FFmpeg简单使用：解封装 ---- 提取aac

#include <stdio.h>
#include "libavutil/log.h"
#include "libavformat/avio.h"
#include "libavformat/avformat.h"

//#define AacHeader

const char* fileName = "believe.flv";
#ifdef AacHeader
const char* outFileName = "outBelieve.aac";
#else
const char* outFileName = "outBelieveNoHeader.aac";
#endif

const int sampling_frequencies[] = {
    96000,  // 0x0
    88200,  // 0x1
    64000,  // 0x2
    48000,  // 0x3
    44100,  // 0x4
    32000,  // 0x5
    24000,  // 0x6
    22050,  // 0x7
    16000,  // 0x8
    12000,  // 0x9
    11025,  // 0xa
    8000   // 0xb
    // 0xc d e f是保留的
};

int adts_header(char * const p_adts_header, const int data_length,
                const int profile, const int samplerate,
                const int channels)
{

    int sampling_frequency_index = 3; // 默认使用48000hz
    int adtsLen = data_length + 7;

    int frequencies_size = sizeof(sampling_frequencies) / sizeof(sampling_frequencies[0]);
    int i = 0;
    for(i = 0; i < frequencies_size; i++)
    {
        if(sampling_frequencies[i] == samplerate)
        {
            sampling_frequency_index = i;
            break;
        }
    }
    if(i >= frequencies_size)
    {
        printf("unsupport samplerate:%d\n", samplerate);
        return -1;
    }

    p_adts_header[0] = 0xff;         //syncword:0xfff                          高8bits
    p_adts_header[1] = 0xf0;         //syncword:0xfff                          低4bits
    p_adts_header[1] |= (0 << 3);    //MPEG Version:0 for MPEG-4,1 for MPEG-2  1bit
    p_adts_header[1] |= (0 << 1);    //Layer:0                                 2bits
    p_adts_header[1] |= 1;           //protection absent:1                     1bit

    p_adts_header[2] = (profile)<<6;            //profile:profile               2bits
    p_adts_header[2] |= (sampling_frequency_index & 0x0f)<<2; //sampling frequency index:sampling_frequency_index  4bits
    p_adts_header[2] |= (0 << 1);             //private bit:0                   1bit
    p_adts_header[2] |= (channels & 0x04)>>2; //channel configuration:channels  高1bit

    p_adts_header[3] = (channels & 0x03)<<6; //channel configuration:channels 低2bits
    p_adts_header[3] |= (0 << 5);               //original：0                1bit
    p_adts_header[3] |= (0 << 4);               //home：0                    1bit
    p_adts_header[3] |= (0 << 3);               //copyright id bit：0        1bit
    p_adts_header[3] |= (0 << 2);               //copyright id start：0      1bit
    p_adts_header[3] |= ((adtsLen & 0x1800) >> 11);           //frame length：value   高2bits

    p_adts_header[4] = (uint8_t)((adtsLen & 0x7f8) >> 3);     //frame length:value    中间8bits
    p_adts_header[5] = (uint8_t)((adtsLen & 0x7) << 5);       //frame length:value    低3bits
    p_adts_header[5] |= 0x1f;                                 //buffer fullness:0x7ff 高5bits
    p_adts_header[6] = 0xfc;      //‭11111100‬       //buffer fullness:0x7ff 低6bits
    // number_of_raw_data_blocks_in_frame：
    //    表示ADTS帧中有number_of_raw_data_blocks_in_frame + 1个AAC原始帧。
    return 0;
}

int main()
{
    printf("Hello World!\n");


    // 1.打开文件
    AVFormatContext *ctx = NULL;
    int ret = avformat_open_input(&ctx, fileName, NULL, NULL);
    if (ret < 0) {
        char buf[1024] = {0};
        av_strerror(ret, buf, sizeof (buf) - 1);
        printf("avformat_open_input %s failed: %s\n", fileName, buf);
        return -1;
    }

    // 2.获取码流信息
    ret = avformat_find_stream_info(ctx, NULL);
    if (ret < 0) {
        char buf[1024] = {0};
        av_strerror(ret, buf, sizeof (buf) - 1);
        printf("avformat_find_stream_info %s failed: %s\n", fileName, buf);
        return -1;
    }

    // 3.获取音频流
    int audioIndex = av_find_best_stream(ctx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);
    if (audioIndex < 0) {
        printf("av_find_best_stream failed: %s\n", av_get_media_type_string(AVMEDIA_TYPE_AUDIO));
        return -1;
    }

    // 4.检查是否是aac
    if (ctx->streams[audioIndex]->codecpar->codec_id != AV_CODEC_ID_AAC) {
        printf("audio codec %d is not AAC.\n", ctx->streams[audioIndex]->codecpar->codec_id);
        goto failed;
    }

    // 5.打开输出aac文件
     FILE *fd = fopen(outFileName, "wb");
     if (fd == NULL) {
         printf("fopen open %s failed.\n", outFileName);
         goto failed;
     }

    // 6.提取aac数据
     AVPacket pkt;
     av_init_packet(&pkt);
     int len = 0;
     while (av_read_frame(ctx, &pkt) >= 0) {
         if (pkt.stream_index == audioIndex) {
#ifdef AacHeader  // 写入aac头
            char adts_header_buf[7] = {0};
            adts_header(adts_header_buf, pkt.size,
                        ctx->streams[audioIndex]->codecpar->profile,
                        ctx->streams[audioIndex]->codecpar->sample_rate,
                        ctx->streams[audioIndex]->codecpar->channels);
            fwrite(adts_header_buf, 1, 7, fd);  // 写adts header , ts流不适用，ts流分离出来的packet带了adts header
#endif
            len = fwrite( pkt.data, 1, pkt.size, fd);   // 写adts data
            if(len != pkt.size)
            {
                printf("warning, length of writed data isn't equal pkt.size(%d, %d)\n", len, pkt.size);
            }
         }
         av_packet_unref(&pkt);
     }

failed:
    if (ctx) {
        avformat_close_input(&ctx);
    }
    if (fd) {
        fclose(fd);
    }

    return 0;
}

这个有个地方要注意，flv提取的aac流没有头，需要自己手动添加：