AVIOContext是FFMPEG管理輸入輸出數(shù)據(jù)的結(jié)構(gòu)體。
參考結(jié)構(gòu)體理解：http://www.itdecent.cn/p/d109e7ef9749

unsigned char *buffer

：緩存開始位置
在解碼的情況下，buffer用于存儲(chǔ)ffmpeg讀入的數(shù)據(jù)。例如打開一個(gè)視頻文件的時(shí)候，先把數(shù)據(jù)從硬盤讀入buffer，然后在送給解碼器用于解碼。

int buffer_size

：緩存大?。J(rèn)32768）

unsigned char *buf_ptr

：當(dāng)前指針讀取到的位置

unsigned char *buf_end

：緩存結(jié)束的位置

void *opaque ：URLContext結(jié)構(gòu)體
typedef struct URLContext {  
    const AVClass *av_class; ///< information for av_log(). Set by url_open().  
    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 URL */  
    int is_connected;  
    AVIOInterruptCB interrupt_callback;  
} URLContext;

URLContext結(jié)構(gòu)體中還有一個(gè)結(jié)構(gòu)體URLProtocol。注：每種協(xié)議（rtp，rtmp，file等）對(duì)應(yīng)一個(gè)URLProtocol。這個(gè)結(jié)構(gòu)體也不在FFMPEG提供的頭文件中。從FFMPEG源代碼中翻出其的定義：
typedef struct URLProtocol {  
    const char *name;  
    int (*url_open)(URLContext *h, const char *url, int flags);  
    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);  
    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);  
    int (*url_get_file_handle)(URLContext *h);  
    int priv_data_size;  
    const AVClass *priv_data_class;  
    int flags;  
    int (*url_check)(URLContext *h, int mask);  
} URLProtocol;

在這個(gè)結(jié)構(gòu)體中，除了一些回調(diào)函數(shù)接口之外，有一個(gè)變量const char *name，該變量存儲(chǔ)了協(xié)議的名稱。每一種輸入?yún)f(xié)議都對(duì)應(yīng)這樣一個(gè)結(jié)構(gòu)體。

URLProtocol ff_rtmp_protocol = {  
    .name                = "rtmp",  
    .url_open            = rtmp_open,  
    .url_read            = rtmp_read,  
    .url_write           = rtmp_write,  
    .url_close           = rtmp_close,  
    .url_read_pause      = rtmp_read_pause,  
    .url_read_seek       = rtmp_read_seek,  
    .url_get_file_handle = rtmp_get_file_handle,  
    .priv_data_size      = sizeof(RTMP),  
    .flags               = URL_PROTOCOL_FLAG_NETWORK,  
};