av_dump_format经验分析,FFmpeg获取媒体文件总时长(FLV获取总时长的误区)

播放器有个功能，当用户打开视频时，需要读取媒体文件的总时长等信息，不巧的时，获取FLV时总失败，下面来具体分析下FLV和MP4获取总时长的原因和区别：
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播放器有个获取MediaInfo的接口，功能如下：

int MediaFFmpeg::DecoderGetMediaInfo(MediaInfo *mi,AVCodecContext *decodec_ctx,AVStream *stream){if(!mi || !stream || !decodec_ctx){return -1;}//videoif(stream->codecpar->codec_type == AVMEDIA_TYPE_VIDEO){//获取视频总时长if(AV_NOPTS_VALUE != stream->duration){mi->duration =  stream->duration * av_q2d(stream->time_base);std::cout << "video_time : " <<(mi->duration / 3600)<< ":" <<(mi->duration % 3600) / 60<< ":" <<(mi->duration % 60) << std::endl;char formatStr[128] = {0,};sprintf(formatStr, "%02d:%02d:%02d",(mi->duration / 3600),((mi->duration % 3600) / 60),(mi->duration % 60));mi->durationFormatStr = formatStr;}else{printf("audio duration unknown ! \n");}mi->width = stream->codecpar->width;mi->height = stream->codecpar->height;}//audioelse if(stream->codecpar->codec_type == AVMEDIA_TYPE_AUDIO){mi->sample_fmt                  =  AV_SAMPLE_FMT_S16;mi->sample_rate                 =   decodec_ctx->sample_rate;  //采样率/*48000; */mi->channels                    =   decodec_ctx->channels;     //通道数/*1;  */mi->nb_samples                  =   decodec_ctx->frame_size;/*1024;  */mi->audio_buffer_size           =   av_samples_get_buffer_size(NULL, mi->channels, mi->nb_samples, (enum AVSampleFormat)mi->sample_fmt, 1);   //输出buff}return 0;
}

有经验的人可能很快就能看出来是否存在问题。

总是打印duration不合法：
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很奇怪的是，使用av_dump_format函数可以看到Duration:

Input #0, flv, from '/home/zhenghui/视频/1080P.flv':Metadata:major_brand     : isomminor_version   : 512compatible_brands: isomiso2avc1mp41description     : Packed by Bilibili XCoder v2.0.2encoder         : Lavf60.3.100Duration: 00:03:46.53, start: 0.000000, bitrate: 3309 kb/sStream #0:0: Video: flv1, yuv420p, 1920x1080, 200 kb/s, 30 fps, 30 tbr, 1k tbnStream #0:1: Audio: adpcm_swf, 44100 Hz, stereo, s16, 352 kb/s

