Operating Systems Lecture 9a
Dr Ronald Grau School of Engineering and Informatics Spring term 2020
Previously 1 Memory management
Virtual memory
Addressing and address spaces Partitioning and segmentation Paging & Page replacement
Recap: Questions 2 Recap questions
1. How can we avoid page faults when a process starts?
2. How can thrashing be controlled?
3. What is a working set?
4. What is the significance of the reference bit in the page table?
5. What is the main problem of the FIFO page replacement policy?
6. What information is required to operate the LRU policy?
Today 3 Files
Persistent storage
Formats, access, operations Attributes & permissions
Implementation
File abstraction 4 Files
Persistent storage of data Named
Byte-wise access
Access protection
Consistency guarantees Operating system duties:
Manage efficient access (e.g. caching)
Protect storage device from being damaged/corrupted Provide uniform interface for heterogeneous devices
File abstraction 5 Basic terminology
Bit: 0, 1
Byte: typically 8 bits
Word: the number of bits a processor can operate on at once
Character: maps one or more bytes to letters, digits, etc
Character set: defines the mapping system (e.g. ASCII, Unicode) Field: a group of characters
Record: a group of fields
File: a group of related records
Volume: a unit of data storage holding multiple files
File system: the way the OS organises files and manages access
File names 6
Human-readable name to identify a file
Maps to an internal identifier in the file system
Length of the file name
MS-DOS: 8 characters + extension Modern OS: 255 characters
Relevance of upper/lower case, allowed characters
Unix distinguishes between upper/lower case, unlike Windows
File name extensions
MS-DOS: separate from the le name
Modern OS: convention, multiple extensions possible
File types 7
Regular files
Text files: interpretation as characters
Binary files: interpretation application-specific e.g. executables, pictures
Directories: hierarchy of files Special files for I/O devices
File formats 8 Define how the data is organised within a file
Fixed-length records
Variable-length records Indexed records
Trees: XML, JSON
File formats 9 Example: ZIP format
File formats 10 Example: ZIP format
File formats 11 Example: GIF format
File formats 12 Example: GIF format
File access 13
File access 14 Sequential access:
Random access:
File operations 15 Typical operations:
Create, Delete, Rename
Open, Close
Read, Write, Append, Truncate Seek
Get Attributes, Set Attributes
Lock, Unlock
File operations 16 Example: POSIX System calls
int open(const char *filename, int flags, mode t mode);
// example:
int fd = open(“myfile.txt”, O WRONLY | O CREAT, 0644); if (fd < 0) switch(errno) { ... }
File operations 17
Example: POSIX System calls
ssize_t read(int fd, void *buf, size_t count);
// example:
char buffer[BUF SIZE];
ssize_t bytes read = read(fd, buffer, BUF_SIZE);
File operations 18 Example: POSIX System calls
ssize_t write(int fd, void *buf, size_t count);
// example:
char buffer[BUF_SIZE];
ssize_t_bytes_written = write(fd, buffer, BUF_SIZE);
File operations 19 Example: POSIX System calls
ssize_t lseek(int fd, off t_offset, int whence);
// example:
off_t pos = lseek(fd, 0, SEEK_CUR);
File operations 20 Example: POSIX System calls
int close(int fd);
int unlink(const char *pathname);
int fcntl(int fd, int cmd, ... /* args */ );
File operations 21
Example: POSIX System calls
int stat(const char *filename, struct stat *buf);
File attributes 22 Typical metadata:
Creator, Owner, Owner Group, Protection, Password
Read-only flag, Write flag, Executable flag, Hidden flag,
System flag, Archive flag, ASCII/binary flag, Random access flag, Temporary flag, Lock flags
Creation time, Time of last access, Time of last change
Record length, Key position, Key length
Current size, Maximum size
File attributes 23 Example: UNIX file attributes
File attributes 24 Example: UNIX file permissions
File attributes 25 Example: UNIX file permissions
File implementation 26 Example:
java.io.File
File(String pathname)
boolean exists()
boolean createNewFile()
boolean renameTo(File dest)
boolean delete()
long length()
long lastModified()
boolean canRead() )
boolean canWrite()
boolean canExecute()
boolean setReadable(boolean readable, boolean ownerOnly)
boolean setWritable(boolean writable, boolean ownerOnly)
boolean setExecutable(boolean executable, boolean ownerOnly)
.. .
File implementation 27 Example:
java.io.RandomAccessFile
RandomAccessFile(String name, String mode)
throws FileNotFoundException
void close()
int read(byte[] b)
int write(byte[] b)
void seek(long pos)
long getFilePointer()
void setLength(long newLength) String readLine()
double readDouble()
.. .
throw
IOException,
EOFException, SecurityException, .. .
File implementation 28 Example:
java.io Streams and Readers/Writers
FileInputStream(File file) FileOutputStream(File file)
FileReader(File file) FileWriter(File file)
int read(byte[] buf) void close()
int write(byte[] buf) void close()
int read(char[] cbuf) void close()
int write(char[] cbuf)
void close()
Other aspects 29
How files are stored
How storage space is managed Performance
Reliability
Summary 30 Files
Naming: human-readable identification
Attributes: permissions, ownership, dates, . . .
Access: open - read / write - close
Unified interface for access to a variety of devices Examples: POSIX, Java
Read 31 Tanenbaum & Bos., Modern Operating Systems
Chapter 4
Silberschatz et al., Operating System Concepts Chapters 10 & 11
Next Lecture
32
Introduction
Operating System Architectures Processes
Threads - Programming
Process Scheduling - Evaluation Process Synchronisation
Deadlocks
Memory Management File Systems (continued) Input / Output
Security