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int frame_size_out_encode = out_fmt_ctx->streams[out_stream->index]->codec->frame_size;			if(!frame_size_out_encode)				frame_size_out_encode = put_into_fifo->nb_samples;

确定采样长度，先取得编码器frame_size，如果该值是0，比如pcm，则用输入Frame的nb_samples 作为采样长度。

AVCodecContext *acout = out_fmt_ctx->streams[0]->codec;	                        AVFrame *frame_fifo;  			frame_fifo = av_frame_alloc();			frame_fifo->nb_samples     = frame_size_out_encode;			frame_fifo->channel_layout = acout->channel_layout;  			frame_fifo->channels = av_get_channel_layout_nb_channels(frame_fifo->channel_layout);			frame_fifo->format         = acout->sample_fmt;  			frame_fifo->sample_rate    = acout->sample_rate;  			av_frame_get_buffer(frame_fifo, 0);			av_samples_set_silence(frame_fifo->data, 0, frame_fifo->nb_samples, frame_fifo->channels, (AVSampleFormat)frame_fifo->format);

按channels，format等参数申请缓存

av_init_packet(&pkt_out);					int ret = avcodec_encode_audio2(out_fmt_ctx->streams[0]->codec, &pkt_out, frame_fifo, &got_picture);					if (got_picture ) 					{						pkt_out.stream_index = out_stream->index;						pkt_out.dts = av_rescale_q_rnd(pkt_out.dts,							out_fmt_ctx->streams[out_stream->index]->codec->time_base,							out_fmt_ctx->streams[out_stream->index]->time_base,							(AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));						pkt_out.pts = av_rescale_q_rnd(pkt_out.pts,							out_fmt_ctx->streams[out_stream->index]->codec->time_base,							out_fmt_ctx->streams[out_stream->index]->time_base,							(AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));						pkt_out.duration = av_rescale_q(pkt_out.duration,							out_fmt_ctx->streams[out_stream->index]->codec->time_base,							out_fmt_ctx->streams[out_stream->index]->time_base);						 						av_log(NULL, AV_LOG_DEBUG, "Muxing frame\n");						/* mux encoded frame */						ret = av_interleaved_write_frame(out_fmt_ctx, &pkt_out);						av_free_packet(&pkt_out);						if (ret < 0)						{							printf("write a null frame failed!\n");							break;						}						printf("success write a null frame:index %d\n", frame_index++);						duration_sum += frame_size_out_encode;						if(duration_sum * av_q2d(out_fmt_ctx->streams[out_stream->index]->codec->time_base) >= g_offset)							break;					}

编码，写入AVPacket，duration_sum记录总时长的信息，g_offset是静音时常

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