Week 09 Tutorial Questions

• develop an understanding of how the UNIX file permissions system works
• understand and practice coding with UTF-8 encoded characters
• introduce environment variables and how programs can access and manipulate them

1. The stat() and lstat() functions both take an argument which is a pointer to a struct stat object, and fill it with the meta-data for a named file.

   On Linux, a struct stat contains the following fields (among others, which have omitted for simplicity):

   struct stat {
       ino_t st_ino;         /* inode number */
       mode_t st_mode;       /* protection */
       uid_t st_uid;         /* user ID of owner */
       gid_t st_gid;         /* group ID of owner */
       off_t st_size;        /* total size, in bytes */
       blksize_t st_blksize; /* blocksize for filesystem I/O */
       blkcnt_t st_blocks;   /* number of 512B blocks allocated */
       time_t st_atime;      /* time of last access */
       time_t st_mtime;      /* time of last modification */
       time_t st_ctime;      /* time of last status change */
   };
   

   Explain what each of the fields represents (in more detail than given in the comment!) and give a typical value for a regular file which appears as follows:

   ls -ls stat.c
   8 -rw-r--r--  1 jas  cs1521  1855  Sep  9 14:24 stat.c
   

   Assume that jas has user id 516, and the cs1521 group has group id 36820.

2. Consider the following (edited) output from the command ls -l ~cs1521:

   drwxr-x--- 11 cs1521 cs1521 4096 Aug 27 11:59 17s2.work
   drwxr-xr-x  2 cs1521 cs1521 4096 Aug 20 13:20 bin
   -rw-r-----  1 cs1521 cs1521   38 Jul 20 14:28 give.spec
   drwxr-xr-x  3 cs1521 cs1521 4096 Aug 20 13:20 lib
   drwxr-x--x  3 cs1521 cs1521 4096 Jul 20 10:58 public_html
   drwxr-xr-x 12 cs1521 cs1521 4096 Aug 13 17:31 spim
   drwxr-x---  2 cs1521 cs1521 4096 Sep  4 15:18 tmp
   lrwxrwxrwx  1 cs1521 cs1521   11 Jul 16 18:33 web -> public_html
   

   1. Who can access the 17s2.work directory?

   2. What operations can a typical user perform on the public_html directory?

   3. What is the file web?

   4. What is the difference between stat("web", &info) and lstat("web", &info)?
      (where info is an object of type (struct stat))

3. Explain how this program uses stat to get information about the permissions and type of a file. The inode man page may he helpful here.

   #include <stdio.h>
   #include <stdlib.h>
   #include <sys/types.h>
   #include <sys/stat.h>
   
   void print_some_file_info(char *filename);
   
   int main(int argc, char *argv[]) {
           // Loop over command line arguments
           for (int arg = 1; arg < argc; arg++) {
                   print_some_file_info(argv[arg]);
           }
   }
   
   void print_some_file_info(char *filename) {
           // Use stat to get file information
           struct stat s;
   
           // check that stat worked
           if (stat(filename, &s) != 0) {
                   perror(filename);
                   return;
           }
   
           // get the mode (permissions) of the file
           mode_t mode = s.st_mode;
   
           // check if the file is a directory
           if ((mode & S_IFDIR) == S_IFDIR) {
                   printf("The directory '%s' has mode 0x%x == 0%o\n", filename, mode, mode);
   
           // check if the file is a regular file
           } else if (S_ISREG(mode)) {
                   printf("The file '%s' has mode 0x%x == 0%o\n", filename, mode, mode);
           }
   
           // check if the file is publically readable
           if (mode & S_IROTH) {
                   printf("%s is publically readable\n", filename);
           }
   }
   

   Could you add another if statement to check if the file is publically writeable? How would you check if the file can be executed by the owner?
4. Write a file print_metadata.c, which is given a file path as a command line argument and prints the file size (in bytes) and the file permissions as a string in a format similar to ls -l (e.g. -rw-r--r--). For file type, only directory d and regular file - cases are required to be handled.
   
