// -*- java -*-
import java.io.*;
/**
*
* An implementation of BlockDevice that uses a file to store data
* Also, point this at /dev/fd0 on a linux system and you can use it
* to read/write from disks!
*
* Comments: sdh300@ecs.soton.ac.uk
*
* @author Simon Howard
*/
public class FileBlockDevice implements BlockDevice {
// default size for a new file
private static final int DEFAULT_FILE_SIZE = 1423 * 1024; // 1.44 megs
// for accessing file
private RandomAccessFile r;
private int numBlocks, blockSize;
/**
* Creates a new FileBlockDevice. The filename is the path to the
* file used to store the data. A blocksize can be specified to
* use (seeing as there is no real block size on arbitrary files)
* If the file does not exist then a new file is created of
* size 1.44 megs
*
* @param filename the file to use to store data
* @param blockSize the size of the blocks to use
*/
public FileBlockDevice(String filename, int blockSize) {
this.blockSize = blockSize;
File f = new File(filename);
boolean newfile = !f.exists();
// open file
try {
r = new RandomAccessFile(f, "rw");
} catch(IOException e) {
System.err.println("FileBlockDevice: error opening "
+ filename);
e.printStackTrace();
}
// if this is a new file, give a warning message and make
// sure we extend the file size
if (newfile) {
System.err.println("FileBlockDevice: Creating new file "
+ filename);
numBlocks = DEFAULT_FILE_SIZE / blockSize;
try {
r.setLength(DEFAULT_FILE_SIZE);
} catch(IOException e) {
System.err.println("FileBlockDevice: " +
"error creating new file");
e.printStackTrace();
}
} else {
// cheap hack
if (filename.equals("/dev/fd0")) {
numBlocks = (int) (DEFAULT_FILE_SIZE / blockSize);
} else {
numBlocks = (int) (f.length() / blockSize);
}
if (f.length() % blockSize != 0) {
System.err.println("Warning: length of "
+ filename
+ " is not a multiple of "
+ blockSize);
}
}
}
/**
* Default of 1024 byte blocks
*
* @param filename the file to use to store data
*/
public FileBlockDevice(String filename) {
this(filename, 1024);
}
/**
* Read a block into memory.
*
* @param b A buffer into which the block data is read.
* The buffer should be large enough to hold
* a block.
* @param block The block number of the block to read in
*/
public void readBlock(byte[] b, int block) {
// sanity check
if (block < 0 || block >= numBlocks) {
System.err.println("FileBlockDevice: Attempt to access "
+ "block " + block);
return;
}
try {
r.seek(block * blockSize);
r.read(b);
} catch(IOException e) {
System.err.println("FileBlockDevice: " +
"Error reading from file");
e.printStackTrace();
}
}
/**
* Write a block to disk
*
* @param b A buffer storing the data to be written.
* The buffer should be large enough to hold
* a block.
* @param block The block number of the block to read in.
*/
public void writeBlock(byte[] b, int block) {
// sanity check
if (block < 0 || block >= numBlocks) {
System.err.println("FileBlockDevice: Attempt to access "
+ "block " + block);
return;
}
try {
r.seek(block * blockSize);
r.write(b);
} catch(IOException e) {
System.err.println("FileBlockDevice: " +
"Error writing to file");
e.printStackTrace();
}
}
/**
* Get the number of blocks in the file
*
* @return The number of blocks in the device
*/
public int getNumberOfBlocks() {
return numBlocks;
}
/**
* Get the block size
*
* @return The size of each block in bytes
*/
public int getBlockSize() {
return blockSize;
}
public static void main(String[] args) {
FileBlockDevice f = new FileBlockDevice("blockfile.bin");
byte[] b = new byte[f.getBlockSize()];
for (int i=0; i<f.getBlockSize(); ++i)
b[i] = (byte) (i % 256);
f.writeBlock(b, 1);
}
}