最近看ffmpeg源码，gdigrab.c中的一段代码如下

#define OFFSET(x) offsetof(struct gdigrab, x)
#define DEC AV_OPT_FLAG_DECODING_PARAM
static const AVOption options[] = {
    { "draw_mouse", "draw the mouse pointer", OFFSET(draw_mouse), AV_OPT_TYPE_INT, {.i64 = 1}, 0, 1, DEC },
    { "show_region", "draw border around capture area", OFFSET(show_region), AV_OPT_TYPE_INT, {.i64 = 0}, 0, 1, DEC },
    { "framerate", "set video frame rate", OFFSET(framerate), AV_OPT_TYPE_VIDEO_RATE, {.str = "ntsc"}, 0, INT_MAX, DEC },
    { "video_size", "set video frame size", OFFSET(width), AV_OPT_TYPE_IMAGE_SIZE, {.str = NULL}, 0, 0, DEC },
    { "offset_x", "capture area x offset", OFFSET(offset_x), AV_OPT_TYPE_INT, {.i64 = 0}, INT_MIN, INT_MAX, DEC },
    { "offset_y", "capture area y offset", OFFSET(offset_y), AV_OPT_TYPE_INT, {.i64 = 0}, INT_MIN, INT_MAX, DEC },
    { NULL },
};

static const AVClass gdigrab_class = {
    .class_name = "GDIgrab indev",
    .item_name  = av_default_item_name,
    .option     = options,
    .version    = LIBAVUTIL_VERSION_INT,
    .category   = AV_CLASS_CATEGORY_DEVICE_VIDEO_INPUT,
};

/** gdi grabber device demuxer declaration */
AVInputFormat ff_gdigrab_demuxer = {
    .name           = "gdigrab",
    .long_name      = NULL_IF_CONFIG_SMALL("GDI API Windows frame grabber"),
    .priv_data_size = sizeof(struct gdigrab),
    .read_header    = gdigrab_read_header,
    .read_packet    = gdigrab_read_packet,
    .read_close     = gdigrab_read_close,
    .flags          = AVFMT_NOFILE,
    .priv_class     = &gdigrab_class,
};

以前一般都是先定义变量，然后用变量加"."号跟着成员逐一赋值，这种方式挺陌生，不过如果在Visual Studio中，会提示有错误