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OpenFile CRC

Name:

 OpenFile

Comment:

 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.

Collaborators:

Responsibilities: 

#ifdef FILESYS_STUB                     // Temporarily implement calls to
                                        // Nachos file system as calls to UNIX!
                                        // See definitions listed under #else
class 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 // FILESYS
class 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,
                                                                                  
    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
};