av_write_trailer()
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首先编码的大致流程如下:
(1)avformat_alloc_output_context2()
(2)avfomat_write_header()
(3)avcodec_send_frame()/avcodec_receive_packet()
(4)av_write_frame()
(5)av_write_trailer()
av_write_trailer()用于输出文件尾,它的声明位于libavformat\avformat.h,如下所示。
/** * Write the stream trailer to an output media file and free the * file private data. * * May only be called after a successful call to avformat_write_header. * * @param s media file handle * @return 0 if OK, AVERROR_xxx on error */ int av_write_trailer(AVFormatContext *s);
它只需要指定一个参数,即用于输出的AVFormatContext。
函数正常执行后返回值等于0。
av_write_trailer()的调用关系如下图所示。
av_write_trailer()的定义位于libavformat\mux.c,如下所示。
int av_write_trailer(AVFormatContext *s) { int ret, i; for (;; ) { AVPacket pkt; ret = interleave_packet(s, &pkt, NULL, 1); if (ret < 0) goto fail; if (!ret) break; //写入AVPacket ret = write_packet(s, &pkt); if (ret >= 0) s->streams[pkt.stream_index]->nb_frames++; av_free_packet(&pkt); if (ret < 0) goto fail; if(s->pb && s->pb->error) goto fail; } fail: //写文件尾 if (s->oformat->write_trailer) if (ret >= 0) { ret = s->oformat->write_trailer(s); } else { s->oformat->write_trailer(s); } if (s->pb) avio_flush(s->pb); if (ret == 0) ret = s->pb ? s->pb->error : 0; for (i = 0; i < s->nb_streams; i++) { av_freep(&s->streams[i]->priv_data); av_freep(&s->streams[i]->index_entries); } if (s->oformat->priv_class) av_opt_free(s->priv_data); av_freep(&s->priv_data); return ret; }
从源代码可以看出av_write_trailer()主要完成了以下两步工作:
(1)循环调用interleave_packet()以及write_packet(),将还未输出的AVPacket输出出来。
(2)调用AVOutputFormat的write_trailer(),输出文件尾。
其中第一步和av_write_frame()中的步骤大致是一样的(interleave_packet()这一部分在并不包含在av_write_frame()中,而是包含在av_interleaved_write_frame()中,这一部分源代码还没有分析)。下面分析一下第二步
AVOutputFormat的write_trailer()是一个函数指针,指向特定的AVOutputFormat中的实现函数。我们以FLV对应的AVOutputFormat为例,看一下它的定义,如下所示。
AVOutputFormat ff_flv_muxer = { .name = "flv", .long_name = NULL_IF_CONFIG_SMALL("FLV (Flash Video)"), .mime_type = "video/x-flv", .extensions = "flv", .priv_data_size = sizeof(FLVContext), .audio_codec = CONFIG_LIBMP3LAME ? AV_CODEC_ID_MP3 : AV_CODEC_ID_ADPCM_SWF, .video_codec = AV_CODEC_ID_FLV1, .write_header = flv_write_header, .write_packet = flv_write_packet, .write_trailer = flv_write_trailer, .codec_tag = (const AVCodecTag* const []) { flv_video_codec_ids, flv_audio_codec_ids, 0 }, .flags = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS | AVFMT_TS_NONSTRICT, };
write_trailer = flv_write_trailer,write_trailer()最终会调用flv_write_trailer()函数。
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