Slides: Files and Directories

class: title
# Files and Directories
## CSC 213 – September 29, 2025

---
# Agenda for Today
1. Department Events
2. Upcoming Work
3. Files and Directories
5. Wrap Up

---
# Department Events

## CS Table: Open Source Software Supply-Chain Security
.indent[
  Tuesday, September 30th at noon in JRC 224C
]

## CS Extra: Welcome Session
.indent[
  *Information for 2nd year intended CS majors*  
  Thursday, October 2nd at 4:15pm in Noyce 3821
]

## Flu Shot Clinic
.indent[
  October 2nd and 3rd from 9am–4pm in the Natatorium Lobby  
  Register with Hy-Vee at <https://tinyurl.com/f25-flu-shot>
]

*To receive 0.25% extra credit, send an email to let me know you attended no more than 48 hours after the event.*

---
class: section, blue
# Upcoming Work

---
# Lab: Malloc
**Due:** Wednesday, October 8th by 11:59pm  
**Late Deadline:** Wednesday, October 15th by 11:59pm

## Questions?

---
# Assignment: Listing Directories
**Due:** Monday, October 6th by 11:59pm

Let's look at the assignment now.

---
class: section, blue
# Files and Directories

---
# Discussion: Filesystem Basics
**Questions for think, pair, share:**

1. What is a file?

2. What is a directory?

3. What could we find on a computer's filesystem other than files and directories?

*Start by thinking of answers on your own. I will let you know when to start discussion.*

---
# Discussion: Filesystem Permissions
**Questions for think, pair, share:**

1. A file has permissions `rwxrw-r--`.
   What can different users do with this file?

2. A directory has permissions `rwxr-xr--`.
   What can different users do with this directory?

*Start by thinking of answers on your own. I will let you know when to start discussion.*

---
class: section, green
# Puzzles

---
# Puzzle Rules
## Format
Each puzzle shows a program fragment.
The program finishes executing and exits without any errors.

## Setup
The first puzzle program runs in an empty directory.
Each following puzzle runs in the same directory, which has all the outputs from the previous puzzles.

## Responses
Your task is to determine (without running the code) what files will be in the directory after each puzzle program runs, and what those files will contain.

---
# Puzzle 1
```c
int fd = open("message.txt", O_RDWR | O_CREAT, 0640);
char* message = "Hello";
write(fd, message, strlen(message));
close(fd);
```

---
# Puzzle 2
```c
int fd = open("message.txt", O_RDWR | O_CREAT | O_APPEND, 0640);
char* message = " world!\n";
write(fd, message, strlen(message));
close(fd);
```

---
# Puzzle 3
```c
int fd = open("message.txt", O_RDWR);
char* message = "J";
write(fd, message, strlen(message));
close(fd);
```

---
# Puzzle 4
```c
link("message.txt", "greeting.txt");
int fd = open("greeting.txt", O_RDWR | O_APPEND);
char* message = "Greetings.\n";
write(fd, message, strlen(message));
close(fd);
```

---
# Puzzle 5
```c
symlink("message.txt", "symlink.txt");
int fd = open("symlink.txt", O_RDWR | O_APPEND);
char* message = "What?\n";
write(fd, message, strlen(message));
close(fd);
```

---
# Puzzle 6
```c
int fd = open("message.txt", O_RDWR | O_TRUNC);
unlink("message.txt");
char* message = "Well, this is awkward.\n";
write(fd, message, strlen(message));
close(fd);
```

---
# Puzzle 7
```c
int fd = open("message.txt", O_RDWR | O_CREAT | O_TRUNC, 0640);
char* message = "Curiouser and curiouser!\n";
write(fd, message, strlen(message));
close(fd);
```

---
# Puzzle 8
```c
mkdir("somedir", 0755);
int fd = open("somedir/message.txt", O_RDWR | O_CREAT | O_TRUNC, 0640);
char* message = "Hello world!\n";
write(fd, message, strlen(message));
close(fd);
```

---
# Puzzle 9
```c
chdir("somedir");
int fd = open("greeting.txt", O_RDWR | O_CREAT | O_TRUNC, 0640);
char* message = "Greetings.\n";
write(fd, message, strlen(message));
close(fd);
```

---
# Puzzle 10
```c
int fd = open("message.txt", O_RDWR, 0640);
char* data = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
data[0] = 'F';
data[14] = 'f';
munmap(data, 4096);
close(fd);
```

---
# Puzzle 11
```c
int fd = open("fork.txt", O_RDWR | O_CREAT, 0640);
pid_t child_id = fork();
if (child_id == 0) {
  char* message = "fork\n";
  write(fd, message, strlen(message));
} else {
  wait(NULL);
  char* message = "spoon\n";
  write(fd, message, strlen(message));
}
close(fd);
```

---
# Puzzle 12
```c
int fd = open("dup.txt", O_RDWR | O_CREAT, 0640);
dup2(fd, STDOUT_FILENO);
char* message = "Hello from write().\n";
write(fd, message, strlen(message));
printf("Hello from printf().\n");
message = "Interesting...\n";
write(fd, message, strlen(message));
close(fd);
```

---
class: section, blue
# Wrap Up

---
# Reminders

## Lab
The malloc lab is due next week.
We will spend all of Wednesday working on the lab, but you should find additional time to work with your group.

## Assignment
The listing directories assignment is due next Monday.

## Need Help?
My regular office hours schedule is now posted and times are bookable.
Don't forget about Tuesday's mentor session from 5–6pm.

## Reading

*No reading for the next class.*