【FFmpeg】AVFormatContext结构体
- 1.AVFormatContext结构体
-
- [1.2 const struct AVInputFormat *iformat](#1.2 const struct AVInputFormat *iformat)
- [1.3 const struct AVOutputFormat *oformat](#1.3 const struct AVOutputFormat *oformat)
参考：
FFMPEG结构体分析：AVFormatContext
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
1.AVFormatContext结构体
AVFormatContext的定义位于libavformat\avformat.h之中，主要包括了格式的上下文信息，定义如下
c
/**
* Format I/O context.
* New fields can be added to the end with minor version bumps.
* Removal, reordering and changes to existing fields require a major
* version bump.
* sizeof(AVFormatContext) must not be used outside libav*, use
* avformat_alloc_context() to create an AVFormatContext.
*
* Fields can be accessed through AVOptions (av_opt*),
* the name string used matches the associated command line parameter name and
* can be found in libavformat/options_table.h.
* The AVOption/command line parameter names differ in some cases from the C
* structure field names for historic reasons or brevity.
*/
// 格式I/O的上下文结构体
// 新字段可以添加到末尾，并伴有轻微的版本变更。
// 删除、重新排序和更改现有字段需要一个主要的版本更新。
// sizeof(AVFormatContext)不能在libav*之外使用，使用avformat_alloc_context()来创建AVFormatContext
// 字段可以通过AVOptions (av_opt*)访问，使用的名称字符串匹配相关的命令行参数名称，可以在libavformat/options_table.h中找到
// 由于历史原因或简洁，AVOption/命令行参数名在某些情况下与C结构字段名不同
typedef struct AVFormatContext {
/**
* A class for logging and @ref avoptions. Set by avformat_alloc_context().
* Exports (de)muxer private options if they exist.
*/
// 一个用于日志记录和@ref avoptions的类。由avformat_alloc_context()设置
// 导出(删除)多个私有选项(如果存在的话)
const AVClass *av_class;
/**
* The input container format.
*
* Demuxing only, set by avformat_open_input().
*/
// 输入容器的格式
// 仅用于解复用，由set by avformat_open_input()设置
const struct AVInputFormat *iformat;
/**
* The output container format.
*
* Muxing only, must be set by the caller before avformat_write_header().
*/
// 输出容器的格式
// 仅用于复用，必须在使用avformat_write_header()之前由调用者设置
const struct AVOutputFormat *oformat;
/**
* Format private data. This is an AVOptions-enabled struct
* if and only if iformat/oformat.priv_class is not NULL.
*
* - muxing: set by avformat_write_header()
* - demuxing: set by avformat_open_input()
*/
// 格式化私有数据。这是一个启用avoptions的结构体，当且仅当format/oformat。priv_class不是NULL
void *priv_data;
/**
* I/O context.
*
* - demuxing: either set by the user before avformat_open_input() (then
* the user must close it manually) or set by avformat_open_input().
* - muxing: set by the user before avformat_write_header(). The caller must
* take care of closing / freeing the IO context.
*
* Do NOT set this field if AVFMT_NOFILE flag is set in
* iformat/oformat.flags. In such a case, the (de)muxer will handle
* I/O in some other way and this field will be NULL.
*/
// 输入输出的上下文
// 解复用：由用户在avformat_open_input()之前设置(然后用户必须手动关闭它)或由avformat_open_input()设置
// 复用：在avformat_write_header()之前由用户设置。调用者必须负责关闭/释放IO上下文
// 如果在iformat/oformat.flags中设置了AVFMT_NOFILE标志，则不要设置此字段。在这种情况下，(de)muxer将以其他方式处理I/O，并且该字段将为NULL
AVIOContext *pb;
/* stream info */
// 下面的变量记录了流的信息
/**
* Flags signalling stream properties. A combination of AVFMTCTX_*.
* Set by libavformat.
*/
// 描述了流信息的标志，是AVFMTCTX_*的组合，由libavformat设置
int ctx_flags;
/**
* Number of elements in AVFormatContext.streams.
*
* Set by avformat_new_stream(), must not be modified by any other code.
*/
// AVFormatContext.streams中的元素数量
unsigned int nb_streams;
/**
* A list of all streams in the file. New streams are created with
* avformat_new_stream().
