Soma: Created page with "==Video== Video from the lecture given on September 28, 2021 is now available: * [https://homeostasis.scs.carleton.ca/~soma/os-2021f/lectures/comp3000-2021f-lec06-20210928.m4..."

2021-09-28T16:37:03Z

Created page with "==Video== Video from the lecture given on September 28, 2021 is now available: * [https://homeostasis.scs.carleton.ca/~soma/os-2021f/lectures/comp3000-2021f-lec06-20210928.m4..."

New page

==Video==

Video from the lecture given on September 28, 2021 is now available:
* [https://homeostasis.scs.carleton.ca/~soma/os-2021f/lectures/comp3000-2021f-lec06-20210928.m4v video]
* [https://homeostasis.scs.carleton.ca/~soma/os-2021f/lectures/comp3000-2021f-lec06-20210928.cc.vtt auto-generated captions]
Video is also available through Brightspace (Resources->Class zoom meetings->Cloud Recordings tab)

==Notes==

<pre>
Lecture 6
---------
- deadlines
- A1 is due Friday, Oct 1
- but will be accepted until October 5th 10 AM
no penalty
(we'll discuss solutions in class)
- but T1 & T2 are due by midnight on October 1st

- Tutorial 3 came out yesterday, officially due in a week
- but will be accepted until the official due date of A2

- missing official deadlines don't have grading penalties
- until the hard deadline
- but if you miss one, you should consider yourself behind

What is the material for the midterm?
- the two assignments
- I literally take the assignment questions as a basis for
the midterm questions (making a question up to cover the concepts of an assignment question or two)
- midterm is all short answer, should be answerable without
referring to notes or using a computer
- but it is open book, open note, open internet
- just NO COLLABORATION
- remember there will be randomized interviews after
- to make sure they were graded fairly
- but yes, if it is clear what you know is different from
what is on the test...we'll have a problem

- My dog's name is Roshi

Topics for today
----------------
- stack vs. heap
- assembly directives
- standard I/O, I/O redirection
- terminals

To try and understand a memory map, helpful to run with setarch -R
- disables ASLR (address space layout randomization),
a security mitigation against code injection attacks

Note that the addresses in a process are consistent from run to run with setarch -R
- because each process has its own address space
- process address spaces are "virtual"
- no direct correspondence to addresses of actual RAM in your
computer (physical addresses)
- OS manages virtual <-> physical address mappings
- memview gives us a view of virtual addresses
- cannot access physical addresses outside of the kernel
- (page tables are the data structure that maintains
virtual <-> physical mappings, but we'll talk about that
later)

- In a classic view of C programs, variables are stored in the
"stack" or the "heap"
- stack is for local variables
- heap is for malloc'd variables
- but the memory map is more complex

- normally there is an area for runtime data storage in memory that holds the stack and the heap
- heap is allocated starting from the bottom (lowest address) and the stack is allocated from the top (highest address)
- sbrk(0) gives you the address of the beginnig of the free space in the heap
- as you allocate things dynamically, sbrk(0) increases

- note that code & static data (data fixed at compile time)
are stored together (i.e., strings embedded in the code)

A segment is just a variable-sized area of memory
- generally with specific semantics (purpose)
.text is normally the code
.rodata is read-only data known at compile time
- main thing to know is the stack and the heap don't have
anything really in the on-disk segments
- at most you have a declaration
- they are then dynamically allocated at runtime
- stack/heap is really one segment used for dynamic allocation
- but nowadays they are logically separate because we don't
want the stack and the heap to ever collide
- we add barriers between them
- a "quad" is a 64-bit quantity
- I think this is historic, quad word, when
a "word" was 16 bits (i.e., when registers only held
16 bits, not 64 bit quantities)

- With static linking, all references are resolved at link time
- With dynamic linking, some references have to be resolved at
runtime (the runtime dynamic linker has to run before main starts)

- this is why dynamically linked programs have many more system calls before main than do statically linked programs
- it is literally loading files from disk, i.e., the C library

- make sure you understand the output of memview
- and try similar things in 3000quiz to see its memory map

I would suggest translating the output of 3000memview into a diagram
- label it with addresses
- see where things are relative to each other
- you should see a clear picture
- and if parts don't make sense, ask!

Standard I/O
- you've learned about standard in, out, and error
- correspond to file descriptors 0, 1, and 2 as we
previously discussed
- but how do we change them, and where do they point by default?

In most shells, you can use <, >, and | to redirect standard in and out
- > redirect standard output
- < redirect standard input
- | redirect the standard output of one program to the
standard input of another

These operators can work with arbitrary file descriptors,
just give the number before them (for < and > at least)

Example, standard error redirection is 2>
(so 1> is the same as >)

If I don't redirect standard in, out, and error, where are they going by default?
- they are going to a file, but what file?

Whether you are in a graphical text window locally or have ssh'd to a remote system, you'll probably see bash's file descriptors referring to /dev/pts/? (where ? is a small number)

What is /dev/pts?
- pseudo TTYs

A teletype is like a fancy telegraph
- you type in things locally, they appear at a remote printer
- remote person types, you see it locally, printed out
- OLD fashion version of texting

When computers came around, early interactive interfaces were teletypes
- computer was one side, rather than a human
- was printing to paper rather than a screen
- could produce A LOT of paper

Eventually the paper was replaced with a CRT (cathode ray tube)
- see the VT100, a "video terminal"
- teletype but with screen output, not paper output
- pure text! (except sometimes for weird fonts with shapes)

There were LOTS of video terminals
- many incompatibilities
- so UNIX developed ways of dealing with different terminals
- terminfo database
- the TERM environment variable says which terminal you're using
- when you connect to your class VM, what's the value of TERM
for you? It may be different from mine

Why does the type of terminal matter?
- because not all text interfaces are the same!
- when I run a program in a terminal that shows colors or positions text at specific locations on the screen, it is using special escape codes to do all that
- it is just sending data to standard out, encoded so
the terminal understands it
- the terminfo files tells common libraries how to use
common terminal capabilities

- but how do you get a program like gnome-terminal to behave like a teletype, and how can ssh do the same?
- they implement the pseudo tty interface via /dev/pts
- any program that implements the pseudo tty interface
can behave like a terminal to a program

So question, why does it have to be a special device?
- because terminals are devices with special capabilities
- special things for interacting with keyboard and screen
- big thing is speed traditionally, we'd connect via
modems and so buffering, local echo were necessary
- different from a regular file

When you interact with a terminal, you're dealing with tech with 50+ years of history
- lots of weird bits if you really dig in

Terminal tricks
- reset state: reset
- disable local echo (say for password entry): stty -echo

man stty to see all the things you can do with a terminal
</pre>

Operating Systems 2021F Lecture 6 - Revision history

Soma: Created page with "==Video== Video from the lecture given on September 28, 2021 is now available: * [https://homeostasis.scs.carleton.ca/~soma/os-2021f/lectures/comp3000-2021f-lec06-20210928.m4..."