Files and Directories

1. Files and Directories • Properties of files • ownership, permissions, etc. • Functions relating to files • stat, fstat, lstat • access • umask • chmod, fchmod • chown, fchown, lchown

2. stat Functions • Three basic stat functions • stat, fstat, lstat • return 0 ok, -1 on error • all three take buffer args • fill with information on files • stat structure system dependent • stat and lstat take pathnames • fstat takes file descriptor of open file • lstat returns info on symbolic link

3. stat Structure • system defined structure which contains file information • ‘man stat’ should return local details • common elements include • st_mode - file type and permissions • st_uid - uid of owner • st_gid - gid of owner • st_size - size in bytes

4. File Types • Regular file - data in some form • Directory - file with data on files • Character special - device file • Block special - device file • FIFO - named pipe for interprocess communication • Socket - interprocess communication • Symbolic link - file that points to a file • st_mode element of stat structure shows type

5. user and group ID’s • properties of processes • real user, real group • set at login • effective user, effective group, supplementary group • establish permissions, changeable • saved set-user-ID, set-group-ID • set to effective when program executes

6. real UID, GID • real UID and GID established at login • normally unchanged in session, but can be changed by root • ‘id’ command will show UID, GID, and groups for specific user (effective if no user given)

7. effective UID, GID • effective ID’s normally match real ID’s • effective ID’s can be changed via flag in st_mode • effective ID’s can be changed under a login session using the ‘su’ (substitute user) command from the terminal

8. set-user-ID, set-group-ID bits • bits in st_mode which force effective user and group ID’s. • programs with set-user-ID bit set will run under the effective user ID of the programs owner • programs with set-group-ID bit set will run under the effective group ID of the programs group

9. Access Permissions • nine permission bits per file • user, group, other • read, write, execute • st_mode contains values

10. Permission rules • opening a file requires execute (search) permissions in all intervening directories, write permission is required for truncation • creating a new file requires write and execute permission on it’s directory • deleting a file requires write and execute permissions on it’s directory (NOT on the file) • execute permissions are required to use the exec function

11. File access checks • EUID of 0 (root) give unlimited access • EUID = owner of file • bit set, action allowed, else denied • EGID/SGID = group of file • bit set, action allowed, else denied • other bit controls all other access • steps tried in sequence until match, then finished

12. New File Ownership • files and directories have same ownership rules • user ID of new file set as effective ID of creating process • group ID of new file has two possible values • effective group ID of creating process • group ID of containing directory • LINUX uses SVR4 rules • SGID bit forces directory matching • no SGID bit forces process EGID matching

13. access Function • Tests for access to file under real user ID • takes pathname, mode • returns 0 OK, -1 on error

14. umask Function • used to set permissions mask for file creation • takes mask • bitwise OR of permission bits • returns previous mask • no error return • shell umask command can set current mask (for shell session)

15. chmod and fchmod Functions • Used to change access permissions and special states • chmod used for file • fchmod used for open file descriptor • both take mode_t values • 9 permissions values plus suid, sgid, and sticky • clears sgid and sticky if permissions incorrect • chmod takes pathname, fchmod FD • both return 0 OK, -1 on error

16. Sticky Bit • historically used to save text to swap • currently used for directories • allows only file/directory owner or root to remove or rename contained file

17. chown, fchown, and lchown Functions • used to change user or group ID of file • chown, lchown take pathnames • fchown takes FD • lchown changes ownership of symbolic link • all take UID and GID • all return 0 OK, -1 on error

18. File Size • meaningful for regular files, directories, symbolic links • size in bytes • directory size is size of directory file • symbolic link size is size of pathname

19. File Truncation • reduce (normally) file size • some systems will extend • truncate, ftruncate functions • truncate takes pathname • ftruncate takes FD • both take new file size in bytes • both return 0 OK, -1 on error