*
* - demuxing: streams are created by libavformat in avformat_open_input().
* If AVFMTCTX_NOHEADER is set in ctx_flags, then new streams may also
* appear in av_read_frame().
* - muxing: streams are created by the user before avformat_write_header().
*
* Freed by libavformat in avformat_free_context().
*/
// 文件中所有流的列表。使用avformat_new_stream()创建新流
// 解复用：流是由libavformat在avformat_open_input中创建的
// 复用：用户在avformat_write_header()之前创建流
AVStream **streams;
/**
* Number of elements in AVFormatContext.stream_groups.
*
* Set by avformat_stream_group_create(), must not be modified by any other code.
*/
// AVFormatContext.stream_groups中的元素数量
// 由avformat_stream_group_create()设置，不能被任何其他代码修改
unsigned int nb_stream_groups;
/**
* A list of all stream groups in the file. New groups are created with
* avformat_stream_group_create(), and filled with avformat_stream_group_add_stream().
*
* - demuxing: groups may be created by libavformat in avformat_open_input().
* If AVFMTCTX_NOHEADER is set in ctx_flags, then new groups may also
* appear in av_read_frame().
* - muxing: groups may be created by the user before avformat_write_header().
*
* Freed by libavformat in avformat_free_context().
*/
// 文件中所有流组的列表。使用avformat_stream_group_create()创建新组，并使用avformat_stream_group_add_stream()填充新组
// 解复用：组可以通过libavformat在avformat_open_input()中创建。如果AVFMTCTX_NOHEADER在ctx_flags中设置，
// 那么av_read_frame()中也可能出现新的组。
// 复用：用户可以在avformat_write_header()之前创建组
// 由libavformat在avformat_free_context()之中释放
AVStreamGroup **stream_groups;
/**
* Number of chapters in AVChapter array.
* When muxing, chapters are normally written in the file header,
* so nb_chapters should normally be initialized before write_header
* is called. Some muxers (e.g. mov and mkv) can also write chapters
* in the trailer. To write chapters in the trailer, nb_chapters
* must be zero when write_header is called and non-zero when
* write_trailer is called.
* - muxing: set by user
* - demuxing: set by libavformat
*/
// AVChapter数组中的章节数
// 当复用时，章节通常写在文件头中，所以nb_章节通常应该在调用write_header之前初始化。
// 一些编码器(例如mov和mkv)也可以在预告片中编写章节。要在尾文件中写入章节，调用write_header时nb_章节必须为零，
// 调用write_trailer时nb_章节必须为非零
unsigned int nb_chapters;
AVChapter **chapters;
/**
* input or output URL. Unlike the old filename field, this field has no
* length restriction.
*
* - demuxing: set by avformat_open_input(), initialized to an empty
* string if url parameter was NULL in avformat_open_input().
* - muxing: may be set by the caller before calling avformat_write_header()
* (or avformat_init_output() if that is called first) to a string
* which is freeable by av_free(). Set to an empty string if it
* was NULL in avformat_init_output().
*
* Freed by libavformat in avformat_free_context().
*/
// 输入或输出URL，与原来使用的filename变量不同，这里没有长度限制，原来的定义为 char filename[1024]
// 解复用：由avformat_open_input()设置，如果avformat_open_input()中的url参数为NULL，则初始化为空字符串
// 复用：可以由调用者在调用avformat_write_header()(或avformat_init_output()如果先调用它)之前设置为av_free()可释放的字符串。
// 如果avformat_init_output()中为NULL，则设置为空字符串
char *url;
/**
* Position of the first frame of the component, in
* AV_TIME_BASE fractional seconds. NEVER set this value directly:
* It is deduced from the AVStream values.
*
* Demuxing only, set by libavformat.
*/
// 组件的第一帧的位置，以AV_TIME_BASE小数秒为单位。千万不要直接设置这个值
// 它是从AVStream的值推导出来的
int64_t start_time;
/**
* Duration of the stream, in AV_TIME_BASE fractional
* seconds. Only set this value if you know none of the individual stream
* durations and also do not set any of them. This is deduced from the
* AVStream values if not set.
*
* Demuxing only, set by libavformat.
