File-System Interface & File System Implementation I Lecture 10 Dr. Cem Özdogan˘ File-System Interface & File System Implementation I File-System Interface Lecture Information File Concept File Attributes File Operations An Example Program Ceng328 Operating Systems at May 04, 2010 Using File System Calls File Types Internal File Structure Access Methods Sequential Access Direct (Random) Access Directory Structure Storage Structure Directory Overview Single-Level Directory Two-Level Directory Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Dr. Cem Özdogan˘ Multiple Users Protection Computer Engineering Department Types of Access Çankaya University Access Control

10.1 File-System Interface & Contents File System Implementation I

1 File-System Interface Dr. Cem Özdogan˘ File Concept File Attributes File Operations An Example Program Using File System Calls File Types Internal File Structure File-System Interface Access Methods File Concept File Attributes Sequential Access File Operations Direct (Random) Access An Example Program Using File System Calls Directory Structure File Types Storage Structure Internal File Structure Access Methods Directory Overview Sequential Access Single-Level Directory Direct (Random) Access Two-Level Directory Directory Structure Storage Structure Tree-Structured Directories Directory Overview Acyclic-Graph Directories Single-Level Directory Two-Level Directory File-System Mounting Tree-Structured Directories Acyclic-Graph Directories File Sharing File-System Mounting Multiple Users File Sharing Multiple Users Protection Protection Types of Access Types of Access Access Control Access Control

10.2 File-System Interface & File-System Interface I File System Implementation I

Dr. Cem Özdogan˘ • Since main memory is usually too small, the computer system must provide secondary storage to back up main memory. • The file system provides the mechanism for storage of and

(multiple) access to both data and programs residing on File-System Interface the disks. File Concept File Attributes • File Management System: Bridges the gap between File Operations An Example Program low-level disk organization (an array of blocks) and the Using File System Calls File Types user’s views (a stream or collection of records) (mapped). Internal File Structure • Access Methods Some devices transfer a character or a block of characters Sequential Access at a time. Direct (Random) Access Directory Structure • Some can be accessed only sequentially, others randomly. Storage Structure • Directory Overview Some transfer data synchronously, others asynchronously. Single-Level Directory • Some are dedicated, some shared. Two-Level Directory • Tree-Structured Directories They can be read-only or read-write. Acyclic-Graph Directories • File-System Mounting Also includes tools outside the kernel; formatting, File Sharing Multiple Users recovery, defrag, consistency, backup utilities (system Protection administration). Types of Access Access Control

10.3 File-System Interface & File-System Interface II File System Implementation I

Dr. Cem Özdogan˘ • In many ways, they are also the slowest major component of the computer. • A file is a collection of related information defined by its creator. File-System Interface • The file system consists of two distinct parts: File Concept File Attributes 1 a collection of files; File Operations An Example Program 2 a directory structure; Using File System Calls File Types • Objectives for a file management system; Internal File Structure Access Methods • Provide a convenient naming system for files. Sequential Access • Direct (Random) Access Provide a standardized set of I/O interface routines and Directory Structure provide access control for multiple users. Storage Structure • Directory Overview Guarantee that the data in the file are valid. Minimize or Single-Level Directory eliminate the potential for lost or destroyed data. Two-Level Directory Tree-Structured Directories • Optimize performance. Acyclic-Graph Directories • File-System Mounting How do you keep one user from reading another’s data? File Sharing • How do you know which blocks are free? Multiple Users Protection Types of Access Access Control

10.4 File-System Interface & File Concept File System Implementation I • Think of a disk as a linear sequence of fixed-size blocks Dr. Cem Özdogan˘ and supporting reading and writing of blocks. • The OS abstracts from the physical properties of its storage devices to define a logical storage unit, the file. • A file is a named collection of related information that is

recorded on secondary storage, with two views: File-System Interface • Logical (programmer) view, File Concept • File Attributes Physical (OS) view, File Operations • An Example Program The information in a file is defined by its creator. Using File System Calls • File Types Data files may be numeric, alphabetic, alphanumeric, or Internal File Structure binary. Access Methods • Sequential Access Files may be free form, such as text files, or may be Direct (Random) Access formatted rigidly. Directory Structure Storage Structure • A file has a certain defined structure, which depends on its Directory Overview Single-Level Directory type. Two-Level Directory • Tree-Structured Directories A text file; (sequence of characters) Acyclic-Graph Directories • A source file; (sequence of subroutines and functions) File-System Mounting • File Sharing An object file is a sequence of bytes organized into blocks Multiple Users understandable by the system’s linker. Protection Types of Access • An executable file is a series of code sections that the Access Control loader can bring into memory and execute. 10.5 File-System Interface & File Attributes I File System Implementation I When a file is named, it becomes independent of the process, Dr. Cem Özdogan˘ the user, and even the system that created it.

