因为Linux是工作在保护模式下,所以用户态进程是无法象DOS那样使用显卡BIOS里

  提供的中断调用来实现直接写屏,故Linux抽象出FrameBuffer这个设备来供用户态

  进程实现直接写屏。

  在继续下面的之前,先说明几个背景知识:

  1、FrameBuffer主要是根据VESA标准的实现的,所以只能实现最简单的功能。

  2、由于涉及内核的问题,FrameBuffer是不允许在系统起来后修改显示模式等一系

  列操作。(好象很多人都想要这样干,这是不被允许的,当然如果你自己与驱动

  的话,是可以实现的)

  3、对FrameBuffer的操作,会直接影响到本机的所有控制台的输出,包括XWIN的图

  形界面。

  好,现在可以让我们开始实现直接写屏:

  1、打开一个FrameBuffer设备

  2、通过mmap调用把显卡的物理内存空间映射到用户空间

  3、直接写内存。

  好象很简单哦~

  fbtools.h

  代码:

  #ifndef _FBTOOLS_H_

  #define _FBTOOLS_H_

  #include <linux/fb.h>

  //a framebuffer device structure;

  typedef struct fbdev{

   int fb;

   unsigned long fb_mem_offset;

   unsigned long fb_mem;

   struct fb_fix_screeninfo fb_fix;

   struct fb_var_screeninfo fb_var;

   char dev[20];

  } FBDEV, *PFBDEV;

  //open & init a frame buffer

  //to use this function,

  //you must set FBDEV.dev="/dev/fb0"

  //or "/dev/fbX"

  //it's your frame buffer.

  int fb_open(PFBDEV pFbdev);

  //close a frame buffer

  int fb_close(PFBDEV pFbdev);

  //get display depth

  int get_display_depth(PFBDEV pFbdev);

  //full screen clear

  void fb_memset(void *addr, int c, size_t len);

  #endif

  fbtools.c

  代码:

  #include <stdio.h>

  #include <stdlib.h>

  #include <fcntl.h>

  #include <unistd.h>

  #include <string.h>

  #include <sys/ioctl.h>

  #include <sys/mman.h>

  #include <asm/page.h>

  #include "fbtools.h"

  #define TRUE 1

  #define FALSE 0

  #define MAX(x,y) ((x)>(y)?(x):(y))

  #define MIN(x,y) ((x)<(y)?(x):(y))

  //open & init a frame buffer

  int fb_open(PFBDEV pFbdev)

  {

   pFbdev->fb = open(pFbdev->dev, O_RDWR);

   if(pFbdev->fb < 0)

   {

   printf("Error opening %s: %m. Check kernel config\\n", pFbdev->dev);

   return FALSE;

   }

   if (-1 == ioctl(pFbdev->fb,FBIOGET_VSCREENINFO,&(pFbdev->fb_var)))

   {

   printf("ioctl FBIOGET_VSCREENINFO\\n");

   return FALSE;

   }

   if (-1 == ioctl(pFbdev->fb,FBIOGET_FSCREENINFO,&(pFbdev->fb_fix)))

   {

   printf("ioctl FBIOGET_FSCREENINFO\\n");

   return FALSE;

   }

  

   //map physics address to virtual address

   pFbdev->fb_mem_offset = (unsigned long)(pFbdev->fb_fix.smem_start) & (~PAGE_MASK);

   pFbdev->fb_mem = (unsigned long int)mmap(NULL, pFbdev->fb_fix.smem_len +

  pFbdev->fb_mem_offset,

   PROT_READ | PROT_WRITE, MAP_SHARED, pFbdev->fb, 0);

   if (-1L == (long) pFbdev->fb_mem)

   {

   printf("mmap error! mem:%d offset:%d\\n", pFbdev->fb_mem,

  pFbdev->fb_mem_offset);

   return FALSE;

   }

  

   return TRUE;

  }

  //close frame buffer

  int fb_close(PFBDEV pFbdev)

  {

   close(pFbdev->fb);

   pFbdev->fb=-1;

  }

  //get display depth

  int get_display_depth(PFBDEV pFbdev);

  {

   if(pFbdev->fb<=0)

   {

   printf("fb device not open, open it first\\n");

   return FALSE;

   }

   return pFbdev->fb_var.bits_per_pixel;

  }

  //full screen clear

  void fb_memset (void *addr, int c, size_t len)

  {

  memset(addr, c, len);

  }

  //use by test

  #define DEBUG

  #ifdef DEBUG

  main()

  {

   FBDEV fbdev;

   memset(&fbdev, 0, sizeof(FBDEV));

   strcpy(fbdev.dev, "/dev/fb0");

   if(fb_open(&fbdev)==FALSE)

   {

   printf("open frame buffer error\\n");

   return;

   }

  

   fb_memset(fbdev.fb_mem + fbdev.fb_mem_offset, 0, fbdev.fb_fix.smem_len);

  

   fb_close(&fbdev);

  }