Disks, Partitions, and Filesystems

1. Disks, Partitions, and Filesystems Multiple systems in use S5, UFS, others Focus on standard S5 system HD’s divide into partitions, each of which can contain a file system

2. Filesystems • Filesystems contain boot block(s), a super block, an i-list, directory blocks and data blocks • i-nodes contain most file info • directory blocks contain directory info • data blocks contain contents of file

3. Directories • tables of filenames and associated i-nodes • directories always contain . and .. as table entries • directories are files, with i-nodes, .etc

4. i-node • contains all info about file • type, access, size, etc. • pointers to data associated with file • count of links to i-node (hard links) • directory entries pointing to i-node • can be pointed to by multiple directory entries • directory entries must point to i-nodes in same file system

5. link, unlink, remove, rename Functions • link creates directory entry for existing file • takes pathname, returns 0 OK, -1 error • unlink removes existing directory entry • takes pathname, returns 0 OK, -1 error • remove identical to unlink for files, rmdir for directories • takes pathname, returns 0 OK, -1 error • rename changes names of files/directories • takes oldpath, newpath; returns 0 OK -1 error

6. link • creates directory entry for existing file • increments link count in i-node • operations atomic • only superuser can create links to directories (avoids filesystem loop problems)

7. unlink • removes directory entry • decrements link count in i-node • must have wx permission in directory • if sticky bit set, must own file, own directory, or be superuser • file not deleted until link count 0 and file not in use

8. remove • calls unlink for files, rmdir for directories • ANSI C standard (replacing unlink)

9. rename Function • renames directory or file • if oldname is file • if newname exists, cannot be directory • newname removed, oldname becomes newname • write permission required for both • if oldname is directory • if newname exists, must be empty directory • newname removed, oldname becomes newname • newname cannot contain oldname in path

10. Symbolic Links • not pointers to i-nodes • symbolic links contain pathname as data • i-node contains flag indicating symbolic link • not constrained to single filesystem, superuser creation of directory links.

11. symlink and readlink Functions • symlink creates symbolic link • takes realpath, sympath; returns 0 OK, -1 error • can cross filesystems • can be used for directories by ordinary users • readlink manipulates symlink file itself • takes pathname, *buffer, bufsize • returns # bytes read, -1 error • combines open, read, close on symlink file

12. File Times • last access time of data • system does not maintain i-node access time • last modification time of data • last modification time of i-node

13. utime Function • changes access and mod times of files • takes pathname, *utimbuf structure • null pointer sets to current time • requires ownership or write permission • non-null pointer requires ownership or root • returns 0 OK, -1 error

14. mkdir and rmdir Functions • mkdir creates directories • takes pathname, mode • usually want some execute permission • automatically creates . and .. entries • rmdir removes empty directories • takes pathname • both return 0 OK, -1 error

15. Reading Directories • format of directories system dependent • opendir - open directory stream • takes pathname, returns DIR *, null on error • readdir - read directory • takes DIR *, returns dirent structure *, null on error or EOD • rewinddir - reset stream to beginning of directory • takes DIR *, returns nothing • closedir - close directory stream • takes DIR *, returns nothing

16. chdir, fchdir, getcwd Functions • chdir, fchdir set current working directory of process • takes pathname, returns 0 OK, -1 on error • getcwd gets absolute pathname of cwd • takes buffer *, size of buffer; returns buff on success, NULL on error

17. Special Device Files • block special, character special • identified by major/minor numbers • block special • ramdom access device (hard disk, cdrom, etc.) • character special • sequential access device (tape drive, modem, etc.)

18. sync and fsync Functions • cause kernal buffers to be flushed • sync schedules a write and returns • Linux waits for completion • takes no arg, returns 0 OK, -1 on error • fsync flushes single file buffer, and waits for return • takes FD, returns 0 OK, -1 on error