   Example:

   echo -n "hello" > demo.txt
   chmod 640 demo.txt
   ls -l demo.txt
   -rw-r-----  1 user  group  5 Sep  4 17:00 demo.txt
   dcc print_metadata.c -o print_metadata
   ./print_metadata demo.txt
   size: 5
   perms: -rw-r-----
   

5. Write a C program, chmod_if_public_write.c, which is given 1+ command-line arguments which are the pathnames of files or directories

   If the file or directory is publically-writeable, it should change it to be not publically-writeable, leaving other permissions unchanged.

   It also should print a line to stdout as in the example below

   dcc chmod_if_public_write.c -o chmod_if_public_write
   ls -ld file_modes.c file_modes file_sizes.c file_sizes
   -rwxr-xrwx 1 z5555555 z5555555 116744 Nov  2 13:00 file_sizes
   -rw-r--r-- 1 z5555555 z5555555    604 Nov  2 12:58 file_sizes.c
   -rwxr-xr-x 1 z5555555 z5555555 222672 Nov  2 13:00 file_modes
   -rw-r--rw- 1 z5555555 z5555555   2934 Nov  2 12:59 file_modes.c
   ./file_modes file_modes file_modes.c file_sizes file_sizes.c
   removing public write from file_sizes
   file_sizes.c is not publically writable
   file_modes is not publically writable
   removing public write from file_modes.c
   ls -ld file_modes.c file_modes file_sizes.c file_sizes
   -rwxr-xr-x 1 z5555555 z5555555 116744 Nov  2 13:00 file_sizes
   -rw-r--r-- 1 z5555555 z5555555    604 Nov  2 12:58 file_sizes.c
   -rwxr-xr-x 1 z5555555 z5555555 222672 Nov  2 13:00 file_modes
   -rw-r--r-- 1 z5555555 z5555555   2934 Nov  2 12:59 file_modes.c
   

   Make sure you handle errors.

6. What does the following printf(3) statement display?

   printf ("%c%c%c%c%c%c", 72, 101, 0x6c, 108, 111, 0x0a);
   

   Try to work it out without simply compiling and running the code. The ascii(7) manual page will help with this; read it by running man 7 ascii. Then, check your answer by compiling and running.

7. Recall the UTF-8 encoding for characters below:

   

   Why did UTF-8 replace the ASCII coding standard? What is the difference in how ASCII and UTF-8 codepoints are represented in bytes?

8. Write a C program that reads a null-terminated UTF-8 string as a command line argument and counts how many Unicode characters (code points) it contains. Assume that all codepoints in the string are valid. Some examples of how your program should work:

       dcc count_utf8.c -o count_utf8
       ./count_utf8 "チョコミント、よりもあなた！"
       there are 14 codepoints in the string
       ./count_utf8 "早上好中国现在我有冰淇淋"
       there are 12 codepoints in the string
       ./count_utf8 "🤓🤓🤓🤓🤓🤓🤓🤓"
       there are 8 codepoints in the string
       

9. Write a C program, recurse_sample.c, that walks a directory tree and prints each matching file’s permissions (ls -l style) and pathname when the filename contains the substring "hello".

   1. Part 1 — Print CWD: Get and print the current working directory pathname.
      Hint: getcwd()
   2. Part 2 — List current directory: Print one line per entry in the current directory (non-recursive):
      <perms> <pathname>
      Hint: use readdir() to recurse through subdirectories
   3. Part 3 — Recurse: Recurse into all subdirectories and print pathnames in the same format as Part 1.
   4. Part 4 — Filter by name: Only print lines for files whose basename contains "hello" (case-sensitive match).

