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。