*/
// 流的持续时间，以AV_TIME_BASE小数秒为单位。只有在您不知道任何单个流持续时间的情况下才设置此值，
// 并且也不要设置它们中的任何一个。如果没有设置，这是从AVStream值推断出来的
int64_t duration;
/**
* Total stream bitrate in bit/s, 0 if not
* available. Never set it directly if the file_size and the
* duration are known as FFmpeg can compute it automatically.
*/
// 总流比特率(以bit/s为单位)，如果不可用则为0。如果file_size和持续时间被称为FFmpeg可以自动计算，则不要直接设置它
int64_t bit_rate;
// pakcet包的大小
unsigned int packet_size;
// 最大延时
int max_delay;
/**
* Flags modifying the (de)muxer behaviour. A combination of AVFMT_FLAG_*.
* Set by the user before avformat_open_input() / avformat_write_header().
*/
// 描述复用器或者解复用器的行为，是一组AVFMT_FLAG_*组成的标志符
int flags;
#define AVFMT_FLAG_GENPTS 0x0001 ///< Generate missing pts even if it requires parsing future frames.
#define AVFMT_FLAG_IGNIDX 0x0002 ///< Ignore index.
#define AVFMT_FLAG_NONBLOCK 0x0004 ///< Do not block when reading packets from input.
#define AVFMT_FLAG_IGNDTS 0x0008 ///< Ignore DTS on frames that contain both DTS & PTS
#define AVFMT_FLAG_NOFILLIN 0x0010 ///< Do not infer any values from other values, just return what is stored in the container
#define AVFMT_FLAG_NOPARSE 0x0020 ///< Do not use AVParsers, you also must set AVFMT_FLAG_NOFILLIN as the fillin code works on frames and no parsing -> no frames. Also seeking to frames can not work if parsing to find frame boundaries has been disabled
#define AVFMT_FLAG_NOBUFFER 0x0040 ///< Do not buffer frames when possible
#define AVFMT_FLAG_CUSTOM_IO 0x0080 ///< The caller has supplied a custom AVIOContext, don't avio_close() it.
#define AVFMT_FLAG_DISCARD_CORRUPT 0x0100 ///< Discard frames marked corrupted
#define AVFMT_FLAG_FLUSH_PACKETS 0x0200 ///< Flush the AVIOContext every packet.
/**
* When muxing, try to avoid writing any random/volatile data to the output.
* This includes any random IDs, real-time timestamps/dates, muxer version, etc.
*
* This flag is mainly intended for testing.
*/
#define AVFMT_FLAG_BITEXACT 0x0400
#define AVFMT_FLAG_SORT_DTS 0x10000 ///< try to interleave outputted packets by dts (using this flag can slow demuxing down)
#define AVFMT_FLAG_FAST_SEEK 0x80000 ///< Enable fast, but inaccurate seeks for some formats
#if FF_API_LAVF_SHORTEST
#define AVFMT_FLAG_SHORTEST 0x100000 ///< Stop muxing when the shortest stream stops.
#endif
#define AVFMT_FLAG_AUTO_BSF 0x200000 ///< Add bitstream filters as requested by the muxer
/**
* Maximum number of bytes read from input in order to determine stream
* properties. Used when reading the global header and in
* avformat_find_stream_info().
*
* Demuxing only, set by the caller before avformat_open_input().
*
* @note this is \e not used for determining the \ref AVInputFormat
* "input format"
* @sa format_probesize
*/
// 为了确定流属性而从输入读取的最大字节数。在读取全局头文件和avformat_find_stream_info()中使用
int64_t probesize;
/**
* Maximum duration (in AV_TIME_BASE units) of the data read
* from input in avformat_find_stream_info().
* Demuxing only, set by the caller before avformat_find_stream_info().
* Can be set to 0 to let avformat choose using a heuristic.
*/
// avformat_find_stream_info()从输入中读取数据的最大持续时间(AV_TIME_BASE单位)
// 可以设置为0，来让avformat选择使用启发式模式？
int64_t max_analyze_duration;
const uint8_t *key;
int keylen;
// 程序描述的结构体，从定义上看似乎是区分和管理不同的程序集
unsigned int nb_programs;
AVProgram **programs;
/**
* Forced video codec_id.
