文件系统（六）

到现在为止，整个文件系统还有最后一个重要功能没有实现，那就是读文件。现在我们就来看看这个功能是怎么实现的。

 

首先自然是FAT16File对象的构造函数，很简单。

FAT16File::FAT16File(System::Boot::FAT16 *fs, struct Entry *pEntry) { m_fs = fs; memcpy(&m_entry, pEntry, sizeof(struct Entry)); }

 

接下来是GetName方法。这个方法的实现和FAT16Directory相同。

wchar_t * FAT16File::GetName() { wchar_t * name = new wchar_t[13]; int i = 0; for(; i < 8; i++) { if(m_entry.filename[i] == 0 || m_entry.filename[i] == 0x20) break; name[i] = m_entry.filename[i]; } name[i++] = '.'; for(int j = 8; j < 11; j++) { if(m_entry.filename[j] == 0 || m_entry.filename[j] == 0x20) break; name[i ++] = m_entry.filename[j]; } name[i] = '/0'; return name;

 

在读文件之前，我们必须知道文件的大小。文件大小信息记录在文件的目录表项中。

size_t FAT16File::GetSize() { return m_entry.Size;

 

好了，终于到了读文件的环节了。不多说了，看代码。

// Read file to the memory address buffer void FAT16File::Read(void * buf) { IBlockDevice *pDevice = m_fs->GetDevice(); char * pBuffer = (char*)buf; char buffer[512]; // 读引导扇区，里面有BPB pDevice->ReadBlock(0, buffer); struct BPB_Block * pBPB = (struct BPB_Block *)(buffer + 0x0b); // 计算数据区位置 int rootSize = (pBPB->NumberOfRootDirEntries * 32 + pBPB->BytesPerSector - 1) / pBPB->BytesPerSector; int firstsector = pBPB->NumberOfFATs * pBPB->SectorsPerFAT + 1 + rootSize; // count is the count of clusters the file has int count = (m_entry.Size + pBPB->BytesPerSector * pBPB->SectorPerCluster- 1) / (pBPB->BytesPerSector * pBPB->SectorPerCluster); int bytesPerSector = pBPB->BytesPerSector; int SectorPerCluster = pBPB->SectorPerCluster; // 从第一簇开始读 WORD cluster = m_entry.FirstClusterLow; // Read the cluster, we'll read it sector by sector while(count > 0 && cluster < 0xFFF8) { // 依次读每簇的各个扇区 for(int sector = 0; sector < SectorPerCluster; sector ++) { // Read a sector pDevice->ReadBlock(firstsector + (cluster - 2) * SectorPerCluster + sector, pBuffer); pBuffer += bytesPerSector; } count --; // 读下一簇 cluster = m_fs->NextCluster(cluster); } return; }