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音视频入门基础：WAV专题系列文章：
音视频入门基础：WAV专题（1）——使用FFmpeg命令生成WAV音频文件
音视频入门基础：WAV专题（3）——FFmpeg源码中，判断某文件是否为WAV音频文件的实现
音视频入门基础：WAV专题（4）——FFmpeg源码中获取WAV文件音频压缩编码格式、采样频率、声道数量、采样位数、码率的实现
音视频入门基础：WAV专题（5）——FFmpeg源码中解码WAV Header的实现
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一、引言
执行FFmpeg命令：
./ffmpeg -i XXX.wavFFmpeg内部会调用wav_probe函数检测该文件是否为WAV格式的音频文件（具体可以参考：《音视频入门基础：WAV专题（3）——FFmpeg源码中，判断某文件是否为WAV音频文件的实现》）。然后如果检测出该文件为WAV格式的音频文件，会调用wav_read_header函数解码WAV Header。
二、wav_read_header函数的定义
wav_read_header函数定义在FFmpeg源码（本文演示用的FFmpeg源码版本为5.0.3）的源文件libavformat/wavdec.c中：
/* wav input */ static int wav_read_header(AVFormatContext *s) { int64_t size, av_uninit(data_size); int64_t sample_count = 0; int rf64 = 0, bw64 = 0; uint32_t tag; AVIOContext *pb = s->pb; AVStream *st = NULL; WAVDemuxContext *wav = s->priv_data; int ret, got_fmt = 0, got_xma2 = 0; int64_t next_tag_ofs, data_ofs = -1; wav->unaligned = avio_tell(s->pb) & 1; wav->smv_data_ofs = -1; /* read chunk ID */ tag = avio_rl32(pb); switch (tag) { case MKTAG('R', 'I', 'F', 'F'): break; case MKTAG('R', 'I', 'F', 'X'): wav->rifx = 1; break; case MKTAG('R', 'F', '6', '4'): rf64 = 1; break; case MKTAG('B', 'W', '6', '4'): bw64 = 1; break; default: av_log(s, AV_LOG_ERROR, "invalid start code %s in RIFF header\n", av_fourcc2str(tag)); return AVERROR_INVALIDDATA; } /* read chunk size */ avio_rl32(pb); /* read format */ if (avio_rl32(pb) != MKTAG('W', 'A', 'V', 'E')) { av_log(s, AV_LOG_ERROR, "invalid format in RIFF header\n"); return AVERROR_INVALIDDATA; } if (rf64 || bw64) { if (avio_rl32(pb) != MKTAG('d', 's', '6', '4')) return AVERROR_INVALIDDATA; size = avio_rl32(pb); if (size < 24) return AVERROR_INVALIDDATA; avio_rl64(pb); /* RIFF size */ data_size = avio_rl64(pb); sample_count = avio_rl64(pb); if (data_size < 0 || sample_count < 0) { av_log(s, AV_LOG_ERROR, "negative data_size and/or sample_count in " "ds64: data_size = %"PRId64", sample_count = %"PRId64"\n", data_size, sample_count); return AVERROR_INVALIDDATA; } avio_skip(pb, size - 24); /* skip rest of ds64 chunk */ } /* Create the audio stream now so that its index is always zero */ st = avformat_new_stream(s, NULL); if (!st) return AVERROR(ENOMEM); for (;;) { AVStream *vst; size = next_tag(pb, &tag, wav->rifx); next_tag_ofs = avio_tell(pb) + size; if (avio_feof(pb)) break; switch (tag) { case MKTAG('f', 'm', 't', ' '): /* only parse the first 'fmt ' tag found */ if (!got_xma2 && !got_fmt && (ret = wav_parse_fmt_tag(s, size, st)) < 0) { return ret; } else if (got_fmt) av_log(s, AV_LOG_WARNING, "found more than one 'fmt ' tag\n"); got_fmt = 1; break; case MKTAG('X', 'M', 'A', '2'): /* only parse the first 'XMA2' tag found */ if (!got_fmt && !got_xma2 && (ret = wav_parse_xma2_tag(s, size, st)) < 0) { return ret; } else if (got_xma2) av_log(s, AV_LOG_WARNING, "found more than one 'XMA2' tag\n"); got_xma2 = 1; break; case MKTAG('d', 'a', 't', 'a'): if (!