File-System Interface File Concept File Attributes File Operations An Example Program Using File System Calls File Types Internal File Structure Access Methods Sequential Access Direct (Random) Access Directory Structure Storage Structure Directory Overview Single-Level Directory Two-Level Directory Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Figure: Some possible file attributes. Protection Types of Access Access Control

10.6 File-System Interface & File Attributes II File System Implementation I • The table of Fig. 1 shows some of the possibilities, but Dr. Cem Özdogan˘ other ones also exist. No existing system has all of these, but each one is present in some system. • A file’s attributes vary from one OS to another but typically consist of these: File-System Interface • Name. File Concept • File Attributes Identifier. This unique tag, usually a number, identifies the File Operations file within the file system (non-human-readable name) An Example Program Using File System Calls • Type. File Types • Internal File Structure Location. This information is a pointer to a device and to Access Methods the location of the file on that device. Sequential Access • Direct (Random) Access Size. The current size of the file (in bytes, words, or blocks) Directory Structure and possibly the maximum allowed size. Storage Structure Directory Overview • Protection. Access-control information determines who can Single-Level Directory Two-Level Directory do reading, writing, executing, and so on. Tree-Structured Directories • Time, date, and user identification. This information may Acyclic-Graph Directories File-System Mounting be kept for creation, last modification, and last use. File Sharing Multiple Users • The information about all files is kept in the directory Protection Types of Access structure, which also resides on secondary storage. Access Control

10.7 File-System Interface & File Operations I File System Implementation I

Dr. Cem Özdogan˘

• A file is an abstract data type. To define a file properly, we need to consider the operations that can be performed on files. • Six basic file operations. The OS can provide system calls File-System Interface to create, write, read, reposition, delete, and truncate files. File Concept • File Attributes Creating a file. Two steps are necessary to create a file. File Operations An Example Program 1 Space in the file system must be found for the file. Using File System Calls 2 An entry for the new file must be made in the directory. File Types Internal File Structure • Writing a file. The write pointer must be updated whenever Access Methods Sequential Access a write occurs. Direct (Random) Access • Reading a file. The system needs to keep a read pointer to Directory Structure Storage Structure the location in the file where the next read is to take place. Directory Overview • Because a process is usually either reading from or writing to Single-Level Directory Two-Level Directory a file, the current operation location can be kept as a Tree-Structured Directories per-process current-file-position pointer. Acyclic-Graph Directories • File-System Mounting Both the read and write operations use this same pointer, File Sharing saving space and reducing system complexity. Multiple Users Protection Types of Access Access Control

10.8 File-System Interface & File Operations II File System Implementation I • Repositioning within a file. The directory is searched for Dr. Cem Özdogan˘ the appropriate entry, and the current-file-position pointer is repositioned to a given value (file seek). • Deleting a file. To delete a file, we search the directory for the named file. Having found the associated directory entry, we release all file space and erase the directory File-System Interface entry. File Concept • File Attributes Truncating a file. The user may want to erase the File Operations An Example Program contents of a file but keep its attributes. The file be reset to Using File System Calls length zero and its file space released. File Types Internal File Structure Access Methods Sequential Access Direct (Random) Access Directory Structure Storage Structure Directory Overview Single-Level Directory Two-Level Directory Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Protection Types of Access Access Control Figure: File operations. 10.9 File-System Interface & File Operations III File System Implementation I

Dr. Cem Özdogan˘ • These primitive operations can then be combined to perform other file operations (i.e., copying). • The OS keeps a small table, called the open-file table, containing information about all open files.

• When a file operation is requested, the file is specified via File-System Interface File Concept an index into this table, so no searching is required. File Attributes • When the file is no longer being actively used, it is closed by File Operations An Example Program the process, and the OS removes its entry from the Using File System Calls File Types open-file table. Internal File Structure • Access Methods Most systems require that the programmer open a file Sequential Access Direct (Random) Access explicitly with the open() system call before that file can be Directory Structure used. Storage Structure Directory Overview • Single-Level Directory The implementation of the open() and close() operations Two-Level Directory is more complicated in an environment where Tree-Structured Directories Acyclic-Graph Directories several processes may open the file at the same time. File-System Mounting File Sharing • This may occur in a system where several different Multiple Users Protection applications open the same file at the same time. Types of Access Access Control

10.10 File-System Interface & File Operations IV File System Implementation I

Dr. Cem Özdogan˘

• Typically, the OS uses two levels of internal tables: 1 A per-process table. The per-process table tracks all files that a process has open. Access rights to the file and accounting information can also be included. File-System Interface File Concept 2 A system-wide table. Each entry in the per-process table File Attributes File Operations in turn points to a system-wide open-file table (also contains An Example Program process-independent information, such as the location of Using File System Calls File Types the file on disk, access dates, and file size). Internal File Structure Access Methods • Typically, the open-file table also has an open count Sequential Access Direct (Random) Access associated with each file to indicate how many processes Directory Structure Storage Structure have the file open. Directory Overview • Each close() decreases this open count, and when the Single-Level Directory Two-Level Directory open count reaches zero, the file is no longer in use, and Tree-Structured Directories Acyclic-Graph Directories the file’s entry is removed from the open-file table. File-System Mounting File Sharing Multiple Users Protection Types of Access Access Control

