Todays topic Access and manipulate meta data for

Today’s topic • Access and manipulate meta data for files – File type, ownership, access permissions, access time, etc • How to determine if a file is not there? • How to find out the access permissions of a file? • How to have the full control of permissions of the files you created in your program? • Operations on directories

• How to determine if a file is not there? – Try ‘ls –l’, ‘ls aa’ – Get the meta data of a file: stat #include <sys/types. h> #include <sys/stat. h> int stat (const char *pathname, struct stat *buf) int fstat(int filedes, struct stat *buf) int lstat(const char *filename, struct stat *buf) – This function fails when the file is not there • There are some other ways for it to fail, – man –a stat – When the function succeeds, buf stores the meta data for the file.

Struct stat { mode_t st_mode; /* file type & mode & permissions */ ino_t st_ino; /* file inode number */ dev_t st_dev; /* device number (file system) */ dev_t st_rdev; /* device number for special files */ nlink_t st_nlinks; /* number of links */ uid_t st_uid; /* owner user ID */ gid_t st_gid; /* owner group ID */ off_t st_size; /* size in bytes, for regular files */ time_t st_atime; /* time of the last access */ time_t st_mtime; /* time of the last modification */ time_t st_ctime; /* time of the last status change */ long st_blksize; /* best I/O block size */ long st_blocks; /* number of 512 byte blocks */ }; You can see most of the fields in the stat data structure with ls.

• Files types (st_mode): – Regular (S_ISREG(buf. st_mode)) – Directory (S_ISDIR(. . )) – Character special file (S_ISCHR(. . )) – Block special file (S_ISBLK(. . )) – FIFO (S_ISFIFO(. . )) – Symbolic link (S_ISLNK(. . )) – Socket (S_ISSOCK(. . )) – See example 1. c

• Set-user-ID and set-group-ID (st_mode): – For executable files only. Relate to what permission should the system gives to a program executing? – What permission should a process get when a program is executed? • The same permission as whoever runs it? • Not sufficient sometimes. Example? – Change effective user ID and group ID when executed. • The main use: allowing regular users to have root access. – Checking these bits (example 1 a. c) • S_ISUID & buf. st_mode, S_ISGID & buf. st_mode

• File access permissions (st_mode) – user-read (S_IRUSR) – user-write (S_IWUSR) – user-execute(S_IXUSR) – group-read (S_IRGRP) – group-write (S_IWGRP) – group-execute(S_IXGRP) – other-read (S_IROTH) – other-write(S_IWOTH) – other-execute(S_IXOTH) • See example 2. c

• Creating file with customized permissions. – Can specify permission in the creat and open calls. – See example 3. c – There is a default file mode creation mask. • Try ‘umask’ • The bits that are 1’s are turned-off. – The mask can be manipulated in the program, see example 4. c. #include <sys/types. h> #include <sys/stat. h> Mode_t umask(mode_t cmask);

• Change mode and change owner – chmod and chown, both as command lines and as system calls. – Remove a file without write permission, see example 5 a. c – Read a file without the read permission, see example 5. c (as long as you are the owner). • Remove and rename files: #include <stdio. h> int remove(const char *path); /* C function, same as unlink */ int rename(const char *oldname, const char *newname) /* also a C function */

• Hard link and symbolic link of a file: – Hard link of a file is to create a new directory entry with the same inode number. – Hard link is not allowed for a directory (to avoid loops in the directory’s tree structure). – In struct stat, the st_nlinks field records the number of directory entries pointing to the inode for the file. – Link/unlink are both command line and system calls #include <unistd. h> int link(const char *existingpath, const char *newpath) /* create a new entry in the directory with the same inode number */ /* similar to copy, what is the difference? */ int unlink(const char *path);

• Symbolic link – The link function creates a “hard link”: • (newname, inode number) in a directory • If not done carefully, can be a problem (destroy the tree structure of the directories) – Not allow to create a hard link on directories – Symbolic link, which is an indirect pointer to a file, gets around the problem • need to be careful whether a function follow the symbolic link – Unlink do not follow symbolic link (only the symbolic link is removed), open follow the symbolic link by default.

• Symbolic link – Try the following ‘ln –s /no/such/file myfile’ ‘ls myfile’ ‘cat myfile’ ‘ls –l myfile’ – System calls: #include <unistd. h> int symlink(const char *actualpath, const char *sympath); int readlink(const char *pathname, char *buf, int bufsize);

• File times – st_atime: last access time – st_mtime: last modification time – st_ctime: last status change time – Try: ‘ls –u -l’, ‘ls –c -l’ – The access time and modification time can be changed with the utime function. See example 6. c.

• Random access files – Current file offset can be manipulated with lseek calls, see example 7. c. • Try ‘more hole 001’ and ‘od –c hole 001’. #include <sys/types. h> #include <unistd. h> off_t lssek(int filedes, off_t offset, int whence) whence = SEEK_SET, SEEK_CUR, SEEK_END

Operating on directories • Only root can write, all other can only read (or write implicitly through system calls). #include <sys/types. h> #include <dirent. h> DIR *opendir(const char *name) struct dirent *readdir(DIR *dp); void rewinddir (DIR *dp); int closedir(DIR *dp)

Operating on directories #include <sys/types. h> #include <dirent. h> struct dirent *readdir(DIR *dp); struct dirent { ino_t d_ino; char *d_name[NAME_MAX+1]; } – Return NULL when reaching the end of the directory. – Example: implementing the ‘find’ command, see example 8. c

Current Working Directory • Can be changed using – int chdir(const char *pathname); – int fchdir(int filedes); • Note that they only affect the current process’ working directory • Obtaining current working directory – char *getcwd(char *buf, size_t size);

Review • What is the most reliable way to check if a file exists? • What other information are stored in the struct stat variable returned from stat()? • What is the purpose of set-user-id and set-group-id in the file meta-data? • Which system call sets the file permission? • Give 2 functions that delete a file. • What is the difference between hard link and symbolic link? • How to create/delete a hard link? • How to create/delete a symbolic link? • Is a directory a file? Can you edit directory? • What does chdir do?