CS207 Introduction to Computer Systems
Wednesday, 17 May 2023

+ From Slack
- "With handling SIGINT, my terminal by default echoes back a “^C” to stderr anytime I press ctrl+C. In my interruption handling function, I’m also printing “^C”, so now I’m getting two of those (“^C^C”) anytime I try to interrupt. Is there a way to prevent this?"

- "When we fork twice, where do the waits happen?"

    if fork() != 0
        if fork() != 0
            wait
            wait
        else
            // child2
            dup2, close, whatever
    else
        // child1
        dup2, close, whatever

- "What does fflush() do and why should we use it? Most sites aren’t super helpful about thhis"
    printf, fprintf, putchar, etc. send output to an output buffer
    when the system feels like it, it sends the output buffer
        contents to stdout
    fflush(stdout) says "send it now"

    printf("debugging message\n"); // maybe you never see this
    some-code-that-crashes();
    printf("other debugging message\n");


    command1 | command2 < file1
    
+ Some questions

- What is a process?
- What does > do?
- What does < do?
- What does | do?

- List some examples of command lines with | that you might actually use
    ls -l | wc -l  # count files in current dir
    cat names.txt | grep Han
    
    python3 scores.py 06-bombs.txt , then copy/paste into my spreadsheet
        the terminal font size,etc. pastes into Excel, which I hate
    python3 scores.py 06-bombs.txt | pbcopy # copies the output to the clipboard

    $ history | grep '|'
    142  cat asdf | pbcopy
    261  cat a | pbcopy
    328  cat something.txt | wc
    344  echo "hi" | wc
    345  echo "hi there" | wc
    346  echo -n "hi there" | wc
    351  ls -l | grep something
    469  history | grep "|"

    465  ps aux | grep shell208
    495  ps aux | grep -c bash

    ps aux | grep bash | less


- DIGRESSION: grep
    grep = global regular expression print
    simplest case
        grep moose file.txt # print all lines from file.txt that contain moose
        grep "[Mm]oose" file.txt
        grep -c moose file.txt # just print how many (count) of lines with moose
        grep -v moose file.txt # print all lines without moose
        grep -r moose some_directory # grep on all the files in this dir tree
        grep -c dak dictionary-web-log # (count how many accesses to my
            online dakota dictionary)

- What do these functions do?
    - fork
    - exec
    - signal
    - pipe
    - dup2
    - wait
    - fflush (and why would you use it?)
    - strtok (and why would you use it?)

- What is a signal?
- What does the "kill" command do?

- When you quit VS Code on mantis, do you first click on the bottom left
    and then select "Close Remote Connection"?

- When one of my sample programs is confusing, what do you do?


+ Processes can be weird
- Race conditions