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 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 example1.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 (example1a.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 example2.c

Creating file with customized permissions. –Can specify permission in the creat and open calls. –See example3.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 example4.c. #include 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 example5a.c –Read a file without the read permission, see example5.c (as long as you are the owner). Remove and rename files: #include 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 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 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 example6.c.

Random access files –Current file offset can be manipulated with lseek calls, see example7.c. Try ‘more hole001’ and ‘od –c hole001’. #include 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 DIR *opendir(const char *name) struct dirent *readdir(DIR *dp); void rewinddir (DIR *dp); int closedir(DIR *dp)

Operating on directories #include 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 example8.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?