https://homeostasis.scs.carleton.ca/wiki/index.php?action=history&feed=atom&title=Operating_Systems_2026F_Lecture_10 2026-10-25T14:32:14Z Revision history for this page on the wiki MediaWiki 1.42.1 https://homeostasis.scs.carleton.ca/wiki/index.php?title=Operating_Systems_2026F_Lecture_10&diff=25209&oldid=prev 2026-10-16T15:04:56Z

<p>Created page with &quot;==Video== Video from the lectures given on October 15th and 16th, 2026 are now available: * [https://homeostasis.scs.carleton.ca/~soma/os-2026f/lectures/comp3000-2026f-lec10a-20261015.mp4 Lecture 10A] * [https://homeostasis.scs.carleton.ca/~soma/os-2026f/lectures/comp3000-2026f-lec10b-20261016.mp4 Lecture 10B] ==Notes== ===Lecture 10A=== &lt;pre&gt; Lecture 10A ----------- What&#039;s going to be on the midterm? - Assignment 1 &amp; 2 - parts of T1-4 related to A1 &amp; A2 Concepts...&quot;</p> <p><b>New page</b></p><div>==Video==<br /> <br /> Video from the lectures given on October 15th and 16th, 2026 are now available:<br /> * [https://homeostasis.scs.carleton.ca/~soma/os-2026f/lectures/comp3000-2026f-lec10a-20261015.mp4 Lecture 10A]<br /> * [https://homeostasis.scs.carleton.ca/~soma/os-2026f/lectures/comp3000-2026f-lec10b-20261016.mp4 Lecture 10B]<br /> <br /> ==Notes==<br /> <br /> ===Lecture 10A===<br /> <br /> &lt;pre&gt;<br /> Lecture 10A<br /> -----------<br /> <br /> What&#039;s going to be on the midterm?<br /> - Assignment 1 &amp; 2<br /> - parts of T1-4 related to A1 &amp; A2<br /> <br /> Concepts<br /> - environment variables<br /> - how to set them<br /> - how to retrieve values<br /> - how they are passed to other processes/programs<br /> - getenv()/setenv()<br /> - how shells parse commands &amp; arguments<br /> - how system() works<br /> - assembly language<br /> - compiler/assembler directives<br /> (metadata about code)<br /> - (high level) how functions work in assembly<br /> - start with manipulating the stack<br /> (save registers/allocate local variables)<br /> - end with a ret instruction<br /> - how C symbols show up in assembly<br /> (function names and variables)<br /> - how C constants show up in assembly<br /> - especially strings<br /> - if/while/for turn into branches (jmp, jne, etc)<br /> - how the compiler will replace some library calls<br /> with other calls (puts for printf)<br /> - system calls vs function calls in assembly<br /> (call vs syscall)<br /> - you can&#039;t make system calls directly from C,<br /> must use assembly/special compiler directives<br /> - fork()<br /> - duplicates the process<br /> - returns &quot;twice&quot;<br /> - parent &amp; child return values<br /> - execve()<br /> - replaces currently running executable<br /> - does NOT create a new process<br /> - command line arguments<br /> - how to make command line arguments<br /> - how they are formatted/arranged in memory<br /> - where they are in memory<br /> - environment variables<br /> - how to set/change them<br /> - where they are in memory<br /> - how they are arranged in memory<br /> - how to get them<br /> (arguments to main, extern var, getenv() )<br /> - background vs foreground processes<br /> - how open files can be shared between processes<br /> - including open terminals (e.g., /dev/pts/0)<br /> - I/O redirection<br /> - how to change stdin, stdout, stderr for a process<br /> (file descriptors 0, 1, 2)<br /> - file descriptors!<br /> - system calls, including wait and read, can fail/<br /> return with no result<br /> - always should check whether system calls<br /> succeeded or not<br /> - PATH environment variable, how used by shells<br /> to map commands onto executables<br /> - signals!<br /> - how to send them<br /> - how to define signal handlers<br /> - how to tell the kernel to use a signal handler<br /> - interaction with system calls<br /> - restarting system calls<br /> - /proc<br /> - how it stores info about all running processes<br /> - how to access values by reading files<br /> - kind of info that is in those files<br /> - relationship with ps, top<br /> - accounts in UNIX<br /> - what makes up an account<br /> - entries in passwd, group, shadow, gshadow<br /> - home directory<br /> - accessing account info with getpwnam() and related<br /> functions<br /> - password hashing, keeping passwords in /etc/shadow<br /> - and why<br /> - root vs regular users<br /> - setuid, dropping privileges<br /> - user vs group vs other permissions<br /> - users having their own shell<br /> - how to find out what that shell is<br /> - how to change it<br /> &lt;/pre&gt;<br /> <br /> ===Lecture 10B===<br /> <br /> &lt;pre&gt;<br /> Lecture 10B<br /> -----------<br /> <br /> To review for the midterm, we&#039;re going to make up questions for the midterm.<br /> <br /> A1Q1<br /> Environment variables<br /> - What is an environment variable?<br /> - What environment variable was used in 3000menu to create a custom command?<br /> - either need to include 3000menu code<br /> - or this is a test of memorization<br /> - what&#039;s the difference between envp and environ?<br /> - Another student was trying to set a custom command in 3000menu, but it wasn&#039;t working for them. This is the command they used:<br /> MENUCMD=&quot;nano&quot;<br /> Another student says &quot;Dummy you defined a shell variable!&quot; What did the second student mean, and what did the first do wrong? Why wouldn&#039;t a shell variable work?<br /> <br /> A1Q2 &amp; A1Q8<br /> - calling system(), which calls /bin/sh -c to execute the command<br /> - there&#039;s a fork/execve in there, but it isn&#039;t of the<br /> command directly<br /> <br /> Possible questions<br /> - What does system() do?<br /> - What system calls does system() make?<br /> - this is a bit tricky because it makes more than<br /> fork/execve/wait<br /> - 3000menu and 3000menu2 use different ways of handling commands with arguments. Which method do you think is more efficient? Why?<br /> <br /> <br /> I/O redirection<br /> - what is file descriptor 1 normally referred to in C?<br /> - when a program first opens a file (using open() or other function that calls open()), it normally gets file descriptor 3 first. Why?<br /> &lt;/pre&gt;</div>

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