FFmpeg 自定义IO CONTEXT实现音频解码，以及seek函数_ffmpeg hls自定义io

 对于从音频流buffer中解码的场景中，我们需要实现自己的io context 去从buffer中解码，参考ffmepg官方实例：doc/examples/avio_reading.c

关于是否要实现avio context中的seek函数，需要看需要解码什么格式，大部分格式不需要seek,但是有些格式需要，比如apple开发的ALAC格式，这个格式的音频有的时候它的头文件moov信息是在文件的结尾，这就很坑，一般都是在开头，所以在获取音频的时候需要先seek到文件的结尾，获取moov的信息，然后再seek回来继续解析格式并解码。

关于moov格式的坑：【开发笔记】终于，我们解决了iOS播放器的一个Bug... - 哔哩哔哩

如果你不想实现seek，有没有办法直接把音频文件的moov信息从结尾提到开头呢？也是有的

ffmpeg -i ./old.mp4 -movflags faststart -c copy new.mp4

通过这个命令转换后再去解码，文件信息就在开头，就可以不用seek了。

可以通过以下命令去查看头文件信息：

ffprobe -v trace filename

直接贴代码：

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavformat/avio.h>
#include <libavutil/file.h>
#include <libavutil/frame.h>
#include <libavutil/mem.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
#define BUF_SIZE 20480
 
FILE *in_file = NULL;
 
struct buffer_data {
  uint8_t *ptr;
  uint8_t *ori_ptr;  // for seek file stream
  size_t size;       ///< size left in the buffer
  size_t file_size;  ///< size of the file to decode
};
 
static char *av_get_err(int errnum) {
  static char err_buf[128] = {0};
  av_strerror(errnum, err_buf, 128);
  return err_buf;
}
 
static void print_sample_format(const AVFrame *frame) {
  printf("ar-samplerate: %uHz\n", frame->sample_rate);
  printf("ac-channel: %u\n", frame->channels);
  printf("f-format: %u\n",
         frame->format);  // 格式需要注意，实际存储到本地文件时已经改成交错模式
}
/*
int read_size;
static int read_packet(void *opaque, uint8_t *buf, int buf_size) {
  //    FILE *in_file = (FILE *)opaque;
  read_size = fread(buf, 1, buf_size, in_file);
   printf("read_packet read_size:%d, buf_size:%d\n", read_size, buf_size);
  if (read_size <= 0) {
    return AVERROR_EOF;  // 数据读取完毕
  }
  return read_size;
}
*/
static int read_packet(void *opaque, uint8_t *buf, int buf_size) {
  struct buffer_data *bd = (struct buffer_data *)opaque;
  buf_size = FFMIN(buf_size, bd->size);
 
  if (!buf_size) return AVERROR_EOF;
  //    printf("ptr:%p size:%zu buf_size: %d\n", bd->ptr, bd->size, buf_size);
 
  /* copy internal buffer data to buf */
  memcpy(buf, bd->ptr, buf_size);
  bd->ptr += buf_size;
  bd->size -= buf_size;
 
  return buf_size;
}
 
// for some format like ALAC (apple format) , which moov partten is located at
// the end of file so we need to implement seek function during demux to seek to
// the end of file for paring the moov info and then seek back to the front
static int64_t seek_packet(void *opaque, int64_t offset, int whence) {
  //    FILE *in_file = (FILE *)opaque;
  struct buffer_data *bd = (struct buffer_data *)opaque;
  int64_t ret = -1;
  printf("whence=%d , offset=%lld \n", whence, offset);
  switch (whence) {
    case AVSEEK_SIZE:
      printf("AVSEEK_SIZE \n");
      ret = bd->file_size;
      break;
    case SEEK_SET:
      printf("SEEK_SET \n");
      bd->ptr = bd->ori_ptr + offset;
      bd->size = bd->file_size - offset;
      ret = bd->ptr;
      break;
 
