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 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 <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

example5a.c– Read a file without the read permission, see

example5.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 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 <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 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?