问帧缓冲区输出图形撕裂，FBIO_WAITFORVSYNC和FBIOPAN_DISPLAY失败

EN

    int zero = 0;
    ioctl(fd, FBIO_WAITFORVSYNC, &zero);

    static int bufferNumber = 0;
    //Assume the frame buffer has more memory than
    //just just 1 screen - either set xoffset or 
    //yoffset to flip to another page. 
    varInfo.yoffset = varInfo.yres * bufferNumber;
    ioctl(fd, FBIOPUT_VSCREENINFO, &varInfo);
    //now flip the buffer to which to render to
    bufferNumber = 1 - bufferNumber;
    //now render to appropriate frame-buffer page
    //assuming we change yoffset to switch pages
    pageBuffer = fixInfo.smem_start + 
                 (bufferNumber * 
                 (fixInfo.line_length / (varInfo.bits_per_pixel / 8) * 
                 varInfo.yres ));