2ab. UNIX files.ppt JSS science and technology university

FILE TYPES
 Regular file
 Directory file
 Fifo file
 Character device file
 Block device file

Regular file
 It mat be text or binary file
 Both the files are executable provided
execution rights are set
 They can be read or written by users with
appropriate permissions
 To remove regular files use rm command

Directory file
 Folder that contains other files and
subdirectories
 Provides a means to organize files in
hierarchical structure
 To create : mkdir command
 To remove : rmdir command

Block device file :
Physical device which transmits data a
block at a time
EX: hard disk drive, floppy disk drive
 Character device file :
Physical device which transmits data
in a character-based manner
EX: line printers, modems, consoles

 To create : mknod command
mknod /dev/cdsk c 115 5
/dev/cdsk : name of the device
c -- character device b -- block device
115 — major device number
5 — minor device number
Major device number : an index to the kernel’s
file table that contains address of all device
drivers
Minor device number : indicates the type of
physical file and I/O buffering scheme used
for data transfer

Fifo file
 Special pipe device file which provides a
temporary buffer for two or more
processes to communicate by writing data
to and reading data from the buffer
 Max size of buffer – PIPE_BUF
 The storage is temporary
 The file exists as long as there is one
process in direct connection to the fifo
for data access

 To create : mkfifo OR mkfifo
mkfifo /usr/prog/fifo_pipe
mknod /usr/prog/fifo_pipe p
 A fifo file can be removed like any other
regular file

Symbolic link file
 A symbolic link file contains a pathname
which references another file in either the
local or a remote file system
 To create : ln command with –s option
ln –s /usr/abc/original /usr/xyz/slink
cat /usr/xyz/slink
/*will print contents of /usr/abc/original file*/

UNIX and POSIX file systems
 They have a tree-like hierarchical
file system
 “/” – denotes the root
 “.” – current directory
 “..” – parent directory
 NAME_MAX – max number of characters
in a file name
 PATH_MAX -- max number of characters
in a path name

Common UNIX files
 /etc : Stores system
administrative files &
programs
 /etc/passwd : Stores all user information
 /etc/shadow : Stores user passwords
 /etc/group : Stores all group
information

 /bin : Stores all system programs
 /dev : Stores all character and
block device files
 /usr/include : Stores standard
header files
 /usr/lib : Stores standard libraries
 tmp : Stores all temporary files
created by programs

UNIX and POSIX file attributes
 File type : type of file
 Access permission : the file access
permission for
owner group and
others
 Hard link count : number of hard
links of a file

 UID : the file owner user
ID
 GID : the file group ID
 File size : the file size in
bytes
 Last access time : the time the file
was last accessed
 Last modify time : the time the file was
last modified

 Last change time : the time the file
access permission
UID ,GID
or hard link count
was last changed

 Inode number : the system inode
number of the file
 File system ID : the file system ID
where the file is
stored

Attributes of a file that remain
unchanged
 File type
 File inode number
 File system ID
 Major and minor device number
Other attributes are changed using UNIX
commands or system calls

UNIX
command
System call Attributes changed
chmod chmod Changes access
permission, last
change time
chown chown Changes UID, last
change time
chgrp chown Changes GID, last
change time

touch utime Changes last access
time, modification time
ln link Increases hard link
count
rm unlink Decreases hard link
count .If the hard link
count is zero ,the file will
be removed from the file
system
vi, emac Changes file size,last
access time, last
modification time

Inodes in UNIX system
 UNIX system V has an inode table which
keeps track of all files
 Each entry in inode table is an inode
record
 Inode record contains all attributes of file
including inode number and physical
address of file
 Information of a file is accessed using its
inode number

 Inode number is unique within a file
system
 A file record is identified by a file system
ID and inode number
 Inode record doesnot contain the name of
the file
 The mapping of filenames to inode
number is done via directory files