(pb->seekable & AVIO_SEEKABLE_NORMAL) && !got_fmt && !got_xma2) { av_log(s, AV_LOG_ERROR, "found no 'fmt ' tag before the 'data' tag\n"); return AVERROR_INVALIDDATA; } if (rf64 || bw64) { next_tag_ofs = wav->data_end = avio_tell(pb) + data_size; } else if (size != 0xFFFFFFFF) { data_size = size; next_tag_ofs = wav->data_end = size ? next_tag_ofs : INT64_MAX; } else { av_log(s, AV_LOG_WARNING, "Ignoring maximum wav data size, " "file may be invalid\n"); data_size = 0; next_tag_ofs = wav->data_end = INT64_MAX; } data_ofs = avio_tell(pb); /* don't look for footer metadata if we can't seek or if we don't * know where the data tag ends */ if (!(pb->seekable & AVIO_SEEKABLE_NORMAL) || (!(rf64 && !bw64) && !size)) goto break_loop; break; case MKTAG('f', 'a', 'c', 't'): if (!sample_count) sample_count = (!wav->rifx ? avio_rl32(pb) : avio_rb32(pb)); break; case MKTAG('b', 'e', 'x', 't'): if ((ret = wav_parse_bext_tag(s, size)) < 0) return ret; break; case MKTAG('S','M','V','0'): if (!got_fmt) { av_log(s, AV_LOG_ERROR, "found no 'fmt ' tag before the 'SMV0' tag\n"); return AVERROR_INVALIDDATA; } // SMV file, a wav file with video appended. if (size != MKTAG('0','2','0','0')) { av_log(s, AV_LOG_ERROR, "Unknown SMV version found\n"); goto break_loop; } av_log(s, AV_LOG_DEBUG, "Found SMV data\n"); wav->smv_given_first = 0; vst = avformat_new_stream(s, NULL); if (!vst) return AVERROR(ENOMEM); wav->vst = vst; avio_r8(pb); vst->id = 1; vst->codecpar->codec_type = AVMEDIA_TYPE_VIDEO; vst->codecpar->codec_id = AV_CODEC_ID_SMVJPEG; vst->codecpar->width = avio_rl24(pb); vst->codecpar->height = avio_rl24(pb); if ((ret = ff_alloc_extradata(vst->codecpar, 4)) < 0) { av_log(s, AV_LOG_ERROR, "Could not allocate extradata.\n"); return ret; } size = avio_rl24(pb); wav->smv_data_ofs = avio_tell(pb) + (size - 5) * 3; avio_rl24(pb); wav->smv_block_size = avio_rl24(pb); if (!wav->smv_block_size) return AVERROR_INVALIDDATA; avpriv_set_pts_info(vst, 32, 1, avio_rl24(pb)); vst->duration = avio_rl24(pb); avio_rl24(pb); avio_rl24(pb); wav->smv_frames_per_jpeg = avio_rl24(pb); if (wav->smv_frames_per_jpeg > 65536) { av_log(s, AV_LOG_ERROR, "too many frames per jpeg\n"); return AVERROR_INVALIDDATA; } AV_WL32(vst->codecpar->extradata, wav->smv_frames_per_jpeg); goto break_loop; case MKTAG('L', 'I', 'S', 'T'): case MKTAG('l', 'i', 's', 't'): if (size < 4) { av_log(s, AV_LOG_ERROR, "too short LIST tag\n"); return AVERROR_INVALIDDATA; } switch (avio_rl32(pb)) { case MKTAG('I', 'N', 'F', 'O'): ff_read_riff_info(s, size - 4); break; case MKTAG('a', 'd', 't', 'l'): if (s->nb_chapters > 0) { while (avio_tell(pb) < next_tag_ofs && !avio_feof(pb)) { char cue_label[512]; unsigned id, sub_size; if (avio_rl32(pb) != MKTAG('l', 'a', 'b', 'l')) break; sub_size = avio_rl32(pb); if (sub_size < 5) break; id = avio_rl32(pb); avio_get_str(pb, sub_size - 4, cue_label, sizeof(cue_label)); avio_skip(pb, avio_tell(pb) & 1); for (int i = 0; i < s->nb_chapters; i++) { if (s->chapters[i]->id == id) { av_dict_set(&s->chapters[i]->metadata, "title", cue_label, 0); break; } } } } break; } break; case MKTAG('I', 