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0x07. C - Even more pointers, arrays and strings

Resources

Learning Objectives

What is a pointer to a pointer and how to use it?

A pointer to a pointer is a type of pointer variable that stores the address of another pointer variable. It is also known as a double pointer.

In C programming, a pointer to a pointer can be useful in situations where we need to pass a pointer as an argument to a function and modify the original pointer. By using a pointer to a pointer, we can indirectly modify the value of the original pointer.

Here's an example:

#include <stdio.h>

void changePointer(int **ptr)
{
    int newNum = 20;
    *ptr = &newNum;
}

int main()
{
    int num = 10;
    int *ptr = &num;
    
    printf("Before function call: %d\n", *ptr); // output: 10
    
    changePointer(&ptr);
    
    printf("After function call: %d\n", *ptr); // output: 20
    
    return 0;
}

What is a multidimensional array and how to use it?

A multidimensional array is an array of arrays. It is a way to represent data in multiple dimensions, such as a table or a matrix.

In C, a multidimensional array can be declared as follows:

data_type array_name[size1][size2]...[sizeN];

where data_type is the type of data to be stored in the array, array_name is the name of the array, and size1, size2, ..., sizeN are the sizes of each dimension of the array.

To access elements of a multidimensional array, you can use nested loops. For example, to initialize a 2D array of integers with the values 0 to 9, you can do:

int arr[3][3];

for (int i = 0; i < 3; i++) {
  for (int j = 0; j < 3; j++) {
    arr[i][j] = i * 3 + j;
  }
}

This will create a 3x3 array with the following values:

0 1 2
3 4 5
6 7 8

You can also create arrays with more than two dimensions, such as 3D arrays, 4D arrays, and so on. The concept is the same, but you need to use nested loops for each dimension.

What are the most common C standard library functions to manipulate strings?

The most common C standard library functions to manipulate strings include:

  • strlen - returns the length of a string.
  • strcpy - copies a string.
  • strncpy - copies a certain number of characters from one string to another.
  • strcat - concatenates two strings.
  • strncat - concatenates a certain number of characters from one string to another.
  • strcmp - compares two strings.
  • strncmp - compares a certain number of characters from two strings.
  • strstr - searches for a substring in a string.
  • strtok - breaks a string into tokens.
  • sprintf - formats and stores a string in a buffer.

There are many other functions in the C standard library that can be used to manipulate strings as well.

Tasks

  • All *-main.c files are provided in the [test_files](./test_files) directory.
  • We will be using function prototypes from the [main.h](./main.h) header file for this project.
0. memset 0-memset 
#include "main.h"
#include <stdio.h>

/**
 * simple_print_buffer - prints buffer in hexa
 * @buffer: the address of memory to print
 * @size: the size of the memory to print
 *
 * Return: Nothing.
 */
void simple_print_buffer(char *buffer, unsigned int size)
{
        unsigned int i;

        i = 0;
        while (i < size)
        {
                if (i % 10)
                {
                        printf(" ");
                }
                if (!(i % 10) && i)
                {
                        printf("\n");
                }
                printf("0x%02x", buffer[i]);
                i++;
        }
        printf("\n");
}

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char buffer[98] = {0x00};

    simple_print_buffer(buffer, 98);
    _memset(buffer, 0x01, 95);
    printf("-------------------------------------------------\n");
    simple_print_buffer(buffer, 98);    
    return (0);
}

Expected output:

0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
-------------------------------------------------
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x01 0x01 0x01 0x01 0x01 0x00 0x00 0x00

Compile with:

gcc -Wall -pedantic -Werror -Wextra -std=gnu89 0-main.c 0-memset.c -o 0-memset
1. memcpy 1-memcpy 
#include "main.h"
#include <stdio.h>

/**
 * simple_print_buffer - prints buffer in hexa
 * @buffer: the address of memory to print
 * @size: the size of the memory to print
 *
 * Return: Nothing.
 */
void simple_print_buffer(char *buffer, unsigned int size)
{
    unsigned int i;

    i = 0;
    while (i < size)
    {
        if (i % 10)
        {
            printf(" ");
        }
        if (!(i % 10) && i)
        {
            printf("\n");
        }
        printf("0x%02x", buffer[i]);
        i++;
    }
    printf("\n");
}

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char buffer[98] = {0};
    char buffer2[98] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14};

    simple_print_buffer(buffer, 98);
    _memcpy(buffer + 50, buffer2, 10);
    printf("-------------------------------------------------\n");
    simple_print_buffer(buffer, 98);    
    return (0);
}

Expected output:

0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
-------------------------------------------------
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x01 0x02 0x03 0x04 0x05 0x07 0x07 0x08 0x09 0x0a
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00