112
67 ..
97 abc
234 a.out
To access a file for example /usr/abc, the
kernel knows ”/” directory inode number of
any process, it will scan “/” directory to
find inode number of “usr” directory it then
checks for inode number of abc in usr .
The entire process in carried out taking
into account the permissions of calling
process

APPLICATION PROGRAM
INTERFACE TO FILES
 Files are indentified by path names
 Files must must be created before they can
be used.
 Files must be opened before they can be
accessed by application programs .
 open system call is for this purpose, which
returns a file descriptor, which is a file
handle used in other system calls to
manipulate the open file

 A process can may open at most
OPEN_MAX number of files
 The read and write system calls can be
used to read and write data to opened
files
 File attributes are queried using stat or
fstat system calls
 File attributes are changed using chmod,
chown, utime and link system calls
 Hard links are removed by unlink system
call

Directory files
 Directory is a record oriented file.
 Record data type is struct dirent in UNIX V
and POSIX.1, and struct dirent in BSD UNIX
 Unix system also provides telldir and seekdir
function
Directoryfunction
opendir
readdir
closedir
rewinddir
Purpose
Opens a directory file
Reads next record from file closes
a directory file
Sets file pointer to beginning of file

 Open Opens a file for data access
 Read Reads data from file
 Write Writes data into file
 Lseek Allows random access of
data in a file
 Close Terminates connection to a
file
 Stat,fstat Queries attributes of a
file
General file APIs

 Chmod Changes access
permissions of a file
 Chown Changes UID and/or GID of a
file
 Utime Changes last modification
time and access time
stamps of a file
 Link creates a hard link to a file
 Ulink Deletes a hard link to a file
 Umask Sets default file creation
mask

Open
 The function establishes connection
between process and a file
 The prototype of the function
#include <sys/types.h>
#include <fcntl.h>
int open (const char *pathname, int access
mode , mode_t permission);

 Pathname : It can be absolute path
name or a relative path
name
 Access_mode : An integer which
specifies how file is to be
accessed by calling
process

 Access mode flag Use
 O_RDONLY Opens file for read-
only
 O_WRONLY Opens file for write
only
 O_RDWR Opens file for read
& write

 Access modifier flag
 O_APPEND
 O_CREAT
 O_EXCL
 O_TRUNC
 O_NONBLOCK
 O_NOCTTY

 O_APPEND : appends data to end of file
 O_TRUNC : if the file already exists,
discards its contents and
sets file size to zero
 O_CREAT : creates the file if it does not
exist
 O_EXCL : used with O_CREAT only.
This flag causes open to
fail if the file exists

 O_NONBLOCK : specifies that any
subsequent read or write
on the file should be non
blocking
 O_NOCTTY : specifies not to use the
named terminal device
file as the calling process
control terminal

Umask
 It specifies some access rights to be masked
off
 Prototype:
mode_t umask ( mode_t new_umask);
mode_t old_mask =
umask (S_IXGRP|S_IWOTH);
/*removes execute permission from group and
write permission from others*/
 Actual_permission =
permission & ~umask_value

 It is used to create new regular files
Creat
#include <unistd.h>
Int creat (const char* pathname,mode_t mode)

Read
 This function fetches a fixed size block of
data from a file referenced by a given file
descriptor
#include <unistd.h>
ssize_t read (int fdesc ,void* buf, size_t size);

Write
 The write function puts a fixed size block
of data to a file referenced by a file
descriptor
#include <unistd.h>
ssize_t write (int fdesc ,void* buf, size_t size);

Close
 Disconnects a file from a process
 Close function will allocate system resources
 It a process terminates without closing all the
files it has opened ,the kernel will close those
files for the process
#include <unistd.h>
int close (int fdesc);

Cat command
#include<fcntl.h>
#include<stdio.h>
#include<stdlib.h>
main(int argc,char*argv[])
{ if(argc!=2)
printf("nThe syntax should as be follows");
printf("nCommandname filetoreadn");
exit(1); }

