openfilewithposition.java

nachos操作系统框架

// PART OF THE MACHINE SIMULATION. DO NOT CHANGE.

package nachos.machine;

/**
 * An <tt>OpenFile</tt> that maintains a current file position.
 */
public abstract class OpenFileWithPosition extends OpenFile {
    /**
     * Allocate a new <tt>OpenFileWithPosition</tt> with the specified name on
     * the specified file system.
     *
     * @param	fileSystem	the file system to which this file belongs.
     * @param	name		the name of the file, on that file system.
     */
    public OpenFileWithPosition(FileSystem fileSystem, String name) {
	super(fileSystem, name);
    }

    /**
     * Allocate a new unnamed <tt>OpenFileWithPosition</tt> that is not
     * associated with any file system.
     */
    public OpenFileWithPosition() {
	super();
    }

    public void seek(int position) {
	this.position = position;
    }

    public int tell() {
	return position;
    }

    public int read(byte[] buf, int offset, int length) {
	int amount = read(position, buf, offset, length);
	if (amount == -1)
	    return -1;
	
	position += amount;
	return amount;
    }

    public int write(byte[] buf, int offset, int length) {
	int amount = write(position, buf, offset, length);
	if (amount == -1)
	    return -1;
	
	position += amount;
	return amount;
    }

    /**
     * The current value of the file pointer.
     */
    protected int position = 0;
}