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• libavcodec/ffmpeg reports two channels when decoding, but segfaults when trying to access the second

  1er décembre 2017, par Astrognome

  I am attempting to decode an audio file into raw pcm data using FFMpeg’s libavcodec. I have the following code so far, but it segfaults when trying to access the second audio channel for some reason.

  #include 
  
  #include 
  
  #include <ao></ao>ao.h>
  
  #include 
  
  
  
  #include <libavutil></libavutil>opt.h>
  
  #include <libavcodec></libavcodec>avcodec.h>
  
  #include <libavformat></libavformat>avformat.h>
  
  #include <libswresample></libswresample>swresample.h>
  
  
  
  #define BUF_SIZE 4096
  
  
  
  
  
  #define AUDIO_INBUF_SIZE 20480
  
  #define AUDIO_REFILL_THRESH 4096
  
  
  
  int decode(AVCodecContext *dec_ctx, AVPacket *pkt, AVFrame *frame, uint8_t* out_data, int* index) {
  
  
  
      int i, ch;
  
      int ret, data_size;
  
  
  
      ret = avcodec_send_packet(dec_ctx, pkt);
  
      if (ret &lt; 0) {
  
          fprintf(stderr, "Error submitting packet to decoder\n");
  
          exit(1);
  
      }
  
  
  
      while (ret >= 0) {
  
          ret = avcodec_receive_frame(dec_ctx, frame);
  
          if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {
  
              return 0;
  
          }
  
          else if (ret &lt; 0) {
  
              fprintf(stderr, "Error during decoding\n");
  
              exit(1);
  
          }
  
  
  
          data_size = av_get_bytes_per_sample(dec_ctx->sample_fmt);
  
          if (data_size &lt; 0) {
  
              fprintf(stderr, "Failed to calculate data size\n");
  
              exit(1);
  
          }
  
          for (i = 0; i &lt; frame->nb_samples; ++i)
  
              for (ch = 0; ch &lt; dec_ctx->channels; ++ch) {
  
  
  
                  //Things go wrong here...
  
                  memcpy(out_data + *index, frame->data[ch] + (data_size*i), data_size);
  
                  *index += data_size;
  
              }
  
      }
  
  
  
      return 0;
  
  }
  
  
  
  int decode_audio_file(const char* path, uint8_t* out_data, int size) {
  
      const AVCodec *codec;
  
      AVCodecContext *c = NULL;
  
      AVCodecParserContext *parser = NULL;
  
      int len, ret;
  
      FILE *f;
  
      uint8_t inbuf[AUDIO_INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];
  
      uint8_t *data;
  
      size_t data_size;
  
      AVPacket *pkt;
  
      AVFrame *decoded_frame = NULL;
  
  
  
  
  
      avcodec_register_all();
  
      pkt = av_packet_alloc();
  
      codec = avcodec_find_decoder(AV_CODEC_ID_FLAC);
  
      if (!codec) {
  
          fprintf(stderr, "Codec not found!\n");
  
          return -1;
  
      }
  
  
  
      parser = av_parser_init(codec->id);
  
      if (!parser) {
  
          fprintf(stderr, "Parser not found!\n");
  
          return -1;
  
      }
  
  
  
      c = avcodec_alloc_context3(codec);
  
      if (!c) {
  
          fprintf(stderr, "Could not allocate audio context!\n");
  
          return -1;
  
      }
  
  
  
      if(avcodec_open2(c, codec, NULL) &lt; 0) {
  
          fprintf(stderr, "Could not open codec\n");
  
          exit(1);
  
      }
  
  
  
      f = fopen(path, "rb");
  
      if(!f) {
  
          fprintf(stderr, "Could not open %s\n", path);
  
          exit(1);
  
      }
  
  
  
      data = inbuf;
  
      data_size = fread(inbuf, 1, AUDIO_INBUF_SIZE, f);
  
  
  
      int index = 0;  
  
      while (data_size > 0) {
  
          if (!decoded_frame) {
  
              if (!(decoded_frame = av_frame_alloc())) {
  
                  fprintf(stderr, "Could not allocate audio frame\n");
  
                  exit(1);
  
              }
  
          }
  
  
  
          ret = av_parser_parse2(parser, c, &amp;pkt->data, &amp;pkt->size, data, data_size, AV_NOPTS_VALUE, AV_NOPTS_VALUE, 0);
  
          if (ret &lt; 0) {
  
              fprintf(stderr, "Error while parsing\n");
  
              exit(1);
  
          }
  
          data += ret;
  
          data_size -= ret;
  
  
  
          if (pkt->size)
  
              decode(c, pkt, decoded_frame, out_data, &amp;index);
  
          if (data_size &lt; AUDIO_REFILL_THRESH) {
  
              memmove(inbuf, data, data_size);
  
              data = inbuf;
  
              len = fread(data + data_size, 1, AUDIO_INBUF_SIZE - data_size, f);
  
              if (len > 0)
  
                  data_size += len;
  
          }
  
      }
  
      avcodec_free_context(&amp;c);
  
      av_parser_close(parser);
  
      av_frame_free(&amp;decoded_frame);
  
      av_packet_free(&amp;pkt);
  
  
  
      return 0;
  
  }
  
  
  
  
  
  int main(int argc, char **argv)
  
  {
  
      int bsize = 1024*1024*60;
  
      uint8_t* audio_buffer = calloc(bsize, sizeof(uint8_t));
  
      decode_audio_file("testsong.flac", audio_buffer, bsize);
  
  }

  Here is the offending line :

  memcpy(out_data + *index, frame->data[ch] + (data_size*i), data_size);

  If you access frame->data[0] it’s fine, but if you attempt to access frame->data[1] it segfaults. The context reports two channels and the file contains two channels (tried with several different tracks).
  But it gets even weirder. If I switch the offending line to

  memcpy(out_data + *index, frame->data[0] + (data_size*i*2 + ch), data_size);

  it will have the left channel with correct data and the right channel is white noise. This tells me (and I could be wrong) that the first channel actually contains 2 channels of audio interleaved, the actual left channel, plus a junk channel of white noise.
  I was wrong, I forgot to multiple ch by data_size. If the line is memcpy(out_data + *index, frame->data[0] + (data_size*i*2 + ch*data_size), data_size); things seem to work as expected. This all still seems very sketchy though so if anyone can thoroughly explain what’s happening here, that would be nice.

  I have been working off the decode_audio example available in the ffmpeg documentation here although modified slightly.

  This ALSO happens if I use the vanilla example with the only change being setting the codec to FLAC, so I assume it’s a bug in the library. In that case, how can I work around it.

• Anomalie #4589 (Fermé) : PHP8 : Warning Trying to access array offset on value of type bool in inc...

  4 novembre 2020

  Merci. Corrigé par : https://git.spip.net/spip/revisions/commit/b6d54178

• Access to folders on Ubuntu-based Rails server (keep getting "No such file..")

  13 avril 2012, par Stpn

  I am setting up Rails+ffmpeg on Ubuntu and I keep getting

   Errno::ENOENT
  
   No such file or directory..

  The setup is as follows :

  /home/username/RailsApp
  /home/username/videos/

  I am trying to run ffmpeg to write to /home/username/videos and I used "/home/username/videos/" and " /videos/" but no luck..

  What am I missing ?

  "www-data" user is included in "username" group..

  Rails app works fine otherwise..

  Any input greatly appreciated !

  Thanks !