CSCE 313 Introduction to Computer Systems Instructor: Dr. Guofei Gu http://courses.cse.tamu.edu/guofei/csce313 The UNIX File System File Systems and Directories Accessing directories UNIX s Understanding links in directories File protection/access control Reading: R&R, Ch 5 1
A Typical File-system Organization Directories Large amounts of data: Partition and structure for easier access. High-level structure: partitions in MS-DOS minidisks in MVS/VM file systems in UNIX Directories: Map file name to directory entry (basically a symbol table). 2
Organize the Directory (Logically) to Obtain Efficiency locating a file quickly Naming convenient to users Two users can have same name for different files The same file can have several different names Grouping logical grouping of files by properties, (e.g., all Java programs, all games, ) Single-level directory: directory Directory Structures file Problems: limited-length file names multiple users? not composable (how do you combine multiple file systems?) 3
Two-Level Directories master directory user1 user2 user3 user4 user directories file Path names Location of system files special directory search path Tree-Structured Directories user bin pub user1 user2 user3... find count ls cp bin mail netsc openw bin demoinclude... create subdirectories new concepts: gcc gdb xmt xinit xman xmh xterm current directory path names: complete vs. relative search paths 4
Generalized Tree Structures share directories and files keep them easily accessible user bin pub user1 user2 user3... find count ls cp xman bin mail opwin netsc bin demo incl... xinit gcc gdb xmt xinit xman xmh xterm Links: File name that, when referred, affects file to which it was linked. (hard links, symbolic links) Problems: consistency, deletion Why links to directories only allowed for system managers? UNIX Directory Navigation: current directory #include <unistd.h> char * getcwd(char * buf, size_t size); /* get current working directory */ Example: void main(void) { char mycwd[path_max]; } if (getcwd(mycwd, PATH_MAX) == NULL) { perror ( Failed to get current working directory ); return 1; } printf( Current working directory: %s\n, mycwd); return 0; 5
UNIX Directory Navigation: directory traversal #include <dirent.h> DIR * opendir(const char * dirname); /* returns pointer to directory object */ struct dirent * readdir(dir * dirp); /* read successive entries in directory dirp */ int closedir(dir *dirp); /* close directory stream */ void rewinddir(dir * dirp); /* reposition pointer to beginning of directory */ #include <dirent.h> Directory Traversal: Example int main(int argc, char * argv[]) { struct dirent * direntp; DIR * dirp; if (argc!= 2) { fprintf(stderr, Usage: %s directory_name\n, argv[0]); return 1; } if ((dirp = opendir(argv[1])) == NULL) { perror( Failed to open directory ); return 1; } } while ((direntp = readdir(dirp))!= NULL) printf(%s\n, direntp->d_name); while((closedir(dirp) == -1) && (errno == EINTR)); return 0; 6
Interlude: Logical View of File Management user file structure directory management access control records access method physical blocks in memory blocking physical blocks on disk disk scheduling file allocation Unix File System Implementation: s file information: - size (in bytes) - owner UID and GID - relevant times (3) - link and block counts - permissions 0 multilevel indexed allocation table multilevel allocation table direct 9 10 11 12 single indirect double indirect triple indirect 7
Directory Implementation file information: - size (in bytes) - owner UID and GID - relevant times (3) - link and block counts - permissions Where is the filename?! Name information is contained in separate Directory File, which contains entries of type: (name of file, 1 number of file) 12345 name myfile.txt 12345 1 23567 block 23567 some text in the file 1 More precisely: Number of block that contains. Hard Links directory entry in /dira name 12345 name1 directory entry in /dirb name 12345 name2 shell command ln /dira/name1 /dirb/name2 12345 21 23567 block 23567 some text in the file is typically implemented using the link system call: #include <stdio.h> #include<unistd.h> if (link( /dira/name1, /dirb/name2 ) == -1) perror( failed to make new link in /dirb ); 8
Hard Links: unlink directory entry in /dira name 12345 name1 directory entry in /dirb name 12345 name2 12345 12 0 23567 block 23567 some text in the file #include <stdio.h> #include<unistd.h> if (unlink( /dira/name1 ) == -1) perror( failed to delete link in /dira ); if (unlink( /dirb/name2 ) == -1) perror( failed to delete link in /dirb ); Symbolic (Soft) Links directory entry in /dira name 12345 name1 directory entry in /dirb name 13579 name2 Only count hard links! 12345 block 23567 13579 block 3546 1 some text in the file 1 /dira/na me1 shell command ln -s /dira/name1 /dirb/name2 23567 3546 is typically implemented using the link system call: #include <stdio.h> #include<unistd.h> if (symlink( /dira/name1, /dirb/name2 ) == -1) perror( failed to create symbolic link in /dirb ); 9
Exercises You execute ln /dira/name1 /dirb/name2 and then use a text editor Open file /dira/name1 Read file into memory Close /dira/name1 Modify the memory image of the file Unlink /dira/name1 Open file /dira/name1 with create and write flags Write the contents of memory to file Close /dira/name1 What exactly happened during the above process? What if the first step is ln s /dira/name1 /dirb/name2? Protection File owner/creator should be able to control: what can be done by whom Types of access Read Write Execute Append Delete List 10
Access Lists and Groups Mode of access: read, write, execute Three classes of users RWX a) owner access 7 Þ 1 1 1 RWX b) group access 6 Þ 1 1 0 RWX c) public access 1 Þ 0 0 1 Ask manager to create a group (unique name), say G, and add some users to the group. For a particular file (say game) or subdirectory, define an appropriate access. owner group public Attach a group to a file chmod 761 game chgrp G game Windows XP Access-control List Management 11
A Sample UNIX Directory Listing 12