将音频流解码为pcm（with avformat）

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#include "stdafx.h"  

extern "C"
{
#include "libavformat\avformat.h"
};

int main(int argc, char* argv[])
{
	AVFormatContext *fmt_ctx;
	AVStream *stream;
	AVCodecContext *codec_ctx;
	AVCodec *codec;
	AVPacket pkt;
	AVFrame *frame = NULL;
	int ret, stream_index, got_frame;
	FILE *infile = NULL, *outfile = NULL;
	
	const char *infilename = "上和下.aac";
	const char *outfilename = "aac2pcm.pcm";

	av_register_all();
	avformat_network_init();

	fmt_ctx = avformat_alloc_context();
	if ((ret = avformat_open_input(&fmt_ctx, infilename, NULL, NULL)) < 0){
		printf("Couldn't open input stream.\n");
		return ret;
	}
	if ((ret = avformat_find_stream_info(fmt_ctx, NULL)) < 0){
		printf("Couldn't find stream information.\n");
		return ret;
	}
	ret = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);
	if (ret < 0) {
		fprintf(stderr, "Could not find %s stream in input file '%s'\n",
			av_get_media_type_string(AVMEDIA_TYPE_AUDIO), infilename);
		return ret;
	}
	else {
		stream_index = ret;
		stream = fmt_ctx->streams[stream_index];

		/* find decoder for the stream */
		codec_ctx = stream->codec;
		codec = avcodec_find_decoder(codec_ctx->codec_id);
		if (!codec) {
			fprintf(stderr, "Failed to find %s codec\n",
				av_get_media_type_string(AVMEDIA_TYPE_AUDIO));
			goto end;
			return AVERROR(EINVAL);
		}

		if ((ret = avcodec_open2(codec_ctx, codec, NULL)) < 0) {
			fprintf(stderr, "Failed to open %s codec\n",
				av_get_media_type_string(AVMEDIA_TYPE_AUDIO));
			goto end;
			return ret;
		}
	}
	
	printf("Decode audio file %s to %s\n", infilename, outfilename);
    infile = fopen(infilename, "rb");
	if (!infile) {
		fprintf(stderr, "Could not open %s\n", infilename);
		goto end;
		exit(1);
	}
	outfile = fopen(outfilename, "wb");
	if (!outfile) {
		goto end;
		exit(1);
	}
	
	av_init_packet(&pkt);
	while (av_read_frame(fmt_ctx, &pkt) >= 0){

		if (pkt.stream_index == stream_index){
			if (!frame) {
				if (!(frame = av_frame_alloc())) {
					fprintf(stderr, "Could not allocate audio frame\n");
					goto end;
					exit(1);
				}
			}

			ret = avcodec_decode_audio4(codec_ctx, frame, &got_frame, &pkt);
			if (ret < 0) {
				printf("Error in decoding audio frame.\n");
				goto end;
				return -1;
			}
			if (got_frame > 0){
				/* if a frame has been decoded, output it */
				int data_size = av_get_bytes_per_sample(codec_ctx->sample_fmt);
				if (data_size < 0) {
					/* This should not occur, checking just for paranoia */
					fprintf(stderr, "Failed to calculate data size\n");
					goto end;
					exit(1);
				}
				for (int i = 0; i < frame->nb_samples; i++)
					for (int ch = 0; ch < codec_ctx->channels; ch++)
						fwrite(frame->data[ch] + data_size*i, 1, data_size, outfile);
			}
		}
		av_free_packet(&pkt);
	}

end:
	if (outfile)
		fclose(outfile);
	if (infile)
		fclose(infile);
	avcodec_close(codec_ctx);
	avformat_close_input(&fmt_ctx);
	if (frame)
		av_frame_free(&frame);
	
	return 0;
}