保存 framebuffer内容到BMP文件

代码是下面的：

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


typedef unsigned short WORD;
typedef unsigned int DWORD;
typedef unsigned long LONG;

typedef struct tagBITMAPFILEHEADER
{
	WORD bfType;  
	DWORD bfSize; 
	WORD bfReserved1; 
	WORD bfReserved2;
	DWORD bfOffBits;
} __attribute__((packed)) BITMAPFILEHEADER;


typedef struct tagBITMAPINFOHEADER
{
	DWORD biSize;    
	LONG biWidth;   
	LONG biHeight;   
	WORD biPlanes;   
	WORD biBitCount;  
	DWORD biCompression; 
	DWORD biSizeImage;  
	LONG biXPelsPerMeter; 
	LONG biYPelsPerMeter; 
	DWORD biClrUsed; 
	DWORD biClrImportant;
} __attribute__((packed)) BITMAPINFOHEADER;


int main(int argc, char **argv)
{
	struct fb_fix_screeninfo finfo;
	struct fb_var_screeninfo vinfo;

	int fb_fd = open("/dev/fb0",O_RDWR);

	ioctl(fb_fd, FBIOGET_VSCREENINFO, &vinfo);
	ioctl(fb_fd, FBIOGET_FSCREENINFO, &finfo);

	long screensize = vinfo.yres_virtual * finfo.line_length;
	uint8_t *fbp = mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED, fb_fd, (off_t)0);
	if(fbp == -1)
	{
		printf("mmap fails\n");
		return -1;
	}

	BITMAPFILEHEADER    bmfh;
	BITMAPINFOHEADER    bmih;


    //create bitmap file header
    ((unsigned char *)&bmfh.bfType)[0] = 'B';
    ((unsigned char *)&bmfh.bfType)[1] = 'M';
    bmfh.bfSize = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER) + vinfo.xres * vinfo.yres * 4;
    bmfh.bfReserved1 = 0;
    bmfh.bfReserved2 = 0;
    bmfh.bfOffBits = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);


    //create bitmap information header
    bmih.biSize = sizeof(BITMAPINFOHEADER);
    bmih.biWidth = vinfo.xres;
    bmih.biHeight = vinfo.yres;
    bmih.biPlanes = 1;
    bmih.biBitCount = 32;
    bmih.biCompression = 0;
    bmih.biSizeImage = 0;
    bmih.biXPelsPerMeter = 3800;
    bmih.biYPelsPerMeter = 3800;
    bmih.biClrUsed = 0;
    bmih.biClrImportant = 0;


	int image_file = open("screen.bmp", O_RDWR|O_CREAT);
	write(image_file, &bmfh, sizeof(bmfh));
	write(image_file, &bmih, sizeof(bmih));
	
	int x, y;
	for(y = vinfo.yres - 1; y >= 0; y--)
	{
		for(x = 0; x < vinfo.xres; x++)
		{
			long location = (x + vinfo.xoffset) * (vinfo.bits_per_pixel/8) + (y + vinfo.yoffset) * finfo.line_length;			
			write(image_file, fbp + location, 4);
		}
	}
　　　　　munmap(fbp, screensize);
	close(image_file);
	close(fb_fd);	
	return 0;
}

运行的时候，需要用 root权限。

但是，上面的代码貌似有问题：

１）　保存的图片颜色不对，像素值都是 FF 00 FF 00 (RGBA).

2)gdb 调试的时候，无法访问 mmap的 framebuffer地址：

(gdb) x /4x fbp
0xb788a000:	Cannot access memory at address 0xb788a000

参考：

1. http://betteros.org/tut/graphics1.php