10.11 File-System Interface & File Operations V File System Implementation I

Dr. Cem Özdogan˘

• In summary, several pieces of information are associated with an open file. • File pointer. • File-open count. File-System Interface • File Concept Disk location of the file. The information needed to locate File Attributes the file on disk is kept in memory so that the system does File Operations An Example Program not have to read it from disk for each operation. Using File System Calls • File Types Access rights. Each process opens a file in an access Internal File Structure mode. This information is stored on the per-process table Access Methods Sequential Access so the OS can allow or deny subsequent I/O requests. Direct (Random) Access Directory Structure • Some OSs provide facilities for locking an open file (or Storage Structure Directory Overview sections of a file). Single-Level Directory Two-Level Directory • File locks are useful for files that are shared by several Tree-Structured Directories Acyclic-Graph Directories processes. File-System Mounting File Sharing Multiple Users Protection Types of Access Access Control

10.12 File-System Interface & An Example Program Using File System Calls I File System Implementation I

Dr. Cem Özdogan˘

• A simple UNIX program that copies one file from its source file to a destination file (see Fig. 3). The program has minimal functionality and even worse error reporting. File-System Interface copyfile abc xyz File Concept File Attributes • The copy loop. It starts by trying to read in 4 KB of data to File Operations An Example Program buffer. It does this by calling the library procedure read, Using File System Calls File Types which actually invokes the read system call. Internal File Structure Access Methods • The call to write outputs the buffer to the destination file. Sequential Access Direct (Random) Access • When the entire file has been processed, the first call Directory Structure Storage Structure beyond the end of file will return 0 to rd_count which will Directory Overview Single-Level Directory make it exit the loop. At this point the two files are closed Two-Level Directory and the program exits with a status indicating normal Tree-Structured Directories Acyclic-Graph Directories termination. File-System Mounting File Sharing Multiple Users Protection Types of Access Access Control

10.13 File-System Interface & An Example Program Using File System Calls II File System Implementation I

Dr. Cem Özdogan˘

File-System Interface File Concept File Attributes File Operations An Example Program Using File System Calls File Types Internal File Structure Access Methods Sequential Access Direct (Random) Access Directory Structure Storage Structure Directory Overview Single-Level Directory Two-Level Directory Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Protection Types of Access Access Control

Figure: A simple program to copy a file. 10.14 File-System Interface & File Types I File System Implementation I

Dr. Cem Özdogan˘ • A common technique for implementing file types is to include the type as part of the file name (see Fig. 4). • The name is split into two parts -a name and an extension, usually separated by a period character. • In this way, the user and the OS can tell from the name File-System Interface File Concept alone what the type of a file is. File Attributes • File Operations Many file systems support names as long as 255 An Example Program Using File System Calls characters. Some file systems distinguish between upper File Types and lower case letters (Case (in)sensitivity). Internal File Structure Access Methods • Sequential Access Windows 95 and Windows 98 both use the MS-DOS file Direct (Random) Access system, and thus inherit many of its properties, such as Directory Structure Storage Structure how file names are constructed. Directory Overview Single-Level Directory • Windows NT and Windows 2000 support the MS-DOS file Two-Level Directory Tree-Structured Directories system and thus also inherit its properties. However, these Acyclic-Graph Directories File-System Mounting OSs also have a native file system (NTFS) that has File Sharing Multiple Users different properties. Protection Types of Access Access Control

10.15 File-System Interface & File Types II File System Implementation I

Dr. Cem Özdogan˘

File-System Interface File Concept File Attributes File Operations An Example Program Using File System Calls File Types Internal File Structure Access Methods Sequential Access Direct (Random) Access Directory Structure Storage Structure Directory Overview Single-Level Directory Two-Level Directory Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Protection Types of Access Figure: Common file types. Access Control

10.16 File-System Interface & File Types III File System Implementation I • The UNIX system uses a crude magic number stored at the beginning of some files to indicate roughly the type of Dr. Cem Özdogan˘ the file -executable program, batchfile (or shell script), PostScript file, and so on. • Not all files have magic numbers, so system features cannot be based solely on this information.

