filehdr.h

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// filehdr.h 
//      Data structures for managing a disk file header.  
//
//      A file header describes where on disk to find the data in a file,
//      along with other information about the file (for instance, its
//      length, owner, etc.)
//
// Copyright (c) 1992-1993 The Regents of the University of California.
// All rights reserved.  See copyright.h for copyright notice and limitation 
// of liability and disclaimer of warranty provisions.

#include "copyright.h"

#ifndef FILEHDR_H
#define FILEHDR_H

#include "disk.h"
#include "pbitmap.h"

#define NumDirect       ((SectorSize - 2 * sizeof(int)) / sizeof(int))
#define MaxFileSize     (NumDirect * SectorSize)

// The following class defines the Nachos "file header" (in UNIX terms,  
// the "i-node"), describing where on disk to find all of the data in the file.
// The file header is organized as a simple table of pointers to
// data blocks. 
//
// The file header data structure can be stored in memory or on disk.
// When it is on disk, it is stored in a single sector -- this means
// that we assume the size of this data structure to be the same
// as one disk sector.  Without indirect addressing, this
// limits the maximum file length to just under 4K bytes.
//
// There is no constructor; rather the file header can be initialized
// by allocating blocks for the file (if it is a new file), or by
// reading it from disk.

class FileHeader {
  public:
    bool Allocate(PersistentBitmap *bitMap, int fileSize);// Initialize a file header, 
                                                //  including allocating space 
                                                //  on disk for the file data
    void Deallocate(PersistentBitmap *bitMap);  // De-allocate this file's 
                                                //  data blocks

    void FetchFrom(int sectorNumber);   // Initialize file header from disk
    void WriteBack(int sectorNumber);   // Write modifications to file header
                                        //  back to disk

    int ByteToSector(int offset);       // Convert a byte offset into the file
                                        // to the disk sector containing
                                        // the byte

    int FileLength();                   // Return the length of the file 
                                        // in bytes

    void Print();                       // Print the contents of the file.

  private:
    int numBytes;                       // Number of bytes in the file
    int numSectors;                     // Number of data sectors in the file
    int dataSectors[NumDirect];         // Disk sector numbers for each data 
                                        // block in the file
};

#endif // FILEHDR_H

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