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

Announcements

• Return short quiz #8. Most students did well -- mean was 8/10.
• Speaker Richard Stallman tomorrow, 4pm in Whitaker Auditorium.
• Friday is Exam #2. Topics include:
  • Coding style
  • Debugging tools and techniques
  • Bash scripting
  • UNIX programming
     
    
  • ... bring questions to Wednesday's class!
• Homework #5 is due tomorrow.

Collaborative Programming

• Last week a few students participated in helping to write versions of cp and ls.
• We will continue that today, by writing touch, and extending the simple ls that we wrote Friday.
• 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)

Extending ls

Now that touch is complete, let's revisit ls and extend it to work like ls -l works. The (working) version of ls that was developed in class is here. This lab will help walk you through the necessary steps. 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