參考這個帖子
http://bbs.chinaunix.net/thread-1932291-1-1.html
朝/dev/fb0中寫入內容的過程主要分爲以下幾部分
0.首先打開文件，這是熟知的
fbFd = open ("/dev/fb0", O_RDWR);
查詢屏幕信息。
struct fb_fix_screeninfo finfo;
ioctl (fbFd, FBIOGET_FSCREENINFO, &finfo）;
則其中finfo.line_length代表屏幕一行的象素點的數目。
struct fb_var_screeninfo vinfo;
ioctl (fbFd, FBIOGET_VSCREENINFO, &vinfo);
則其中vinfo.bits_per_pixel代表一個象素點有幾位(bit)；其值有以下幾種情況
(a).爲32,此時每個象素點的數據依次爲 8alpha+8reds+8greens+8blues
(b).16, 此時每個象素點的數據依次代表 5reds+6greens+5blues
(c).15, 此時每個象素點的數據依次代表5reds+5greens+5blues
在下面我們以情況(a)爲例說明一下怎麼對屏幕進行寫。
首先通過mmap把文件映射到內存。
char *frameBuffer = 0;
frameBuffer =
(char *) mmap (0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED,
fbFd, 0);
由於此時每個象素點是32位，也就是4字節，所以我們可以用整數表示它。
int a;一個象素點的alpha,red,green,blue值若分別爲b1,b2,b3,b4;則
a=b1<<24+b2<<16+b3<<8+b4;
若要將該值寫入座標爲x,y的象素點，則只要賦值
int *dest = (int *) (frameBuffer)
+ (y + vinfo.yoffset) * finfo.line_length / 4 + (x + vinfo.xoffset);
*dest=a;
便可，
最後注意munmap和close文件。
附上原文代碼如下
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <linux/kd.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
#include <sys/time.h>
#include <string.h>
#include <errno.h>
struct fb_var_screeninfo vinfo;
struct fb_fix_screeninfo finfo;
char *frameBuffer = 0;
//打印fb驅動中fix結構信息，注：在fb驅動加載後，fix結構不可被修改。
void
printFixedInfo ()
{
printf ("Fixed screen info:\n"
"\tid: %s\n"
"\tsmem_start: 0x%lx\n"
"\tsmem_len: %d\n"
"\ttype: %d\n"
"\ttype_aux: %d\n"
"\tvisual: %d\n"
"\txpanstep: %d\n"
"\typanstep: %d\n"
"\tywrapstep: %d\n"
"\tline_length: %d\n"
"\tmmio_start: 0x%lx\n"
"\tmmio_len: %d\n"
"\taccel: %d\n"
"\n",
finfo.id, finfo.smem_start, finfo.smem_len, finfo.type,
finfo.type_aux, finfo.visual, finfo.xpanstep, finfo.ypanstep,
finfo.ywrapstep, finfo.line_length, finfo.mmio_start,
finfo.mmio_len, finfo.accel);
}
//打印fb驅動中var結構信息，注：fb驅動加載後，var結構可根據實際需要被重置
void
printVariableInfo ()
{
printf ("Variable screen info:\n"
"\txres: %d\n"
"\tyres: %d\n"
"\txres_virtual: %d\n"
"\tyres_virtual: %d\n"
"\tyoffset: %d\n"
"\txoffset: %d\n"
"\tbits_per_pixel: %d\n"
"\tgrayscale: %d\n"
"\tred: offset: %2d, length: %2d, msb_right: %2d\n"
"\tgreen: offset: %2d, length: %2d, msb_right: %2d\n"
"\tblue: offset: %2d, length: %2d, msb_right: %2d\n"
"\ttransp: offset: %2d, length: %2d, msb_right: %2d\n"
"\tnonstd: %d\n"
"\tactivate: %d\n"
"\theight: %d\n"
"\twidth: %d\n"
"\taccel_flags: 0x%x\n"
"\tpixclock: %d\n"
"\tleft_margin: %d\n"
"\tright_margin: %d\n"
"\tupper_margin: %d\n"
"\tlower_margin: %d\n"
"\thsync_len: %d\n"
"\tvsync_len: %d\n"
"\tsync: %d\n"
"\tvmode: %d\n"
"\n",
vinfo.xres, vinfo.yres, vinfo.xres_virtual, vinfo.yres_virtual,
vinfo.xoffset, vinfo.yoffset, vinfo.bits_per_pixel,
vinfo.grayscale, vinfo.red.offset, vinfo.red.length,
