【FFmpeg】AVIOContext结构体

参考：
FFMPEG结构体分析：AVIOContext

示例工程：
【FFmpeg】调用ffmpeg库实现264软编
【FFmpeg】调用ffmpeg库实现264软解
【FFmpeg】调用ffmpeg库进行RTMP推流和拉流
【FFmpeg】调用ffmpeg库进行SDL2解码后渲染

流程分析：
【FFmpeg】编码链路上主要函数的简单分析
【FFmpeg】解码链路上主要函数的简单分析

结构体分析：
【FFmpeg】AVCodec结构体
【FFmpeg】AVCodecContext结构体
【FFmpeg】AVStream结构体
【FFmpeg】AVFormatContext结构体

1.AVIOContext结构体的定义

AVIOContext定义了输入输出上下文之中的信息，涉及到码流的输入输出，是非常重要的结构体，位于libavformat\avio.h中，定义如下

/*** Bytestream IO Context.* New public fields can be added with minor version bumps.* Removal, reordering and changes to existing public fields require* a major version bump.* sizeof(AVIOContext) must not be used outside libav*.** @note None of the function pointers in AVIOContext should be called*       directly, they should only be set by the client application*       when implementing custom I/O. Normally these are set to the*       function pointers specified in avio_alloc_context()*/
// 码流输入输出的上下文
// @note AVIOContext中的所有函数指针都不应该被直接调用，它们只应该由客户端应用程序在实现自定义I/O时设置
// 通常将这些设置为avio_alloc_context()中指定的函数指针
typedef struct AVIOContext {/*** A class for private options.** If this AVIOContext is created by avio_open2(), av_class is set and* passes the options down to protocols.** If this AVIOContext is manually allocated, then av_class may be set by* the caller.** warning -- this field can be NULL, be sure to not pass this AVIOContext* to any av_opt_* functions in that case.*/// 用于私有配置的选项// 如果这个AVIOContext是由avio_open2创建的，设置av_class并将选项向下传递给协议// 如果这个AVIOContext是手动分配的，那么av_class可以由调用者设置// warning: 这个字段可以为NULL，在这种情况下，请确保不要将这个AVIOContext传递给任何av_opt_*函数const AVClass *av_class;/** The following shows the relationship between buffer, buf_ptr,* buf_ptr_max, buf_end, buf_size, and pos, when reading and when writing* (since AVIOContext is used for both):************************************************************************************                                   READING************************************************************************************                            |              buffer_size              |*                            |---------------------------------------|*                            |                                       |**                         buffer          buf_ptr       buf_end*                            +---------------+-----------------------+*                            |/ / / / / / / /|/ / / / / / /|         |*  read buffer:              |/ / consumed / | to be read /|         |*                            |/ / / / / / / /|/ / / / / / /|         |*                            +---------------+-----------------------+**                                                         pos*              +-------------------------------------------+-----------------+*  input file: |                                           |                 |*              +-------------------------------------------+-----------------+*************************************************************************************                                   WRITING************************************************************************************                             |          buffer_size                 |*                             |--------------------------------------|*                             |                                      |**                                                buf_ptr_max*                          buffer                 (buf_ptr)       buf_end*                             +-----------------------+--------------+*                             |/ / / / / / / / / / / /|              |*  write buffer:              | / / to be flushed / / |              |*                             |/ / / / / / / / / / / /|              |*                             +-----------------------+--------------+*                               buf_ptr can be in this*                               due to a backward seek**                            pos*               +-------------+----------------------------------------------+*  output file: |             |                                              |*               +-------------+----------------------------------------------+**/// buffer的起始地址unsigned char *buffer;  /**< Start of the buffer. */// 最大的buffer大小int buffer_size;        /**< Maximum buffer size */// 当前buffer指针指向的位置unsigned char *buf_ptr; /**< Current position in the buffer */// 如果read函数返回的数据少于请求，则数据的结束值可能小于buffer+buffer_size，例如，对于尚未接收到更多数据的流// 当前buffer指针的末尾unsigned char *buf_end; /**< End of the data, may be less thanbuffer+buffer_size if the read function returnedless data than requested, e.g. for streams whereno more data has been received yet. */// 私有指针，传递给read/write/seek/… 函数void *opaque;           /**< A private pointer, passed to the read/write/seek/...functions. */// 读取packetint (*read_packet)(void *opaque, uint8_t *buf, int buf_size);// 写入packetint (*write_packet)(void *opaque, const uint8_t *buf, int buf_size);// 查找int64_t (*seek)(void *opaque, int64_t offset, int whence);// 当前缓冲区在文件中的位置int64_t pos;            /**< position in the file of the current buffer */// 如果由于错误或eof而无法读取，则为Trueint eof_reached;        /**< true if was unable to read due to error or eof */// 错误码int error;              /**< contains the error code or 0 if no error happened */// 如果是写入状态，则置为trueint write_flag;         /**< true if open for writing */// 最大的packet大小int max_packet_size;// 最小的packet大小(在刷新数据之前，尝试至少缓冲这么多的数据)int min_packet_size;    /**< Try to buffer at least this amount of databefore flushing it. */// checksum用于验证数据完整性// 存储了校验值，以验证数据的完整性。通过将数据块的大小累加到checksum，然后对结果进行按位反转来实现的// 这个机制主要用于检测数据传输过程中可能发生的错误，尤其是在网络传输或文件读写中unsigned long checksum;unsigned char *checksum_ptr;unsigned long (*update_checksum)(unsigned long checksum, const uint8_t *buf, unsigned int size);/*** Pause or resume playback for network streaming protocols - e.g. MMS.*/// 暂停或恢复网络流协议的播放int (*read_pause)(void *opaque, int pause);/*** Seek to a given timestamp in stream with the specified stream_index.* Needed for some network streaming protocols which don't support seeking* to byte position.*/// 使用指定的stream_index在流中查找给定的时间戳// 需要一些网络流协议，不支持寻求字节位置int64_t (*read_seek)(void *opaque, int stream_index,int64_t timestamp, int flags);/*** A combination of AVIO_SEEKABLE_ flags or 0 when the stream is not seekable.*//*流查找的行为，有两个定义* Seeking works like for a local file.#define AVIO_SEEKABLE_NORMAL (1 << 0)* Seeking by timestamp with avio_seek_time() is possible.#define AVIO_SEEKABLE_TIME   (1 << 1)*/int seekable;/*** avio_read and avio_write should if possible be satisfied directly* instead of going through a buffer, and avio_seek will always* call the underlying seek function directly.*/// 如果可能的话，应该直接满足Avio_read和avio_write，而不是通过缓冲区，avio_seek将始终直接调用底层的seek函数int direct;/*** ',' separated list of allowed protocols.*/// 协议的白名单const char *protocol_whitelist;/*** ',' separated list of disallowed protocols.*/// 协议的黑名单const char *protocol_blacklist;/*** A callback that is used instead of write_packet.*/// 一个用来代替write_packet的回调int (*write_data_type)(void *opaque, const uint8_t *buf, int buf_size,enum AVIODataMarkerType type, int64_t time);/*** If set, don't call write_data_type separately for AVIO_DATA_MARKER_BOUNDARY_POINT,* but ignore them and treat them as AVIO_DATA_MARKER_UNKNOWN (to avoid needlessly* small chunks of data returned from the callback).*/// 如果设置了，不要为AVIO_DATA_MARKER_BOUNDARY_POINT单独调用write_data_type，// 而是忽略它们并将它们视为AVIO_DATA_MARKER_UNKNOWN(以避免从回调返回不必要的小块数据)int ignore_boundary_point;/*** Maximum reached position before a backward seek in the write buffer,* used keeping track of already written data for a later flush.*/// 写缓冲区中向后寻道之前达到的最大位置，用于跟踪已写入的数据，以便以后刷新unsigned char *buf_ptr_max;/*** Read-only statistic of bytes read for this AVIOContext.*/// 只读统计为该AVIOContext读取的字节数int64_t bytes_read;/*** Read-only statistic of bytes written for this AVIOContext.*/// 只读统计写入该AVIOContext的字节数。int64_t bytes_written;
} AVIOContext;

