MKV与MatroskaExtractor
mkv封装格式相关内容可以参考：MKV 文件格式解析_Martyn哥的博客-CSDN博客_mkv 格式解析
本篇主要是用来记录MatroskaExtractor是如何解析EBML的，如何解析EBML header以及SEGMENT的
构造函数
先看构造函数，android MKV文件的解析需要依赖外部库mkvparser，首先会去解析EBML header（这个部分在sniff中会先调用一次判断是否为mkv文件）
MatroskaExtractor::MatroskaExtractor(DataSourceHelper *source) : mDataSource(source), mReader(new DataSourceBaseReader(mDataSource)), mSegment(NULL), mExtractedThumbnails(false), mIsWebm(false), mSeekPreRollNs(0) { off64_t size; mIsLiveStreaming = (mDataSource->flags() & (DataSourceBase::kWantsPrefetching | DataSourceBase::kIsCachingDataSource)) && mDataSource->getSize(&size) != OK; // 1、解析EBML header mkvparser::EBMLHeader ebmlHeader; long long pos; if (ebmlHeader.Parse(mReader, pos) < 0) { return; } if (ebmlHeader.m_docType && !strcmp("webm", ebmlHeader.m_docType)) { mIsWebm = true; } // 2、创建Segment long long ret = mkvparser::Segment::CreateInstance(mReader, pos, mSegment); if (ret) { CHECK(mSegment == NULL); return; } if (mIsLiveStreaming) { // from mkvparser::Segment::Load(), but stop at first cluster ret = mSegment->ParseHeaders(); if (ret == 0) { long len; ret = mSegment->LoadCluster(pos, len); if (ret >= 1) { // no more clusters ret = 0; } } else if (ret > 0) { ret = mkvparser::E_BUFFER_NOT_FULL; } } else { // 3、parse Segment header ret = mSegment->ParseHeaders(); if (ret < 0) { ALOGE("Segment parse header return fail %lld", ret); delete mSegment; mSegment = NULL; return; } else if (ret == 0) { // 找到Cues const mkvparser::Cues* mCues = mSegment->GetCues(); // 找到SeekHead const mkvparser::SeekHead* mSH = mSegment->GetSeekHead(); if ((mCues == NULL) && (mSH != NULL)) { size_t count = mSH->GetCount(); const mkvparser::SeekHead::Entry* mEntry; for (size_t index = 0; index < count; index++) { mEntry = mSH->GetEntry(index); if (mEntry->id == libwebm::kMkvCues) { // Cues ID long len; long long pos; mSegment->ParseCues(mEntry->pos, pos, len); mCues = mSegment->GetCues(); ALOGV("find cue data by seekhead"); break; } } } if (mCues) { long len; ret = mSegment->LoadCluster(pos, len); ALOGV("has Cue data, Cluster num=%ld", mSegment->GetCount()); } else { long status_Load = mSegment->Load(); ALOGW("no Cue data,Segment Load status:%ld",status_Load); } } else if (ret > 0) { ret = mkvparser::E_BUFFER_NOT_FULL; } } if (ret < 0) { char uri[1024]; if(!mDataSource->getUri(uri, sizeof(uri))) { uri[0] = '\0'; } ALOGW("Corrupt %s source: %s", mIsWebm ? "webm" : "matroska", uriDebugString(uri).c_str()); delete mSegment; mSegment = NULL; return; } // 获取tracks addTracks(); }
EBMLHeader::Parse
下图是EMBL Tree解析的mkv文件，EBML_ReadVersion是element id（0x47），对应红色部分；后面的s(1):0x1，括号里的1表示数据长度，冒号后面的0x01表示数据内容的长度为1，对应绿色部分（这里的计算比较复杂一些，使用的是ReadUInt）；最后面的0x01表示数据内容；