vinfo.red.msb_right, vinfo.green.offset, vinfo.green.length,
vinfo.green.msb_right, vinfo.blue.offset, vinfo.blue.length,
vinfo.blue.msb_right, vinfo.transp.offset, vinfo.transp.length,
vinfo.transp.msb_right, vinfo.nonstd, vinfo.activate,
vinfo.height, vinfo.width, vinfo.accel_flags, vinfo.pixclock,
vinfo.left_margin, vinfo.right_margin, vinfo.upper_margin,
vinfo.lower_margin, vinfo.hsync_len, vinfo.vsync_len,
vinfo.sync, vinfo.vmode);
}
//畫大小爲width*height的同色矩陣，8alpha+8reds+8greens+8blues
void
drawRect_rgb32 (int x0, int y0, int width, int height, int color)
{
const int bytesPerPixel = 4;
const int stride = finfo.line_length / bytesPerPixel;
int *dest = (int *) (frameBuffer)
+ (y0 + vinfo.yoffset) * stride + (x0 + vinfo.xoffset);
int x, y;
for (y = 0; y < height; ++y)
{
for (x = 0; x < width; ++x)
{
dest[x] = color;
}
dest += stride;
}
}
//畫大小爲width*height的同色矩陣，5reds+6greens+5blues
void
drawRect_rgb16 (int x0, int y0, int width, int height, int color)
{
const int bytesPerPixel = 2;
const int stride = finfo.line_length / bytesPerPixel;
const int red = (color & 0xff0000) >> (16 + 3);
const int green = (color & 0xff00) >> (8 + 2);
const int blue = (color & 0xff) >> 3;
const short color16 = blue | (green << 5) | (red << (5 + 6));
short *dest = (short *) (frameBuffer)
+ (y0 + vinfo.yoffset) * stride + (x0 + vinfo.xoffset);
int x, y;
for (y = 0; y < height; ++y)
{
for (x = 0; x < width; ++x)
{
dest[x] = color16;
}
dest += stride;
}
}
//畫大小爲width*height的同色矩陣，5reds+5greens+5blues
void
drawRect_rgb15 (int x0, int y0, int width, int height, int color)
{
const int bytesPerPixel = 2;
const int stride = finfo.line_length / bytesPerPixel;
const int red = (color & 0xff0000) >> (16 + 3);
const int green = (color & 0xff00) >> (8 + 3);
const int blue = (color & 0xff) >> 3;
const short color15 = blue | (green << 5) | (red << (5 + 5)) | 0x8000;
short *dest = (short *) (frameBuffer)
+ (y0 + vinfo.yoffset) * stride + (x0 + vinfo.xoffset);
int x, y;
for (y = 0; y < height; ++y)
{
for (x = 0; x < width; ++x)
{
dest[x] = color15;
}
dest += stride;
}
}
void
drawRect (int x0, int y0, int width, int height, int color)
{
switch (vinfo.bits_per_pixel)
{
case 32:
drawRect_rgb32 (x0, y0, width, height, color);
break;
case 16:
drawRect_rgb16 (x0, y0, width, height, color);
break;
case 15:
drawRect_rgb15 (x0, y0, width, height, color);
break;
default:
printf ("Warning: drawRect() not implemented for color depth %i\n",
vinfo.bits_per_pixel);
break;
}
}
#define PERFORMANCE_RUN_COUNT 5
void
performSpeedTest (void *fb, int fbSize)
{
int i, j, run;
struct timeval startTime, endTime;
unsigned long long results[PERFORMANCE_RUN_COUNT];
unsigned long long average;
unsigned int *testImage;
unsigned int randData[17] = {