• UNIX does allow file-name-extension hints, but these File-System Interface extensions are neither enforced nor depended on by the OS File Concept File Attributes (interpreted by tools). File Operations An Example Program • In contrast, Windows is aware of the extensions and Using File System Calls File Types assigns meaning to them. Users (or processes) can register Internal File Structure extensions with the operating system (Interpreted by OS). Access Methods • Sequential Access UNIX also has character and block special files (Device Direct (Random) Access Files). Directory Structure Storage Structure • Character special files are related to input/output and used Directory Overview Single-Level Directory to model serial I/O devices such as terminals, printers, and Two-Level Directory networks. Tree-Structured Directories • Acyclic-Graph Directories Block special files are used to model disks. File-System Mounting • File Sharing Other files are binary files, which just means that they are Multiple Users not ASCII files. Protection Types of Access • Usually, they have some internal structure known to Access Control programs that use them (see Fig. 5). 10.17 File-System Interface & File Types IV File System Implementation I

Dr. Cem Özdogan˘

Module Magic number name Header Text size Data size Date BSS size Object Symbol table size Owner

Header module Entry point Protection File-System Interface Size File Concept Flags Header File Attributes File Operations Text An Example Program Object module Using File System Calls File Types Data Internal File Structure Header Access Methods Sequential Access Relocation bits Direct (Random) Access Object Directory Structure module Symbol Storage Structure table Directory Overview Single-Level Directory

(a) (b) Two-Level Directory Tree-Structured Directories Acyclic-Graph Directories Figure: (a) An executable file. (b) An archive. File-System Mounting File Sharing Multiple Users Protection Types of Access Every OS must recognize at least one file type; its own Access Control executable file. 10.18 File-System Interface & File Types V File System Implementation I

Dr. Cem Özdogan˘

• A simple executable binary file taken from a version of UNIX is seen in Fig. 5a . • Although technically the file is just a sequence of bytes, the operating system will only execute a file if it has the proper File-System Interface format. File Concept • It has five sections: header, text, data, relocation bits, and File Attributes File Operations symbol table. An Example Program • Using File System Calls Try the following commands: File Types Internal File Structure readelf -S exe_file Access Methods objdump -h exe_file Sequential Access Direct (Random) Access • Second example of a binary file is an archive, also from Directory Structure Storage Structure UNIX (see Fig. 5b). Directory Overview • Single-Level Directory It consists of a collection of library procedures (modules) Two-Level Directory compiled but not linked. Tree-Structured Directories Acyclic-Graph Directories • Each one is prefaced by a header telling its name, creation File-System Mounting File Sharing date, owner, protection code, and size. Multiple Users Protection Types of Access Access Control

10.19 File-System Interface & Internal File Structure I File System Implementation I • Three common possibilities for structuring are depicted in Dr. Cem Özdogan˘ Fig. 6. 1 Byte 1 Record

Ant Fox Pig

Cat Cow Dog Goat Lion Owl Pony Rat Worm File-System Interface File Concept File Attributes File Operations Hen Ibis Lamb An Example Program Using File System Calls (a) (b) (c) File Types Internal File Structure Access Methods Figure: Three kinds of files. (a) Byte sequence. (b) Record Sequential Access sequence. (c) Tree. Direct (Random) Access Directory Structure Storage Structure • Directory Overview Stream of Bytes. The file in Fig. 6a is an unstructured Single-Level Directory sequence of bytes. All it sees are bytes. Two-Level Directory • Tree-Structured Directories Records. The first step up in structure is shown in Fig. 6b. Acyclic-Graph Directories A file is a sequence of fixed-length records, each with some File-System Mounting File Sharing internal structure. Multiple Users • Protection Tree of Records. The third kind of file structure is shown in Types of Access Fig. 6c. In this organization, a file consists of a tree of Access Control records, not necessarily all the same length (a key field). 10.20 File-System Interface & Internal File Structure II File System Implementation I • Internally, locating an offset within a file can be Dr. Cem Özdogan˘ complicated for the OS. • All disk I/O is performed in units of one block (physical record), and all blocks are the same size. • It is unlikely that the physical record size will exactly match File-System Interface the length of the desired logical record. File Concept File Attributes • File Operations Packing a number of logical records into physical blocks is An Example Program Using File System Calls a common solution to this problem. File Types • Internal File Structure The file may be considered to be a sequence of blocks. All Access Methods Sequential Access the basic I/O functions operate in terms of blocks. Direct (Random) Access • Directory Structure Because disk space is always allocated in blocks, some Storage Structure portion of the last block of each file is generally wasted. Directory Overview Single-Level Directory • Two-Level Directory The waste incurred to keep everything in units of blocks Tree-Structured Directories (instead of bytes) is internal fragmentation. Acyclic-Graph Directories File-System Mounting • File Sharing All file systems suffer from internal fragmentation; the Multiple Users larger the block size, the greater the internal Protection Types of Access fragmentation. Access Control

10.21 File-System Interface & Sequential Access I File System Implementation I

Dr. Cem Özdogan˘

• The simplest access method is sequential access. • Information in the file is processed in order, one record File-System Interface after the other. File Concept File Attributes • Reads and writes make up the bulk of the operations on a File Operations An Example Program file. Using File System Calls File Types • A read operation read next reads the next portion of the file Internal File Structure Access Methods and automatically advances a file pointer, which tracks the Sequential Access I/O location. Direct (Random) Access • Directory Structure Similarly, the write operation write next appends to the end Storage Structure of the file and advances to the end of the newly written Directory Overview Single-Level Directory material (the new end of file). Two-Level Directory Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Protection Types of Access Access Control

