学步园

/*URLProtocol 是类似COM 接口的数据结构，表示广义的输入文件，着重于功能函数*/
typedef struct URLProtocol {
	const char *name;
	int (*url_open)(URLContext *h, const char *filename, int flags);
	int (*url_read)(URLContext *h, unsigned char *buf, int size);
	int (*url_write)(URLContext *h, unsigned char *buf, int size);
	int64_t (*url_seek)(URLContext *h, int64_t pos, int whence);
	int (*url_close)(URLContext *h);
	struct URLProtocol *next;
	int (*url_read_pause)(URLContext *h, int pause);
	int64_t (*url_read_seek)(URLContext *h, int stream_index,
		int64_t timestamp, int flags);
} URLProtocol;

//用file 协议相应函数初始化URLProtocol 结构。
URLProtocol file_protocol = {
	"file",
	file_open,
	file_read,
	file_write,
	file_seek,
	file_close,
};
//注册协议
int av_register_protocol(URLProtocol *protocol)
{
	URLProtocol **p;
	p = &first_protocol;
	while (*p != NULL) p = &(*p)->next;
	*p = protocol;
	protocol->next = NULL;
	return 0;
}

#define REGISTER_PROTOCOL(X,x) { \
	extern URLProtocol x##_protocol; \
	if(CONFIG_##X##_PROTOCOL) av_register_protocol(&x##_protocol); }

//url_open中识别并打开协议
up = first_protocol;
while (up != NULL) {
	if (!strcmp(proto_str, up->name))
		return url_open_protocol (puc, up, filename, flags);
	up = up->next;
}

/*URLContext 结构表示程序运行的当前广义输入文件使用的上下文，着重于所有广义输入文件共有的属性(并
且是在程序运行时才能确定其值)和关联其他结构的字段。prot 字段关联相应的广义输入文件；priv_data 字段关
联各个具体广义输入文件的句柄。*/
struct URLContext {
	struct URLProtocol *prot;
	int flags;
	int is_streamed;  /**< true if streamed (no seek possible), default = false */
	int max_packet_size;  /**< if non zero, the stream is packetized with this max packet size */
	void *priv_data;
	char *filename; /**< specified filename */
};