Kernel Directory Interface

1. Kernel Directory Interface

2. operating systems Recall that in Unix your hard drive is organized into a hierarchical system of directories and files. Root Directories B A C B A C C B B B C C C C

3. operating systems Directories All of the directory functions are defined in the header file dirent.h A pointer to a DIR structure is used to manipulate directory information, much as a FILE structure is used to manipulate files.

4. operating systems Opening a Directory #include <dirent.h> #include <sys/types.h> DIR *opendir (const char *name); If the open succeeds, a pointer to a directory structure is returned. Otherwise a NULL pointer is returned. see the man page for opendir!

5. operating systems Reading from a Directory #include <dirent.h> #include <sys/types.h> struct dirent *readdir (DIR *dirp); readdir reads the next entry from the directory and returns a pointer to a dirent structure. see the man page for readdir!

6. operating systems On OS/X The dirent structure is defined as (see dirent man page) struct dirent { u_int32_t d_fileno; // files inode number u_int16_t d_reclen; // length of this record u_int8_t d_type; // file type u_int8_t d_namlen; // length of name string char d_name[255 + 1]; // name }; The dirent structure types defined in <sys/types.h> This is the only field guaranteed by Posix

7. operating systems DT_UNKNOWN 0 DT_FIFO 1 DT_CHR 2 DT_DIR 4 DT_BLK 6 DT_REG 8 DT_LNK 10 DT_SOCK 12 DT_WHT 14 dirent File Types

8. What Else Can You Do with a Directory? operating systems telldir( ) returns the current location in the directory stream seekdir( ) sets the position in the directory stream rewinddir( ) sets the position to the beginning of the directory stream closedir( ) closes a directory

9. operating systems Look at an Example

10. links The ls Command $ ls -al total 104 drwxr-xr-x 25 debryro staff 850 Aug 3 14:50 . drwxr-xr-x 13 debryro staff 442 Jun 15 13:09 3060 drwxr-xr-x 2 debryro staff 68 Jun 15 13:08 CNS3060 -rw-r--r-- 1 debryro staff 36 Jun 14 14:09 afile -rw-r--r-- 2 debryro staff 16 Jun 14 15:08 file-a -rw-r--r-- 2 debryro staff 16 Jun 14 15:08 file-b lrwxr-xr-x 1 debryro staff 6 Jun 14 15:11 file-c -> file-a -r-xr-xr-x 1 debryro staff 1809 Jun 14 11:34 monthly drwxr-xr-x 4 debryro staff 136 Aug 1 14:59 more owner group size last modified name permissions file type

11. operating systems file attributes are returned in a stat structure. The header file <sys/stat.h> declares all of the symbols used in this structure. File attributes are returned in the following three functions: int stat (const char *filename, struct stat *buf); int lstat (const char *filename, struct stat *buf); int fstat (int filedes, struct stat *buf); Where does ls get its information? when the file is a symbolic link, lstat returns information about the link, while stat returns information about the file the link points to! returns 0 when successful -1 when fails fstat returns information about a file that is already open.

12. operating systems struct stat { dev_t st_dev; /* ID of device containing file */ ino_t st_ino; /* File serial number */ mode_t st_mode; /* Mode of file */ nlink_t st_nlink; /* Number of hard links */ uid_t st_uid; /* User ID of the file */ gid_t st_gid; /* Group ID of the file */ dev_t st_rdev; /* Device ID */ time_t st_atime; /* Time of last access */ long st_atimensec; /* nsec of last access */ time_t st_mtime; /* Last data modification time */ long st_mtimensec; /* last data modification nsec */ time_t st_ctime; /* Time of last status change */ long st_ctimensec; /* nsec of last status change */ off_t st_size; /* file size, in bytes */ blkcnt_t st_blocks; /* blocks allocated for file */ blksize_t st_blksize; /* optimal blocksize for I/O */ __uint32_t st_flags; /* user defined flags for file */ __uint32_t st_gen; /* file generation number */ __int32_t st_lspare; /* RESERVED: DO NOT USE! */ __int64_t st_qspare[2]; /* RESERVED: DO NOT USE! */ }; stat structure (os/X) see /usr/include/sys.stat.h

13. operating systems Reading st_mode bits Use the following macros to test the file type int S_ISDIR (mode_t m); // returns non-zero if a directory int S_ISCHR (mode_t m); // returns non-zero if a char device int S_ISBLK (mode_t m); // returns non-zero if a block device int S_ISREG (mode_t m); // returns non-zero if a regular file int S_ISFIFO (mode_t m); // returns non-zero if a pipe int S_ISLNK (mode_t m) returns non-zero if a symbolic link int S_ISSOCK (mode_t m); // returns non-zero if a socket Example usage: if (S_ISREG(buf.st_mode) ) printf(“regular file”);

14. operating systems Use these masks for the permission bits S_IRUSR read bit for file owner S_IWUSR write bit for file owner S_IXUSR execute but for file owner S_IRGRP read bit for group S_IWGRP write bit for group S_IXGRP execute bit for group S_IROTH read bit for other S_IWOTH write bit for other S_IXOTH execute bit for other Example: if ( buf.st_mode & S_IRUSR) printf (“user read bit is on”);

15. operating systems Look at Some Examples

16. operating systems Every process has six or more IDs associated with it. These identify who we really are. They are taken from the password file when we log in. These are properties of the file. a real user id a real group id File Ownership an effective user id an effective group id When we execute a program, the effective user id is usually the same as the real user id and the effective group id is usually the same as the real group id. These are properties of the process. It is possible to set a flag that says “when this file is executed, set the effective user ID of the process to be the owner of the file. Example: if the file owner is root, and the set-user-ID bit is set, then while that file is running, it has root privileges (example – passwd) Test the set-User-ID bit with the constant S_ISUID

