URLContext中是对具体资源文件进行操作的上下文，URLProtocol则是在将资源进行分类的基础上，对某一类资源操作的函数集。

typedef struct URLContext {
    const AVClass *av_class;    /**< information for av_log(). Set by url_open(). */
    const struct URLProtocol *prot;
    void *priv_data;
    char *filename;             /**< specified URL */
    int flags;
    int max_packet_size;        /**< if non zero, the stream is packetized with this max packet size */
    int is_streamed;            /**< true if streamed (no seek possible), default = false */
    int is_connected;
    AVIOInterruptCB interrupt_callback;
    int64_t rw_timeout;         /**< maximum time to wait for (network) read/write operation completion, in mcs */
    const char *protocol_whitelist;
    const char *protocol_blacklist;
    int min_packet_size;        /**< if non zero, the stream is packetized with this min packet size */
} URLContext;

typedef struct URLProtocol {
    const char *name;
    int     (*url_open)( URLContext *h, const char *url, int flags);
    /**
     * This callback is to be used by protocols which open further nested
     * protocols. options are then to be passed to ffurl_open()/ffurl_connect()
     * for those nested protocols.
     */
    int     (*url_open2)(URLContext *h, const char *url, int flags, AVDictionary **options);
    int     (*url_accept)(URLContext *s, URLContext **c);
    int     (*url_handshake)(URLContext *c);

    /**
     * Read data from the protocol.
     * If data is immediately available (even less than size), EOF is
     * reached or an error occurs (including EINTR), return immediately.
     * Otherwise:
     * In non-blocking mode, return AVERROR(EAGAIN) immediately.
     * In blocking mode, wait for data/EOF/error with a short timeout (0.1s),
     * and return AVERROR(EAGAIN) on timeout.
     * Checking interrupt_callback, looping on EINTR and EAGAIN and until
     * enough data has been read is left to the calling function; see
     * retry_transfer_wrapper in avio.c.
     */
    int     (*url_read)( URLContext *h, unsigned char *buf, int size);
    int     (*url_write)(URLContext *h, const unsigned char *buf, int size);
    int64_t (*url_seek)( URLContext *h, int64_t pos, int whence);
    int     (*url_close)(URLContext *h);
    int (*url_read_pause)(URLContext *h, int pause);
    int64_t (*url_read_seek)(URLContext *h, int stream_index,
                             int64_t timestamp, int flags);
    int (*url_get_file_handle)(URLContext *h);
    int (*url_get_multi_file_handle)(URLContext *h, int **handles,
                                     int *numhandles);
    int (*url_get_short_seek)(URLContext *h);
    int (*url_shutdown)(URLContext *h, int flags);
    int priv_data_size;
    const AVClass *priv_data_class;
    int flags;
    int (*url_check)(URLContext *h, int mask);
    int (*url_open_dir)(URLContext *h);
    int (*url_read_dir)(URLContext *h, AVIODirEntry **next);
    int (*url_close_dir)(URLContext *h);
    int (*url_delete)(URLContext *h);
    int (*url_move)(URLContext *h_src, URLContext *h_dst);
    const char *default_whitelist;
} URLProtocol;

在代码中搜索url_open，很容易能够找到ffmpeg中支持的几个协议：
ff_bluray_protocol ff_data_protocol ff_file_protocol ff_pipe_protocol ff_ftp_protocol ff_hls_protocol ff_httpproxy_protocol .. 还有很多协议，不全部列出了，例如tcp, udp, …支持这些协议而定义的结构体在av_register_all()函数被调用时而注册。
avio_open2—>ffurl_open—>ffurl_alloc—>static struct URLProtocol *url_find_protocol(const char *filename) //会根据filename判断协议先判断具体的协议，然后根据判读出来的协议，调用ffurl_protocol_next,在之前注册的first_protocol链表上找到相应的协议结构体，然后返回给函数url_alloc_for_protocol，在这个函数中，则是根据判断的协议，生成URLContext结构体，

在ffurl_open函数中，ffurl_alloc函数返回之后，URLContext **puc结构体就已经被设置完毕，然后在ffurl_open函数中，再调用ret = ffurl_connect(*puc, options)，ffurl_connect会调用url_open2，函数指针。