* Demuxing: Set by user.
*/
// 视频的codec ID
enum AVCodecID video_codec_id;
/**
* Forced audio codec_id.
* Demuxing: Set by user.
*/
// 音频的codec ID
enum AVCodecID audio_codec_id;
/**
* Forced subtitle codec_id.
* Demuxing: Set by user.
*/
// 字幕的codec ID
// 使用何种codec来编码和解码字幕
enum AVCodecID subtitle_codec_id;
/**
* Forced Data codec_id.
* Demuxing: Set by user.
*/
// 数据流的codec ID
// 用于处理非音视频的数据内容，比如字幕、加密秘钥或者自定义的元数据
enum AVCodecID data_codec_id;
/**
* Metadata that applies to the whole file.
*
* - demuxing: set by libavformat in avformat_open_input()
* - muxing: may be set by the caller before avformat_write_header()
*
* Freed by libavformat in avformat_free_context().
*/
// 应用于整个文件的元数据
AVDictionary *metadata;
/**
* Start time of the stream in real world time, in microseconds
* since the Unix epoch (00:00 1st January 1970). That is, pts=0 in the
* stream was captured at this real world time.
* - muxing: Set by the caller before avformat_write_header(). If set to
* either 0 or AV_NOPTS_VALUE, then the current wall-time will
* be used.
* - demuxing: Set by libavformat. AV_NOPTS_VALUE if unknown. Note that
* the value may become known after some number of frames
* have been received.
*/
// 流在现实世界时间中的开始时间，自Unix纪元(1970年1月1日00:00)以来以微秒为单位。
// 也就是说，流中的pts=0是在这个真实世界时间被捕获的
int64_t start_time_realtime;
/**
* The number of frames used for determining the framerate in
* avformat_find_stream_info().
* Demuxing only, set by the caller before avformat_find_stream_info().
*/
// 在avformat_find_stream_info()中用于确定帧率的帧数
int fps_probe_size;
/**
* Error recognition; higher values will detect more errors but may
* misdetect some more or less valid parts as errors.
* Demuxing only, set by the caller before avformat_open_input().
*/
// 错误识别;较高的值将检测到更多的错误，但可能会将一些或多或少有效的部分误检测为错误
int error_recognition;
/**
* Custom interrupt callbacks for the I/O layer.
*
* demuxing: set by the user before avformat_open_input().
* muxing: set by the user before avformat_write_header()
* (mainly useful for AVFMT_NOFILE formats). The callback
* should also be passed to avio_open2() if it's used to
* open the file.
*/
// 为I/O层定制中断回调
AVIOInterruptCB interrupt_callback;
/**
* Flags to enable debugging.
*/
// 用于debug的flag
int debug;
#define FF_FDEBUG_TS 0x0001
/**
* The maximum number of streams.
* - encoding: unused
* - decoding: set by user
*/
// 流的最大数
int max_streams;
/**
* Maximum amount of memory in bytes to use for the index of each stream.
* If the index exceeds this size, entries will be discarded as
* needed to maintain a smaller size. This can lead to slower or less
* accurate seeking (depends on demuxer).
* Demuxers for which a full in-memory index is mandatory will ignore
* this.
* - muxing: unused
* - demuxing: set by user
*/
// 用于每个流索引的最大内存量(以字节为单位)
// 如果索引超过此大小，则根据需要丢弃条目以保持较小的索引大小。这可能导致搜索速度变慢或准确性降低(取决于demuxer)
// 强制使用完整内存索引的解压缩器将忽略这一点
unsigned int max_index_size;
/**
* Maximum amount of memory in bytes to use for buffering frames
* obtained from realtime capture devices.
*/
// 用于缓冲从实时捕获设备获得的帧的最大内存量(以字节为单位)
unsigned int max_picture_buffer;
/**
* Maximum buffering duration for interleaving.
*
* To ensure all the streams are interleaved correctly,
* av_interleaved_write_frame() will wait until it has at least one packet
* for each stream before actually writing any packets to the output file.
* When some streams are "sparse" (i.e. there are large gaps between
* successive packets), this can result in excessive buffering.
*
* This field specifies the maximum difference between the timestamps of the
* first and the last packet in the muxing queue, above which libavformat
* will output a packet regardless of whether it has queued a packet for all
* the streams.