   Output format (one match per line):
   -rw-r--r-- ./examples/hello.txt

   Example (setup & run):

   mkdir -p examples/sub
   : > examples/hello.txt
   : > examples/sub/nothe11o.c
   : > examples/sub/hello_world.c
   dcc -o recurse_sample recurse_sample.c
   ./recurse_sample .
   -rw-r--r-- ./examples/sub/hello_world.c
   -rw-r--r-- ./examples/hello.txt
   

10. Write a C program, print_diary.c, which prints the contents of the file $HOME/.diary to stdout

    The lecture example getstatus.c shows how to get the value of an environment variable.

    snprintf is a convenient function for constructing the pathname of the diary file.

Extra questions

The following questions are extra content not necessarily needed to cover this week's content.
Your tutor may still choose to cover some of these questions, time permitting.

1. The following code will compile with

   gcc   void-pointer.c -o void-pointer
   clang void-pointer.c -o void-pointer
   dcc   void-pointer.c -o void-pointer
   

   It will even compile with

   gcc   -Werror -Wall -Wextra void-pointer.c -o void-pointer
   clang -Werror -Wall -Wextra void-pointer.c -o void-pointer
   dcc   -Werror -Wall -Wextra void-pointer.c -o void-pointer
   

   But it wont compile with

   gcc   -Werror -Wall -Wextra              -Wpedantic void-pointer.c -o void-pointer
   clang -Werror -Wall -Wextra -Weverything -Wpedantic void-pointer.c -o void-pointer
   dcc   -Werror -Wall -Wextra              -Wpedantic void-pointer.c -o void-pointer
   

   How can the code be modified to compile with the -Wpedantic option?

   #include <stdio.h>
   #include <stdlib.h>
   #include <stdint.h>
   
   static uint16_t a;
   
   int main(void)
   {
       uint16_t b;
   
       uint16_t *pa = &a;
       uint16_t *pb = &b;
       uint16_t *pc = malloc(sizeof(uint16_t));
   
       *pa = 0xAAAA;
       *pb = 0xBBBB;
       *pc = 0xCCCC;
   
       printf("a:\n\tvalue: 0x%X\n\taddress: %16p\n", *pa, pa);
       printf("b:\n\tvalue: 0x%X\n\taddress: %16p\n", *pb, pb);
       printf("c:\n\tvalue: 0x%X\n\taddress: %16p\n", *pc, pc);
   }
   

2. The following code will compile with

   gcc   pointer-arithmetic.c -o pointer-arithmetic
   clang pointer-arithmetic.c -o pointer-arithmetic
   dcc   pointer-arithmetic.c -o pointer-arithmetic
   

   It will even compile with

   gcc   -Werror -Wall -Wextra pointer-arithmetic.c -o pointer-arithmetic
   clang -Werror -Wall -Wextra pointer-arithmetic.c -o pointer-arithmetic
   dcc   -Werror -Wall -Wextra pointer-arithmetic.c -o pointer-arithmetic
   

   But it wont compile with

   gcc   -Werror -Wall -Wextra              -Wpedantic pointer-arithmetic.c -o pointer-arithmetic
   clang -Werror -Wall -Wextra -Weverything -Wpedantic pointer-arithmetic.c -o pointer-arithmetic
   dcc   -Werror -Wall -Wextra              -Wpedantic pointer-arithmetic.c -o pointer-arithmetic
   

   How can the code be modified to compile with the -Wpedantic option?

   #include <stdio.h>
   #include <stdlib.h>
   #include <stdint.h>
   
   void *next_byte(void *byte){
       return byte + 1;
   }
   
   int main(void)
   {
       uint8_t  chars[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
       uint32_t ints[10]  = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
   
       printf("%16p - %16p\n", (void *)&chars[0], (void *)next_byte(&chars[0]));
       printf("%16p - %16p\n", (void *)&ints[0],  (void *)next_byte(&ints[0]));
   }
   

Revision questions

The following questions are primarily intended for revision, either this week or later in session.
Your tutor may still choose to cover some of these questions, time permitting.