L5/6: Files and Directories

Question 1

Six Main Topics

in the

Systems Programming Class

Answer 1

Program Development

Files and Directories

Tool Building

Processes

Networking

OS Implementation

Question 2

Shell Command Equivalents:

mv

Answer 2

From stdio.h library:

rename( old_filepath, new_filepath);

Equivalent to:

%mv old new

Question 3

Shell Command Equivalents:

ln

Answer 3

From unistd.h library:

link( old_filepath, new_filepath);

Equivalent to:

%ln old new

Question 4

Shell Command Equivalents:

rm

Answer 4

From unistd.h library:

unlink( “old_file” );

Equivalent to:

%rm old_file

Question 5

Shell Command Equivalents:

rm -r

Answer 5

From stdio.h library:

remove( “old_file” );

Equivalent to:

%rm -r old_file

Question 6

Shell Command Equivalents:

cp

Answer 6

No direct method.

Use read/write File I/O operations

to copy old file to new.

Equivalent to:

%cp old new

Question 7

Shell Command Equivalents:

cd

Answer 7

From unistd.h library:

chdir( “path” );

Equivalent to:

%cd path

Question 8

Shell Command Equivalents:

mkdir

Answer 8

From unistd.h library:

mkdir( “newpath”, 0777);

Note: Not sure what the 0777 flag does

Equivalent to:

%mkdir newpath

Question 9

Shell Command Equivalents:

rmdir

Answer 9

From unistd.h library:

rmdir( “path”);

Equivalent to:

%rmdir path

Question 10

unistd.h

Methods that are equivalent to

common shell commands (5)

Answer 10

• link()
  
  • equivalent to “ln”
• unlink()
  
  • equivalent to “rm”
• chdir()
  
  • equivalent to “chdir”
• mkdir()
  
  • equivalent to “mkdir”
• rmdir()
  
  • equivalent to “rmdir”

Question 11

Physical Disk Organization:

Partition Sections

Answer 11

• boot and superblocks
• ilist
  
  • contains inodes
• datablocks

Question 12

Physical Disk Organization:

inode structure

Answer 12

An inode consists of

• a status block
• addresses to multiple data blocks

Question 13

Physical Disk Organization:

Key Points

Answer 13

• Disk is arranged into blocks
  
  • some for data
  • some for management
• Each inode corresponds to one file and the data blocks associated with that file
• Physical layout is not consecutive, file blocks can be all over the place

Question 14

How many data blocks can an inode point to?

Answer 14

13

Question 15

How do inodes handle larger file sizes?

Answer 15

• The inode only has 13 pointers, so an indirect approach is used
• File blocks can each contain many pointers
• Pointers are classified into levels L0, L1, L2, …
• This allows a hierarchy of blocks to be created to handle arbitrary file sizes
• Level L0 is a pointer that points directly to an actual datablock
• L1 points to a block of L0 pointers, L2 to a block of L1 pointers, and so on.

Question 16

Directory Access Rules (3)

Answer 16

• Search Rule
  
  • to open a file, the user or program must have execute permission on each directory in the full pathname
• Write Rule
  
  • to create or delete a file, must have write access to directory
• Read Rule
  
  • to read file names, must have read access on the directory

Question 17

inodes:

Design Consequences

for

Move Operations

Answer 17

• Move speed is independent of the file size
  
  • data blocks do not have to be moved around
  • only the pointers change(unless moving partitions/drives)
• Moves are easy to implement as atomic operations

Question 18

Structure of a

Directory

Answer 18

• The directory is a specific type of file
• Contains a table of inodes paired with filenames
• inodes are identified with inode numbers
• The table contains a reference to the directory itself (.)
• and to it’s parent directory( .. )

Question 19

Directory Structure:

How do Link(ln) operations work?

Answer 19

• Works similarly to move operation, but old reference is kept
• The inode number is copied from the first directory’s table and added to the second specified directory’s table with a new filename
• Both directories now have a reference to the same inode, but may have different names for the same file.
• File data is not duplicated
• Example:

ln dir_1/myFile dir_2/myFile2

Question 20

Directory Structure:

How Move (mv)

operations work

Answer 20

• Physical data is not moved around on the disk, and will keep the same inode
• The inode number is copied from the table in the source directory
• Then added to the table in the destination directory with a new filename
• The old reference is then deleted from the table in the source directory
• Example:

mv dir_1/derp.txt dir_2/samederp.txt

Question 21

Physical Disk Organization:

Common Data Block Size

Pointer Size

Answer 21

Block Size: 512 bytes

Pointer Size: 4 bytes

Question 22

Disk Structure:

How data is accessed by a programmer

Answer 22

• A programmer can get to data only through an inode.
• Data is “seen” as a simple, expandable array of bytes
• Physical and “housekeeping” details are invisible to the programmer

Question 23

Which Operations( Access Modes)

are files protected for?