10.22 File-System Interface & Sequential Access II File System Implementation I

Dr. Cem Özdogan˘

File-System Interface File Concept File Attributes File Operations An Example Program Using File System Calls File Types Internal File Structure Figure: Sequential-access file. Access Methods Sequential Access Direct (Random) Access Directory Structure Sequential access, which is depicted in Fig. 7, is based on a Storage Structure Directory Overview tape model of a file and works as well on sequential-access Single-Level Directory Two-Level Directory devices as it does on random-access ones. Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Protection Types of Access Access Control

10.23 File-System Interface & Direct (Random) Access File System Implementation I • Another method is direct access (or relative access). Dr. Cem Özdogan˘ • A file is made up of fixed-length logical records that allow programs to read and write records rapidly in no particular order. • There are no restrictions on the order of reading or writing for a direct-access file. File-System Interface File Concept • The direct-access method is based on a disk model of a File Attributes File Operations file, since disks allow random access to any file block. An Example Program Using File System Calls • Direct-access files are of great use for immediate access File Types Internal File Structure to large amounts of information (Databases). Access Methods • Sequential Access The block number provided by the user to the OS is Direct (Random) Access normally a relative block number. Directory Structure Storage Structure • A relative block number is an index relative to the beginning Directory Overview Single-Level Directory of the file. Two-Level Directory • Thus, the first relative block of the file is 0, the next is 1, and Tree-Structured Directories Acyclic-Graph Directories so on, even though the actual absolute disk address of the File-System Mounting block may be 14703 for the first block and 3192 for the File Sharing Multiple Users second. Protection Types of Access • Modern OSs have all their files are automatically random Access Control access. 10.24 File-System Interface & Storage Structure I File System Implementation I

Dr. Cem Özdogan˘ • Sometimes, it is desirable to place multiple file systems on a disk or to use parts of a disk for a file system and other parts for other things, such as swap space or unformatted (raw) disk space. • These parts are known variously as partitions, slices, or File-System Interface File Concept (in the IBM world) minidisks. File Attributes File Operations • An Example Program A file system can be created on each of these parts of the Using File System Calls disk. We simply refer to a chunk of storage that holds a file File Types Internal File Structure system as a volume. Access Methods Sequential Access • Each volume that contains a file system must also contain Direct (Random) Access Directory Structure information about the files in the system. This information Storage Structure Directory Overview is kept in entries in a device directory or volume table of Single-Level Directory Two-Level Directory contents. Tree-Structured Directories • Acyclic-Graph Directories The device directory (more commonly known simply as a File-System Mounting directory) records information-such as name, location, File Sharing Multiple Users size, and type-for all files on that volume. Protection Types of Access Access Control

10.25 File-System Interface & Storage Structure II File System Implementation I

Dr. Cem Özdogan˘

Figure 8 shows a typical file-system organization.

File-System Interface File Concept File Attributes File Operations An Example Program Using File System Calls File Types Internal File Structure Access Methods Sequential Access Direct (Random) Access Directory Structure Storage Structure Directory Overview Single-Level Directory Two-Level Directory Figure: A typical file-system organization. Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Protection Types of Access Access Control

10.26 File-System Interface & Directory Overview File System Implementation I • To keep track of files, file systems normally have Dr. Cem Özdogan˘ directories or folders. • Usually, a directory is itself a file. • The directory can be viewed as a symbol table that translates file names into their directory entries. File-System Interface • File Concept A typical directory entry contains information (attributes, File Attributes location, ownership) about a file. File Operations An Example Program • Using File System Calls When considering a particular directory structure, we need File Types to keep in mind the operations that are to be performed on Internal File Structure Access Methods a directory: Sequential Access • Direct (Random) Access Search for a file. Directory Structure • Create a file. Storage Structure • Directory Overview Delete a file. Single-Level Directory • List a directory. Two-Level Directory Tree-Structured Directories • Rename a file. Acyclic-Graph Directories • File-System Mounting Traverse the file system. We may wish to access every File Sharing directory and every file within a directory structure. For Multiple Users Protection reliability, it is a good idea to save the contents and structure Types of Access of the entire file system at regular intervals (backup copy). Access Control

10.27 File-System Interface & Single-Level Directory File System Implementation I • The simplest directory structure is the single-level Dr. Cem Özdogan˘ directory. • All files are contained in the same directory, which is easy to support and understand (see Fig. 9).