'D', '3', ' '): case MKTAG('i', 'd', '3', ' '): { ID3v2ExtraMeta *id3v2_extra_meta; ff_id3v2_read_dict(pb, &ffformatcontext(s)->id3v2_meta, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta); if (id3v2_extra_meta) { ff_id3v2_parse_apic(s, id3v2_extra_meta); ff_id3v2_parse_chapters(s, id3v2_extra_meta); ff_id3v2_parse_priv(s, id3v2_extra_meta); } ff_id3v2_free_extra_meta(&id3v2_extra_meta); } break; case MKTAG('c', 'u', 'e', ' '): if (size >= 4 && got_fmt && st->codecpar->sample_rate > 0) { AVRational tb = {1, st->codecpar->sample_rate}; unsigned nb_cues = avio_rl32(pb); if (size >= nb_cues * 24LL + 4LL) { for (int i = 0; i < nb_cues; i++) { unsigned offset, id = avio_rl32(pb); if (avio_feof(pb)) return AVERROR_INVALIDDATA; avio_skip(pb, 16); offset = avio_rl32(pb); if (!avpriv_new_chapter(s, id, tb, offset, AV_NOPTS_VALUE, NULL)) return AVERROR(ENOMEM); } } } break; } /* seek to next tag unless we know that we'll run into EOF */ if ((avio_size(pb) > 0 && next_tag_ofs >= avio_size(pb)) || wav_seek_tag(wav, pb, next_tag_ofs, SEEK_SET) < 0) { break; } } break_loop: if (!got_fmt && !got_xma2) { av_log(s, AV_LOG_ERROR, "no 'fmt ' or 'XMA2' tag found\n"); return AVERROR_INVALIDDATA; } if (data_ofs < 0) { av_log(s, AV_LOG_ERROR, "no 'data' tag found\n"); return AVERROR_INVALIDDATA; } avio_seek(pb, data_ofs, SEEK_SET); if (data_size > (INT64_MAX>>3)) { av_log(s, AV_LOG_WARNING, "Data size %"PRId64" is too large\n", data_size); data_size = 0; } if ( st->codecpar->bit_rate > 0 && data_size > 0 && st->codecpar->sample_rate > 0 && sample_count > 0 && st->codecpar->channels > 1 && sample_count % st->codecpar->channels == 0) { if (fabs(8.0 * data_size * st->codecpar->channels * st->codecpar->sample_rate / sample_count /st->codecpar->bit_rate - 1.0) < 0.3) sample_count /= st->codecpar->channels; } if ( data_size > 0 && sample_count && st->codecpar->channels && (data_size << 3) / sample_count / st->codecpar->channels > st->codecpar->bits_per_coded_sample + 1) { av_log(s, AV_LOG_WARNING, "ignoring wrong sample_count %"PRId64"\n", sample_count); sample_count = 0; } /* G.729 hack (for Ticket4577) * FIXME: Come up with cleaner, more general solution */ if (st->codecpar->codec_id == AV_CODEC_ID_G729 && sample_count && (data_size << 3) > sample_count) { av_log(s, AV_LOG_WARNING, "ignoring wrong sample_count %"PRId64"\n", sample_count); sample_count = 0; } if (!sample_count || av_get_exact_bits_per_sample(st->codecpar->codec_id) > 0) if ( st->codecpar->channels && data_size && av_get_bits_per_sample(st->codecpar->codec_id) && wav->data_end <= avio_size(pb)) sample_count = (data_size << 3) / (st->codecpar->channels * (uint64_t)av_get_bits_per_sample(st->codecpar->codec_id)); if (sample_count) st->duration = sample_count; if (st->codecpar->codec_id == AV_CODEC_ID_PCM_S32LE && st->codecpar->block_align == st->codecpar->channels * 4 && st->codecpar->bits_per_coded_sample == 32 && st->codecpar->extradata_size == 2 && AV_RL16(st->codecpar->extradata) == 1) { st->codecpar->codec_id = AV_CODEC_ID_PCM_F16LE; st->codecpar->bits_per_coded_sample = 16; } else if (st->codecpar->codec_id == AV_CODEC_ID_PCM_S24LE && st->codecpar->block_align == st->codecpar->channels * 4 && st->codecpar->bits_per_coded_sample == 24) { st->codecpar->codec_id = AV_CODEC_ID_PCM_F24LE; } else if (st->codecpar->codec_id == AV_CODEC_ID_XMA1 || st->codecpar->codec_id == AV_CODEC_ID_XMA2) { st->codecpar->block_align = 2048; } else if (st->codecpar->codec_id == AV_CODEC_ID_ADPCM_MS && st->codecpar->channels > 2 && st->codecpar->block_align < INT_MAX / st->codecpar->channels) { st->codecpar->block_align *= st->codecpar->channels; } ff_metadata_conv_ctx(s, NULL, wav_metadata_conv); ff_metadata_conv_ctx(s, NULL, ff_riff_info_conv); set_spdif(s, wav); return 0; }形参s：既是输入型参数也是输出型参数，指向AVFormatContext类型的变量。s->pb包含整个WAV Header的二进制数据。执行wav_read_header函数后，(WAVDemuxContext *)(s->priv_data)->vst->codecpar中的成员变量会被赋值为WAV Header中Format chunk中的信息（包括音频压缩编码格式、采样频率、声道数量、采样位数、码率）。
返回值：返回0表示解码WAV Header成功。返回一个负数表示解码WAV Header失败。
三、wav_read_header函数的内部实现分析
（一）读取WAV Header中的“区块编号”
WAV Header中的第一个区块为“区块编号”，通过下面语句读取“区块编号”的内容（关于avio_rXXX系列函数的用法可以参考：《FFmpeg源码：avio_r8、avio_rl16、avio_rl24、avio_rl32、avio_rl64函数分析》）：
/* read chunk ID */ tag = avio_rl32(pb);有的同学看到这里可能会有疑问：根据文章《音视频入门基础：WAV专题（2）——WAV格式简介》的描述，“区块编号”不是大端字节序的吗？而avio_rl32函数是按照小端模式读取四个字节数据。所以为什么是用avio_rl32函数而不是avio_rb32函数读取呢？avio_rb32函数才是按照大端模式读取四个字节数据的吧？
原因是这样的，在语句tag = avio_rl32(pb)之后，又通过MKTAG函数（关于MKTAG和MKBETAG宏定义的用法可以参考：《FFmpeg源码：MKTAG和MKBETAG宏定义分析》）将字符（比如'R', 'I', 'F', 'F'）转换为整形，按小端模式存贮，让其跟tag进行比较。所以这就是所谓的“负负得正”，“用avio_rl32函数读取然后跟MKTAG转换出来的整数进行比较”，这个跟“用avio_rb32函数读取然后用MKBETAG转换出来的整数进行比较”，效果是一样的：
switch (tag) { case MKTAG('R', 'I', 'F', 'F'): break; case MKTAG('R', 'I', 'F', 'X'): wav->rifx = 1; break; case MKTAG('R', 'F', '6', '4'): rf64 = 1; break; case MKTAG('B', 'W', '6', '4'): bw64 = 1; break; default: av_log(s, AV_LOG_ERROR, "invalid start code %s in RIFF header\n", av_fourcc2str(tag)); return AVERROR_INVALIDDATA; }所以下面代码块：
/* read chunk ID */ tag = avio_rl32(pb); switch (tag) { case MKTAG('R', 'I', 'F', 'F'): break; case MKTAG('R', 'I', 'F', 'X'): wav->rifx = 1; break; case MKTAG('R', 'F', '6', '4'): rf64 = 1; break; case MKTAG('B', 'W', '6', '4'): bw64 = 1; break; default: av_log(s, AV_LOG_ERROR, "invalid start code %s in RIFF header\n", av_fourcc2str(tag)); return AVERROR_INVALIDDATA; }等价于：
/* read chunk ID */ tag = avio_rb32(pb); switch (tag) { case MKBETAG('R', 'I', 'F', 'F'): break; case MKBETAG('R', 'I', 'F', 'X'): wav->rifx = 1; break; case MKBETAG('R', 'F', '6', '4'): rf64 = 1; break; case MKBETAG('B', 'W', '6', '4'): bw64 = 1; break; default: av_log(s, AV_LOG_ERROR, "invalid start code %s in RIFF header\n", av_fourcc2str(tag)); return AVERROR_INVALIDDATA; }读取到“区块编号”后，如果值等于“RIFF”，表示该文件遵守RIFF格式的规则，按默认处理；如果值等于“RIFX”，让wav->rifx赋值为1，表示该文件遵守RIFX格式的规则；如果值等于“RF64”，让变量rf64赋值为1，表示这是WAVE 64位扩展格式中的一种：WAV RF64；如果值等于“BW64”，让变量bw64赋值为1，表示这是WAVE 64位扩展格式中的一种：WAV BW64；如果值不为上述，打印日志："invalid start code XXX in RIFF header"并返回AVERROR_INVALIDDATA表示WAV Header中的数据不合法。
