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AVFormatContext对开发者开放，是连接开发者与FFmpeg内部的桥梁。结构体内部包含有AVInputFormat、AVOutputFormat、AVCodec、AVStream、AVDictionary 、AVClass等。支持设置自定义IO、监听网络中断状态、设置options、直播秒开调优等。
AVFormatContext结构体的源码位于libavformat/avformat.h，具体代码如下：
typedef struct AVFormatContext {// 用于打印日志和设置选项的类const AVClass *av_class;// 输入容器格式ff_const59 struct AVInputFormat *iformat;// 输出容器格式ff_const59 struct AVOutputFormat *oformat;// 私有数据void *priv_data;/*** I/O 上下文** - demuxing: avformat_open_input()之前设置* - muxing: avformat_write_header()之前设置*/AVIOContext *pb;// 关于stream属性的Flagsint ctx_flags;// 码流的数量unsigned int nb_streams;// 码流数组AVStream **streams;// 输入或输出的URLchar *url;// 第一帧的开始时间int64_t start_time;// 码流的时长int64_t duration;// 码流的码率，单位为bit/sint64_t bit_rate;unsigned int packet_size;int max_delay;int flags;
#define AVFMT_FLAG_GENPTS 0x0001 ///< Generate missing pts
#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
#define AVFMT_FLAG_NOFILLIN 0x0010 ///< Do not infer any values from other values
#define AVFMT_FLAG_NOPARSE 0x0020 ///< Do not use AVParsers
#define AVFMT_FLAG_NOBUFFER 0x0040 ///< Do not buffer frames when possible
#define AVFMT_FLAG_CUSTOM_IO 0x0080 ///< The caller has supplied a custom AVIOContext
#define AVFMT_FLAG_DISCARD_CORRUPT 0x0100 ///< Discard frames marked corrupted
#define AVFMT_FLAG_FLUSH_PACKETS 0x0200 ///< Flush the AVIOContext every packet.
#define AVFMT_FLAG_BITEXACT 0x0400
#if FF_API_LAVF_MP4A_LATM
#define AVFMT_FLAG_MP4A_LATM 0x8000 ///< Deprecated, does nothing.
#endif
#define AVFMT_FLAG_SORT_DTS 0x10000 ///< try to interleave outputted packets by dts
#if FF_API_LAVF_PRIV_OPT
#define AVFMT_FLAG_PRIV_OPT 0x20000 ///< Enable use of private options by delaying codec open (deprecated, will do nothing once av_demuxer_open() is removed)
#endif
#if FF_API_LAVF_KEEPSIDE_FLAG
#define AVFMT_FLAG_KEEP_SIDE_DATA 0x40000 ///< Deprecated, does nothing.
#endif
#define AVFMT_FLAG_FAST_SEEK 0x80000 ///< Enable fast, but inaccurate seeks for some formats
#define AVFMT_FLAG_SHORTEST 0x100000 ///< Stop muxing when the shortest stream stops.