File-System Interface File Concept File Attributes File Operations An Example Program Using File System Calls Figure: Single-level directory. File Types Internal File Structure Access Methods Sequential Access • A single-level directory has significant limitations, when Direct (Random) Access Directory Structure the number of files increases or when the system has Storage Structure Directory Overview more than one user. Single-Level Directory • Two-Level Directory Since all files are in the same directory, they must have Tree-Structured Directories unique names. If two users call their data file test, then the Acyclic-Graph Directories File-System Mounting unique-name rule is violated. File Sharing Multiple Users • Even a single user on a single-level directory may find it Protection Types of Access difficult to remember the names of all the files as the Access Control number of files increases. 10.28 File-System Interface & Two-Level Directory I File System Implementation I

Dr. Cem Özdogan˘

• The standard solution to limitations of single-level directory is to create a separate directory for each user. • In the two-level directory structure, each user has his own

user file directory (UFD). File-System Interface • File Concept The UFDs have similar structures, but each lists File Attributes File Operations only the files of a single user. An Example Program Using File System Calls • When a user job starts or a user logs in, the system’s File Types Internal File Structure master file directory (MFD) is searched. Access Methods Sequential Access • The MFD is indexed by user name or account number, and Direct (Random) Access Directory Structure each entry points to the UFD for that user (see Fig. 10). Storage Structure Directory Overview • When a user refers to a particular file, only his own UFD is Single-Level Directory Two-Level Directory searched (create a file, delete a file?). Tree-Structured Directories • Acyclic-Graph Directories Although the two-level directory structure solves the File-System Mounting File Sharing name-collision problem, it still has disadvantages. Multiple Users Protection Types of Access Access Control

10.29 File-System Interface & Two-Level Directory II File System Implementation I

Dr. Cem Özdogan˘

File-System Interface File Concept File Attributes Figure: Two-level directory structure. File Operations An Example Program Using File System Calls File Types Internal File Structure • This structure effectively isolates one user from another. Access Methods Sequential Access • Direct (Random) Access Isolation is an advantage when the users are completely Directory Structure independent but is a disadvantage when the users want to Storage Structure Directory Overview cooperate on some task and to access one another’s files. Single-Level Directory Two-Level Directory • A two-level directory can be thought of as a tree, or an Tree-Structured Directories Acyclic-Graph Directories inverted tree, of height 2. File-System Mounting • File Sharing The root of the tree is the MFD. Multiple Users • Its direct descendants are the UFDs. Protection • Types of Access The descendants of the UFDs are the files themselves. The Access Control files are the leaves of the tree. 10.30 File-System Interface & Two-Level Directory III File System Implementation I

Dr. Cem Özdogan˘

• Specifying a user name and a file name defines a path in the tree from the root (the MFD) to a leaf (the specified File-System Interface file). File Concept • File Attributes Thus, a user name and a file name define a path name. File Operations An Example Program • To name a file uniquely, a user must know the path name Using File System Calls File Types of the file desired. Internal File Structure Access Methods • Additional syntax is needed to specify the volume of a file. Sequential Access Direct (Random) Access • For instance, in MS-DOS a volume is specified by a letter Directory Structure Storage Structure followed by a colon. Thus, a file specification might be Directory Overview Single-Level Directory C:\userb\test Two-Level Directory Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Protection Types of Access Access Control

10.31 File-System Interface & Tree-Structured Directories I File System Implementation I • Once we have seen how to view a two-level directory as a Dr. Cem Özdogan˘ two-level tree, the natural generalization is to extend the directory structure to a tree of arbitrary height ( see Fig. 11).

File-System Interface File Concept File Attributes File Operations An Example Program Using File System Calls File Types Internal File Structure Access Methods Sequential Access Direct (Random) Access Directory Structure Storage Structure Directory Overview Single-Level Directory Two-Level Directory Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Protection Figure: Tree-structured directory structure. Types of Access Access Control

10.32 File-System Interface & Tree-Structured Directories II File System Implementation I • This generalization allows users to create their own Dr. Cem Özdogan˘ subdirectories and to organize their files accordingly. • A tree is the most common directory structure. • The tree has a root directory, and every file in the system has a unique path name. • A directory is simply another file, but it is treated in a File-System Interface File Concept special way. File Attributes File Operations • One bit in each directory entry defines the entry An Example Program Using File System Calls • as a file (0), File Types • Internal File Structure as a subdirectory (1). Access Methods • Sequential Access Path names can be of two types: absolute and relative Direct (Random) Access • Directory Structure With a tree-structured directory system, users can be Storage Structure allowed to access, in addition to their files, the files of Directory Overview Single-Level Directory other users. Two-Level Directory • Tree-Structured Directories For example, user B can access a file of user A by Acyclic-Graph Directories specifying its path names. File-System Mounting • File Sharing User B can specify either an absolute or a relative path Multiple Users name. Protection Types of Access • Alternatively, user B can change her current directory to be Access Control user A’s directory and access the file by its file names. 10.33 File-System Interface & Acyclic-Graph Directories I File System Implementation I • The acyclic graph is a natural generalization of the Dr. Cem Özdogan˘ tree-structured directory scheme. • The common subdirectory should be shared. • A tree structure prohibits the sharing of files or directories. • An acyclic graph (a graph with no cycles) allows directories to share subdirectories and files (see Fig. 12). File-System Interface File Concept File Attributes File Operations An Example Program Using File System Calls File Types Internal File Structure Access Methods Sequential Access Direct (Random) Access Directory Structure Storage Structure Directory Overview Single-Level Directory Two-Level Directory Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Protection Types of Access Figure: Acyclic-graph directory structure. Access Control

