FFmpeg5.0源码阅读——URLContext和URLProtocol

  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callback
    int64_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URLProtocolurl_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http
    int     (*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);               //seek
    int     (*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handle
    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);                              //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"
};

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