long long EBMLHeader::Parse(IMkvReader* pReader, long long& pos) { if (!pReader) return E_FILE_FORMAT_INVALID; long long total, available; long status = pReader->Length(&total, &available); if (status < 0) // error return status; pos = 0; // 检查0x1A的位置 while (pos < kMaxScanBytes) { status = pReader->Read(pos, 1, &scan_byte); if (status < 0) // error return status; else if (status > 0) return E_BUFFER_NOT_FULL; if (scan_byte == kEbmlByte0) break; ++pos; } long len = 0; // 读取ID const long long ebml_id = ReadID(pReader, pos, len); if (ebml_id == E_BUFFER_NOT_FULL) return E_BUFFER_NOT_FULL; // 检查ID是否为0x1A45DFA3，mkv文件会以这四个字节为开头 if (len != 4 || ebml_id != libwebm::kMkvEBML) return E_FILE_FORMAT_INVALID; // Move read pos forward to the EBML header size field. // 修改文件读取偏移量 pos += 4; // Read length of size field. // 接下来的一个字节表示数据长度，下面这部分到ReadUInt之前都是在检查数据长度是否正常 long long result = GetUIntLength(pReader, pos, len); if (result < 0) // error return E_FILE_FORMAT_INVALID; else if (result > 0) // need more data return E_BUFFER_NOT_FULL; if (len < 1 || len > 8) return E_FILE_FORMAT_INVALID; if ((total >= 0) && ((total - pos) < len)) return E_FILE_FORMAT_INVALID; if ((available - pos) < len) return pos + len; // try again later // 真正读取数据长度 // Read the EBML header size. result = ReadUInt(pReader, pos, len); if (result < 0) // error return result; pos += len; // consume size field // pos now designates start of payload if ((total >= 0) && ((total - pos) < result)) return E_FILE_FORMAT_INVALID; if ((available - pos) < result) return pos + result; const long long end = pos + result; Init(); while (pos < end) { long long id, size; status = ParseElementHeader(pReader, pos, end, id, size); if (status < 0) // error return status; if (size == 0) return E_FILE_FORMAT_INVALID; if (id == libwebm::kMkvEBMLVersion) { m_version = UnserializeUInt(pReader, pos, size); if (m_version <= 0) return E_FILE_FORMAT_INVALID; } else if (id == libwebm::kMkvEBMLReadVersion) { m_readVersion = UnserializeUInt(pReader, pos, size); if (m_readVersion <= 0) return E_FILE_FORMAT_INVALID; } else if (id == libwebm::kMkvEBMLMaxIDLength) { m_maxIdLength = UnserializeUInt(pReader, pos, size); if (m_maxIdLength <= 0) return E_FILE_FORMAT_INVALID; } else if (id == libwebm::kMkvEBMLMaxSizeLength) { m_maxSizeLength = UnserializeUInt(pReader, pos, size); if (m_maxSizeLength <= 0) return E_FILE_FORMAT_INVALID; } else if (id == libwebm::kMkvDocType) { if (m_docType) return E_FILE_FORMAT_INVALID; status = UnserializeString(pReader, pos, size, m_docType); if (status) // error return status; } else if (id == libwebm::kMkvDocTypeVersion) { m_docTypeVersion = UnserializeUInt(pReader, pos, size); if (m_docTypeVersion <= 0) return E_FILE_FORMAT_INVALID; } else if (id == libwebm::kMkvDocTypeReadVersion) { m_docTypeReadVersion = UnserializeUInt(pReader, pos, size); if (m_docTypeReadVersion <= 0) return E_FILE_FORMAT_INVALID; } pos += size; } if (pos != end) return E_FILE_FORMAT_INVALID; // Make sure DocType, DocTypeReadVersion, and DocTypeVersion are valid. if (m_docType == NULL || m_docTypeReadVersion <= 0 || m_docTypeVersion <= 0) return E_FILE_FORMAT_INVALID; // Make sure EBMLMaxIDLength and EBMLMaxSizeLength are valid. if (m_maxIdLength <= 0 || m_maxIdLength > 4 || m_maxSizeLength <= 0 || m_maxSizeLength > 8) return E_FILE_FORMAT_INVALID; return 0; } long long ReadID(IMkvReader* pReader, long long pos, long& len) { if (pReader == NULL || pos < 0) return E_FILE_FORMAT_INVALID; // Read the first byte. The length in bytes of the ID is determined by // finding the first set bit in the first byte of the