scan chained oggflac file with libFLAC 1.5+

main.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <FLAC/stream_decoder.h>
#include <FLAC/format.h>

typedef struct context_t {
  FLAC__StreamMetadata_StreamInfo si;
  FLAC__StreamDecoderErrorStatus err;
  char **vc;
} context_t;

FLAC__StreamDecoderWriteStatus
write_callback(const FLAC__StreamDecoder *decoder, const FLAC__Frame *frame,
               const FLAC__int32 *const buffer[], void *client_data) {
  return FLAC__STREAM_DECODER_WRITE_STATUS_CONTINUE;
}

void error_callback(const FLAC__StreamDecoder *decoder,
                    FLAC__StreamDecoderErrorStatus status, void *client_data) {
  ((context_t *)client_data)->err = status;
}

void metadata_callback(const FLAC__StreamDecoder *decoder,
                       const FLAC__StreamMetadata *metadata,
                       void *client_data) {
  context_t *ctx = (context_t *)client_data;
  switch (metadata->type) {
  case FLAC__METADATA_TYPE_STREAMINFO: {
    ctx->si = metadata->data.stream_info;
    break;
  }
  case FLAC__METADATA_TYPE_VORBIS_COMMENT: {
    FLAC__StreamMetadata_VorbisComment vc = metadata->data.vorbis_comment;
    unsigned ptr_area_size = (vc.num_comments + 1)* sizeof(char*);
    int all_len = 0;
    for (int i = 0; i < vc.num_comments; ++i) {
      all_len += vc.comments[i].length + 1;
    }
    void *buffer = malloc(ptr_area_size + all_len);
    ctx->vc = buffer;
    char *bp = (char*)buffer + ptr_area_size;

    for (int i = 0; i < vc.num_comments; ++i) {
      memcpy(bp, vc.comments[i].entry, vc.comments[i].length);
      bp[vc.comments[i].length] = 0;
      ctx->vc[i] = bp;
      bp += vc.comments[i].length + 1;
    }
    ctx->vc[vc.num_comments] = 0;
    break;
  }
  default:
    break;
  }
}

int main(int argc, char **argv) {
  if (argc == 1) {
    fprintf(stderr, "usage: %s FILE\n", argv[0]);
    return 1;
  }
  FLAC__StreamDecoder *decoder = FLAC__stream_decoder_new();
  context_t ctx;
  memset(&ctx, 0, sizeof ctx);
  FLAC__StreamDecoderInitStatus res = FLAC__stream_decoder_init_ogg_file(
      decoder, argv[1], write_callback, metadata_callback, error_callback,
      &ctx);
  if (res != FLAC__STREAM_DECODER_INIT_STATUS_OK) {
    fprintf(stderr, "ERROR: cannot open %s: error=%d", argv[1], res);
    FLAC__stream_decoder_delete(decoder);
    return 2;
  }
  FLAC__stream_decoder_set_decode_chained_stream(decoder, 1);
  FLAC__stream_decoder_set_metadata_respond(decoder,
                                            FLAC__METADATA_TYPE_STREAMINFO);
  FLAC__stream_decoder_set_metadata_respond(decoder,
                                            FLAC__METADATA_TYPE_VORBIS_COMMENT);

  for (int track = 1;; ++track) {
    memset(&ctx, 0, sizeof ctx);
    if (!FLAC__stream_decoder_process_until_end_of_metadata(decoder) || ctx.err)
      break;
    printf("stream #%d sample rate: %d, channels: %d, bits: %d, duration: %g\n",
           track,
           ctx.si.sample_rate, ctx.si.channels, ctx.si.bits_per_sample,
           (double)ctx.si.total_samples / ctx.si.sample_rate);
    if (ctx.vc) {
      for (int i = 0; ctx.vc[i] != NULL; i++) {
        printf("%s\n", ctx.vc[i]);
      }
      free(ctx.vc);
    }
    if (!FLAC__stream_decoder_skip_single_link(decoder))
      break;
    if (FLAC__stream_decoder_get_state(decoder) == FLAC__STREAM_DECODER_END_OF_STREAM)
      break;
    putchar('\n');
  }
  FLAC__stream_decoder_delete(decoder);
  return 0;
}