17. operating systems Whenever we want to open any type of file by name, we must have execute permissions in each directory mentioned in the path to the file, including the current directory. Read permission on a directory only means that you can read the directory – obtaining a list of file names in the directory. We cannot create a new file in a directory unless we have both write and execute permissions for the directory. To delete a file we need both write and execute permissions in the directory. Note that you do not need read and write permissions for the file! Some File Access Permission Rules

18. operating systems The kernel performs the following access tests each time a process opens, creates, or deletes a file: File access tests If the effective user ID of the process is 0, access is allowed. If the effective user ID of the process equals the owner ID of the file, access is allowed if the appropriate permission bit is set. If the effective group ID of the process equals the group ID of The file, access is allowed if the appropriate permission bit is set. If the appropriate other permission bit is set access is allowed.

19. operating systems #include <sys/types.h> #include <sys/stat.h> int chmod (const char *pathname, mode_t mode); int fchmod(int fd, mode_t mode); Changing File Permissions If (chmod(“foo”, S_IRUSR | S_IWUSR | S_IXUSR) < 0) // error code …

20. operating systems #include <sys/stat.h> #include <unistd.h> int chown (const char *pathname, uid_t owner, gid_t group); int fchown ( int fd, uid_t owner, gid_t group); int lchown (const char *pathname, uid_t owner, gid_t group); Changing File Ownership Changes the owner of the symbolic link, not The owner of the file linked to. Normally, only the file owner can change ownership of the file.

21. operating systems Time Values Field Description Example ls option st_atime last access time of file read -u st_mtime last modification time write default st_ctime last change time (inode) chmod -c

22. operating systems Unix Time Functions A digression …

23. operating systems The basic Linux (and Unix) time service counts the number of seconds that have passed since January 1, 1970 (UTC). These seconds are normally stored in a variable whose type is time_t. This is called calendar time.

24. operating systems We can get calendar time by using the call time_t time (time_t *tptr); returns the time, and stores the value here if the argument is non-null.

25. operating systems string formatted string asctime strftime struct tm broken down time ctime localtime gmtime mktime time_t calendar time time kernel takes time zone into account

26. operating systems Broken down time struct tm { int tm_sec; // seconds after the minute [0-61] int tm_min; // minutes after the hour [0-59] int tm_hour; // hours after midnight [0-23] int tm_mday; // day of the month [1-31] int tm_mon; // month of the year [0-11] int tm_year; // years since 1900 int tm_wday; // day of the week [0-6] int tm_yday; // days since Jan 1 [0-365] int tm_isdst // daylight savings flag [pos, 0, neg] }; allows leap seconds struct tm

27. operating systems #include <time.h> struct tm *gmtime (const time_t *tptr); struct tm *localtime (const time_t *tptr); Converting time_t to tm broken down time is in UTC broken down time is in local time, given time zone and daylight savings

28. operating systems Converting tm to time_t time_t mktime (struct tm *tptr);

29. operating systems char *asctime (const struct tm *tmptr); char *ctime (const time_t *tptr); generating time strings Takes time zone into account both generate a 26 character string of the form Tue Sep 26 16:49:05 2002\n\0

30. operating systems Generating a formatted string size_t strftime (char *tbuf, size_t maxsize, const char *format, const struct tm *tptr); the buffer to store the string in, and its max size a broken down time the format string – works like printf returns the size of the formatted string if successful 0 if fails

31. Format Description Example %a abbreviated weekday name Tue %A full weekday name Tuesday %b abbreviated month name Jan %B full month name January %c date and time Tue Jan 14 19:40:05 2002 %d day of the month [00-31] 14 %H hour of 24 hour day [00-23] 19 %I hour of 24 hour day [00-12] 07 %j day of the year [001-366] 014 %m month [01-12] 01 %M minute [00-59] 40 %p am or pm PM %S second [00-61] 05 %U Sunday week number [00-53] 02 %w weekday [0=Sunday – 6] 5 %W Monday week number [00-53] 07 %x date 01/14/02 %X time 19:40:04 %y year without century[00-99] 02 %Y year with century 2002 %Z time zone name MST

32. operating systems Getting the Directory Name #include <unistd.h> extern char *getcwd (char *buf, size_t size); size is max size of the path name that can be returned.

33. operating systems du command with no arguments Your du command should use the current working directory. Print out the number of bytes in the directory entry itself. For each file in the directory, print out the size of the file in bytes, followed by the file name. For each sub-directory in the directory, print out the number of bytes in the directory entry, then open the directory and repeat this process for each entry in that sub-directory. Repeat this recursively until everything in the directory has been handled. Project #2du command

34. operating systems du command with a filename print out the size of the file in bytes, followed by the file name. Project #2du command

35. operating systems du command with a directory name Print out the number of bytes in the directory entry itself. For each file in the directory, print out the size of the file in bytes, followed by the file name. For each sub-directory in the directory, print out the number of bytes in the directory entry, then open the directory and repeat this process for each entry in that sub-directory. Continue to repeat this recursively until everything in the directory has been handled. Project #2du command

36. operating systems Hints To get the full pathname of the current directory, use the getcwd( ) function. Use the d_type field in the dirent structure to get the type of file (a regular file or a directory) Use the d_name field in the dirent structure to get the name of the file Read chapter 3 in Molay for more ideas. Project #2du command