Cat command
int fdold, count;
char buffer[2048];
fdold=open(argv[1], O_RDONLY);
if(fdold==-1)
{ printf("cannot open file"); exit(1); }
while((count=read(fdold,buffer,sizeof(buffer)))>0)
{ printf("%s",buffer); }
exit(0); }

Cp command
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
main(int argc,char*argv[])
{
if(argc!=3)
printf("nThe syntax should as be follows");
printf("nCommandname filetoread filetowriten");
exit(1); }

Cp command
char buf;
int fd_one, fd_two;
fd_one = open(argv[1], O_RDONLY);
if (fd_one == -1)
{ printf("Error opening first_filen");
close(fd_one);
return; }

Cp command
fd_two = open(argv[2], O_WRONLY | O_CREAT);
if (fd_two == -1)
{ printf("Error opening second_filen");
close(fd_one);
return; }
while(read(fd_one, &buf, 1))
{ write(fd_two, &buf, 1); }
printf("Successful copy");
close(fd_one);
close(fd_two);
}

lseek
 the lseek system call is used to change the
file offset to a different value
 Prototype :
#include <unistd.h>
Off_t lseek(int fdesc , off_t pos, int whence)

 Pos :
 specifies a byte offset to be added to a
reference location in deriving the new file
offset value
 Whence location reference
 SEEK_CUR current file pointer
address
 SEEK_SET the beginning of a
file
 SEEK_END the end of a file

link
 The link function creates a new link for
existing file
 Prototype :
#include <unistd.h>
int link (const char* cur_link ,const char*
new_link)

unlink
 Deletes link of an existing file
 Cannot link a directory unless the
calling function has the superuser
privilege
#include <unistd.h>
int unlink (const char* cur_link );

Stat fstat
 Stat and fstat retrieve arguments of a
given file
#include <unistd.h>
int stat (const char* path_name,struct
stat* statv)
int fstat (const int fdesc,struct stat* statv)

 Struct stat
{
dev_ts t_dev;
ino_t st_ino;
mode_t st_mode;
nlink_t st_nlink;
uid_t st_uid;
gid_t st_gid;
dev_t st_rdev;
off_t st_size;
time_t st_mtime
time_t st_ctime
};

 If pathname specified in stast is a
symbolic link then the attributes of the
non-symbolic file is obtained
 To avoid this lstat system call is used
 It is used to obtain attribites of the
symbolic link file
int lstat (const char* path_name , struct stat*
statv);

/* Program to emulate the UNIX ls -l
command */
#include <iostream.h>
#include <sys/stat.h>
#include <unistd.h>
#include <pwd.h>
#include <grp.h>
static char xtbl[10] = "rwxrwxrwx";

#ifndef MAJOR
#define MINOR_BITS 8
#define MAJOR(dev) ((unsigned)dev >>
MINOR_BITS)
#define MINOR(dev)( dev &
MINOR_BITS)
#endif
/* Show file type at column 1 of an output line
*/

static void display_file_type ( ostream& ofs, int
st_mode )
{
switch (st_mode &S_IFMT)
{
case S_IFDIR: ofs << 'd'; return;
/* directory file
*/
case S_IFCHR: ofs << 'c'; return;
/* character device file */
case S_IFBLK: ofs << 'b'; return;
/* block device file */

case S_IFREG: ofs << ' '; return;
/* regular file */
case S_IFLNK: ofs << 'l'; return;
/* symbolic link file */
case S_IFIFO: ofs << 'p'; return;
/* FIFO file */
}
}

/* Show access permission for owner, group,
others, and any special flags */
static void display_access_perm ( ostream&
ofs, int st_mode )
{
char amode[10];
for (int i=0, j= (1 << 8); i < 9; i++, j>>=1)
amode[i] = (st_mode&j) ? xtbl[i] : '-';
/* set access permission */

/* set access permission */
if (st_mode&S_ISUID)
amode[2] = (amode[2]=='x') ? 'S' : 's';
if (st_mode&S_ISGID)
amode[5] = (amode[5]=='x') ? 'G' : 'g';
if (st_mode&S_ISVTX)
amode[8] = (amode[8]=='x') ? 'T' : 't';
ofs << amode << ' ';
}