（二）读取WAV Header中的“总区块大小”
WAV Header中的第二个区块为“总区块大小”，通过下面语句读取“总区块大小”。由于该值可以由WAV Header中的其它值推导出来，所以对于FFmpeg来讲它没有意义，故FFmpeg没有将其存贮到内部的成员变量中：
/* read chunk size */ avio_rl32(pb);（三）读取WAV Header中的“档案格式”
WAV Header中的第三个区块为“档案格式”。通过下面语句读取该值并进行比较，如果值不为“WAVE”，日志打印"invalid format in RIFF header"表示WAV Header中的数据不合法：
/* read format */ if (avio_rl32(pb) != MKTAG('W', 'A', 'V', 'E')) { av_log(s, AV_LOG_ERROR, "invalid format in RIFF header\n"); return AVERROR_INVALIDDATA; } （四）WAVE 64位扩展格式处理
FFmpeg源码内部将WAV和WAVE 64位扩展格式都放到一起处理，下面代码处理文件格式为WAVE 64位时的情况：
if (rf64 || bw64) { if (avio_rl32(pb) != MKTAG('d', 's', '6', '4')) return AVERROR_INVALIDDATA; size = avio_rl32(pb); if (size < 24) return AVERROR_INVALIDDATA; avio_rl64(pb); /* RIFF size */ data_size = avio_rl64(pb); sample_count = avio_rl64(pb); if (data_size < 0 || sample_count < 0) { av_log(s, AV_LOG_ERROR, "negative data_size and/or sample_count in " "ds64: data_size = %"PRId64", sample_count = %"PRId64"\n", data_size, sample_count); return AVERROR_INVALIDDATA; } avio_skip(pb, size - 24); /* skip rest of ds64 chunk */ }（五）读取WAV Header中的子区块
WAV Header中包含“Format chunk”、“Data chunk”这种必须存在的子区块，也包含Fact chunk、Cue points chunk、Playlist chunk、Associated data list chunk等可选区块。通过下面语句中的for循环和switch case语句来判断是哪种子区块，然后循环处理：
for (;;) { AVStream *vst; size = next_tag(pb, &tag, wav->rifx); next_tag_ofs = avio_tell(pb) + size; if (avio_feof(pb)) break; switch (tag) { case MKTAG('f', 'm', 't', ' '): //... break; case MKTAG('X', 'M', 'A', '2'): //... break; case MKTAG('d', 'a', 't', 'a'): //... break; case MKTAG('f', 'a', 'c', 't'): //... break; case MKTAG('b', 'e', 'x', 't'): //... break; case MKTAG('S','M','V','0'): //... goto break_loop; case MKTAG('L', 'I', 'S', 'T'): case MKTAG('l', 'i', 's', 't'): //... break; case MKTAG('I', 'D', '3', ' '): case MKTAG('i', 'd', '3', ' '): { //... break; case MKTAG('c', 'u', 'e', ' '): //... break; } } }（六）读取WAV Header中“Format chunk”子区块的信息
下面以读取“Format chunk”子区块中的信息为例子进行讲解如何解码子区块。ff_get_wav_header函数中通过wav_parse_fmt_tag函数解析“Format chunk”子区块：
int ff_get_wav_header(AVFormatContext *s, AVIOContext *pb, AVCodecParameters *par, int size, int big_endian) { //... for (;;) { switch (tag) { case MKTAG('f', 'm', 't', ' '): /* only parse the first 'fmt ' tag found */ if (!got_xma2 && !got_fmt && (ret = wav_parse_fmt_tag(s, size, st)) < 0) { return ret; } else if (got_fmt) av_log(s, AV_LOG_WARNING, "found more than one 'fmt ' tag\n"); got_fmt = 1; break; } //... } }而wav_parse_fmt_tag函数内部又调用了ff_get_wav_header函数来获取获取音频压缩编码格式、采样频率、声道数量、采样位数、码率信息（具体可以参考：《音视频入门基础：WAV专题（4）——FFmpeg源码中获取WAV文件音频压缩编码格式、采样频率、声道数量、采样位数、码率的实现》）：
static int wav_parse_fmt_tag(AVFormatContext *s, int64_t size, AVStream *st) { AVIOContext *pb = s->pb; WAVDemuxContext *wav = s->priv_data; int ret; /* parse fmt header */ ret = ff_get_wav_header(s, pb, st->codecpar, size, wav->rifx); if (ret < 0) return ret; handle_stream_probing(st); ffstream(st)->need_parsing = AVSTREAM_PARSE_FULL_RAW; avpriv_set_pts_info(st, 64, 1, st->codecpar->sample_rate); return 0; }