Slides: Files and Directories

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# Files and Directories
## CSC 213 – September 25, 2026

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

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# Department Events

## CS Picnic (RSVP Required)
.indent[
  Today at 5pm, Merrill Park West Shelter
]

## Flu and COVID Shot Clinic
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  October 1st and 2nd from 9am–4pm in the Harris Center  
  Register with Hy-Vee at <https://tinyurl.com/f26-flu-shot>
]

## CS Table: Free and Open Source Software
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  Tuesday, September 29th at noon in JRC 224C
]

*To receive a token, send an email to let me know you attended no more than 48 hours after the event.*

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# Upcoming Work

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# Lab: Malloc
**Due:** Wednesday, October 7th by 11:59pm  
**Late Deadline:** Monday, October 12th by 11:59pm

## Questions
_No new questions since last class_

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# Files and Directories

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# 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.*

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# 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.*

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# Puzzles

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# 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.

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

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# 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);
```

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

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# 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);
```

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

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

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# Puzzle 7
```c
int fd = open("symlink.txt", O_RDWR | O_CREAT | O_TRUNC, 0640);
char* message = "I love the malloc lab\n";
write(fd, message, strlen(message));
close(fd);
```

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# 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);
```

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# 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);
```

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# 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[2] = 'f';
data[3] = 'e';
data[4] = 'a';
data[5] = 'r';
munmap(data, 4096);
close(fd);
```

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# 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);
```

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# 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);
```

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# Wrap Up

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# Reminders

## Upcoming Deadlines
- **Malloc Lab:** due two Wednesdays from now

## Need help? Use your resources!
- Book an appointment during office hours
- Attend mentor office hours
- Review slides, readings, and other course resources
- Use tools like `gdb` and AddressSanitizer to catch and diagnose issues

## Reading

- **File System Implementation** (OSTEP, Chapter 40)