    case SEEK_CUR:
      printf("SEEK_cur \n");
      break;
    case SEEK_END:
      printf("SEEK_end \n");
      break;
  }
  return ret;
}
 
static void decode(AVCodecContext *dec_ctx, AVPacket *packet, AVFrame *frame,
                   FILE *outfile) {
  int ret = 0;
  ret = avcodec_send_packet(dec_ctx, packet);
  if (ret == AVERROR(EAGAIN)) {
    printf(
        "Receive_frame and send_packet both returned EAGAIN, which is an API "
        "violation.\n");
  } else if (ret < 0) {
    printf("Error submitting the packet to the decoder, err:%s\n",
           av_get_err(ret));
    return;
  }
 
  while (ret >= 0) {
    ret = avcodec_receive_frame(dec_ctx, frame);
    if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {
      return;
    } else if (ret < 0) {
      printf("Error during decoding\n");
      exit(1);
    }
    if (!packet) {
      printf("get flush frame\n");
    }
    int out_sample_bytes = av_get_bytes_per_sample(dec_ctx->sample_fmt);
    int out_sample_is_plannar = av_sample_fmt_is_planar(dec_ctx->sample_fmt);
    // printf("debug %d is out_sample_is_plannar : %d \n", __LINE__,
    //       out_sample_is_plannar);
    // print_sample_format(frame);
    if (out_sample_bytes < 0) {
      /* This should not occur, checking just for paranoia */
      fprintf(stderr, "Failed to calculate data size\n");
      exit(1);
    }
 
    //    printf("debug %d out_sample_bytes: %d samples: %d ch:%d\n", __LINE__,
    //           out_sample_bytes, frame->nb_samples,
    //           dec_ctx->ch_layout.nb_channels);
    if (out_sample_is_plannar) {  // plannar frames
      /**
         P表示Planar（平面），其数据格式排列方式为 :
         LLLLLLRRRRRRLLLLLLRRRRRRLLLLLLRRRRRRL...（每个LLLLLLRRRRRR为一个音频帧）
         而不带P的数据格式（即交错排列）排列方式为：
         LRLRLRLRLRLRLRLRLRLRLRLRLRLRLRLRLRLRL...（每个LR为一个音频样本）
      播放范例：   ffplay -ar 48000 -ac 2 -f f32le believe.pcm
          并不是每一种都是这样的格式
      */
      // 这里的写法不是通用，通用要调用重采样的函数去实现
      // 这里只是针对解码出来是planar格式的转换
 
      for (int i = 0; i < frame->nb_samples; i++) {
        for (int ch = 0; ch < frame->channels; ch++) {
          // for(int ch = 0; ch < 1; ch++) {
          fwrite(frame->data[ch] + out_sample_bytes * i, 1, out_sample_bytes,
                 outfile);
        }
      }
    } else  // packed frame
      fwrite(frame->data[0],
             frame->nb_samples * out_sample_bytes * frame->channels, 1,
             outfile);
  }
}
 
int main(int argc, char **argv) {
  if (argc != 3) {
    printf("usage: %s <intput file> <out file>\n", argv[0]);
    return -1;
  }
  av_log_set_level(AV_LOG_TRACE);
  const char *in_file_name = argv[1];
  const char *out_file_name = argv[2];
  //    FILE *in_file = NULL;
  FILE *out_file = NULL;
 
  // 1. 打开参数文件
  in_file = fopen(in_file_name, "rb");
  if (!in_file) {
    printf("open file %s failed\n", in_file_name);
    return -1;
  }
  out_file = fopen(out_file_name, "wb+");
  if (!out_file) {
    printf("open file %s failed\n", out_file_name);
    return -1;
  }
 
  struct buffer_data bd = {0};
  uint8_t *buffer = NULL;
  size_t buffer_size;
  int ret = av_file_map(in_file_name, &buffer, &buffer_size, 0, NULL);
  printf("file size: %d\n", buffer_size);
  bd.ptr = buffer;
  bd.ori_ptr = buffer;
  bd.file_size = buffer_size;
  bd.size = buffer_size;
 
