Lab: File System Utilities

CSC 213 - Operating Systems and Parallel Algorithms - Weinman

Summary:

You will work at the system level to rebuild common command line file utilities.

Assigned:

Tuesday 4 December

Due:

10:30 PM Monday 10 December

Objectives:

 

• Practice defensive programming (you can wreck your data with this assignment)
• Experience new system calls, especially those for manipulating files and directories.

Collaboration:

You will complete this lab in teams as assigned by the instructor.

Exercises

Preliminaries

$ cp -R ~weinman/public_html/courses/CSC213/2012F/labs/code/filesystems ~/somewhere/

Part A: Improving Tanenbaum's file copy

1. Review the file mycp.c. Compile it using make and try using it as you would use the standard cp(1) program.
2. You may have noticed that the program's output is the same-nothing-regardless of whether the copy is successful. Wherever the code exits with an error status, modify the code to use perror(3) whenever possible, or fprintf(3) otherwise, to print a friendly error message.
3. Rather than always creating the new file with the protection mode 0700, use fstat(2) to obtain the protection mode (i.e., file permissions) of the original file.
   Note that creat(2) automatically applies the process's umask to the protection mode.
4. You may have also noticed that mycp.c does not let you copy a file to a different directory except by typing the path and filename for the new file:

   $ ./mycp mycp ../mycp
5. Modify mycp.c so that you can specify just a directory for the destination:

   $ ./mycp mycp ..

   $ ls ../mycp

   ../mycp

   Hint:

   If creating the destination file (out_fd) fails, then check the errno system variable. If it is equal to EISDIR, indicating the destination path names not a file but a directory, then do the following instead of immediately exiting with an error.

   • Get the name of the original file by calling basename(3) on argv[1].
   • Build the destination file name: Use strcpy(3) and either strcat(3) or pointer arithmetic to concatenate the destination path, /, and the source file name.
   • Try again to create the destination file using that new file name.
   • If successful, proceed with copying blocks from the source file to the destination file. Otherwise, report the error and exit.

Part B: More file system utilities

1. Write a program myrm.c, which takes as its sole command-line argument the name of a file to remove. Use unlink(2) to remove the file. In this and all of the programs you write for this lab, be sure to report errors to the user as appropriate. Add myrm to the PROGRAMS variable of the Makefile.
2. Write a program mytouch.c, which will be similar to the touch(1) program. Add mytouch to the PROGRAMS variable of the Makefile.

   $ ls -l

   total 32

   -rw------- 1 davisjan mathfac    6 2010-10-08 21:36 foo

   -rwx------ 1 davisjan mathfac  217 2010-10-08 20:02 Makefile

   -rwx------ 1 davisjan mathfac 7867 2010-10-08 21:28 mycp

   -rw------- 1 davisjan mathfac 1611 2010-10-08 21:01 mycp.c

   -rwx------ 1 davisjan mathfac 7964 2010-10-08 21:35 mytouch

   -rw------- 1 davisjan mathfac  756 2010-10-08 21:35 mytouch.c

   $ cat foo

   blah!

   $ touch foo

   $ cat foo

   blah!

   $ touch bar

   $ cat bar

   $ ls -l

   total 32

   -rw------- 1 davisjan mathfac    0 2010-10-08 21:37 bar

   -rw------- 1 davisjan mathfac    6 2010-10-08 21:37 foo

   -rwx------ 1 davisjan mathfac  217 2010-10-08 20:02 Makefile

   -rwx------ 1 davisjan mathfac 7867 2010-10-08 21:28 mycp

   -rw------- 1 davisjan mathfac 1611 2010-10-08 21:01 mycp.c

   -rwx------ 1 davisjan mathfac 7964 2010-10-08 21:35 mytouch

   -rw------- 1 davisjan mathfac  756 2010-10-08 21:35 mytouch.c

   $

   Here's the basic algorithm:
   • Take a filename as the sole command-line argument.
   • Try to use utime(2) to update the named file's modification time to the current system time. You can get the current system time using time(2).
   • If that fails, and the errno variable indicates that the file does not exist, then create a file with the given name and then immediately close it. Use a reasonable default protection mode.
3. Consider how to implement a version of the mv(1) program, which moves or renames a file. There are three different approaches: (1) copy the file and then use unlink(2) to delete the original; (2) use link(2) to create a hard link from the destination path to the original file, and then use unlink(2) to delete the original; (3) use rename(2) to rename the file.
   1. What are the advantages and disadvantages of these three approaches? Here are three issues you should consider: performance, system crashes, and renaming across the mounted file systems in a virtual file system.
   2. Using whichever approach you prefer, write a program mymv.c, which will be similar to the mv(1) program. As in mycp.c, take as command-line arguments a original file name followed by a destination path, which is either a file or a directory. Add mymv to the PROGRAMS variable of the Makefile.
4. Write a program myls.c, which will be a simple version of the ls(1) program that lists the names of the files in a directory, along with each file's size (in bytes).
   • If given a command-line argument, your program should list the contents of the specified directory. Otherwise, list the contents of the current working directory. Giving more than one command-line argument should result in an error.
   • The kernel interface for reading directory entries does not guarantee that you will read whole entries, which is rather yucky for the programmer. Don't use the kernel interface. Instead, use the POSIX library functions opendir(3), readdir(3), and closedir(3). To find information about a file, use the system call stat(2).
   • Print the name of each file in the directory. For simplicity, print one filename per line, followed by a tab, followed by the file's size in bytes.
   Add myls to the PROGRAMS variable of the Makefile.

What to turn in

• Your source code (five C files) and Makefile
• A single PDF containing (merged)
  • Your enscripted C programs
  • A transcript of your programs' compilations and test runs (including error handling)

Acknowledgment