[ffmpeg]deocde audio(v3.3.2)

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/*
 * Copyright (c) 2001 Fabrice Bellard
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

/**
 * @file
 * audio decoding with libavcodec API example
 *
 * @example decode_audio.c
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <libavutil/frame.h>
#include <libavutil/mem.h>

#include <libavcodec/avcodec.h>

#define AUDIO_INBUF_SIZE 20480
#define AUDIO_REFILL_THRESH 4096

int main(int argc, char **argv)
{
  //定义输入输出文件名字
const char *outfilename, *filename;
  //定义编码器和编码器上下文
const AVCodec *codec; AVCodecContext *c= NULL; int len; FILE *f, *outfile; uint8_t inbuf[AUDIO_INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE]; AVPacket avpkt; AVFrame *decoded_frame = NULL; if (argc <= 2) { fprintf(stderr, "Usage: %s <input file> <output file>\n", argv[0]); exit(0); } filename = argv[1]; outfilename = argv[2]; /* register all the codecs */ avcodec_register_all(); av_init_packet(&avpkt); /* find the MPEG audio decoder */ codec = avcodec_find_decoder(AV_CODEC_ID_MP2); if (!codec) { fprintf(stderr, "Codec not found\n"); exit(1); } c = avcodec_alloc_context3(codec); if (!c) { fprintf(stderr, "Could not allocate audio codec context\n"); exit(1); } /* open it */ if (avcodec_open2(c, codec, NULL) < 0) { fprintf(stderr, "Could not open codec\n"); exit(1); } f = fopen(filename, "rb"); if (!f) { fprintf(stderr, "Could not open %s\n", filename); exit(1); } outfile = fopen(outfilename, "wb"); if (!outfile) { av_free(c); exit(1); } /* decode until eof */ avpkt.data = inbuf; avpkt.size = fread(inbuf, 1, AUDIO_INBUF_SIZE, f);
  //解码直到所有的都完成
while (avpkt.size > 0) { int i, ch; int got_frame = 0;
     //如果未定义解码帧,则申请一个用来存储解码后的数据
if (!decoded_frame) { if (!(decoded_frame = av_frame_alloc())) { fprintf(stderr, "Could not allocate audio frame\n"); exit(1); } }      //解码失败,或者解码接受,解码到了文件EOF位置 len = avcodec_decode_audio4(c, decoded_frame, &got_frame, &avpkt); if (len < 0) { fprintf(stderr, "Error while decoding\n"); exit(1); } if (got_frame) { /* if a frame has been decoded, output it */ int data_size = av_get_bytes_per_sample(c->sample_fmt); if (data_size < 0) { /* This should not occur, checking just for paranoia */ fprintf(stderr, "Failed to calculate data size\n"); exit(1); }
       //将解码后的数据写入到文件中,按通道写入,data_size是一个音频sample的大小
for (i=0; i<decoded_frame->nb_samples; i++) for (ch=0; ch<c->channels; ch++) fwrite(decoded_frame->data[ch] + data_size*i, 1, data_size, outfile); } avpkt.size -= len; avpkt.data += len; avpkt.dts = avpkt.pts = AV_NOPTS_VALUE; if (avpkt.size < AUDIO_REFILL_THRESH) { /* Refill the input buffer, to avoid trying to decode * incomplete frames. Instead of this, one could also use * a parser, or use a proper container format through * libavformat. */ //将一个packet后剩余的内容移动到缓冲区的头部
       memmove(inbuf, avpkt.data, avpkt.size); avpkt.data
= inbuf;
       //在移动后将文件中的内容继续拷贝到packet中以提供下次解码 len
= fread(avpkt.data + avpkt.size, 1, AUDIO_INBUF_SIZE - avpkt.size, f); if (len > 0) avpkt.size += len; } } fclose(outfile); fclose(f); avcodec_free_context(&c); av_frame_free(&decoded_frame); return 0; }

 

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