  //    AVInputFormat* in_fmt = av_find_input_format("flac");
  // 2自定义 io
  uint8_t *io_buffer = av_malloc(BUF_SIZE);
  // AVIOContext *avio_ctx = avio_alloc_context(io_buffer, BUF_SIZE, 0, (void
  // *)in_file,
  AVIOContext *avio_ctx = avio_alloc_context(io_buffer, BUF_SIZE, 0, &bd,
                                             read_packet, NULL, seek_packet);
  // avio_alloc_context(io_buffer, BUF_SIZE, 0, &bd, read_packet, NULL, NULL);
  AVFormatContext *format_ctx = avformat_alloc_context();
  format_ctx->pb = avio_ctx;
  format_ctx->flags = AVFMT_FLAG_CUSTOM_IO;
  // int ret = avformat_open_input(&format_ctx, NULL, in_fmt, NULL);
  // 从输入源读取封装格式文件头
  ret = avformat_open_input(&format_ctx, NULL, NULL, NULL);
  if (ret < 0) {
    printf("avformat_open_input failed:%s\n", av_err2str(ret));
    return -1;
  }
 
  // 从输入源读取一段数据，尝试解码，以获取流信息
  if ((ret = avformat_find_stream_info(format_ctx, NULL)) < 0) {
    av_log(NULL, AV_LOG_ERROR, "Cannot find stream information\n");
    return ret;
  }
 
  av_dump_format(format_ctx, 0, NULL, 0);
 
  int audioStreamIndex =
      av_find_best_stream(format_ctx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);
 
  AVStream *st = format_ctx->streams[audioStreamIndex];
  // 编码器查找
  // AVCodec *codec = avcodec_find_decoder(AV_CODEC_ID_ALAC);
  // set codec id from famat params
  AVCodec *codec = avcodec_find_decoder(st->codecpar->codec_id);
  if (!codec) {
    printf("avcodec_find_decoder failed\n");
    return -1;
  }
 
  AVCodecContext *codec_ctx = avcodec_alloc_context3(codec);
  if (!codec_ctx) {
    printf("avcodec_alloc_context3 failed\n");
    return -1;
  }
 
  // copy params from format to codec
  ret = avcodec_parameters_to_context(
      codec_ctx, format_ctx->streams[audioStreamIndex]->codecpar);
  if (ret < 0) {
    printf("Failed to copy in_stream codecpar to codec context\n");
  }
  ret = avcodec_open2(codec_ctx, codec, NULL);
  if (ret < 0) {
    printf("avcodec_open2 failed:%s\n", av_err2str(ret));
    return -1;
  }
 
  printf("%d debug codec_ctx->sample_rate: %d\n", __LINE__,
         codec_ctx->sample_rate);
  AVPacket *packet = av_packet_alloc();
  AVFrame *frame = av_frame_alloc();
 
  while (1) {
    ret = av_read_frame(format_ctx, packet);
    if (ret < 0) {
      printf("av_read_frame failed:%s\n", av_err2str(ret));
      break;
    }
    decode(codec_ctx, packet, frame, out_file);
  }
 
  printf("read file finish\n");
  decode(codec_ctx, NULL, frame, out_file);
 
  fclose(in_file);
  fclose(out_file);
 
  av_frame_free(frame);
  av_packet_free(packet);
 
  avformat_close_input(&format_ctx);
  avcodec_free_context(&codec_ctx);
 
  printf("main finish\n");
 
  return 0;
}

m4a moov 格式解析：MP4格式解析---M4A是MP4中的音频部分_m4a格式解析_一个专研技术的小蜜蜂的博客-CSDN博客
mp4文件格式解析 - 知乎
参考：ffmpeg 利用AVIOContext自定义IO 输出结果写buffer - 知乎

Creating Custom FFmpeg IO-Context - CodeProject

ffmpeg AVIOContext 自定义 IO 及 seek