CSE271 Lab 9: Files and Directories (ls and touch)

Announcements

Next Friday is Exam #2. Topics include:
- Coding style
- Debugging tools and techniques
- Bash scripting
- UNIX programming
- ... bring questions to Monday's class!
Homework #5 is due Saturday.

Collaborative Programming

Recently a few students participated in helping to write versions of cp and who.
We will continue that today, by writing touch, and implementing the simple ls that we discussed Monday.
Please feel free to work and discuss today's labwork in teams (of any number of students)

touch

What is touch(1)? What does it do?
How does it work?
Task 1: implement touch(1)

Writing ls

Task 2: Now that touch is complete, let's revisit ls and implement a simple version. You might review the notes from Monday, but in the end, we want to implement something like:


main()
    opendir()
    while ( readdir() )
       print d_name

    closedir()
    return 0

This version would only print out the names of files (e.g., as ls does, and not anything further).

Extending ls

Task 3: Now we can extend ls to work like ls -l works. A (working) version of ls is here (in case you have questions about task 2). This lab will help walk you through the necessary steps to extend it. Our goal, at the end, is to have an equivalent to ls -l.

How do we add -l to our ls?

Q: What does -l display?
A: modtime, size, owner, group, links, type, permissions
Q: Where can we learn about these?
A: As always, search the manual
Result: the stat() system call

How does stat() work?

int stat(char *name, struct stat *buf)
struct contains
- st_mode
- st_uid
- st_gid
- st_size
- st_nlink
- st_mtime
So we can modify ls to provide -l features using stat() on the filename

How well should your ls -l work?

Perfectly for filenames, filesizes, but we can fix the rest.
We can use ctime() to convert the mod time.
Owner and group info are stored as numbers, so we need to map them to names (shortly below).
What about type? permissions?

Where are type and permissions stored?

st_mode is a 16 bit value. Within it, we have
- four bits for the file type, followed by
- three bits for suid, sgid, and sticky bit, followed by
- nine bits for owner, group and world rwx permissions
So we will use subfield coding.
- Much like telephone numbers, IP addresses, etc.

Bits and subfields

Q: How to examine a bit or subfield?
A: Use bitwise AND (&) with a value to MASK the rest

So we can test permissions, e.g.

if (st_mode & 004)
   printf("Readable by others\n");

Typical masks can be found in </sys/stat.h>

UIDs and GIDs

UIDs and GIDs from stat() are just numbers
How can we convert them to names?
Then we can finish our ls -l
First, search manual pages

Converting UIDs

From the man pages, we learn about /etc/passwd and ypcat passwd
The getpwuid() function provides access to user information.
A similar function is available to convert gid information.
Your ls should now be able to generate correct permissions, types, owner, group, etc., just like ls -l

Last revised: 18 March 2009, Prof. Davison.