Week 09 Tutorial Questions

Objectives

introduce environment variables and how programs can access and manipulate them
develop an understanding of how the UNIX file permissions system works
introduce the concept of a process and how they are created via posix_spawn()

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.

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.

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

Who can access the 17s2.work directory?
What operations can a typical user perform on the public_html directory?
What is the file web?
What is the difference between stat("web", &info) and lstat("web", &info)?
(where info is an object of type (struct stat))

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.

Write a C program, print_file_bits.c, which given as a command line arguments the name of a file contain 32-bit hexadecimal numbers, one per line, prints the low (least significant) bytes of each number as a signed decimal number (-128..127).

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);
}

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.