AVIOContext当中比较重要的信息包括：
（1）unsigned char *buffer：buffer的起始地址
（2）int buffer_size：buffer的大小
（3）unsigned char *buf_ptr：buffer当前的指针
（4）unsigned char *buf_end：buffer末尾的地址
（5）void *opaque：URLContext
（6）int (*read_packet)(void *opaque, uint8_t *buf, int buf_size)：读取packet
（7）int (*write_packet)(void *opaque, const uint8_t *buf, int buf_size)：写入packet
（8）int64_t (*seek)(void *opaque, int64_t offset, int whence)：定位查找
（9）unsigned long (*update_checksum)(unsigned long checksum, const uint8_t *buf, unsigned int size)：更新校验信息checksum
（10）int (*read_pause)(void *opaque, int pause)：暂停流的读取
（11）const char *protocol_XXXlist：协议的黑白名单

相对比于雷博记录的内容，新版本的AVIOContext增加的比较重要的是协议的黑白名单，白名单当中的protocol表示可用，黑名单当中的protocol表示不可用。在解码的时候，buffer会存储FFmpeg读取的数据，例如解码已经编码好的.h264文件。

URLContext的定义如下，位于libavformat\url.h中

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;

相对于老版本的FFmpeg，这里一个比较大的改变是增加了protocol的黑白名单。URLProtocol的定义如下，同样位于libavformat\url.h中

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_whitelist()* or ffurl_connect() for those nested protocols.*/// 此回调将由打开进一步嵌套协议的协议使用。然后将这些嵌套协议的选项传递给ffurl_open_whitelist()或ffurl_connect()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.*/// 从protocol读取数据// 如果数据立即可用(甚至小于大小)，达到EOF或发生错误(包括EINTR)，则立即返回// 在非阻塞模式下，立即返回AVERROR(EAGAIN)。在阻塞模式下，等待数据/EOF/错误有一个短超时(0.1s)，并在超时时返回AVERROR(EAGAIN)// 检查interrupt_callback，在EINTR和EAGAIN上循环，直到有足够的数据被读取给调用函数;参见avio.c中的retry_transfer_wrapperint     (*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)(void *urlcontext, int pause);int64_t (*url_read_seek)(void *urlcontext, 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);const AVClass *priv_data_class;int priv_data_size;int flags;// 相对比于老版本的FFmpeg，这里还增加了dir的操作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;

在实际的结构体定义如下所示，定义是file的protocol，其中还定义了白名单为"file,crypto,data"

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"
};

又比如tcp的protocol，不过这里没有定义默认的白名单

const URLProtocol ff_tcp_protocol = {.name                = "tcp",.url_open            = tcp_open,.url_accept          = tcp_accept,.url_read            = tcp_read,.url_write           = tcp_write,.url_close           = tcp_close,.url_get_file_handle = tcp_get_file_handle,.url_get_short_seek  = tcp_get_window_size,.url_shutdown        = tcp_shutdown,.priv_data_size      = sizeof(TCPContext),.flags               = URL_PROTOCOL_FLAG_NETWORK,.priv_data_class     = &tcp_class,
};

CSDN : https://blog.csdn.net/weixin_42877471
Github : https://github.com/DoFulangChen