                char *t_fb_addr = framebuf;
                int bytew= width<<1;
                while(--height >= 0)
                {
                        memcpy(t_fb_addr,t_data,bytew);
                        t_fb_addr += width;
                        t_data += width;
                }
}
编译后，运行看一下结果，尽量在字符模式下运行
vi /etc/inittab将运行级别改为3
重启后，运行一下看看结果。