Operating Systems 2026F Lecture 10

Video

Video from the lectures given on October 15th and 16th, 2026 are now available:

• Lecture 10A
• Lecture 10B

Notes

Lecture 10A

Lecture 10A
-----------

What's going to be on the midterm?
 - Assignment 1 & 2
 - parts of T1-4 related to A1 & A2

Concepts
 - environment variables
   - how to set them
   - how to retrieve values
   - how they are passed to other processes/programs
   - getenv()/setenv()
 - how shells parse commands & arguments
   - how system() works
 - assembly language
   - compiler/assembler directives
     (metadata about code)
   - (high level) how functions work in assembly
     - start with manipulating the stack
       (save registers/allocate local variables)
     - end with a ret instruction
   - how C symbols show up in assembly
     (function names and variables)
   - how C constants show up in assembly
     - especially strings
   - if/while/for turn into branches (jmp, jne, etc)
   - how the compiler will replace some library calls
     with other calls (puts for printf)
   - system calls vs function calls in assembly
     (call vs syscall)
   - you can't make system calls directly from C,
     must use assembly/special compiler directives
 - fork()
   - duplicates the process
   - returns "twice"
   - parent & child return values
 - execve()
   - replaces currently running executable
   - does NOT create a new process
 - command line arguments
   - how to make command line arguments
   - how they are formatted/arranged in memory
   - where they are in memory
 - environment variables
   - how to set/change them
   - where they are in memory
   - how they are arranged in memory
   - how to get them
     (arguments to main, extern var, getenv() )
 - background vs foreground processes
 - how open files can be shared between processes
   - including open terminals (e.g., /dev/pts/0)
 - I/O redirection
   - how to change stdin, stdout, stderr for a process
     (file descriptors 0, 1, 2)
 - file descriptors!
 - system calls, including wait and read, can fail/
   return with no result
    - always should check whether system calls
      succeeded or not
 - PATH environment variable, how used by shells
   to map commands onto executables
 - signals!
   - how to send them
   - how to define signal handlers
   - how to tell the kernel to use a signal handler
   - interaction with system calls
     - restarting system calls
 - /proc
   - how it stores info about all running processes
   - how to access values by reading files
   - kind of info that is in those files
   - relationship with ps, top
 - accounts in UNIX
   - what makes up an account
     - entries in passwd, group, shadow, gshadow
     - home directory
 - accessing account info with getpwnam() and related
   functions
 - password hashing, keeping passwords in /etc/shadow
   - and why
 - root vs regular users
 - setuid, dropping privileges
 - user vs group vs other permissions
 - users having their own shell
   - how to find out what that shell is
   - how to change it

Lecture 10B

Lecture 10B
-----------

To review for the midterm, we're going to make up questions for the midterm.

A1Q1
Environment variables
 - What is an environment variable?
 - What environment variable was used in 3000menu to create a custom command?
   - either need to include 3000menu code
   - or this is a test of memorization
 - what's the difference between envp and environ?
 - Another student was trying to set a custom command in 3000menu, but it wasn't working for them. This is the command they used:
    MENUCMD="nano"
Another student says "Dummy you defined a shell variable!" What did the second student mean, and what did the first do wrong? Why wouldn't a shell variable work?

A1Q2 & A1Q8
 - calling system(), which calls /bin/sh -c to execute the command
 - there's a fork/execve in there, but it isn't of the
   command directly

Possible questions
 - What does system() do?
 - What system calls does system() make?
   - this is a bit tricky because it makes more than
     fork/execve/wait
 - 3000menu and 3000menu2 use different ways of handling commands with arguments. Which method do you think is more efficient? Why?
 

I/O redirection
 - what is file descriptor 1 normally referred to in C?
 - when a program first opens a file (using open() or other function that calls open()), it normally gets file descriptor 3 first. Why?