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 buffervoid FAT16File::Read(void * buf){IBlockDevice *pDevice = m_fs->GetDevice();char * pBuffer = (char*)buf;char buffer[512];// 读引导扇区，里面有BPBpDevice->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 hasint 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 sectorwhile(count > 0 && cluster < 0xFFF8){ // 依次读每簇的各个扇区for(int sector = 0; sector < SectorPerCluster; sector ++){// Read a sectorpDevice->ReadBlock(firstsector + (cluster - 2) * SectorPerCluster + sector, pBuffer);pBuffer += bytesPerSector;}count --; // 读下一簇cluster = m_fs->NextCluster(cluster);}return;}