*
* Muxing only, set by the caller before avformat_write_header().
*/
// 用于interleaving的最大缓冲时间
// 1. 为了确保所有流都正确交错，av_interleaved_write_frame()将等待，直到每个流至少有一个数据包，
// 然后才实际将任何数据包写入输出文件。当一些流是"稀疏的"(即在连续的数据包之间有很大的间隙)，这可能会导致过度的缓冲
//
// 2.该字段指定muxing队列中第一个和最后一个数据包的时间戳之间的最大差异，高于此值的libavformat将输出一个数据包，
// 而不管它是否为所有流排队
int64_t max_interleave_delta;
/**
* Maximum number of packets to read while waiting for the first timestamp.
* Decoding only.
*/
// 等待第一个时间戳时要读取的最大数据包数
int max_ts_probe;
/**
* Max chunk time in microseconds.
* Note, not all formats support this and unpredictable things may happen if it is used when not supported.
* - encoding: Set by user
* - decoding: unused
*/
// 最大块时间(以微秒为单位)
// 注意，并非所有格式都支持此功能，如果在不支持的情况下使用此功能，可能会发生不可预测的事情
int max_chunk_duration;
/**
* Max chunk size in bytes
* Note, not all formats support this and unpredictable things may happen if it is used when not supported.
* - encoding: Set by user
* - decoding: unused
*/
// 最大块大小(以字节为单位)
int max_chunk_size;
/**
* Maximum number of packets that can be probed
* - encoding: unused
* - decoding: set by user
*/
// 可以探测的最大数据包数
int max_probe_packets;
/**
* Allow non-standard and experimental extension
* @see AVCodecContext.strict_std_compliance
*/
// 允许非标准和实验性扩展
int strict_std_compliance;
/**
* Flags indicating events happening on the file, a combination of
* AVFMT_EVENT_FLAG_*.
*
* - demuxing: may be set by the demuxer in avformat_open_input(),
* avformat_find_stream_info() and av_read_frame(). Flags must be cleared
* by the user once the event has been handled.
* - muxing: may be set by the user after avformat_write_header() to
* indicate a user-triggered event. The muxer will clear the flags for
* events it has handled in av_[interleaved]_write_frame().
*/
// 指示文件上发生的事件的标志，AVFMT_EVENT_FLAG_*的组合
int event_flags;
/**
* - demuxing: the demuxer read new metadata from the file and updated
* AVFormatContext.metadata accordingly
* - muxing: the user updated AVFormatContext.metadata and wishes the muxer to
* write it into the file
*/
#define AVFMT_EVENT_FLAG_METADATA_UPDATED 0x0001
/**
* Avoid negative timestamps during muxing.
* Any value of the AVFMT_AVOID_NEG_TS_* constants.
* Note, this works better when using av_interleaved_write_frame().
* - muxing: Set by user
* - demuxing: unused
*/
// 在muxing期间避免负时间戳
// AVFMT_AVOID_NEG_TS_*常量的任何值
int avoid_negative_ts;
#define AVFMT_AVOID_NEG_TS_AUTO -1 ///< Enabled when required by target format
#define AVFMT_AVOID_NEG_TS_DISABLED 0 ///< Do not shift timestamps even when they are negative.
#define AVFMT_AVOID_NEG_TS_MAKE_NON_NEGATIVE 1 ///< Shift timestamps so they are non negative
#define AVFMT_AVOID_NEG_TS_MAKE_ZERO 2 ///< Shift timestamps so that they start at 0
/**
* Audio preload in microseconds.
* Note, not all formats support this and unpredictable things may happen if it is used when not supported.
* - encoding: Set by user
* - decoding: unused
*/
// 以微秒计的音频预加载
int audio_preload;
/**
* forces the use of wallclock timestamps as pts/dts of packets
* This has undefined results in the presence of B frames.
* - encoding: unused
* - decoding: Set by user
*/
// 强制使用时钟时间戳作为包的pts/dts
// 这在存在B帧的情况下会产生未定义的结果
int use_wallclock_as_timestamps;
/**
* Skip duration calcuation in estimate_timings_from_pts.
