给aac音频添加adts头，函数实现

#include <stdio.h>
#include <libavutil/log.h>
#include <libavformat/avio.h>
#include <libavformat/avformat.h>
#define ADTS_HEADER_LEN  7;
void adts_header(char *szAdtsHeader, int dataLen){
    int audio_object_type = 2;
    int sampling_frequency_index = 7;
    int channel_config = 2;
    int adtsLen = dataLen + 7;
    szAdtsHeader[0] = 0xff;         //syncword:0xfff                          高8bits
    szAdtsHeader[1] = 0xf0;         //syncword:0xfff                          低4bits
    szAdtsHeader[1] |= (0 << 3);    //MPEG Version:0 for MPEG-4,1 for MPEG-2  1bit
    szAdtsHeader[1] |= (0 << 1);    //Layer:0                                 2bits 
    szAdtsHeader[1] |= 1;           //protection absent:1                     1bit
    szAdtsHeader[2] = (audio_object_type - 1)<<6;            //profile:audio_object_type - 1                      2bits
    szAdtsHeader[2] |= (sampling_frequency_index & 0x0f)<<2; //sampling frequency index:sampling_frequency_index  4bits 
    szAdtsHeader[2] |= (0 << 1);                             //private bit:0                                      1bit
    szAdtsHeader[2] |= (channel_config & 0x04)>>2;           //channel configuration:channel_config               高1bit
    szAdtsHeader[3] = (channel_config & 0x03)<<6;     //channel configuration:channel_config      低2bits
    szAdtsHeader[3] |= (0 << 5);                      //original：0                               1bit
    szAdtsHeader[3] |= (0 << 4);                      //home：0                                   1bit
    szAdtsHeader[3] |= (0 << 3);                      //copyright id bit：0                       1bit  
    szAdtsHeader[3] |= (0 << 2);                      //copyright id start：0                     1bit
    szAdtsHeader[3] |= ((adtsLen & 0x1800) >> 11);           //frame length：value   高2bits
    szAdtsHeader[4] = (uint8_t)((adtsLen & 0x7f8) >> 3);     //frame length:value    中间8bits
    szAdtsHeader[5] = (uint8_t)((adtsLen & 0x7) << 5);       //frame length:value    低3bits
    szAdtsHeader[5] |= 0x1f;                                 //buffer fullness:0x7ff 高5bits
    szAdtsHeader[6] = 0xfc;
}
int main(int argc, char *argv[])
{
    int err_code;
    char errors[1024];
    char *src_filename = NULL;
    char *dst_filename = NULL;
    FILE *dst_fd = NULL;
    int audio_stream_index = -1;
    int len;
    AVFormatContext *ofmt_ctx = NULL;
    AVOutputFormat *output_fmt = NULL;
    AVStream *out_stream = NULL;
    AVFormatContext *fmt_ctx = NULL;
    AVFrame *frame = NULL;
    AVPacket pkt;
    av_log_set_level(AV_LOG_DEBUG);
    if(argc < 3){
        av_log(NULL, AV_LOG_DEBUG, "the count of parameters should be more than three!\n");
        return -1;
    }
    src_filename = argv[1];
    dst_filename = argv[2];
    if(src_filename == NULL || dst_filename == NULL){
        av_log(NULL, AV_LOG_DEBUG, "src or dts file is null, plz check them!\n");
        return -1;
    }
    /*register all formats and codec*/
    av_register_all();
    /*
    ofmt_ctx = avformat_alloc_context();
    output_fmt = av_guess_format(NULL, dst_filename, NULL);
    if(!output_fmt){
        av_log(NULL, AV_LOG_DEBUG, "Cloud not guess file format \n");
        exit(1);
    }
    ofmt_ctx->oformat = output_fmt;
    out_stream = avformat_new_stream(ofmt_ctx, NULL);
    if(!out_stream){
        av_log(NULL, AV_LOG_DEBUG, "Failed to create out stream!\n");
        exit(1);
    }
    if((err_code = avio_open(&ofmt_ctx->pb, dst_filename, AVIO_FLAG_WRITE)) < 0) {
        av_strerror(err_code, errors, 1024);
        av_log(NULL, AV_LOG_DEBUG, "Could not open file %s, %d(%s)\n",
               dst_filename,
               err_code,
               errors);
        exit(1);
    }
    */
    dst_fd = fopen(dst_filename, "wb");
    if (!dst_fd) {
        av_log(NULL, AV_LOG_DEBUG, "Could not open destination file %s\n", dst_filename);
        return -1;
    }
    /*open input media file, and allocate format context*/
    if((err_code = avformat_open_input(&fmt_ctx, src_filename, NULL, NULL)) < 0){
        av_strerror(err_code, errors, 1024);
        av_log(NULL, AV_LOG_DEBUG, "Could not open source file: %s, %d(%s)\n",
               src_filename,
               err_code,
               errors);
        return -1;
    }
    /*retrieve audio stream*/
    if((err_code = avformat_find_stream_info(fmt_ctx, NULL)) < 0) {
        av_strerror(err_code, errors, 1024);
        av_log(NULL, AV_LOG_DEBUG, "failed to find stream information: %s, %d(%s)\n",
               src_filename,
               err_code,
               errors);
        return -1;
    }
    /*dump input information*/
    av_dump_format(fmt_ctx, 0, src_filename, 0);
    /*dump output information*/
    //av_dump_format(ofmt_ctx, 0, dst_filename, 1);
    frame = av_frame_alloc();
    if(!frame){
        av_log(NULL, AV_LOG_DEBUG, "Could not allocate frame\n");
        return AVERROR(ENOMEM);
    }
    /*initialize packet*/
    av_init_packet(&pkt);
    pkt.data = NULL;
    pkt.size = 0;
    /*find best audio stream*/
    audio_stream_index = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);
    if(audio_stream_index < 0){
        av_log(NULL, AV_LOG_DEBUG, "Could not find %s stream in input file %s\n",
               av_get_media_type_string(AVMEDIA_TYPE_AUDIO),
               src_filename);
        return AVERROR(EINVAL);
    }
    /*
    if (avformat_write_header(ofmt_ctx, NULL) < 0) {
        av_log(NULL, AV_LOG_DEBUG, "Error occurred when opening output file");
        exit(1);
    }
    */
    /*read frames from media file*/
    while(av_read_frame(fmt_ctx, &pkt) >=0 ){
        if(pkt.stream_index == audio_stream_index){
            /*
            pkt.stream_index = 0;
            av_write_frame(ofmt_ctx, &pkt);
            av_free_packet(&pkt);
            */
            char adts_header_buf[7];
            adts_header(adts_header_buf, pkt.size);
            fwrite(adts_header_buf, 1, 7, dst_fd);
            len = fwrite( pkt.data, 1, pkt.size, dst_fd);
            if(len != pkt.size){
                av_log(NULL, AV_LOG_DEBUG, "warning, length of writed data isn't equal pkt.size(%d, %d)\n",
                       len,
                       pkt.size);
            }
        }
    }
    //av_write_trailer(ofmt_ctx);
    /*close input media file*/
    avformat_close_input(&fmt_ctx);
    if(dst_fd) {
        fclose(dst_fd);
    }
    //avio_close(ofmt_ctx->pb);
    return 0;
}