CS代考 CSCI 4061 Introduction to Operating Systems – cscodehelp代写

CSCI 4061 Introduction to Operating Systems
Instructor:
Outline
 File Systems
 Directories
 File and directory operations  Inodes and metadata
 Links
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File Systems
 An organized collection of data
 Think of your book rack or music collection
 Think of how libraries organize books/magazines
 Discussion: Why do we need file systems?
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File System Structure
 A file system typically consists of:
 Files: Data objects
 Directories or folders: Collection of files
 Other objects specific to how data is organized in
the file system
 Different file systems implement these entities in different ways
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Unix File System
/
bin dev etc home lib usr tmp var
cp ls
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chandra
bin myprog
bin include lib
cc which libc.so libm.so
Unix File System Structure
 Files: Data objects
 Regular files, device files, FIFOs
 Directories: Collections of files  Contain information about files
 Links: Pointers to other files  Pseudonyms/nicknames
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Directories
 Directory is a special file
 A directory is a list of entries about files and
directories that it contains  Each entry contains:
 Name of file and file type
 Pointer to file inode (metadata)
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Directory Structure
filename inode no.
file1
12345
file2
98765
inode 12345
inode 98765
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Directory Operations
 Different operations from the ones used for regular files. Why?
 Cannot use:
 open, read, write, close
 Instead use:
 opendir, readdir, closedir
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Opening a Directory
 Returns a directory stream of type DIR
 Contains an ordered sequence of directory entries
 Order need not be alphabetical
 Order is implementation-dependent
DIR *opendir(char *dirname);
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Reading Directory Entries
 Returns the next directory entry  NULL if end of directory or error
 Directory entry contains information about a file or subdirectory
 Filename
 File’s inode number
 Optional fields: E.g.: File type
struct dirent *readdir(DIR *dirp);
Closing and Repositioning a Directory
 Closes an open directory
 Repositions the directory to the beginning
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int closedir(DIR *dirp);
void rewinddir(DIR *dirp);
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Modifying Directory Entries
 No writedir: Cannot write to a directory entry directly. Why not?
 Directory entries changed through operations on files:
 File creation/deletion: creat, remove, open  Filenames, symbolic links: rename, link,
unlink
 Directory creation and removal: mkdir, rmdir
 Some of the operations differ based on whether the argument is a file or a directory
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File System Traversal
 Change directory to a given path  cd command
 Get the current working directory  pwd command
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int chdir(char *path);
char *getcwd(char *buf, size_t size);
Inodes
 Inodes are kernel structures that contain file information
 File properties
 File data location: pointers to disk blocks
containing file information
 Directory entry contains filename and pointer to
file inode
 Filename is information provided by file system
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 Actual file is independent of its filename
File Properties
 File type
 Size
 Owner, permissions
 Times: Last access, last modification, last status change
 Number of hard links
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Reading File Properties
 Parameters:
 path: File path
 buf: structure containing file properties
 struct stat:
 Contains file properties and inode no.
 Variations:
 lstat: Distinguishes between links and files  fstat: Takes file descriptor instead of path
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int stat(char *path, struct stat *buf);
File Operations
 The following change the file properties in the inode:
 chmod: Mode change
 chown: Ownership change
 The following changes the directory entry in the file system
 rename: Move/rename
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Inode Structure
File Properties
Direct Data Block pointers
Single Indirect ptr
Double Indirect ptr
Triple Indirect ptr
Data Blocks Data Blocks
Data Blocks
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Links
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 Pointers to files
 Pseudonyms/nicknames
 Example: ln /home/user/file1 file2  Creates a link (file2) that points to file1
 Links provide access to the original files  Hard links: Direct access to file
 Symbolic links: Indirect access to file
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Hard Links
 Pointer to the inode of a file  Each link is equivalent
 No “original” filename
 File is deleted only when all hard links are deleted
inode 12345
file1 12345
file2 12345
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Symbolic Links
 Pointer to another directory entry
 A symbolic link is different from original file
 If original file is moved or deleted, the link becomes hanging
 inode does not keep count of symbolic links
inode “file1” inode 12345 13579
file1 12345
file2 13579
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Hard Link vs. Symbolic Link
 What happens if we delete the original file?  What happens if we delete the link?
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