Answer 23

Each file is protected for:

• read
• write
• execute

Question 24

3 Identities for File Access

Answer 24

Access (bits)permissions are defined for:

• user
• group
• other

Question 25

How do Unix/Linux systems

distinguish

Users and Groups?

Answer 25

Each User has a unique identifier

• uid
• The etc/passwd file matches the user name and uid

Each User belongs to one or more groups

Each Group is identified by a unique identifier

• gid
• The etc/groups file matches group names and gid

Question 26

Where are uid’s and user names stored?

Answer 26

etc/passwd

This matches user names to uid

Question 27

Where are group id’s (gid) and names stored?

Answer 27

etc/groups

matches group names and gid

Question 28

What is the “stat” structure?

Answer 28

A structure that can be obtained at runtime,

it contains statistics and information about a specific file.

Often used to check the access mode bits on a file before attempting to use it.

Question 29

Important fields of the “stat” structure (9)

Answer 29

Given a stat structure “s”

Not comprehensive.

• st_ino - inode
• st_nlink - link count
• st_uid - user id of the file
• st_gid - group id of the file
• • st_mode - mode bits
• • st_atime - last access time
• st_ctime - creation time
• st_mtime - last modification time
• st_size - size of the file

Question 30

Difference between

real uid

and

effective uid

Answer 30

Real uid

• the uid of the actual user who is logged in

Effective uid

• uid temporarily used for the lifetime of a process

Most of the time, the effective uid and real uid are the same,

but Administrative programs are able to operate with a different uid than the invoker.

Question 31

How to get the

Status of a file

(in code)

Answer 31

Need to use the “stat” struct

• located in the “sys/stat.h” library

Use one of the functions:

• stat( “pathname”, stat s)
• fstat()
• lstat()

To return a stat structure, placed in s

Then check s->st_mode to determine the file type and if the program has the correct permissions to access

Question 32

File Status:

Helpful operations defined in

sys/stat.h

(2)

Answer 32

S_ISREG(m)

• Check if the file is a regular file
• pass the stat struct as the parameter “m”
• if( S_ISREG( s->st_mode) ) …

S_ISDIR(m)

• Check if the file is a directory
• if ( S_ISDIR( s->st_mode) ) …

Question 33

stat structure:

How to check permissions

Answer 33

the sys/stat.h header file defines some useful values to use as bitmasks to check against s->st_mode:

• User permissions:
  
  • S_IRUSR, S_IWUSR, S_IXUSR
• Group permissions:
  
  • S_IRGRP, S_IWGRP, S_IXGRP
• Other permissions:
  
  • S_IROTH, S_IWOTH, S_IXOTH

These can be “anded” against s->st_mode:

• if (s->st_mode && S_IRUSR) …

Question 34

inodes and Datablocks

are reclaimed only

when ________

Answer 34

Two conditions are met:

• The Link Count becomes 0
• The number of “opens” becomes 0

Question 35

How to change the

Effective uid

of a program?

Answer 35

• Must use the “set-uid” bit on executables
• The Super User must turn on the set-uid bit for trusted programs
• The effective uid becomes that of the executable’s owner for the duration of the process

Turning on the set-uid bit:

Use chmod, as super user. It is the 4th of the permission bits:

-rws –x –x

Question 36

Symbolic Links:

Overview

Answer 36

• A special file type that simply contains the name of another file
• Different from regular links, called “hard links”
• All file operations attempted on a symbolic link will follow the link and affect the destination file
• Created using command:
  
  • ln -s
• Also called sym links

Question 37

How to list inodes on the shell

Answer 37

ls -i

Question 38

Design Consequences of Inodes

Answer 38

• Directories reference files only through inode numbers
  
  • moving files between directories is very fast
  • and, the operation is atomic (as are link and remove)
• Two directories can include the same inode number
  
  • a file can be shared, appearing in several places with same or different name
  • also, no special cases for “shortcuts” etc.
• Space is freed only when inode link & open counts are both 0
  
  • removing a file may not create free space
  • files can’t disappear while a process is working on them
  • also, no special cases for “sharing violation” etc.
• An inode number is unique only within a file system
  
  • files can’t span partitions or disks, or be moved to new disks
  • and, files can’t be bigger than a single physical disk
• Files don’t have “type” (beyond regular vs. directory)
  
  • no optimizations for databases, no self-identifying objects, no new types
  • but, easy to code general programs such as “cp”, “mv”
• Plus, as we will see, file security information is in the inode
  
  • data is protected even when files are shared (details later)