/* List attributes of one file */
static void long_list (ostream& ofs, char*
path_name)
{
struct stat statv;
struct group *gr_p;
struct passwd *pw_p;
if (stat (path_name, &statv))
{
perror( path_name );
return;
}

display_file_type( ofs, statv.st_mode );
display_access_perm( ofs, statv.st_mode );
ofs << statv.st_nlink; /*
display hard link count */
gr_p = getgrgid(statv.st_gid); /* convert GID
to group name */
pw_p = getpwuid(statv.st_uid); /*convert UID
to user name */
ofs << ' ' << pw_p->pw_name << ' ' <<
gr_p->gr_name << ' ';

if ((statv.st_mode&S_IFMT) == S_IFCHR ||
(statv.st_mode&S_IFMT)==S_IFBLK)
ofs << MAJOR(statv.st_rdev) << ','
<< MINOR(statv.st_rdev);
else ofs << statv.st_size;
/* show file size or major/minor no. */
ofs << ' ' << ctime (&statv.st_mtime);
/* print last modification time */
ofs << ' ' << path_name << endl;
/* show file name */
}

/* Main loop to display file attributes one file
at a time */
int main (int argc, char* argv[])
{
if (argc==1)
cerr << "usage: " << argv[0] << " <file
path name> ...n";
else while (--argc >= 1) long_list( cout,
*++argv);
return 0;
}

Access
 The access function checks the existence
and/or access permission of user to a
named file
#include <unistd.h>
int access (const char* path_name, int flag);

 The flag contains one or more bit flags
 Bit flags USE
 F_OK checks whether the file exists
 R_OK checks whether calling
process has read permission
 W_OK checks whether calling
process has write permission
 X_OK checks whether calling
process has read permission

 The chmod and fchmod functions change
file access permissions for owner, group
and others and also set-UID ,set-GID and
sticky bits
Chmod fchmod
#include <sys/stat.h>
#include <unistd.h>
int chmod (const char* path_name, mode_t flag);
int fchmod (int fdsec, mode_t flag);

 Flag argument contains new access
permissions and any special flags to be
set on the file
 Flag value can be specified as an octal
integer value in UNIX, or constructed from
the manifested constants defined in
<sys/stat.h>

Chown fchown lchown
 The chown and fchown function change
the user ID and group ID of files
 lchown changes the ownership of
symbolic link file
#include <unistd.h>
int chown (const char* path_name,
uid_t uid, gid_t gid);
int fchown (int fdesc, uid_t uid, gid_t gid);
int lchown (const char* path_name,
uid_t uid, gid_t gid);

utime
#include <unistd.h>
#include <utime.h>
int utime (const char* path_name,
struct utimbuf* times);
• The function modifies the access and
modification time stamps of a file

 Struct utimbuf
{
time_t actime;
time_t modtime;
};

fcntl
 The function helps to query or set access
control flags and the close-on-exec flag of
any file descriptor
 cmd argument specifies which operation to
perform on a file referenced by the fdesc
argument
#include <fcntl.h>
Int fcntl (int fdesc ,int cmd);

 cmd value
 F_GETFL : returns the access control flags of
a file descriptor fdesc
 F_SETFL : sets or clears control flags that
are specified
 F_GETFD : returns the close-on-exec flag of
a file referenced by fdesc
 F_SETFD : sets or clears close-on-exec flag
of a file descriptor fdesc
 F_DUPFD : duplicates the file descriptor
fdesc with another file descriptor

FILE AND RECORD LOCKING
 UNIX systems allow multiple processes to
read and write the same file concurrently.
 It is a means of data sharing among
processes.
 Why the need to lock files?
It is needed in some applications like
database access where no other process
can write or read a file while a process is
accessing a file-locking mechanism.
 File locking is applicable only to regular files

Shared and exclusive locks
 A read lock is also called a shared lock and a
write lock is called an exclusive lock.
 These locks can be imposed on the whole
file or a portion of it.
 A write lock prevents other processes from
setting any overlapping read or write locks
on the locked regions of the file. The
intention is to prevent other processes from
both reading and writing the locked region
while a process is modifying the region.

