Structure gets the address of the main structure (struct) through member variables

I saw a clever usage when looking at the code today ：
Two structures , One is V4L2Context, The other is V4L2m2mContext;

typedef struct V4L2m2mContext {
    
    char devname[PATH_MAX];
    int fd;

    /* the codec context queues */
    V4L2Context capture;
    V4L2Context output;

    /* dynamic stream reconfig */
    AVCodecContext *avctx;
    sem_t refsync;
    atomic_uint refcount;
    int reinit;
...
}

If we know one capture perhaps output Can you push out the address of V4L2m2mContext What's the address? , In this way, we can V4L2Context One pointer variable is missing .
The answer is yes ：

#define container_of(ptr, type, member) ({
       \ const __typeof__(((type *)0)->member ) *__mptr = (ptr); \ (type *)((char *)__mptr - offsetof(type,member) );})

About its analysis ： analysis

The actual use ：

static inline V4L2m2mContext *buf_to_m2mctx(V4L2Buffer *buf)
{
    
    return V4L2_TYPE_IS_OUTPUT(buf->context->type) ?
        container_of(buf->context, V4L2m2mContext, output) :
        container_of(buf->context, V4L2m2mContext, capture);
}