10.34 File-System Interface & Acyclic-Graph Directories II File System Implementation I • The same file or subdirectory may be in two different Dr. Cem Özdogan˘ directories. • It is important to note that a shared file (or directory) is not the same as two copies of the file. • With two copies, each programmer can view the copy rather than the original, but if one programmer changes the file, the changes will not appear in the other’s copy. File-System Interface • File Concept With a shared file, only one actual file exists, so any File Attributes changes made by one person are immediately visible to the File Operations An Example Program other. Using File System Calls File Types • A common way, exemplified by many of the UNIX Internal File Structure Access Methods systems, is to create a new directory entry called a link. Sequential Access • Direct (Random) Access A link is effectively a pointer to another file or subdirectory. Directory Structure • We resolve the link by using that path name to locate the Storage Structure Directory Overview real file. Single-Level Directory Two-Level Directory • Links are easily identified by their format in the directory Tree-Structured Directories Acyclic-Graph Directories entry and are effectively named indirect pointers. File-System Mounting • File Sharing Another common approach to implementing shared files is Multiple Users simply to duplicate all information about them in both Protection Types of Access sharing directories. Access Control • Thus, both entries are identical and equal. 10.35 File-System Interface & Acyclic-Graph Directories III File System Implementation I • A link is clearly different from the original directory entry; Dr. Cem Özdogan˘ thus, the two are not equal. • Several problems must be considered carefully for an acyclic-graph directory structure. • A file may now have multiple absolute path names. • Another problem involves deletion. When can the space allocated to a shared file be deallocated and reused? File-System Interface • One possibility is to remove the file whenever anyone deletes File Concept File Attributes it, but this action may leave dangling pointers to the now File Operations nonexistent file. An Example Program • Using File System Calls Worse, if the remaining file pointers contain actual disk File Types addresses, and the space is subsequently reused for other Internal File Structure files, these dangling pointers may point into the middle of Access Methods Sequential Access other files. Direct (Random) Access • In a system where sharing is implemented by Directory Structure Storage Structure symbolic links, this situation is somewhat easier to handle. Directory Overview • Single-Level Directory The deletion of a link need not affect the original file; only Two-Level Directory the link is removed. Tree-Structured Directories Acyclic-Graph Directories • If the file entry itself is deleted, the space for the file is File-System Mounting File Sharing deallocated, leaving the links dangling. Multiple Users • In the case of UNIX, symbolic links are left when a file is Protection Types of Access deleted. Microsoft Windows (all flavours) uses the same Access Control approach. 10.36 File-System Interface & Acyclic-Graph Directories IV File System Implementation I

Dr. Cem Özdogan˘ • Another approach to deletion is to preserve the file until all references to it are deleted. • To implement this approach, we must have some mechanism for determining that the last reference to the file has been deleted. File-System Interface File Concept • The trouble with this approach is the variable and File Attributes File Operations potentially large size of the file-reference list. An Example Program Using File System Calls • However, we really do not need to keep the entire list -we File Types Internal File Structure need to keep only a count of the number of references. Access Methods • Sequential Access Adding a new link or directory entry increments the Direct (Random) Access reference count; Directory Structure Storage Structure • Deleting a link or entry decrements the count. Directory Overview • Single-Level Directory When the count is 0, the file can be deleted; there are no Two-Level Directory remaining references to it. Tree-Structured Directories Acyclic-Graph Directories • The UNIX OS uses this approach for non-symbolic links File-System Mounting File Sharing (or hard links), keeping a reference count in the file Multiple Users Protection information block (or inode). Types of Access Access Control

10.37 File-System Interface & File-System Mounting I File System Implementation I

Dr. Cem Özdogan˘ • Just as a file must be opened before it is used, a file system must be mounted before it can be available to processes on the system. • The mount procedure is straightforward. The OS is given

the name of the device and the mount point. File-System Interface File Concept • Typically, a mount point is an empty directory. File Attributes File Operations • An Example Program Next, the OS verifies that the device Using File System Calls contains a valid file system. File Types Internal File Structure • Finally, the OS notes in its directory structure that a file Access Methods Sequential Access system is mounted at the specified mount point. Direct (Random) Access Directory Structure • To illustrate file mounting, consider the file system Storage Structure Directory Overview depicted in Fig. 13. Single-Level Directory Two-Level Directory • At this point, only the files on the existing file system can Tree-Structured Directories Acyclic-Graph Directories be accessed. File-System Mounting File Sharing • Figure 14 shows the effects of mounting the volume Multiple Users Protection residing on /dev/disk over /users. Types of Access Access Control

10.38 File-System Interface & File-System Mounting II File System Implementation I

Dr. Cem Özdogan˘

File-System Interface File Concept File Attributes File Operations Figure: File system. (a) Existing system. (b) Unmounted volume. An Example Program Using File System Calls File Types Internal File Structure Access Methods Sequential Access Direct (Random) Access Directory Structure Storage Structure Directory Overview Single-Level Directory Two-Level Directory Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Protection Types of Access Figure: Mount point. Access Control

