// openfile.h //	Data structures for opening, closing, reading and writing to //	individual files.  The operations supported are similar to//	the UNIX ones -- type 'man open' to the UNIX prompt.////	There are two implementations.  One is a "STUB" that directly//	turns the file operations into the underlying UNIX operations.//	(cf. comment in filesys.h).////	The other is the "real" implementation, that turns these//	operations into read and write disk sector requests. //	In this baseline implementation of the file system, we don't //	worry about concurrent accesses to the file system//	by different threads -- this is part of the assignment.//// 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.#ifndef OPENFILE_H#define OPENFILE_H#include "copyright.h"#include "utility.h"#ifdef FILESYS_STUB			// Temporarily implement calls to 					// Nachos file system as calls to UNIX!					// See definitions listed under #elseclass OpenFile {  public:    OpenFile(int f) { file = f; currentOffset = 0; }	// open the file    ~OpenFile() { Close(file); }			// close the file    int ReadAt(char *into, int numBytes, int position) {     		Lseek(file, position, 0); 		return ReadPartial(file, into, numBytes); 		}	    int WriteAt(char *from, int numBytes, int position) {     		Lseek(file, position, 0); 		WriteFile(file, from, numBytes); 		return numBytes;		}	    int Read(char *into, int numBytes) {		int numRead = ReadAt(into, numBytes, currentOffset); 		currentOffset += numRead;		return numRead;    		}    int Write(char *from, int numBytes) {		int numWritten = WriteAt(from, numBytes, currentOffset); 		currentOffset += numWritten;		return numWritten;		}    int Length() { Lseek(file, 0, 2); return Tell(file); }      private:    int file;    int currentOffset;};#else // FILESYSclass FileHeader;class OpenFile {  public:    OpenFile(int sector);		// Open a file whose header is located					// at "sector" on the disk    ~OpenFile();			// Close the file    void Seek(int position); 		// Set the position from which to 					// start reading/writing -- UNIX lseek    int Read(char *into, int numBytes); // Read/write bytes from the file,					// starting at the implicit position.					// Return the # actually read/written,					// and increment position in file.    int Write(char *from, int numBytes);    int ReadAt(char *into, int numBytes, int position);    					// Read/write bytes from the file,					// bypassing the implicit position.    int WriteAt(char *from, int numBytes, int position);    int Length(); 			// Return the number of bytes in the					// file (this interface is simpler 					// than the UNIX idiom -- lseek to 					// end of file, tell, lseek back       private:    FileHeader *hdr;			// Header for this file     int seekPosition;			// Current position within the file};#endif // FILESYS#endif // OPENFILE_H