* - encoding: unused
* - decoding: set by user
*/
// 跳过estimate_timings_from_pts中的持续时间计算
int skip_estimate_duration_from_pts;
/**
* avio flags, used to force AVIO_FLAG_DIRECT.
* - encoding: unused
* - decoding: Set by user
*/
// avio标志，用于强制AVIO_FLAG_DIRECT
int avio_flags;
/**
* The duration field can be estimated through various ways, and this field can be used
* to know how the duration was estimated.
* - encoding: unused
* - decoding: Read by user
*/
// duration字段可以通过各种方式进行估计，并且可以使用该字段了解如何估计持续时间
enum AVDurationEstimationMethod duration_estimation_method;
/**
* Skip initial bytes when opening stream
* - encoding: unused
* - decoding: Set by user
*/
// 打开流时跳过初始字节
int64_t skip_initial_bytes;
/**
* Correct single timestamp overflows
* - encoding: unused
* - decoding: Set by user
*/
// 正确的单时间戳溢出
unsigned int correct_ts_overflow;
/**
* Force seeking to any (also non key) frames.
* - encoding: unused
* - decoding: Set by user
*/
// 强制查找任何帧(也是非关键帧)
int seek2any;
/**
* Flush the I/O context after each packet.
* - encoding: Set by user
* - decoding: unused
*/
// 在每个数据包之后刷新I/O上下文
int flush_packets;
/**
* format probing score.
* The maximal score is AVPROBE_SCORE_MAX, its set when the demuxer probes
* the format.
* - encoding: unused
* - decoding: set by avformat, read by user
*/
// 格式探测分数
// 最大分数是AVPROBE_SCORE_MAX，它是在解解器探测格式时设置的
int probe_score;
/**
* Maximum number of bytes read from input in order to identify the
* \ref AVInputFormat "input format". Only used when the format is not set
* explicitly by the caller.
*
* Demuxing only, set by the caller before avformat_open_input().
*
* @sa probesize
*/
// 为了识别\ref AVInputFormat"输入格式"，从输入中读取的最大字节数。仅在调用方未显式设置格式时使用
int format_probesize;
/**
* ',' separated list of allowed decoders.
* If NULL then all are allowed
* - encoding: unused
* - decoding: set by user
*/
// 解码器的白名单
char *codec_whitelist;
/**
* ',' separated list of allowed demuxers.
* If NULL then all are allowed
* - encoding: unused
* - decoding: set by user
*/
// 解码格式的白名单
char *format_whitelist;
/**
* ',' separated list of allowed protocols.
* - encoding: unused
* - decoding: set by user
*/
// 解码协议的白名单
char *protocol_whitelist;
/**
* ',' separated list of disallowed protocols.
* - encoding: unused
* - decoding: set by user
*/
// 解码协议的黑名单
char *protocol_blacklist;
/**
* IO repositioned flag.
* This is set by avformat when the underlaying IO context read pointer
* is repositioned, for example when doing byte based seeking.
* Demuxers can use the flag to detect such changes.
*/
// IO重定位标志
// 当底层IO上下文读指针被重新定位时，这是由avformat设置的，例如在进行基于字节的查找时
// 解复用器可以使用这个标志来检测这种变化
int io_repositioned;
/**
* Forced video codec.
* This allows forcing a specific decoder, even when there are multiple with
* the same codec_id.
* Demuxing: Set by user
*/
// 允许强制一个特定的视频解码器，即使有多个具有相同的codec_id
const struct AVCodec *video_codec;
/**
* Forced audio codec.
* This allows forcing a specific decoder, even when there are multiple with
* the same codec_id.
* Demuxing: Set by user
*/
// 允许强制一个特定的音频解码器，即使有多个具有相同的codec_id
const struct AVCodec *audio_codec;
/**
* Forced subtitle codec.
* This allows forcing a specific decoder, even when there are multiple with
* the same codec_id.
* Demuxing: Set by user
*/
// 允许强制一个特定的字幕解码器，即使具有多个相同的codec_id
const struct AVCodec *subtitle_codec;
/**
* Forced data codec.
* This allows forcing a specific decoder, even when there are multiple with
* the same codec_id.