 A read lock allows processes to set
overlapping read locks but not write
locks. Other processes are allowed to lock
and read data from the locked regions.
 A mandatory locks can cause problems: If
a runaway process sets a mandatory
exclusive lock on a file and never unlocks
it, no other processes can access the
locked region of the file until either a
runaway process is killed or the system is
rebooted.

 If a file lock is not mandatory it is advisory.
An advisory lock is not enforced by a
kernel at the system call level
 The following procedure is to be followed
 Try to set a lock at the region to be
accessed. if this fails, a process can either
wait for the lock request to become
successful or go do something else and try
to lock the file again.
 After a lock is acquired successfully, read
or write the locked region
 Release the lock

Advisory locks
 A process should always release any lock
that it imposes on a file as soon as it is
done.
 An advisory lock is considered safe, as no
runaway processes can lock up any file
forcefully. It can read or write after a fixed
number of failed attempts to lock the file
 Drawback: the programs which create
processes to share files must follow the
above locking procedure to be
cooperative.

FCNTL file locking
 int fcntl (int fdesc, int cmd_flag, …);
Use
Sets a file lock. Do not block if this
cannot succeed immediately.
Sets a file lock and blocks the
calling process until the lock is
acquired.
Queries as to which process locked
a specified region of a file.
Cmd_flag
F_SETLK
F_SETLKW
F_GETLK

For file locking the third argument is struct
flock-typed variable.
struct flock
{
short l_type;
short l_whence;
off_t l_start;
off_t l_len;
pid_t l_pid;
};

l_type and l_whence fields of flock
l_type value Use
F_RDLCK Sets as a read (shared) lock
on a specified region
F_WRLCK Sets a write (exclusive) lock
on a specified region
F_UNLCK Unlocks a specified region

l_whence value Use
SEEK_CUR The l_start value is
added to the current file
pointer address
SEEK_SET The l_start value is
added to byte 0 of file
SEEK_END The l_start value is
added to the end
(current size) of the file

 The l_len specifies the size of a locked
region beginning from the start address
defined by l_whence and l_start. If l_len is
0 then the length of the locked is imposed
on the maximum size and also as it
extends. It cannot have a –ve value.
 When fcntl is called, the variable contains
the region of the file locked and the ID of
the process that owns the locked region.
This is returned via the l_pid field of the
variable.

LOCK PROMOTION AND SPLITTING
 If a process sets a read lock and then
a write lock the read lock is now
covered by a write lock. This process
is called lock promotion.
 If a process unlocks any region in
between the region where the lock
existed then that lock is split into two
locks over the two remaining regions.

Mandatory locks can be achieved by setting
the following attributes of a file.
 Turn on the set-GID flag of the file.
 Turn off the group execute right of the file.
All file locks set by a process will be
unlocked terminates.
If a process locks a file and then creates a
child process via fork, the child process
will not inherit the lock.
The return value of fcntl is 0 if it succeeds
or -1 if it fails.

#include <iostream.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
int main (int argc, char* argv[])
{
struct flock fvar;
int fdesc;

while (--argc > 0) { /* do the
following for each file */
if ((fdesc=open(*++argv,O_RDWR))==-1)
{
perror("open"); continue;
}
fvar.l_type = F_WRLCK;
fvar.l_whence = SEEK_SET;
fvar.l_start = 0;
fvar.l_len = 0;

/* Attempt to set an exclusive (write) lock on the
entire file */
while (fcntl(fdesc, F_SETLK,&fvar)==-1)
{
/* Set lock fails, find out who has locked the file
*/
while (fcntl(fdesc,F_GETLK,&fvar)!=-1 &&
fvar.l_type!=F_UNLCK)
{
cout << *argv << " locked by " << fvar.l_pid<<
" from " << fvar.l_start << " for "<<
fvar.l_len << " byte for " <<

