File I/O and File Systems

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1 File I/O and File Systems System calls for Þles UNIX File Manipulation and protection creat Create a new ordinary (data) Þle mknod Create a new directory link Add an existing Þle to another directory unlink Remove a Þle from a directory, deleting the Þle if it is now in 0 directories chmod Change protection matrix: Owner Group Others Read T T F Write T F F Execute T F F chown Change owner or group owner File I/O open Prepare a Þle to be accessed read Read bytes from a Þle write Write bytes to a Þle seek Change current read/write position close File is no longer needed 1

2 Attributes of Þles UNIX Associated with each Þle Owner Group Protection matrix Time of last read Time of last write Last time attributes were changed Kind of Þle ordinary, directory, special (devices) Size Contents array of bytes All the above except the contents are stored in a small record on disk called an Inode. Directories map Þle names to Þles. So one Þle could be in several directories, or in one directory under multiple names. 2

3 Example After open, Þles are referred to by integer Þle descriptor int infile = open( fred, O_RDONLY ) ; // returns Þle descriptor // 0666 encodes the protection matrix int newfile = creat( ginger, 0666 ) ; // returns a Þle descriptor if( infile!= -1 && newfile!= -1 ) { char buffer[100] ; while( true ) { int count = read( infile, buffer, sizeof(buffer) ) ; if( count == 0 ) break ; write( newfile, buffer, count ) ; } } Associated with each Þle opening A Þle pointer an integer indicating the position of the next read or write. So one Þle can be opened by multiple processes (or one process more than once) with different Þle pointers. Open Þles are inherited by children on fork. 3

4 Object oriented I/O UNIX In UNIX the read, write, seek, & close system calls apply to Ordinary data Þles Directories Tape drives Terminals Pseudo-terminals Other devices Pipes For interprocess communication. Sockets For network communication. Most of the above can also appear in directories and hence be opened with the open system call. 4

5 Object oriented I/O applied Programs are usually written with the assumption that three Þles are already open, with Þle descriptors 0, 1, & 2. These are standard input (stdin/cin/0) standard output (stdout/cout/1), and standard error (stderr/cerr/2). The process that starts a process (usually the shell) can decide which 3 Þle-like objects are open. For example, the sort program sorts stdin to stdout. The shell command sort <somefile >/dev/tty1 reads an ordinary Þle and writes to terminal 1. The shell command extract <f1 sort uniq -c >feature creates two pipes and three child processes. The sort process reads from and writes to the pipes. Thus sort is polymorphic and interacts with various classes of objects that support the same interface of the read and write system calls. You can easily create subroutines or programs that operate on various devices. 5

6 File system implementation UNIX Each disk is divided into concentric tracks. Each track is divided into blocks. Typically blocks are 512 to 8192 bytes. Each disk level read or write reads or writes an entire block. Each disk can be seen as an array of blocks. UNIX disk organization (assuming one partition): {z} Boot block {z} Super block {z } {z } Array of inodes Content blocks Each Þle is represented by a small (64 byte) record on disk called an Inode and 0 or more of the content blocks. 0 or more indirection blocks. Each Þle is known by its Inode number, which is simply an index into the inode array. For example, if blocks are 512 bytes and 1000 blocks are allocated to the Inode array, then the disk can hold 8000 Þles, numbered 0 to

7 What s in an Inode. Each Inode contains all the Þles attributes see attribute slide not its name. Each Inode also contains 13 block addresses. (Details may vary this is typical) The Þrst 10 point to 10 blocks containing data. 0 1 Inode kind,protection, size,times, etc Data Blocks Indirection Block If block size is 512, works for Þles up to 5120 bytes in size. The 11th index points to a content block full of block addresses. (An indirection block) 7

8 For example if block addresses are 3 bytes long, a 512 byte block holds 170 address, so we can represent Þles up to 5, = 92, 160 bytes The 12th index is similar, but uses double indirection The 13th index is similar, but uses triple indirection good up to ( ) 512 = 2, 530, 344, 960 ' 2.3GB Inode Triple Indirection Block Double Indirection Blocks Indirection Blocks Data Blocks x x170 Small Þles can be accessed quickly. approx 170^3 8

9 Large Þles take more time. 9

10 Directories So far each Þle is known only by its Inode number. A directory is a Þle that maps names to Inode numbers. Each Þle is known by 1 or more path names /usr/dmr/hello.c is Inode 99 /bin/../usr/. is Inode 35 /bin ex 31 vi 31 sh bin 33 usr 35 /usr The root directory: / dmr 77 bwk Editor /bin/vi or /bin/ex Shell /bin/sh /usr/dmr hello.c 99 a.out world.c 99 a.out /usr/bwk Executable /usr/dmr/a.out #include<std main() { /usr/drm/hello.c or /usr/bwk/world.c Executable /usr/bwk/a.out 10

11 Kernel data structures MTUNIS The kernel must keep track of information for each open Þle. In user processes: open Þles are known by Þle descriptors small ints Þle descriptors are used in system calls In the User layer: File descriptors are mapped to open Þle numbers on a per process basis Open Þle numbers are used to communicate with the Þle layer In the File layer: Open Þle numbers are mapped to records containing The Þle pointer an int the current read/write position A pointer to an in-core Inode Each in-core Inode is a copy of the Þle s Inode and a reference count. When the count reaches 0, the Inode is written to disk. Hope the power doesn t fail! 11

12 File Protection For each Þle (or other object) there are certain rights: read, write, execute, use directory, append, delete, change attribute, Domains. Each user is a domain Each group of users is a domain All users may be a domain. A protection matrix speciþes the rights on each object for each domain. Each process is associated with one or more domains. Each operation on an object requires that the process be in a domain that holds the applicable right. 12

13 File Protection in UNIX Rights are read, write, execute/use directory Domains: Each user, designated groups of users, all users. Each Inode (i.e. Þle, directory, device) is associated with one user (the owner) and one group (the group owner). Each Inode has a protection matrix of 9 bits: In Octal Which column applies? Owner Group Others Read T T F Write T F F Execute/Use dir T F F if the owner of the process owns the Þle, use Þrst column else if the owner of the process is in the group of the Þle use the second column else use the last column 13

14 The SetUID bit Can users execute processes that have other users as the owner. Yes. A 10th bit is called the SetUID bit. If the SetUID bit is set on an executable Þle, then the owner of the Þle is considered the owner of the process, for the purpose of security. For example: I want to let you add Þles to my directory, but not delete them. So: I write a program to add Þles. I set the SetUID bit to true. I own the executable, but let you execute it. Thus while UNIX protection is crude, it is extensible. 14

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