* Demuxing: Set by user
*/
// 允许强制一个特定的数据解码器，即使具有多个相同的codec_id
const struct AVCodec *data_codec;
/**
* Number of bytes to be written as padding in a metadata header.
* Demuxing: Unused.
* Muxing: Set by user.
*/
// 在元数据头中写入填充的字节数
int metadata_header_padding;
/**
* User data.
* This is a place for some private data of the user.
*/
// 用户数据，这里能够存放一些用户的私有数据
void *opaque;
/**
* Callback used by devices to communicate with application.
*/
// 设备用于与应用程序通信的回调
av_format_control_message control_message_cb;
/**
* Output timestamp offset, in microseconds.
* Muxing: set by user
*/
// 输出时间戳偏移量，以微秒为单位
int64_t output_ts_offset;
/**
* dump format separator.
* can be ", " or "\n " or anything else
* - muxing: Set by user.
* - demuxing: Set by user.
*/
// 输出格式分隔符
uint8_t *dump_separator;
/**
* A callback for opening new IO streams.
*
* Whenever a muxer or a demuxer needs to open an IO stream (typically from
* avformat_open_input() for demuxers, but for certain formats can happen at
* other times as well), it will call this callback to obtain an IO context.
*
* @param s the format context
* @param pb on success, the newly opened IO context should be returned here
* @param url the url to open
* @param flags a combination of AVIO_FLAG_*
* @param options a dictionary of additional options, with the same
* semantics as in avio_open2()
* @return 0 on success, a negative AVERROR code on failure
*
* @note Certain muxers and demuxers do nesting, i.e. they open one or more
* additional internal format contexts. Thus the AVFormatContext pointer
* passed to this callback may be different from the one facing the caller.
* It will, however, have the same 'opaque' field.
*/
// 打开新的IO流的回调函数
// 每当muxer或demuxer需要打开IO流时(对于demuxer通常来自avformat_open_input()，
// 但对于某些格式也可能在其他时间发生)，它将调用此回调以获取IO上下文
// @note 某些复用器和解复用器会嵌套，也就是说，它们会打开一个或多个额外的内部格式上下文。
// 因此，传递给这个回调的AVFormatContext指针可能与面向调用者的指针不同。但是，它将具有相同的"opaque"字段
int (*io_open)(struct AVFormatContext *s, AVIOContext **pb, const char *url,
int flags, AVDictionary **options);
/**
* A callback for closing the streams opened with AVFormatContext.io_open().
*
* Using this is preferred over io_close, because this can return an error.
* Therefore this callback is used instead of io_close by the generic
* libavformat code if io_close is NULL or the default.
*
* @param s the format context
* @param pb IO context to be closed and freed
* @return 0 on success, a negative AVERROR code on failure
*/
// 用于关闭使用AVFormatContext.io_open()打开的流的回调
// 使用这个函数比使用io_close更可取，因为这会返回一个错误
// 因此，如果io_close为NULL或默认值，则通用libavformat代码将使用此回调来代替io_close
int (*io_close2)(struct AVFormatContext *s, AVIOContext *pb);
} AVFormatContext;从AVFormatContext的定义来看，主要的信息包括：
（1）const struct AVInputFormat *iformat：输入容器的格式
（2）const struct AVOutputFormat *oformat：输出容器的格式
（3）AVIOContext *pb：输入输出的上下文结构体
（4）AVStream **streams：文件中流的list
（5）char *url：输入或者输出的地址
（6）int64_t bit_rate：整体流的码率
（7）enum AVCodecID video_codec_id：视频codec ID
（8）enum AVCodecID audio_codec_id：音频codec ID
（9）enum AVCodecID subtitle_codec_id：字幕codec ID
（10）enum AVCodecID data_codec_id：数据流codec ID
（11）unsigned int max_picture_buffer：最大的帧buffer
（12）char *XXX_whitelist：各类信息的白名单（位于白名单当中的工具表示可用，黑名单表示不可用）
（13）const struct AVCodec *XXX_codec：所定义的编解码器（音频、视频、字幕或者数据流）
（14）int (*io_open)(struct AVFormatContext *s, AVIOContext **pb, const char *url,int flags, AVDictionary **options)：打开新的IO流
（15）int (*io_close2)(struct AVFormatContext *s, AVIOContext *pb)：关闭流
从AVFormatContext的定义来看，除了定义了AVInputFormat和AVOutputFormat两种format结构体之外，还定义了输入输出上下文结构体AVIOContext、文件中流的list结构体AVStream、黑白名单list和流的打开关闭函数，这里与老版本的FFmpeg不同的地方包括：（1）将以前的char filename[1024]改成了现在的char* url，不再限制长度；（2）将AVCodec存放在AVFormatContext之中而不是AVStream之中，这里有一点将AVStream和AVCodec并列的思想，因为AVStream之中只包含了AVCodecParameters这个结构体，即只能访问部分codec的变量
下面先看看AVInputFormat和AVOutputFormat的内容（这两个定义比较简单），后续再看AVIOContext
1.2 const struct AVInputFormat *iformat
该结构体描述了输入音视频的格式，定义位于libavformat\avformat.h中
c
/**
* @addtogroup lavf_decoding
* @{
*/
typedef struct AVInputFormat {
/**
* A comma separated list of short names for the format. New names
* may be appended with a minor bump.