#define AVFMT_FLAG_AUTO_BSF 0x200000 ///< Add bitstream filters as requested by the muxer// 从输入流探测数据包的大小int64_t probesize;// 最大的分析时长，单位为AV_TIME_BASEint64_t max_analyze_duration;const uint8_t *key;int keylen;unsigned int nb_programs;AVProgram **programs;enum AVCodecID video_codec_id;enum AVCodecID audio_codec_id;enum AVCodecID subtitle_codec_id;unsigned int max_index_size;unsigned int max_picture_buffer;unsigned int nb_chapters;AVChapter **chapters;// 文件元数据媒体信息AVDictionary *metadata;int64_t start_time_realtime;int fps_probe_size;int error_recognition;// 自定义的I/O层中断回调AVIOInterruptCB interrupt_callback;int debug;int64_t max_interleave_delta;int strict_std_compliance;int event_flags;int max_ts_probe;// 避免负数的时间戳int avoid_negative_ts;
#define AVFMT_AVOID_NEG_TS_AUTO -1 ///< Enabled when required by target format
#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 0int ts_id;int audio_preload;int max_chunk_duration;int max_chunk_size;int use_wallclock_as_timestamps;int avio_flags;enum AVDurationEstimationMethod duration_estimation_method;int64_t skip_initial_bytes;unsigned int correct_ts_overflow;// 强制seek到任意帧，包括非关键帧位置int seek2any;int flush_packets;// 格式探测分数int probe_score;int format_probesize;// codec的白名单char *codec_whitelist;// format的白名单char *format_whitelist;AVFormatInternal *internal;int io_repositioned;// 视频codecAVCodec *video_codec;// 音频codecAVCodec *audio_codec;// 字幕codecAVCodec *subtitle_codec;// 数据codecAVCodec *data_codec;int metadata_header_padding;// 用户数据void *opaque;av_format_control_message control_message_cb;int64_t output_ts_offset;uint8_t *dump_separator;enum AVCodecID data_codec_id;// 协议白名单char *protocol_whitelist;// 打开新IO流的回调int (*io_open)(struct AVFormatContext *s, AVIOContext **pb, const char *url,int flags, AVDictionary **options);// 关闭IO流的回调void (*io_close)(struct AVFormatContext *s, AVIOContext *pb);// 协议黑名单char *protocol_blacklist;int max_streams;int skip_estimate_duration_from_pts;int max_probe_packets;
} AVFormatContext;打开输入流的流程比较简单，先分配AVFormatContext，然后再打开输入流：
AVFormatContext *format_ctx = avformat_alloc_context();
int ret = avformat_open_input(&format_ctx, url, NULL, NULL);如果要设置avoptions，使用av_dict_set()来设置参数键值对：
AVDictionary *options = NULL;
av_dict_set(&options, "video_size", "640x480", 0);
av_dict_set(&options, "pixel_format", "rgb24", 0);
AVFormatContext *format_ctx = avformat_alloc_context();
int ret = avformat_open_input(&format_ctx, url, NULL, &options);如果要监听网络中断状态，使用AVIOInterruptCB进行监听：
static int custom_interrupt_callback(void *arg) {......
}AVFormatContext *format_ctx = avformat_alloc_context();
format_ctx->interrupt_callback.callback = custom_interrupt_callback;
format_ctx->interrupt_callback.opaque= NULL;
int ret = avformat_open_input(&format_ctx, url, NULL, NULL);其中AVIOInterruptCB的结构体定义如下，包含callback函数指针和opaque：
typedef struct AVIOInterruptCB {int (*callback)(void*);void *opaque;
} AVIOInterruptCB;在使用FFmpeg拉流直播时，做直播秒开调优，可以调节probesize和max_analyze_duration。其中probesize表示探测数据包的大小，max_analyze_duration表示最大分析时长，比如这样：
probesize = 5 * 1024 * 1024;
max_analyze_duration = 500;如果要自定义IO读取缓冲区数据，使用AVIOContext，分配缓冲区，实现read_packet、write_packet、seek方法：
#define IO_BUFFER_SIZE (4 * 1024 * 1024)uint8_t *io_buffer = av_malloc(IO_BUFFER_SIZE);int read_packet(void *opaque, uint8_t *buf, int buf_size) {......
}int64_t seek(void *opaque, int64_t offset, int whence) {......
}AVIOContext *avio_ctx = avio_alloc_context(io_buffer, IO_BUFFER_SIZE, 0, &user_data, &read_packet, NULL, &seek);
AVFormatContext *format_ctx = avformat_alloc_context();
format_ctx .pb = avio_ctx;
format_ctx->flags |= AVFMT_FLAG_CUSTOM_IO;这篇关于FFmpeg结构体分析：AVFormatContext连接FFmpeg的桥梁的文章就介绍到这儿，希望我们推荐的文章对编程师们有所帮助！
