Soma: Created page with "==Video== Video from the lecture given on January 22, 2020 [https://homeostasis.scs.carleton.ca/~soma/os-2020w/lectures/comp3000-2020w-lec05-20200122.m4v is now available]...."

2020-03-20T02:30:50Z

Created page with "==Video== Video from the lecture given on January 22, 2020 [https://homeostasis.scs.carleton.ca/~soma/os-2020w/lectures/comp3000-2020w-lec05-20200122.m4v is now available]...."

New page

==Video==

Video from the lecture given on January 22, 2020 [https://homeostasis.scs.carleton.ca/~soma/os-2020w/lectures/comp3000-2020w-lec05-20200122.m4v is now available].

==Notes==

===Connecting to Openstack===

To connect to an openstack instance off campus, either:
* Connect to the [https://carleton.ca/its/help-centre/remote-access/ Carleton VPN], then ssh to the instance, or
* If you instance is at 134.117.30.30 and your username is carletonstudent, do the following (note that you can replace the 1200 with another number between 1024 and 65535):
ssh -L 1200:134.117.30.30:22 carletonstudent@access.scs.carleton.ca
[stay logged in, and in another window do the following]
ssh -p 1200 student@localhost

===Using gdb to view assembly code===

To compile for debugging with gdb:
gcc -O -g myprogram.c -o myprogram
The -O option is for optimization (could use -O2, -O3 for more optimization, but -O is sufficient normally). You want to optimize the code because otherwise the assembly code will be harder to follow.

To run a program under gdb:
gdb ./myprogram

The following commands in gdb should be useful:
* '''break x''': set a breakpoint at function x. Set a breakpoint for main if you want to see program execution starting with your code.
* '''tui enable''': enable display of source code
* '''layout asm''': switch the display to assembly code
* '''layout split''': switch display to assembly and source code
* '''layout src''': show source code
* '''n''' or '''next''': next source-level statement, staying within the current function
* '''step''': next source-level statement, going into called functions
* '''nexti''': next assembly-language statement, staying within the current function
* '''stepi''': next assembly-language statement, going into called functions
To repeat a command, just hit enter.

To see a system call be executed, single-step through a program until you see a "syscall" instruction (on x86-64). Note how even with a program that directly makes a system call such as [http://homeostasis.scs.carleton.ca/~soma/os-2017f/code/tut1/syscall-hello.c syscall-hello.c], the program first makes a function call (using <tt>callq</tt>) to the syscall function, and that function makes the actual system call.

For more information on gdb in tui mode, see [https://www-users.cs.umn.edu/~kauffman/ Chris Kauffman's] [https://www-users.cs.umn.edu/~kauffman/2021/ UMN CSCI 2021] [https://www-users.cs.umn.edu/~kauffman/2021/gdb.html Quick guide to GDB].

===In-Class Notes===

<pre>
Lecture 5
---------
Topics:
- Assignment 1
- syscall-hello.c
- gdb and assembly
- showing a system call instruction

- creating processes with fork
- running programs with execve
- returning values with exit

Process is a running program

Could have a system call to create a new process and run a program in it
- this is how Windows does it with CreateProcess(), other calls?

But UNIX divides this into two steps
- fork: make a copy of the current process
- execve: run a binary in a process, replacing the currently running program

So normally, to run a program in a separate process
- create a "child" process using fork
- child process runs execve to load new program
- parent process waits for child to finish (optional)

Basic flow of a UNIX shell (or anything that wants to run external programs)
- setup execution context
- fork
- parent waits for child to finish, or calls wait when told
the child has finished
- child does an execve to load new program
</pre>

==Code==

<source lang="c" line>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
pid_t fork_pid = -1;
pid_t mypid;

fork_pid = fork();
mypid = getpid();
printf("My pid is %d\n", mypid);

if (fork_pid == 0) {
printf("Fork done! I am the child.\n");
} else {
printf("I am the parent, my child's PID is %d\n", fork_pid);
}

return 0;
}
</source>

Operating Systems 2020W Lecture 5 - Revision history

Soma: Created page with "==Video== Video from the lecture given on January 22, 2020 [https://homeostasis.scs.carleton.ca/~soma/os-2020w/lectures/comp3000-2020w-lec05-20200122.m4v is now available]...."