COMP 3000
Operating Systems
Facilities for Users/Programmers
Lianying Zhao
Talking to the Computer
1. Connect to the computer
2. Log in
3. Send commands (run programs)
4. Control programs
5. Feed data to programs
6. Programs work with OS services
COMP 3000 2
What is a Terminal?
“An electronic or electromechanical hardware device that can be used for
entering data into, and transcribing data from, a computer or a computing
E.g., IBM 2741 (1960s)
IBM 3270, VT102/100 (1970s-80s)
Pseudo terminals
COMP 3000 3
Source: Wikipedia Source:
Users and Access Control
Largely coming down to file protection mechanisms
Username -> UID, User group -> GID
The login process: e.g., /usr/bin/login
To establish a session
The password file:
/etc/passwd -> $HOME, $SHELL, $PATH, …
COMP 3000 4
What about GUI?
And, remote (e.g., SSH)?

Permissions (File Modes)
Very basic, mostly file-based access control
The idea is: I am who/what I am (subject identity). So, what is your decision?
Each object has an owner (e.g., user)
— and is assigned security attributes (e.g., file modes)
In the form of running processes
Because the username is just a label
UID, EUID (“effective” UID), FSUID (file system UID)
read, write, execute
setuid, setgid, sticky bit
COMP 3000 5
The Shell
Command interpreter
Technically not part of the OS
Many shells exist (bash, dash, ksh, csh, tcsh, etc.)
Typical steps:
1. Initialize the environment (e.g., env variables)
2. Display a prompt
3. Parse user input, and perform the task in the case of internal commands
4. fork(), and wait in the parent process
5. Try to find the program (external command)
6. Manipulate fds as needed
7. exec()to attempt execution
8. Report errors (if any) and terminate child
COMP 3000 6
The GUI is also like a shell, but in a
complex form

Controlling Running Programs
All processes have a parent process (except init)
As a result of a fork()-like system call
The PPID of a process
Hence forming the process tree
The wait() System Call
A process must be waited on (or it enters a zombie state after it exits)
A zombie process can’t be killed
Where the return code (exit status) is passed
COMP 3000 7
Controlling Running Programs: Signals
The user may want to notify one program of something
The OS may …
Another program may …
Signals: a limited form of IPC (inter-process communication)
Predefined: e.g., SIGINT (interrupt, Ctrl-C), SIGCHLD (child process terminates)
No new one can be defined. Use SIGUSR1 and SIGUSR2 for custom purposes
An OS artifact
Signals are again a POSIX thing. MS Windows: Messages
COMP 3000 8
Signal Handling
The OS interrupts the signalled process, and calls the handler function
All processes are listening to all signals
Signal handlers are defined by the process (except for a few signals)
Just regular functions
The C runtime (e.g., registers default signal handlers
Specified with sigaction() (or signal(), not recommended)
Can be invoked at any time by the OS kernel
What if the handler function modifies shared data
Do as little as possible
COMP 3000 9
Pipeline and redirections
Pipe: unidirectional data channel
Command-1 | Command-2 | …| Command-N
(shell pipeline)
We will leave programming with pipes to later
discussions, e.g., pipe()
| stdout -> stdin
|& stdout+stderr -> stdin (shorthand for 2>&1 |)
Redirecting: stdin, stdout or stderr
>, <, >>
All these are only possible with the separation of
exec() from fork()
COMP 3000 10
Ways to send input and receive
output from programs:
Computer <-> programs
Human <-> computer
1. Command line arguments
2. Standard I/O
3. Files
4. Network
5. Combination (cf. IPC)

The File Abstraction and File Systems
File: A linear array of bytes, stored persistently*
Ways of organizing data
File system: Ways of organizing files
The user’s perspective of files:
Identifier: filename
Further identified by: path + filename
Can be read from or written to
Meaning of a filename
COMP 3000 11
More on Pathnames
Paths are hierarchical, and there is a root (“/”)
Absolute pathname
E.g., /home/student/comp3000
Relative pathname
(Current) working directory (CWD): per process
E.g., tut1/abc
COMP 3000 12
Operations on Files (POSIX)
E.g., int fd = open(“/path/to/dir”,
For existing files, still a file descriptor (fd) is needed for any operations
Seek: there exists a current location
E.g., lseek()
There’s something “out-of-band”
E.g., ioctl()
COMP 3000 13
File Systems (Implementation)
File system types are determined by purposes
Optical discs, Flash memory, Union/overlay, etc.
Not just a choice: various implications
E.g., FAT32 has a file size limit of 4GB
Special file systems (thanks to VFS)
devfs, udev
COMP 3000 14
Mounting File Systems
To be connected to the uniform file-system tree
Mount point – an existing directory
Pathnames become relative
COMP 3000 15
COMP 3000 16
COMP 3000
Operating Systems
Memory Allocation
Ways to ask for memory at runtime
From the OS: via system calls
In-process memory management (already in address space)
A C runtime library thing
malloc() and free() – from the heap?
Write a custom memory allocator, if interested
COMP 3000 17
The Program Break – brk()
What is the break?
The address of the first byte beyond the data segment — the location of the
end of the “heap”
Actually reflecting the size of the data segment
Two system calls:
COMP 3000 18
Very powerful
Map files or devices into memory
File mapping – access files like memory
Device access, depending on the driver
Anonymous mapping – allocate new space for the process
Independent of the original heap, but can be treated as a heap by the
C runtime
COMP 3000 19
Address Space Layout Randomization (ASLR)
A concept advanced from later security topics
Memory corruption vulnerabilities?
Now the process address space has been contained*
What can an adversary do without direct manipulation?
Return Oriented Programming (ROP)
Jump Oriented Programming (JOP)
Or simply buffer overflow
All require predictable memory layout
The idea is to apply a randomly generated offset to the bases of critical
segments, e.g., heap, stack, and loaded libraries.
COMP 3000 20
File Mode Notation (Permission Bits)
Symbolic mode: e.g., rwx
Octal (numeric) mode: e.g., 7 111
Just conversion from octal to binary
Counting in setuid and setgid
Mask for setuid: 4000
When there’s x for u: lower case s
When there’s no x for u: upper case S
Mask for setgid: 2000
When there’s x for g: lower case s
When there’s no x for g: upper case S
COMP 3000 21