Unix files

UNIX FILES
By : Sunil Kumar R.M
1Sunil Kumar R.M Assistant Professor RLJIT

UNIX FILES
• Files are the building blocks of any operating
system.
user executes a command in UNIX
the UNIX kernel fetches the corresponding
executable file and loads its instruction text to
memory
 and creates a process to execute the
command on your behalf.

File Types
• A file in a UNIX or POSIX system may be one of
the following types:
– Regular file
– Directory file
-- FIFO file
– Character device file
-- Block device file

Regular File
– Text file or binary file
i.e both can be executable files (.exe) with
suitable permissions. Ex :pdf, .txt etc
How to create?
using editors(vi editor)
How to delete?
using rm command.

Directory files
• Folder.
• Tree structure.
• HOW TO CREATE?
– mkdir command
» Example mkdir /usr/sunil/file1
HOW TO delete?
-- rmdir /usr/sunil/file1
We can also display the contents of directory using ls
command

FIFO files
• Used in Interprocess communication (IPC)
buffer
Process1 ------------------------------------------ Process2
(pipe/named pipe)
CREATE?
• mkfifo /usr/sunil/fifo_pipe
• mknod /usr/sunil/fifo_pipe p

DEVICE FILES
• Block device files.
ex: floppy disk, hard disk.
• Character device files.
• Ex: line printer, keyboard, modem etc.
CREATE?
Syntax:
$mknod path c/b major minor
name device # device #
Ex: $ mknod /usr/sunil/file1 C 115 20

Device files 2
• Major device no :
– Index to Kernel table
• Device driver function addresses.
• Minor device no:
– Integer no that is a parameter to the device driver
function

Symbolic link files
• Supported by Unix but not by posix
– It contains a path name that refers to another file
/file system that can be local/remote.
– Creating symbolic link file
• $ ln -s usr/sunil/abc usr/rljit/xyz

The UNIX and POSIX File Systems
• tree-like hierarchical file system.
• Name MAX
• PathMAX
• Posix name and path
• Hard link

The UNIX and POSIX File Attributes
• 1) File type - specifies what type of file it is.
• 2) Access permission - the file access
permission for owner, group and others.
• 3) Hard link count - number of hard link of
the file
• 4) Uid - the file owner user id.
• 5) Gid - the file group id
• . 6) File size - the file size in bytes.

The UNIX and POSIX File Attributes
• 7) Inode no - the system inode no of the file.
8) File system id - the file system id where the
file is stored.
• 9) Last access time - the time, the file was last
accessed.
• 10) Last modified time - the file, the file was
last modified.
• 11) Last change time - the time, the file was
last changed.

Inodes in UNIX System V
• inode table, which keeps tracks of all files
– Inode record(attributes,inode no, physical
address)
– It’s a u i ue id.
– Why os does ’t sto e file a e
– Directory file content.
– Who gives inode no and when?

• Directory is a record oriented file.
• Record data type is struct dirent in UNIX V and
POSIX.1, and struct dirent in BSD UNIX
Directory files

• 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
Hard and symbolic links

• 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

• Files are indentified by path names
• Files must be created before they can be
used.
• open system call  returns a file descriptor
APPLICATION PROGRAM INTERFACE
TO FILES(API)

• OPEN_MAX
• READ and WRITE
• Stat and fstat file attributes
• File attributes are changed using chmod,
chown, utime and link system calls
• Hard links are removed by unlink system call

• user executes a command -> process(kernel)
command execution
• process data structures: file descriptor table is
one among them
• File descriptor table OPEN_MAX entries,
and it records all files opened by the process
UNIX KERNEL SUPPORT FOR FILES

• Whenever an open function is called the
kernel will resolve the pathname to file inode
• Open call fails and returns -1 (file inode not exist
/the process lacks appropriate permissions)
Kernel support to open system call

rc=1
rc=2
r
rc=1
rw
rc=1
w
rc=1
File descriptor table
File table Inode table

• Else a series of steps follow
• The kernel will search the file descriptor
table and look for first unused entry, which is
returned as file descriptor of the opened file
• The kernel will scan the file table, which is
used to reference the file If an unused entry
is found then

1. The p o ess’s file des ipto ta le e t y will e set
to point to file table entry
2. The file table entry will be set to point to inode
table entry where the inode record of file is present
3. The file table entry will contain the current file
pointer of the open file
4. The file table entry will contain an open mode
which specifies the file is opened for read- only
,write-only etc.
5. Reference count of file table is set to 1.
6. The reference count of the in-memory inode of file
is increased by 1.

• The kernel will use the file descriptor to index
the p o ess’s file des ipto ta le to fi d file
table entry to opened file
• It checks the file table entry to make sure that
the file is opened with appropriate mode
• The kernel will use the file pointer used in file
table entry to determine where the read
operation should occur in file
Kernel support : read system call

• The kernel will check the type of file in the
inode record and invokes an appropriate
driver function to initiate the actual data
transfer with a physical file
• If the process calls lseek function then the
changes are made provided the file is not a
character device file, a FIFO file, or a symbolic
link file as they follow only sequential read
and write operations

1. The kernel will set the corresponding
descriptor table entry to unused
2. It decrements the reference count in file
table entry by 1.if reference count !=0 then
go to 6
3. File table entry is marked unused
4. The reference count in file inode table entry
by 1.if reference count !=0 then go to 6
Kernel support : close system call

5. If hard link count is non zero, it returns a
success status, otherwise marks the inode
table entry as unused and and deallocates all
the physical disk storage
6. It returns the process with a 0 (success )
status

rc=1
rc=2
r
rc=1
rw
rc=1
w
rc=1
File descriptor table
File table Inode table

Unix files

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