| 4.1 | Modify the program in Figure 4.3 to use stat instead of lstat. What changes if one of the command-line arguments is a symbolic link? |
| 4.2 | What happens if the file mode creation mask is set to 777 (octal)? Verify the results using your shell's umask command. |
| 4.3 | Verify that turning off user-read permission for a file that you own denies your access to the file. |
| 4.4 | Run the program in Figure 4.9 after creating the files foo and bar. What happens? |
| 4.5 | In Section 4.12, we said that a file size of 0 is valid for a regular file. We also said that the st_size field is defined for directories and symbolic links. Should we ever see a file size of 0 for a directory or a symbolic link? |
| 4.6 | Write a utility like cp(1) that copies a file containing holes, without writing the bytes of 0 to the output file. |
| 4.7 | Note in output from the ls command in Section 4.12 that the files core and core.copy have different access permissions. If the umask value didn't change between the creation of the two files, explain how the difference could have occurred. |
| 4.8 | When running the program in Figure 4.16, we check the available disk space with the df(1) command. Why didn't we use the du(1) command? |
| 4.9 | In Figure 4.20, we show the unlink function as modifying the changed-status time of the file itself. How can this happen? |
| 4.10 | In Section 4.21, how does the system's limit on the number of open files affect the myftw function? |
| 4.11 | In Section 4.21, our version of ftw never changes its directory. Modify this routine so that each time it encounters a directory, it does a chdir to that directory, allowing it to use the filename and not the pathname for each call to lstat. When all the entries in a directory have been processed, execute chdir(".."). Compare the time used by this version and the version in the text. |
| 4.12 | Each process also has a root directory that is used for resolution of absolute pathnames. This root directory can be changed with the chroot function. Look up the description for this function in your manuals. When might this function be useful? |
| 4.13 | How can you set only one of the two time values with the utime function? |
| 4.14 | Some versions of the finger(1) command output "New mail received ..." and "unread since ..." where ... are the corresponding times and dates. How can the program determine these two times and dates? |
| 4.15 | Examine the archive formats by the cpio(1) and tar(1) commands. (These descriptions are usually found in Section 5 of the UNIX Programmer's Manual.) How many of the three possible time values are saved for each file? When a file is restored, what value do you think the access time is set to, and why? |
| 4.16 | Does the UNIX System have a fundamental limitation on the depth of a directory tree? To find out, write a program that creates a directory and then changes to that directory, in a loop. Make certain that the length of the absolute pathname of the leaf of this directory is greater than your system's PATH_MAX limit. Can you call getcwd to fetch the directory's pathname? How do the standard UNIX System tools deal with this long pathname? Can you archive the directory using either tar or cpio? |
| 4.17 | In Section 3.16, we described the /dev/fd feature. For any user to be able to access these files, their permissions must be rw-rw-rw-. Some programs that create an output file delete the file first, in case it already exists, ignoring the return code:
unlink(path);
if ((fd = creat(path, FILE_MODE)) < 0)
err_sys(...);
What happens if path is /dev/fd/1?
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