(fvar.l_type==F_WRLCK ? 'w' : 'r')
<< endl;
if (!fvar.l_len) break;
fvar.l_start += fvar.l_len;
fvar.l_len = 0;
} /* while there are locks set by other
processes */
} /* while set lock un-successful */

// Lock the file OK. Now process data in the file
/* Now unlock the entire file */
fvar.l_type = F_UNLCK;
fvar.l_whence = SEEK_SET;
fvar.l_start = 0;
fvar.l_len = 0;
if (fcntl(fdesc, F_SETLKW,&fvar)==-1)
perror("fcntl");
}
return 0;
} /* main */

Directory File APIs
 Why do need directory files? Uses?
 To aid users in organizing their files
into some structure based on the
specific use of files
 They are also used by the operating
system to convert file path names to
their inode numbers

 To create
int mkdir (const char* path_name , mode_t
mode);
 The mode argument specifies the access
permission for the owner,group, and
others to be assigned to the file.

Difference between mkdir and
mknod
 Directory created by mknod API does not
contain the “.” and “..” links. These links
are accessible only after the user
explicitly creates them.
 Directory created by mkdir has the “.” and
“..” links created in one atomic operation,
and it is ready to be used.
 One can create directories via system
API’s as well.

 A newly created directory has its user ID
set to the effective user ID of the process
that creates it.
 Directory group ID will be set to either the
effective group ID of the calling process
or the group ID of the parent directory that
hosts the new directory.

FUNCTIONS
Opendir:
DIR*opendir (const char* path_name);
This opens the file for read-only
Readdir:
Dirent* readdir(DIR* dir_fdesc);
The dir_fdesc value is the DIR* return
value from an opendir call.

Closedir :
int closedir (DIR* dir_fdesc);
It terminates the connection between the
dir_fdesc handler and a directory file.
Rewinddir :
void rewinddir (DIR* dir_fdesc);
Used to reset the file pointer associated
with a dir_fdesc.

 Rmdir API:
int rmdir (const char* path_name);
Used to remove the directory files. Users
may also use the unlink API to remove
directories provided they have superuser
privileges.
These API’s require that the directories to
be removed be empty, in that they contain
no files other than “.” and “..” links.

Device file APIs
 Device files are used to interface physical
devices (ex: console, modem) with
application programs.
 Device files may be character-based or
block-based
 The only differences between device files
and regular files are the ways in which
device files are created and the fact that
lseek is not applicable for character
device files.

To create:
int mknod(const char* path_name, mode_t
mode,int device_id);
 The mode argument specifies the access
permission of the file
 The device_id contains the major and
minor device numbers. The lowest byte of
a device_id is set to minor device number
and the next byte is set to the major
device number.

MAJOR AND MINOR NUMBERS
 When a process reads from or writes to a
device file, the file’s major device number
is used to locate and invoke a device
driver function that does the actual data
transmission.
 The minor device number is an argument
being passed to a device driver function
when it is invoked. The minor device
number specifies the parameters to be
used for a particular device type.

 A device file may be removed via the
unlink API.
 If O_NOCTTY flag is set in the open call,
the kernel will not set the character device
file opened as the controlling terminal in
the absence of one.
 The O_NONBLOCK flag specifies that the
open call and any subsequent read or
write calls to a device file should be
nonblocking to the process.

FIFO File APIs
 These are special device files used for
interprocess communication.
 These are also known as named files
 Data written to a FIFO file are stored in a
fixed-size buffer and retrieved in a
first-in-first-out order.
 To create:
int mkfifo( const char* path_name, mode_t
mode);

How is synchronization provided?
 When a process opens a FIFO file for
read-only, the kernel will block the
process until there is another process that
opens the same file for write.
 If a process opens a FIFO for write, it will
be blocked until another process opens
the FIFO for read.
This provides a method for process
synchronization

 If a process writes to a FIFO that is full,
the process will be blocked until another
process has read data from the FIFO to
make room for new data in the FIFO.
 If a process attempts to read data from a
FIFO that is empty, the process will be
blocked until another process writes data
to the FIFO.
 If a process does not desire to be blocked
by a FIFO file, it can specify the
O_NONBLOCK flag in the open call to the
FIFO file.

