Examples&Exercises:

• Compile and run the code.
• Analyze the code and output.

1. File Types; stat, fstat, and lstat Functions; http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code48.c code48.c (do not forget to download also the file, http://siber.cankaya.edu.tr/OperatingSystems/cfiles/apue.h apue.h)
   • Given a pathname, the stat function returns a structure of information about the named file. The fstat function obtains information about the file that is already open on the descriptor (file descriptor). The lstat function is similar to stat, but when the named file is a symbolic link, lstat returns information about the symbolic link, not the file referenced by the symbolic link.
   • Study the following command in detail;

     $ man 2 stat
     

   • The type of a file is encoded in the st_mode member of the stat structure.
   • We can determine the file type with the macros in $<sys/stat.h>$ (S_IS***()).
   • Compile the code and execute as the following;

     ./code48 /etc/passwd /etc /dev/initctl /dev/log /dev/tty \
     /dev/hda1 /dev/cdrom
     

   • Study the output.
   • Modify the code such that
     • it will be able to print out the information (present status, values, etc.) about all the fields in the stat structure (man 2 stat; see struct stat) not only the field protection.
2. File Access; chmod and fchmod Functions; http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code49.c code49.c
   • These two functions allow us to change the file access permissions for an existing file. The chmod function operates on the specified file, whereas the fchmod function operates on a file that has already been opened.
   • To change the permission bits of a file, the effective user ID of the process must be equal to the owner ID of the file, or the process must have superuser permissions (we will not discuss the effective user ID and superuser concepts).
   • The mode constants for chmod functions, from $<sys/stat.h>$ are S_IRWXU, S_IRUSR, S_IWUSR, S_IXUSR, S_IRWXG, S_IRGRP, S_IWGRP, S_IXGRP, S_IRWXO, S_IROTH, S_IWOTH, S_IXOTH.
   • They are self-explanatory, but as an example S_IRWXU stands for read, write, and execute by user (owner).
   • Compile the code
   • Before execution create two files as the following;

     -rw------- 1 ozdogan ozdogan 0 May  8 02:27 foo
     -rw-rw-rw- 1 ozdogan ozdogan 0 May  8 02:27 bar
     

     by using touch and chmod commands.
   • Execute the code as

     $./code49
     

   • Observe how the final state of the two files are changed after running the program.
   • Modify the code to use some other mode constants, meanwhile make a search for the S_ISGID.
   • Note that the time and date listed by the ls command did not change after we ran the program.By default, the ls -l lists the time when the contents of the file were last modified.
3. Reading Directories; File tree walk http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code50.c code50.c (do not forget to download also the files, http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code51.c code51.c, http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code52.c code52.c)
   • Directories can be read by anyone who has access permission to read the directory. (But only the kernel can write to a directory, to preserve file system sanity)
   • We'll use these directory routines (opendir, readdir, etc.) to write a program that traverses a file hierarchy. The goal is to produce the count of the various types of files.
   • The program takes a single argument the starting pathname and recursively descends the hierarchy from that point.
   • Apply the following commands;

     $ gcc -c code52.c
     $ gcc -c code51.c
     $ gcc -o code50 code50.c code51.o code52.o
     $ ./code50 .
     $ ./code50 ..
     

     also try with different paths.
   • You can also use this program to see the file profile in your system by executing with root privileges and with the pathname "/".
   • Also see

     $ man nftw