ID. unsigned char temp_byte = 0; // 先读取一个字节，读到得应该是0x1A int read_status = pReader->Read(pos, 1, &temp_byte); if (read_status < 0) return E_FILE_FORMAT_INVALID; else if (read_status > 0) // No data to read. return E_BUFFER_NOT_FULL; if (temp_byte == 0) // ID length > 8 bytes; invalid file. return E_FILE_FORMAT_INVALID; int bit_pos = 0; const int kMaxIdLengthInBytes = 4; const int kCheckByte = 0x80; // Find the first bit that's set. // 这里会对应到博文里面的 ID数据长度等于起始0的个数加1 bool found_bit = false; for (; bit_pos < kMaxIdLengthInBytes; ++bit_pos) { if ((kCheckByte >> bit_pos) & temp_byte) { // 用0x80不断向右移位，并且和读到数据与运算，计算0的个数 found_bit = true; break; } } if (!found_bit) { // The value is too large to be a valid ID. return E_FILE_FORMAT_INVALID; } // Read the remaining bytes of the ID (if any). const int id_length = bit_pos + 1; // ID长度等于起始0的个数加1 long long ebml_id = temp_byte; for (int i = 1; i < id_length; ++i) { ebml_id <<= 8; read_status = pReader->Read(pos + i, 1, &temp_byte); if (read_status < 0) return E_FILE_FORMAT_INVALID; else if (read_status > 0) return E_BUFFER_NOT_FULL; ebml_id |= temp_byte; // 继续往后读取ID，并做与运算，返回真正的ID } len = id_length; return ebml_id; } long long GetUIntLength(IMkvReader* pReader, long long pos, long& len) { if (!pReader || pos < 0) return E_FILE_FORMAT_INVALID; long long total, available; int status = pReader->Length(&total, &available); if (status < 0 || (total >= 0 && available > total)) return E_FILE_FORMAT_INVALID; len = 1; if (pos >= available) return pos; // too few bytes available unsigned char b; // 读取下一个字节 0xA3 status = pReader->Read(pos, 1, &b); if (status != 0) return status; if (b == 0) // we can't handle u-int values larger than 8 bytes return E_FILE_FORMAT_INVALID; unsigned char m = 0x80; // 数据长度为起始0的个数 + 1 while (!(b & m)) { m >>= 1; ++len; } return 0; // success } long long ReadUInt(IMkvReader* pReader, long long pos, long& len) { if (!pReader || pos < 0) return E_FILE_FORMAT_INVALID; len = 1; unsigned char b; int status = pReader->Read(pos, 1, &b); if (status < 0) // error or underflow return status; if (status > 0) // interpreted as "underflow" return E_BUFFER_NOT_FULL; if (b == 0) // we can't handle u-int values larger than 8 bytes return E_FILE_FORMAT_INVALID; unsigned char m = 0x80; // 数据长度为起始0的个数 + 1（这个长度为数据长度的长度） while (!(b & m)) { m >>= 1; ++len; } // result为数据内容的长度，第一个字节需要用m取反并做与运算 long long result = b & (~m); ++pos; for (int i = 1; i < len; ++i) { status = pReader->Read(pos, 1, &b); if (status < 0) { len = 1; return status; } if (status > 0) { len = 1; return E_BUFFER_NOT_FULL; } result <<= 8; result |= b; ++pos; } return result; }
Segment::CreateInstance
看完上面的内容，Segment是如何创建的就很简单了
long long Segment::CreateInstance(IMkvReader* pReader, long long pos, Segment*& pSegment) { if (pReader == NULL || pos < 0) return E_PARSE_FAILED; pSegment = NULL; long long total, available; const long status = pReader->Length(&total, &available); if (status < 0) // error return status; if (available < 0) return -1; if ((total >= 0) && (available > total)) return -1; for (;;) { if ((total >= 0) && (pos >= total)) return E_FILE_FORMAT_INVALID; // Read ID long len; long long