音视频入门基础：WAV专题（5）——FFmpeg源码中解码WAV Header的实现

 =================================================================

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（2）——WAV格式简介

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

=================================================================

一、引言

执行FFmpeg命令：

./ffmpeg -i XXX.wav

FFmpeg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;
}