就翻了翻ffmpeg的源码，找到了av_dump_format的源码：

void av_dump_format(AVFormatContext *ic, int index,const char *url, int is_output)
{int i;uint8_t *printed = ic->nb_streams ? av_mallocz(ic->nb_streams) : NULL;if (ic->nb_streams && !printed)return;av_log(NULL, AV_LOG_INFO, "%s #%d, %s, %s '%s':\n",is_output ? "Output" : "Input",index,is_output ? ic->oformat->name : ic->iformat->name,is_output ? "to" : "from", url);dump_metadata(NULL, ic->metadata, "  ");if (!is_output) {av_log(NULL, AV_LOG_INFO, "  Duration: ");if (ic->duration != AV_NOPTS_VALUE) {int64_t hours, mins, secs, us;int64_t duration = ic->duration + (ic->duration <= INT64_MAX - 5000 ? 5000 : 0);secs  = duration / AV_TIME_BASE;us    = duration % AV_TIME_BASE;mins  = secs / 60;secs %= 60;hours = mins / 60;mins %= 60;av_log(NULL, AV_LOG_INFO, "%02"PRId64":%02"PRId64":%02"PRId64".%02"PRId64"", hours, mins, secs,(100 * us) / AV_TIME_BASE);} else {av_log(NULL, AV_LOG_INFO, "N/A");}if (ic->start_time != AV_NOPTS_VALUE) {int secs, us;av_log(NULL, AV_LOG_INFO, ", start: ");secs = llabs(ic->start_time / AV_TIME_BASE);us   = llabs(ic->start_time % AV_TIME_BASE);av_log(NULL, AV_LOG_INFO, "%s%d.%06d",ic->start_time >= 0 ? "" : "-",secs,(int) av_rescale(us, 1000000, AV_TIME_BASE));}av_log(NULL, AV_LOG_INFO, ", bitrate: ");if (ic->bit_rate)av_log(NULL, AV_LOG_INFO, "%"PRId64" kb/s", ic->bit_rate / 1000);elseav_log(NULL, AV_LOG_INFO, "N/A");av_log(NULL, AV_LOG_INFO, "\n");}if (ic->nb_chapters)av_log(NULL, AV_LOG_INFO, "  Chapters:\n");for (i = 0; i < ic->nb_chapters; i++) {const AVChapter *ch = ic->chapters[i];av_log(NULL, AV_LOG_INFO, "    Chapter #%d:%d: ", index, i);av_log(NULL, AV_LOG_INFO,"start %f, ", ch->start * av_q2d(ch->time_base));av_log(NULL, AV_LOG_INFO,"end %f\n", ch->end * av_q2d(ch->time_base));dump_metadata(NULL, ch->metadata, "      ");}if (ic->nb_programs) {int j, k, total = 0;for (j = 0; j < ic->nb_programs; j++) {const AVProgram *program = ic->programs[j];const AVDictionaryEntry *name = av_dict_get(program->metadata,"name", NULL, 0);av_log(NULL, AV_LOG_INFO, "  Program %d %s\n", program->id,name ? name->value : "");dump_metadata(NULL, program->metadata, "    ");for (k = 0; k < program->nb_stream_indexes; k++) {dump_stream_format(ic, program->stream_index[k],index, is_output);printed[program->stream_index[k]] = 1;}total += program->nb_stream_indexes;}if (total < ic->nb_streams)av_log(NULL, AV_LOG_INFO, "  No Program\n");}for (i = 0; i < ic->nb_streams; i++)if (!printed[i])dump_stream_format(ic, i, index, is_output);av_free(printed);
}

av_dump_format函数中使用的是AVFormatContext中的duration，而我使用的是AVStream的duration。
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Debug了一下：AVFormatContext中的duration确实存在：
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继续跟踪到AVStream的调用位置，确实不存在：
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最终修改如下得已解决：

int MediaFFmpeg::DecoderGetMediaInfo(MediaInfo *mi,AVFormatContext *ic,AVCodecContext *decodec_ctx,AVStream *stream){if(!mi || !stream || !decodec_ctx){return -1;}//videoif(stream->codecpar->codec_type == AVMEDIA_TYPE_VIDEO){//获取视频总时长if(AV_NOPTS_VALUE != ic->duration){int64_t hours, mins, secs, us;int64_t duration = ic->duration + (ic->duration <= INT64_MAX - 5000 ? 5000 : 0);secs  = duration / AV_TIME_BASE;us    = duration % AV_TIME_BASE;mins  = secs / 60;secs %= 60;hours = mins / 60;mins %= 60;mi->duration = duration;char formatStr[128] = {0,};sprintf(formatStr, "%02ld:%02ld:%02ld",hours,mins,secs);
//            mi->duration =  ic->duration * av_q2d(stream->time_base);
//            std::cout << "video_time : " <<
//                    (mi->duration / 3600)<< ":" <<
//                    (mi->duration % 3600) / 60<< ":" <<
//                    (mi->duration % 60) << std::endl;//            char formatStr[128] = {0,};
//                    sprintf(formatStr, "%02d:%02d:%02d",
//                    (mi->duration / 3600),
//                    ((mi->duration % 3600) / 60),
//                    (mi->duration % 60));std::cout << "video_time : " << formatStr;mi->durationFormatStr = formatStr;}else{printf("audio duration unknown ! \n");}mi->width = stream->codecpar->width;mi->height = stream->codecpar->height;}.........return 0;
}