10.39 File-System Interface & Multiple Users File System Implementation I • When an OS accommodates multiple users, the issues of Dr. Cem Özdogan˘ file sharing, file naming, and file protection become important. • Most systems have evolved to use the concepts of file (or directory) owner (or user) and group. • The owner is the user who can change attributes and grant File-System Interface access and who has the most control over the file. File Concept • The group attribute defines a subset of users who can share File Attributes File Operations access to the file. An Example Program Using File System Calls • For example, the owner of a file on a UNIX system can File Types Internal File Structure issue all operations on a file, while members of the file’s Access Methods Sequential Access group can execute one subset of those operations, and all Direct (Random) Access Directory Structure other users can execute another subset of operations. Storage Structure • Directory Overview The owner and group IDs of a given file (or directory) are Single-Level Directory stored with the other file attributes. Two-Level Directory Tree-Structured Directories • When a user requests an operation on a file, the user ID Acyclic-Graph Directories File-System Mounting can be compared with the owner attribute to determine if File Sharing Multiple Users the requesting user is the owner of the file. Protection • Types of Access Likewise, the group IDs can be compared. Access Control • The result indicates which permissions are applicable. 10.40 File-System Interface & Protection File System Implementation I

Dr. Cem Özdogan˘

• When information is stored in a computer system, we want to keep it safe from physical damage (reliability) and improper access (protection). File-System Interface • File Concept Reliability is generally provided by duplicate copies of files File Attributes (copy disk files to tape). File Operations An Example Program • Using File System Calls File systems can be damaged by hardware problems File Types Internal File Structure (such as errors in reading or writing), power surges or Access Methods failures, head crashes, dirt, temperature extremes, and Sequential Access Direct (Random) Access vandalism. Directory Structure Storage Structure • Files may be deleted accidentally. Directory Overview Single-Level Directory • Bugs in the file-system software can also cause file Two-Level Directory Tree-Structured Directories contents to be lost. Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Protection Types of Access Access Control

10.41 File-System Interface & Types of Access File System Implementation I • The need to protect files is a direct result of the ability to Dr. Cem Özdogan˘ access files. • Systems that do not permit access to the files of other users do not need protection. • Alternatively, we could provide free access with no

protection. File-System Interface • File Concept Both approaches are too extreme for general use. What is File Attributes needed is controlled access. File Operations An Example Program • Using File System Calls Several different types of operations may be controlled: File Types • Internal File Structure Read. Access Methods • Write. Sequential Access • Direct (Random) Access Execute. Load the file into memory and execute it. Directory Structure • Append. Write new information at the end of the file. Storage Structure Directory Overview • Delete. Single-Level Directory • Two-Level Directory List. List the name and attributes of the file. Tree-Structured Directories • Acyclic-Graph Directories Other operations, such as renaming, copying, and editing File-System Mounting File Sharing the file, may also be controlled. Multiple Users • Protection These higher-level functions may be implemented by a Types of Access system program that makes lower-level system calls. Access Control

10.42 File-System Interface & Access Control I File System Implementation I • The most common approach to the protection problem is Dr. Cem Özdogan˘ to make access dependent on the identity of the user. • The most general scheme to implement identity-dependent access is to associate with each file and directory an access-control list (ACL) specifying user names and the types of access allowed for each user. File-System Interface • This approach has the advantage of enabling complex File Concept File Attributes access methodologies. File Operations An Example Program • The main problem with access lists is their length. If we Using File System Calls File Types want to allow everyone to read a file, we must list all users Internal File Structure Access Methods with read access. Sequential Access • These problems can be resolved by use of a condensed Direct (Random) Access Directory Structure version of the access list. Storage Structure • Directory Overview To condense the length of the access-control list, many Single-Level Directory systems recognize three classifications of users in Two-Level Directory Tree-Structured Directories connection with each file: Acyclic-Graph Directories • File-System Mounting Owner. The user who created the file is the owner. File Sharing • Group. A set of users who are sharing the file and need Multiple Users Protection similar access is a group, or work group. Types of Access • Universe. All other users in the system constitute the Access Control

universe. 10.43 File-System Interface & Access Control II File System Implementation I

Dr. Cem Özdogan˘

• With this more limited protection classification, only three

fields are needed to define protection. File-System Interface File Concept • Often, each field is a collection of bits, and each bit either File Attributes File Operations allows or prevents the access associated with it. An Example Program Using File System Calls • For example, the UNIX system defines three fields of 3 bits File Types Internal File Structure each−rwx, where r controls read access, w controls write Access Methods Sequential Access access, and x controls execution. Direct (Random) Access • Directory Structure In this scheme, nine bits per file are needed to record Storage Structure Directory Overview protection information. Single-Level Directory Two-Level Directory Tree-Structured Directories Acyclic-Graph Directories File-System Mounting File Sharing Multiple Users Protection Types of Access Access Control

10.44