摘要:
通过一个一个framebuffer例子,复习了内存分配的应用。其中的framebuffer例子为网上流行的(确实有bug的),在编译运行的过程中又重新温习了好多差不多已经被遗忘的知识点,写出来和大家分享!
---------------------------------------------------------------------------------------------------------------------
声明:
此文为原创,欢迎转载,转载请保留如下信息
作者:聂飞(afreez)
联系方式:afreez@sina.com (欢迎与作者交流)
初次发布时间:2006-06-08
不经本人同意,不得用语商业或赢利性质目的,否则,作者有权追究相关责任!
-----------------------------------------------------------------------------
例子实现了直接写屏的功能,即把屏幕清空(变黑),程序的流程大致为:打开一个FrameBuffer设备;通过mmap调用把显卡的物理内存空间映射到用户空间;通过映射关系直接写内存。
头文件
fbtools.h
#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
测试文件,其中深颜色的注释部分为在我机器上测得的结果
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);// pFbdev->fb==3
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_fix.smem_start=f0000000
pFbdev->fb_mem_offset = (unsigned long)(pFbdev->fb_fix.smem_start) & (~PAGE_MASK);
// pFbdev->fb_fix.smem_len=100 0000 pFbdev->fb_mem_offset=0
// pFbdev->fb_mem =0
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;
}
//注意,下面一行有bug
fb_memset(fbdev.fb_mem + fbdev.fb_mem_offset, 0, fbdev.fb_fix.smem_len);
fb_close(&fbdev);
}
#endif
编译
如果对上述代码直接进行编译的话,是不能成功的,即会出现类似下面的编译错误
#gcc –o fbtools fbtools.c
fbtools.c: In function `main`
fbtools.c:89:warning:passing arg 1 of `fb_memset` makes pointer from integer without a cast
对有问题的fbtools.c中的第89行代码(即加粗的有注释的那一行)进行如下操作
修改为:
fb_memset((void *)(fbdev.fb_mem+fbdev.fb_mem_offset), 0, fbdev.fb_fix.smem_len);
或者
unsigned long temp;
temp= fbdev.fb_mem+fbdev.fb_mem_offset;
fb_memset((void *)temp, 0, fbdev.fb_fix.smem_len);
可以成功编译成功
而修改为:
fb_memset((&)(fbdev.fb_mem+fbdev.fb_mem_offset), 0, fbdev.fb_fix.smem_len);
或者
unsigned long temp;
temp= fbdev.fb_mem+fbdev.fb_mem_offset;
fb_memset((&)temp, 0, fbdev.fb_fix.smem_len);
会输出:段错误
分析
函数原形为:void fb_memset (void *addr, int c, size_t len)
而fbtools.c:89调用时传递的参数为:
fb_memset(fbdev.fb_mem+fbdev.fb_mem_offset, 0, fbdev.fb_fix.smem_len);
fbdev.fb_mem和fbdev.fb_mem_offset都是unsigned long类型的变量,它们的计算结果保存在一个临时的栈空间,传递调用时,其临时的地址是不能够传递到被调用的函数的,所以编译是同不过的。具体的可以参考内存分配的相关知识,记得《effective c++》一书里讲的很详细,可以参考。
至于修改后的:
fb_memset((&)temp, 0, fbdev.fb_fix.smem_len);
编译出现段错误,也是很好理解的,因为(&)temp不等于(void *)temp,也不等于
(void *)(fbdev.fb_mem+fbdev.fb_mem_offset),具体原因读者可以对照《effective c++》思考。