本文主要是介绍FFMPeg代码分析：AVFormatContext结构体，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
从先前的demo中可以看到，进入main函数所定义的第一个变量就是AVFormatContext的指针：
int main(int argc, char *argv[])
{AVFormatContext *pFormatCtx = NULL; ....
}typedef struct AVFormatContext
{ const AVClass *av_class;struct AVInputFormat *iformat;struct AVOutputFormat *oformat; void *priv_data;AVIOContext *pb;int ctx_flags;unsigned int nb_streams;AVStream **streams;char filename[1024];int64_t start_time;int64_t duration;int bit_rate;unsigned int packet_size;int max_delay;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.
#define AVFMT_FLAG_MP4A_LATM 0x8000 ///< Enable RTP MP4A-LATM payload
#define AVFMT_FLAG_SORT_DTS 0x10000 ///< try to interleave outputted packets by dts (using this flag can slow demuxing down)
#define AVFMT_FLAG_PRIV_OPT 0x20000 ///< Enable use of private options by delaying codec open (this could be made default once all code is converted)
#define AVFMT_FLAG_KEEP_SIDE_DATA 0x40000 ///< Don't merge side data but keep it separate.unsigned int probesize;int 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;AVDictionary *metadata;int64_t start_time_realtime;int fps_probe_size;int error_recognition;AVIOInterruptCB interrupt_callback;int debug;
#define FF_FDEBUG_TS 0x0001int ts_id;int audio_preload;int max_chunk_duration;int max_chunk_size;int use_wallclock_as_timestamps;int avoid_negative_ts;int avio_flags;enum AVDurationEstimationMethod duration_estimation_method;unsigned int skip_initial_bytes;unsigned int correct_ts_overflow;int seek2any;int flush_packets;int probe_score;
} AVFormatContext;const AVClass *av_class：用于记录日志和设置AVOption的类，由avformat_alloc_context()设置。AVClass类中保存了包含该成员的类名、对象名和AVOption实例等信息；
AVInputFormat *iformat / AVOutputFormat *oformat：输入/输出格式，或为输入或为输出，这两个不能同时存在；分别用于编码和解码。AVOutputFormat类保存了文件名、扩展名、默认音视频和字幕解码器等信息；
void *priv_data：私有数据，只有iformat/oformat.priv_class非0时有效；
AVIOContext *pb：指向IO数据的缓存的指针；
int ctx_flags：媒体流信息；
unsigned int nb_streams：包含的媒体流的数量；
AVStream **streams：指向实际输入/输出数据的指针；
char filename[1024]：输入或输出的文件名；
int64_t start_time / int64_t duration：媒体的开始时间和持续时长，一般由AVStream **streams导出；
int bit_rate：视频码率，由ffmpeg自动计算；
unsigned int packet_size：包尺寸；
int max_delay：最大延迟；
unsigned int probesize：在解码时用于探测的数据的大小，编码时不使用；
int max_analyze_duration：允许输入数据在avformat_find_stream_info()中分析的最长时间；
const uint8_t *key：关键字；
int keylen：关键字长；
unsigned int nb_programs：包含节目的数量；
AVProgram **programs：指向实际节目的指针；
enum AVCodecID video_codec_id：指定的视频解码器ID；由用户指定；
enum AVCodecID audio_codec_id：指定的音频解码器ID；由用户指定；
enum AVCodecID subtitle_codec_id：指定的字幕解码器ID；由用户指定；
unsigned int max_index_size：用于检索码流的索引值的最大字长；
unsigned int max_picture_buffer：图像缓存的最大尺寸；
unsigned int nb_chapters：章节数目；
AVChapter **chapters：指向章节数据的指针；
AVDictionary *metadata：元数据；
int64_t start_time_realtime;：码流开始的实际时间，以毫秒为单位；
int fps_probe_size：表明使用了多少帧用于格式检测；
int error_recognition：错误检测的阈值；
AVIOInterruptCB interrupt_callback：在IO层用户自定义的回调结构；
int ts_id：传输流的ID；
unsigned int skip_initial_bytes：打开媒体流时跳过的字节数；
int seek2any：是否可以移动到任意一帧的位置；
int flush_packets：每处理一个packet之后刷新io上下文；
int probe_score：格式探测的分值，上限为AVPROBE_SCORE_MAX；
指向元数据的结构体AVDictionary由两部分组成：统计计数和指向AVDictionaryEntry的指针，也就是说AVDictionary可以说是AVDictionaryEntry的封装。