 UNIX System V defines the O_NDELAY flag
which is similar to the O_NONBLOCK flag.
In case of O_NDELAY flag the read and
write functions will return a zero value
when they are supposed to block a
process.
 If a process writes to a FIFO file that has
no other process attached to it for read,
the kernel will send a SIGPIPE signal to the
process to notify it of the illegal operation.

 If Two processes are to communicate
via a FIFO file, it is important that the
writer process closes its file
descriptor when it is done, so that the
reader process can see the end-of-file
condition.
 Pipe API
Another method to create FIFO files for
interprocess communications
int pipe (int fds[2]);

 Uses of the fds argument are:
 fds[0] is a file descriptor to read
data from the FIFO file.
 fds[1] is a file descriptor to write
data to a FIFO file.
 The child processes inherit the FIFO file
descriptors from the parent, and they
can communicate among themselves
and the parent via the FIFO file.

Hard and symbolic links
 A hard link is a UNIX path name for a file
 To create hard link ln command is used
ln /usr/abc/old.c /usr/xyz/new.c
 Symbolic link is also a means of
referencing a file
 To create symbolic link ln command is
used with option –s
ln –s /usr/abc/old.c /usr/xyz/new.c

 Cp command creates a duplicated copy of
file to another file with a different path
name
 Where as ln command saves space by not
duplicating the copy here the new file will
have same inode number as original file
Difference : cp and ln command

Difference : hard link and symbolic
link
Hard link Symbolic link
Does not create a new
inode
Create a new inode
Cannot link directories
unless it is done by root
Can link directories
Cannot link files across
file systems
Can link files across file
system
Increase hard link count
of the linked inode
Does not change hard
link count of the linked
inode

Symbolic Link File APIs
 These were developed to overcome several
shortcomings of hard links:
 Symbolic links can link from across file
systems
 Symbolic links can link directory files
 Symbolic links always reference the latest
version of the file to which they link
 Hard links can be broken by removal of one
or more links. But symbolic link will not be
broken.

To create :
int symlink (const char* org_link, const
char* sym_link);
int readlink (const char* sym_link, char* buf,
int size);
int lstat (const char* sym_link, struct stat*
statv);

 To QUERY the path name to which a
symbolic link refers, users must use the
readlink API. The arguments are:
 sym_link is the path name of the symbolic
link
 buf is a character array buffer that holds
the return path name referenced by the
link
 size specifies the maximum capacity of
the buf argument

QUESTIONS
 Explain the access mode flags and
access modifier flags. Also explain how
the permission value specified in an
‘Open’ call is modified by its calling
process ‘unmask, value. Illustrate with
an example (10)
 Explain the use of following APIs (10)
i) fcntl ii) lseek iii) write iv) close

 With suitable examples explain various
directory file APIs (10)
 Write a C program to illustrate the use of
mkfifo ,open ,read & close APIs for a FIFO
file (10)
 Differentiate between hard link and
symbolic link files with an example (5)
 Describe FIFO and device file classes (5)
 Explain process of changing user and
group ID of files (5)

 What are named pipes? Explain with an
example the use of lseek, link, access with
their prototypes and argument values (12)
 Explain how fcntl API can be used for file
record locking (8)
 Describe the UNIX kernel support for a
process . Show the related data
structures (10)

 Give and explain the APIs used for the
following (10)
1. To create a block device file called
SCS15 with major and minor device
number 15 and 3 respectively and
access rights read-write-execute for
everyone
2. To create FIFO file called FIF05 with
access permission of read-write-execute
for everyone

2ab. UNIX files.ppt JSS science and technology university

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