Files and Directories File types stat functions for file information File permissions suid and sgid Sticky bit Hard link and soft link Directory operations Device special files File system
File Types Regular file Directory file Character special file (unbuffered I/O
access) Block special file (buffered I/O) Named Pipe (a type of IPC) - FIFO Socket (a network form of IPC) Symbolic Link (a file that just points to
another file)
stat, fstat and lstat
int stat(const char *path, struct stat *buf);int fstat(int filedes, struct stat *buf);int lstat(const char *path, struct stat *buf); All three return 0 on success or -1 on failure All three set buf to point to a structure of
information stat get info about the file fstat gets info about an already open file lstat same as stat but if given a symbolic link will
get info about the link instead of what the link points to
stat structure Definition on page
88 st_mode st_ino st_dev st_rdev st_nlink st_uid st_gid
st_size st_blocks st_blksize st_atime st_mtime st_ctime
Determining the type of a file
Set of macros defined in <sys/stat.h> can be used.
They all accept the st_mode field of the stat struct as their only parameter
S_ISREG(mode_t mode) S_ISDIR(mode_t mode) S_ISCHR(mode_t mode) S_ISBLK(mode_t mode) S_ISFIFO(mode_t mode) S_ISLNK(mode_t mode) S_ISSOCK(mode_t mode)
Set-User-ID & Set-Group-ID
Every process has 6 or more IDs Real user ID Real group ID Effective user ID Effective group ID Supplementary group IDs Saved set-user-ID Saved set-group-ID
Set-User-ID & Set-Group-ID
Normally, effective IDs are the same as real user IDs
Sometimes, we need to run a process with the permissions of the user or group that owns the file (ex: passwd)
Two bits in the file’s mode word (st_mode) allow this. set-user-ID bit set-group-ID bit
Set-User-ID & Set-Group-ID
When creating a file with the creat function, the new file’s UID is the effective UID of the process, and the new file’s GID is the effective GID of the process
Checking set-user-ID and set-group-ID bits
Can use S_ISUID and S_ISGID masks
Ex:if(S_ISUID & st_mode)if(S_ISGID & st_mode)
Setting SUID and GUID
SUID has a value of 4 GUID has a value of 2chmod 4755 afile -rwsr-xr-xchmod 2755 afile -rwxr-sr-xchmod 6755 afile -rwsr-sr-x
File access permissions
Read section 4.5 on page 92. Note the different permissions required for operations on regular files versus directories
access function Tests to see if the real user ID and real
group ID allow access to a file Useful if a program that may be running as
another user (ex root) wants to be sure that the person that started the program has permissions to access some file
int access(const char *pathname, int mode); Returns 0 if ok, -1 otherwise
access function (cont)
In this context, mode is not the same as in previous functions. Here, mode is the bitwise OR of several constants R_OK - test for read permission W_OK - test for write permission X_OK - test for execute permission F_OK - test for file existence
umask function
mode_t umask(mode_t mask); Sets the “file mode creation mask” “file mode creation mask” is used to
specify the default permissions of newly created files
Since we are dealing with a mask it is the complement of what we want. So we set the bits corresponding to the permissions we don’t want.
umask function
When using open or creat, any bits in mode that are also set in the file creation mask are turned off.
All the UNIX shells also define a shell command called umask that displays the default umask set at login. Each process can change its own mask without interfering with each other
File Size st_size member of stat structure holds
the size of a file Only meaningful for regular files,
directories and symbolic links Regular file – number of bytes in the file Directories – multiple of a number (more on
this later) Symbolic Links – number of chars in
filename
Sticky Bit
Historically Used to tell the system to keep a
copy of the “text” portion of a program in the swap space even after the program exits. (text = instructions, not data)
Swap file is contiguous faster start on next execution.
Sticky Bit Currently
Used on directories, indicates that in order to rename or delete a file in that directory a user must both have write permissions to the directory and one of the following must be true
User owns the file User owns the directory User is the superuser
To set the sticky bit chmod +t mydir
chmod and fchmod Allows us to change the existing
permissions for a fileint chmod(const char *path, mode_t mode);int fchmod(int fildes, mode_t mode); Returns 0 on success or -1 on failure mode is bitwise OR of constants shown
on page 99 fig 4.11 Effective UID of process must be the
same as the UID of the file or the process must have superuser permissions
link, unlink, remove and rename
linkint link(const char *oldpath, const char *newpath); Creates a new hard link within the file
system oldpath is the file to be linked. Newpath
must already exist except for the last portion which will be created
If all of newpath already exists, an error will be returned
Returns 0 if ok, -1 otherwise
link, unlink, remove and rename
unlinkint unlink(const char *pathname); Removes directory entry indicated by
pathname Decrements the link count of the file
Must have write and execute on containing directory
Only when link count reaches 0 and no process has the file open will it be deleted
link, unlink, remove and rename
removeint remove(const char *pathname); Calls unlink for files and rmdir for
directories rename
int rename(const char *oldpath, const char *newpath);
Equivelant to mv shell command
Symbolic Links
Indirect pointer to a file or directory Created to get around limitations of
hard links Hard links must live in the same file
system that the think it links to is in Only superuser can hard link to a
directory
symlink and readlink symlink
int symlink(const char *oldpath, const char *newpath); Creates the symbolic link file newpath that
contains the text contained in oldpath oldpath and newpath may be on different
file systems readlink
ssize_t readlink(const char *path, char *buf, size_t bufsiz); open function follows symbolic links.
readlink operates on the link file itself String from file placed in buf, with number
of bytes in bufsiz. String is not null terminated
File Times 3 times are kept for any file
Last access time Last modified time Last status change time
These are stored in the following fields of the stat struct st_atime st_mtime st_ctime
utimeint utime(const char *filename, const struct utimbuf *buf);struct utimbuf { time_t actime; time_t modtime;}
actime and modtime are calendar times marking seconds since Epoch
buf may be NULL, in which case the time is updated to current system time
Unless using NULL for buf, just having write permissions is not enough. Effective user ID must equal owner ID or process must have superuser privileges
Directory Operations mkdir and rmdir
int mkdir(const char *pathname, mode_t mode);int rmdir(const char *pathname); Create and remove the specified directory For mkdir, the mode parameter is the same
as those used for open. However, recall that interactions with the umask may occur!
When creating directories, remember that we usually want execute permissions so that the files within the directory can be listed
Reading DirectoriesDIR *opendir(const char *name);struct dirent *readdir(DIR *dir);void rewinddir(DIR *dir);int closedir(DIR *dir);off_t telldir(DIR *dir);void seekdir(DIR *dir, off_t offset);
struct dirent { ino_t d_ino; char d_name[NAME_MAX + 1];}
chdir fchdir and getcwd
int chdir(const char *path);int fchdir(int fd);
Change the current working directory of the process
long getcwd(char *buf, unsigned long size); Get the current working directory of
the process
Device Special Files Many devices are represented as
files within /dev directory Each device has major number and
minor number for the Kernel to identify it Major number usually specifies the
driver to use for the device Minor number may be the specific sub
device or options for that device
Device Special Files ls –lL on a device file will show major
and minor numbers instead of size of file
st_dev and st_rdev st_dev for every file is the device number
under the filesystem containing that filename and its associated i-node
st_rdev (only for character and block special files) contains actual device number
Device Special Files
Obtaining the major and minor numbers Use the major and minor macros
major(st_dev) minor(st_dev)
For block and character special devices use st_rdev instead to obtain real device number