0x3A428472, 0x724B84D3, 0x26B898AB, 0x7D980E3C, 0x5345A084,
0x6779B66B, 0x791EE4B4, 0x6E8EE3CC, 0x63AF504A, 0x18A21B33,
0x0E26EB73, 0x022F708E, 0x1740F3B0, 0x7E2C699D, 0x0E8A570B,
0x5F2C22FB, 0x6A742130
};
printf ("Frame Buffer Performance test...\n");
for (run = 0; run < PERFORMANCE_RUN_COUNT; ++run)
{
/* Generate test image with random(ish) data: */
testImage = (unsigned int *) malloc (fbSize);
j = run;
for (i = 0; i < (int) (fbSize / sizeof (int)); ++i)
{
testImage[i] = randData[j];
j++;
if (j >= 17)
j = 0;
}
gettimeofday (&startTime, NULL);
memcpy (fb, testImage, fbSize);
gettimeofday (&endTime, NULL);
long secsDiff = endTime.tv_sec - startTime.tv_sec;
results[run] =
secsDiff * 1000000 + (endTime.tv_usec - startTime.tv_usec);
free (testImage);
}
average = 0;
for (i = 0; i < PERFORMANCE_RUN_COUNT; ++i)
average += results[i];
average = average / PERFORMANCE_RUN_COUNT;
printf (" Average: %llu usecs\n", average);
printf (" Bandwidth: %.03f MByte/Sec\n",
(fbSize / 1048576.0) / ((double) average / 1000000.0));
printf (" Max. FPS: %.03f fps\n\n",
1000000.0 / (double) average);
/* Clear the framebuffer back to black again: */
memset (fb, 0, fbSize);
}
int
main (int argc, char **argv)
{
const char *devfile = "/dev/fb0";
long int screensize = 0;
int fbFd = 0;
/* Open the file for reading and writing */
fbFd = open (devfile, O_RDWR);
if (fbFd == -1)
{
perror ("Error: cannot open framebuffer device");
exit (1);
}
//獲取finfo信息並顯示
if (ioctl (fbFd, FBIOGET_FSCREENINFO, &finfo) == -1)
{
perror ("Error reading fixed information");
exit (2);
}
// printFixedInfo ();
//獲取vinfo信息並顯示
if (ioctl (fbFd, FBIOGET_VSCREENINFO, &vinfo) == -1)
{
perror ("Error reading variable information");
exit (3);
}
// printVariableInfo ();
/* Figure out the size of the screen in bytes */
screensize = finfo.smem_len;
/* Map the device to memory */
frameBuffer =
(char *) mmap (0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED,
fbFd, 0);
if (frameBuffer == MAP_FAILED)
{
perror ("Error: Failed to map framebuffer device to memory");
exit (4);
}
//測試virt fb的性能
// performSpeedTest (frameBuffer, screensize);
memset(frameBuffer,'\0',screensize);
sleep(5);
// printf ("Will draw 3 rectangles on the screen,\n" "they should be colored red, green and blue (in that order).\n");
drawRect (vinfo.xres / 8, vinfo.yres / 8,
vinfo.xres / 4, vinfo.yres / 4, 0xffff0000);
drawRect (vinfo.xres * 3 / 8, vinfo.yres * 3 / 8,
vinfo.xres / 4, vinfo.yres / 4, 0xff00ff00);
drawRect (vinfo.xres * 5 / 8, vinfo.yres * 5 / 8,
vinfo.xres / 4, vinfo.yres / 4, 0xff0000ff);
sleep (5);
// printf (" Done.\n");
memset(frameBuffer,'\0',screensize);
munmap (frameBuffer, screensize); //解除內存映射，與mmap對應
close (fbFd);
return 0;
}