result = GetUIntLength(pReader, pos, len); if (result) // error, or too few available bytes return result; if ((total >= 0) && ((pos + len) > total)) return E_FILE_FORMAT_INVALID; if ((pos + len) > available) return pos + len; const long long idpos = pos; // 读取element id const long long id = ReadID(pReader, pos, len); if (id < 0) return E_FILE_FORMAT_INVALID; pos += len; // consume ID // Read Size // 下面是老套的数据长度检查 result = GetUIntLength(pReader, pos, len); if (result) // error, or too few available bytes return result; if ((total >= 0) && ((pos + len) > total)) return E_FILE_FORMAT_INVALID; if ((pos + len) > available) return pos + len; // 读取数据内容长度 long long size = ReadUInt(pReader, pos, len); if (size < 0) // error return size; pos += len; // consume length of size of element // Pos now points to start of payload // Handle "unknown size" for live streaming of webm files. const long long unknown_size = (1LL << (7 * len)) - 1; // Segment的id为0x18 53 80 67 if (id == libwebm::kMkvSegment) { if (size == unknown_size) size = -1; else if (total < 0) size = -1; else if ((pos + size) > total) size = -1; // 创建Segment，传入参数为Segment element起始位置、数据内容开始位置、数据长度 pSegment = new (std::nothrow) Segment(pReader, idpos, pos, size); if (pSegment == NULL) return E_PARSE_FAILED; return 0; // success } if (size == unknown_size) return E_FILE_FORMAT_INVALID; if ((total >= 0) && ((pos + size) > total)) return E_FILE_FORMAT_INVALID; if ((pos + size) > available) return pos + size; pos += size; // consume payload } }
Segment::ParseHeaders
这个方法用于parse Segment中的track、seekhead等信息，比较重要，它会找到对应的element id以及数据，如何解析数据的这里就不贴代码了，如果碰到问题可以去查看相关的parse方法
long long Segment::ParseHeaders() { // Outermost (level 0) segment object has been constructed, // and pos designates start of payload. We need to find the // inner (level 1) elements. long long total, available; const int status = m_pReader->Length(&total, &available); if (status < 0) // error return status; if (total > 0 && available > total) return E_FILE_FORMAT_INVALID; const long long segment_stop = (m_size < 0) ? -1 : m_start + m_size; if ((segment_stop >= 0 && total >= 0 && segment_stop > total) || (segment_stop >= 0 && m_pos > segment_stop)) { return E_FILE_FORMAT_INVALID; } for (;;) { if ((total >= 0) && (m_pos >= total)) break; if ((segment_stop >= 0) && (m_pos >= segment_stop)) break; long long pos = m_pos; const long long element_start = pos; // Avoid rolling over pos when very close to LLONG_MAX. unsigned long long rollover_check = pos + 1ULL; if (rollover_check > LLONG_MAX) return E_FILE_FORMAT_INVALID; if ((pos + 1) > available) return (pos + 1); // 很熟悉，检查数据长度 long len; long long result = GetUIntLength(m_pReader, pos, len); if (result < 0) // error return result; if (result > 0) { // MkvReader doesn't have enough data to satisfy this read attempt. return (pos + 1); } if ((segment_stop >= 0) && ((pos + len) > segment_stop)) return E_FILE_FORMAT_INVALID; if ((pos + len) > available) return pos + len; const long long idpos = pos; // 读取ID const long long id = ReadID(m_pReader, idpos, len); if (id < 0) return E_FILE_FORMAT_INVALID; // 如果ID 为Cluster则结束本次循环 if (id == libwebm::kMkvCluster) break; pos += len; // consume ID if ((pos + 1) > available) return (pos + 1); // Read Size // 检查数据长度 result = GetUIntLength(m_pReader, pos, len); if (result < 0) // error return result; if (result > 0) { // MkvReader doesn't have enough data to satisfy this read attempt. return (pos + 1); } if ((segment_stop >= 0) && ((pos + len) > segment_stop)) return E_FILE_FORMAT_INVALID; if ((pos + len) > available) return pos + len; // 读取数据内容长度 const long long size = ReadUInt(m_pReader, pos, len); if (size < 0 || len < 1 || len > 8) { // TODO(tomfinegan): ReadUInt should return an error when len is < 1 or // len > 8 is true instead of checking this _everywhere_. return size; } pos += len; // consume length of size of element // Avoid rolling over pos when very close to LLONG_MAX. rollover_check = static_cast<unsigned long long>(pos) + size; if (rollover_check > LLONG_MAX) return E_FILE_FORMAT_INVALID; const long long element_size = size + pos - element_start; // Pos now points to start of payload if ((segment_stop >= 0) && ((pos + size) > segment_stop)) return E_FILE_FORMAT_INVALID; // We read EBML elements either in total or nothing at all. if ((pos + size) > available) return pos + size; // 如果ID为Info，则去创建SegmentInfo if (id == libwebm::kMkvInfo) { if (m_pInfo) return E_FILE_FORMAT_INVALID; m_pInfo = new (std::nothrow) SegmentInfo(this, pos, size, element_start, element_size); if (m_pInfo == NULL) return -1; const long status = m_pInfo->Parse(); if (status) return status; // 如果ID为Track，则去创建Track } else if (id == libwebm::kMkvTracks) { if (m_pTracks) return E_FILE_FORMAT_INVALID; m_pTracks = new (std::nothrow) Tracks(this, pos, size, element_start, element_size); if (m_pTracks == NULL) return -1; // 调用Tracks.parse方法来解析Track信息 const long status = m_pTracks->Parse(); if (status) return status; // 如果ID为Cues，则创建Cues对象 } else if (id == libwebm::kMkvCues) { if (m_pCues == NULL) { m_pCues = new (std::nothrow) Cues(this, pos, size, element_start, element_size); if (m_pCues == NULL) return -1; } // 如果id为SeekHead，那么创建SeekHead对象 } else if (id == libwebm::kMkvSeekHead) { if (m_pSeekHead == NULL) { m_pSeekHead = new (std::nothrow) SeekHead(this, pos, size, element_start, element_size); if (m_pSeekHead == NULL) return -1; const long status = m_pSeekHead->Parse(); if (status) return status; } } else if (id == libwebm::kMkvChapters) { if (m_pChapters == NULL) { m_pChapters = new (std::nothrow) Chapters(this, pos, size, element_start, element_size); if (m_pChapters == NULL) return -1; const long status = m_pChapters->Parse(); if (status) return status; } } else if (id == libwebm::kMkvTags) { if (m_pTags == NULL) { m_pTags = new (std::nothrow) Tags(this, pos, size, element_start, element_size); if (m_pTags == NULL) return -1; const long status = m_pTags->Parse(); if (status) return status; } } m_pos = pos + size; // consume payload } if (segment_stop >= 0 && m_pos > segment_stop) return E_FILE_FORMAT_INVALID; if (m_pInfo == NULL) // TODO: liberalize this behavior return E_FILE_FORMAT_INVALID; if (m_pTracks == NULL) return E_FILE_FORMAT_INVALID; return 0; // success }
本篇大致就先到这，了解了MKV是如何解析EBML的，后续如果有问题就很容易去定位了。