*/
// format的名称
const char *name;
/**
* Descriptive name for the format, meant to be more human-readable
* than name. You should use the NULL_IF_CONFIG_SMALL() macro
* to define it.
*/
// format的全名
const char *long_name;
/**
* Can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER, AVFMT_SHOW_IDS,
* AVFMT_NOTIMESTAMPS, AVFMT_GENERIC_INDEX, AVFMT_TS_DISCONT, AVFMT_NOBINSEARCH,
* AVFMT_NOGENSEARCH, AVFMT_NO_BYTE_SEEK, AVFMT_SEEK_TO_PTS.
*/
// 定义了format的行为
int flags;
/**
* If extensions are defined, then no probe is done. You should
* usually not use extension format guessing because it is not
* reliable enough
*/
// 如果定义了扩展，则不执行探测。通常不应该使用扩展格式猜测，因为它不够可靠
const char *extensions;
// AVCodecTag标识了媒体文件中的编解码格式
const struct AVCodecTag * const *codec_tag;
const AVClass *priv_class; ///< AVClass for the private context
/**
* Comma-separated list of mime types.
* It is used check for matching mime types while probing.
* @see av_probe_input_format2
*/
// mime(Multipurpose Internet Mail Extensions)的类型
const char *mime_type;
} AVInputFormat;其中AVCodecTag标识了媒体文件中的编解码格式，定义如下
c
typedef struct AVCodecTag {
enum AVCodecID id;
unsigned int tag;
} AVCodecTag;1.3 const struct AVOutputFormat *oformat
AVOutputFormat之中记录了输出格式的类型，定义位于libavformat\avformat.h中
c
/**
* @addtogroup lavf_encoding
* @{
*/
typedef struct AVOutputFormat {
// 名称
const char *name;
/**
* Descriptive name for the format, meant to be more human-readable
* than name. You should use the NULL_IF_CONFIG_SMALL() macro
* to define it.
*/
// 全名
const char *long_name;
// mime的类型
const char *mime_type;
const char *extensions; /**< comma-separated filename extensions */
/* output support */
// 输出codec的ID
enum AVCodecID audio_codec; /**< default audio codec */
enum AVCodecID video_codec; /**< default video codec */
enum AVCodecID subtitle_codec; /**< default subtitle codec */
/**
* can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER,
* AVFMT_GLOBALHEADER, AVFMT_NOTIMESTAMPS, AVFMT_VARIABLE_FPS,
* AVFMT_NODIMENSIONS, AVFMT_NOSTREAMS,
* AVFMT_TS_NONSTRICT, AVFMT_TS_NEGATIVE
*/
// 输出格式的行为
int flags;
/**
* List of supported codec_id-codec_tag pairs, ordered by "better
* choice first". The arrays are all terminated by AV_CODEC_ID_NONE.
*/
// 支持的codec_id-codec_tag对列表，按"优先选择更好"排序。数组都以AV_CODEC_ID_NONE结束
const struct AVCodecTag * const *codec_tag;
const AVClass *priv_class; ///< AVClass for the private context
} AVOutputFormat;CSDN : https://blog.csdn.net/weixin_42877471
Github : https://github.com/DoFulangChen