fileS - File System Chapters 10, 11, 12 1 Chapter 10:...

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1 1 File System Chapters 10, 11, 12 Chapter 10: File-System Interface ! File Concept ! Access Methods ! Directory Structure ! File-System Mounting ! File Sharing ! Protection 2
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2 File Concept ! Contiguous logical address space ! Types: " Data # numeric # character # binary " Program 3 File Structure ! None - sequence of words, bytes ! Simple record structure " Lines " Fixed length " Variable length ! Complex Structures " Formatted document " Relocatable load file 4
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3 File Attributes ! Name – only information kept in human-readable form ! Identifier – unique tag (number) identifies file within file system ! Type – needed for systems that support different types ! Location – pointer to file location on device ! Size – current file size ! Protection – controls who can do reading, writing, executing ! Time, date, and user identification – data for protection, security, and usage monitoring ! Information about files are kept in the directory structure, which is maintained on the disk 5 File Operations ! File is an abstract data type ! Create ! Write ! Read ! Reposition within file ! Delete ! Truncate ! Open(F i ) – search the directory structure on disk for entry F i , and move the content of entry to memory ! Close (F i ) – move the content of entry F i in memory to directory structure on disk 6
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4 Open Files ! Several pieces of data are needed to manage open files: " File pointer: pointer to last read/write location, per process that has the file open " File-open count: counter of number of times a file is open – to allow removal of data from open-file table when last processes closes it " Disk location of the file: cache of data access information " Access rights: per-process access mode information 7 Open File Locking ! Provided by some operating systems and file systems ! Mediates access to a file ! Mandatory or advisory: " Mandatory – access is denied depending on locks held and requested " Advisory – processes can find status of locks and decide what to do 8
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5 File Locking Example – Java API import java.io.*; import java.nio.channels.*; public class LockingExample { public static final boolean EXCLUSIVE = false; public static final boolean SHARED = true; public static void main(String arsg[]) throws IOException FileLock sharedLock = null; FileLock exclusiveLock = null; try RandomAccessFile raf = new RandomAccessFile("file.txt", "rw"); // get the channel for the file FileChannel ch = raf.getChannel(); // this locks the first half of the file - exclusive exclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE); /** Now modify the data . . . */ // release the lock exclusiveLock.release(); 9 File Locking Example – Java API (cont’d) // this locks the second half of the file - shared sharedLock = ch.lock(raf.length()/2+1, raf.length(), SHARED); /** Now read the data . . . */ // release the lock sharedLock.release(); } catch (java.io.IOException ioe) { System.err.println(ioe); }finally { if (exclusiveLock != null) exclusiveLock.release(); if (sharedLock != null) sharedLock.release(); } } } 10
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This note was uploaded on 03/01/2012 for the course CMP 426 taught by Professor Gwangs.jung during the Spring '12 term at CUNY Lehman.

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fileS - File System Chapters 10, 11, 12 1 Chapter 10:...

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