摘要：本文描述FFmpeg中URLContext和URLProtocal的实现。
  关键字：URLContext、URLProtocal

  FFmpeg中URLProtocol是具体的协议的抽象，其中定义了对应协议的抽象，其中包含了具体协议的操作函数指针。而URLContext是对协议操作的抽象，描述了当前协议的读写状态。和其他结构体一样，FFmpeg内部针对每一个协议都有一个static的结构体，该结构体描述了对应协议的操作。
  另外FFmpeg中有个list保存了所有URLProtocol的指针，类似AVCodec都是定义好的静态变量。
  该list存储在libavformat\protocol_list.c文件中，需要注意的是该文件是通过脚本生成的，如果新拉的代码应该看不到，configure再make之后就能看到改文件了。

static const URLProtocol * const url_protocols[] = {&ff_async_protocol,//...&ff_file_protocol,&ff_ftp_protocol,//...&ff_unix_protocol,NULL 
};

1 URLContext

1.1 URLContext

  URLContext是对IO操作的抽象，类似以AVCodecContext，其中了当前媒体操作包含的基本信息 ，描述了当前IO操作的参数。使用过程中，URLContext作为AVIO的一个成员用来操作文件流。

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;           //是否连接，网络流AVIOInterruptCB interrupt_callback;     //io终止时的callbackint64_t rw_timeout;         /**< maximum time to wait for (network) read/write operation completion, in mcs */const char *protocol_whitelist;     //白名单const char *protocol_blacklist;     //黑名单int min_packet_size;        /**< if non zero, the stream is packetized with this min packet size */
} URLContext;

1.2 操作API概要

  下面简单列举一些操作API来说明URLContext如何在FFmpeg中使用：

• int ffurl_alloc(URLContext **puc, const char *filename, int flags, const AVIOInterruptCB *int_cb)：根据当前的文件名创建URLContext，第一步在url_protocols中根据文件名搜索对应的URLProtocol，然后初始化URLContext的默认参数，不会做额外多余的动作，简单说就是search->malloc->init（仅仅参数）；

•              const AVIOInterruptCB *int_cb, AVDictionary **options,const char *whitelist, const char* blacklist,URLContext *parent);
  

• int ffurl_connect(URLContext *uc, AVDictionary **options);：打开链接，内部主要就是调用URLProtocol的url_open2函数打开链接然后将is_connected设置为1，根据流的类型设置is_stream；
• int ffurl_accept(URLContext *s, URLContext **c);：accept链接，直接调用的协议的url_accept，一般只会用于网络请求，对于普通的本地IO基本没用；
• int ffurl_write(URLContext *h, const unsigned char *buf, int size);：调用协议的url_write写文件。

  从上面的流程大概能够看出URLContext大部分操作都是直接调用的URLProtocol的函数指针，额外做一些参数检查与匹配。

2 URLProtocal

  URLProtocal是具体的IO的描述类，类似于具体的AVCodec指针，每个类型的IO操作都有其对应的静态对象指针。URLProtocal中用函数指针来表示当前文件操作需要调用的具体操作。

typedef struct URLProtocol {const char *name;           //协议名，比如httpint     (*url_open)( URLContext *h, const char *url, int flags);/*** 下面的函数就是协议连接的api* This callback is to be used by protocols which open further nested* protocols. options are then to be passed to ffurl_open_whitelist()* or 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);/*** 下面就是直接读写数据操作的接口* 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);          //读数据int     (*url_write)(URLContext *h, const unsigned char *buf, int size);    //写数据int64_t (*url_seek)( URLContext *h, int64_t pos, int whence);               //seekint     (*url_close)(URLContext *h);                                        //关闭int (*url_read_pause)(URLContext *h, int pause);                            //暂停读，网络流int64_t (*url_read_seek)(URLContext *h, int stream_index, int64_t timestamp, int flags);int (*url_get_file_handle)(URLContext *h);                                  //获取文件的handleint (*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);                              //关闭const AVClass *priv_data_class;                                             //私有数据int priv_data_size;                                                         //私有数据的大小int flags;                                                                  //标志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;

  FFmpeg中每个IO协议都有一个对应的URLProtocol描述该协议IO的具体操作，比如文件IO就定义在libavformat/file.c中，最终实际调用的都是文件IO那套接口。

const URLProtocol ff_file_protocol = {.name                = "file",.url_open            = file_open,.url_read            = file_read,.url_write           = file_write,.url_seek            = file_seek,.url_close           = file_close,.url_get_file_handle = file_get_handle,.url_check           = file_check,.url_delete          = file_delete,.url_move            = file_move,.priv_data_size      = sizeof(FileContext),.priv_data_class     = &file_class,.url_open_dir        = file_open_dir,.url_read_dir        = file_read_dir,.url_close_dir       = file_close_dir,.default_whitelist   = "file,crypto,data"
};