struct AVDictionary
{int count;AVDictionaryEntry *elems;
};
typedef struct AVDictionaryEntry{char *key;char *value;
} AVDictionaryEntry;了解了AVFormatContext的构造之后，可以再来看看它是如何被初始化的了。在函数avformat_open_input中，AVFormatContext的初始化由avformat_alloc_context()及其调用的函数实现：
AVFormatContext *avformat_alloc_context(void)
{AVFormatContext *ic;ic = av_malloc(sizeof(AVFormatContext));if (!ic) return ic;avformat_get_context_defaults(ic);return ic;
}
static void avformat_get_context_defaults(AVFormatContext *s)
{memset(s, 0, sizeof(AVFormatContext));s->av_class = &av_format_context_class;av_opt_set_defaults(s);
}
void av_opt_set_defaults(void *s)
{
#if FF_API_OLD_AVOPTIONSav_opt_set_defaults2(s, 0, 0);
}
void av_opt_set_defaults2(void *s, int mask, int flags)
{
#endifconst AVClass *class = *(AVClass **)s;const AVOption *opt = NULL;while ((opt = av_opt_next(s, opt)) != NULL) {
#if FF_API_OLD_AVOPTIONSif ((opt->flags & mask) != flags)continue;
#endifswitch (opt->type) {case AV_OPT_TYPE_CONST:/* Nothing to be done here */break;case AV_OPT_TYPE_FLAGS:case AV_OPT_TYPE_INT:case AV_OPT_TYPE_INT64:case AV_OPT_TYPE_DURATION:case AV_OPT_TYPE_CHANNEL_LAYOUT:av_opt_set_int(s, opt->name, opt->default_val.i64, 0);break;case AV_OPT_TYPE_DOUBLE:case AV_OPT_TYPE_FLOAT: {double val;val = opt->default_val.dbl;av_opt_set_double(s, opt->name, val, 0);}break;case AV_OPT_TYPE_RATIONAL: {AVRational val;val = av_d2q(opt->default_val.dbl, INT_MAX);av_opt_set_q(s, opt->name, val, 0);}break;case AV_OPT_TYPE_COLOR:case AV_OPT_TYPE_STRING:case AV_OPT_TYPE_IMAGE_SIZE:case AV_OPT_TYPE_VIDEO_RATE:av_opt_set(s, opt->name, opt->default_val.str, 0);break;case AV_OPT_TYPE_PIXEL_FMT:
#if LIBAVUTIL_VERSION_MAJOR < 53if (class->version && class->version < AV_VERSION_INT(52, 10, 100))av_opt_set(s, opt->name, opt->default_val.str, 0);else
#endifav_opt_set_pixel_fmt(s, opt->name, opt->default_val.i64, 0);break;case AV_OPT_TYPE_SAMPLE_FMT:
#if LIBAVUTIL_VERSION_MAJOR < 53if (class->version && class->version < AV_VERSION_INT(52, 10, 100))av_opt_set(s, opt->name, opt->default_val.str, 0);else
#endifav_opt_set_sample_fmt(s, opt->name, opt->default_val.i64, 0);break;case AV_OPT_TYPE_BINARY:/* Cannot set default for binary */break;default:av_log(s, AV_LOG_DEBUG, "AVOption type %d of option %s not implemented yet\n", opt->type, opt->name);}}
}可以看到，在av_opt_set_defaults2()函数中，首先会调用av_opt_next(s, opt)，并根据opt->type的不同值来进行下一步的设置。av_opt_next()的代码：
const AVOption *av_opt_next(void *obj, const AVOption *last)
{AVClass *class = *(AVClass**)obj;if (!last && class && class->option && class->option[0].name)return class->option;if (last && last[1].name)return ++last;return NULL;
}int av_opt_set_int(void *obj, const char *name, int64_t val, int search_flags)
{return set_number(obj, name, 1, 1, val, search_flags);
}
static int set_number(void *obj, const char *name, double num, int den, int64_t intnum, int search_flags)
{void *dst, *target_obj;const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);if (!o || !target_obj)return AVERROR_OPTION_NOT_FOUND;dst = ((uint8_t*)target_obj) + o->offset;return write_number(obj, o, dst, num, den, intnum);
}
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