Compile with:

gcc -Wall -pedantic -Werror -Wextra -std=gnu89 1-main.c 1-memcpy.c -o 1-memcpy
2. strchr 2-strchr 
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char *s = "hello";
    char *f;

    f = _strchr(s, 'l');

    if (f != NULL)
    {
        printf("%s\n", f);
    }
    return (0);
}

Compiled with:

gcc -Wall -pedantic -Werror -Wextra -std=gnu89 2-main.c 2-strchr.c -o 2-strchr

Expected output:

llo
3. strspn 3-strspn 
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char *s = "hello, world";
    char *f = "oleh";
    unsigned int n;

    n = _strspn(s, f);
    printf("%u\n", n);
    return (0);
}

Compiled with:

gcc -Wall -pedantic -Werror -Wextra -std=gnu89 3-main.c 3-strspn.c -o 3-strspn

Expected output:

5
4. strpbrk 4-strpbrk 
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char *s = "hello, world";
    char *f = "world";
    char *t;

    t = _strpbrk(s, f);
    printf("%s\n", t);
    return (0);
}

Compiled with:

gcc -Wall -pedantic -Werror -Wextra -std=gnu89 4-main.c 4-strpbrk.c -o 4-strpbrk

Expected output:

llo, world
5. strstr 5-strstr 
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char *s = "hello, world";
    char *f = "world";
    char *t;

    t = _strstr(s, f);
    printf("%s\n", t);
    return (0);
}

Compiled with:

gcc -Wall -pedantic -Werror -Wextra -std=gnu89 5-main.c 5-strstr.c -o 5-strstr

Expected output:

world
6. Chess is mental torture 7-print_chessboard 
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char board[8][8] = {
        {'r', 'k', 'b', 'q', 'k', 'b', 'k', 'r'},
        {'p', 'p', 'p', 'p', 'p', 'p', 'p', 'p'},
        {' ', ' ', ' ', ' ', ' ', ' ', ' ', ' '},
        {' ', ' ', ' ', ' ', ' ', ' ', ' ', ' '},
        {' ', ' ', ' ', ' ', ' ', ' ', ' ', ' '},
        {' ', ' ', ' ', ' ', ' ', ' ', ' ', ' '},
        {'P', 'P', 'P', 'P', 'P', 'P', 'P', 'P'},
        {'R', 'K', 'B', 'Q', 'K', 'B', 'K', 'R'},
    };
    print_chessboard(board);
    return (0);
}

Compiled with:

gcc -Wall -pedantic -Werror -Wextra -std=gnu89 _putchar.c 7-main.c 7-print_chessboard.c -o 7-print_chessboard

Expected output:

rkbqkbkr
pppppppp




PPPPPPPP
RKBQKBKR
7. The line of life is a ragged diagonal between duty and desire 8-print_diagsums 
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    int c3[3][3] = {
        {0, 1, 5},
        {10, 11, 12},
        {1000, 101, 102},
    };
    int c5[5][5] = {
        {0, 1, 5, 12124, 1234},
        {10, 11, 12, 123521, 12512},
        {1000, 101, 102, 12545, 214543435},
        {100, 1012451, 11102, 12545, 214543435},
        {10, 12401, 10452, 11542545, 1214543435},
    };
    print_diagsums((int *)c3, 3);
    print_diagsums((int *)c5, 5);
    return (0);
}

Compiled with:

gcc -Wall -pedantic -Werror -Wextra -std=gnu89 8-main.c 8-print_diagsums.c -o 8-print_diagsums

Expected output:

113, 1016
1214556093, 1137318
8. Double pointer, double fun 100-set_string 
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char *s0 = "Bob Dylan";
    char *s1 = "Robert Allen";

    printf("%s, %s\n", s0, s1);
    set_string(&s1, s0);
    printf("%s, %s\n", s0, s1);
    return (0);
}

Compiled with:

gcc -Wall -pedantic -Werror -Wextra -std=gnu89 100-main.c 100-set_string.c -o 100-set_string

Expected output:

Bob Dylan, Robert Allen
Bob Dylan, Bob Dylan
9. My primary goal of hacking was the intellectual curiosity, the seduction of adventure 101-crackme_password

How I cracked the password

  • Step 1: I ran the program with random inputs to see what the output was. (it was always Access denied)
  • Step 2: I opened the program in gdb to see what the program was doing.
    • I used info functions to see what functions were in the program.
101-crackme_password_1
  • Step 3: I used disassemble main to see where the the strcmp and strncmp functions were being called.
101-crackme_cmp_1 101-crackme_cmp_2
  • Step 4: I examined the compared strings to see what the password was. 101-crackme_cmp_2
  • Step 5: After that I tried the using one of the passwords in the program but didn't work!
  • Step 6: I then I noticed that there's MD5 hash functions from the OpenSSL library being called.
  • Step 7: I Used an online decoder to decode the hash to the password.
    • hash: e99a18c428cb38d5f260853678922e03
    • password: abc123
  • Step 8: I ran the program with the password and still didn't work!
  • Step 9: After examining the assembly code I noticed that the program printing Access denied when argc is not equal to 1.
    • That mean that the program use other method to check the password.
  • Step 10: After examining the assembly more I noticed that the program use envp param and checks
    • They iterat through the envp array and check if the envp[i] is equal to jennieandjayloveasm=
    • If it is equal to jennieandjayloveasm= then the program compute its MD5 hash and compare it to the hash e99a18c428cb38d5f260853678922e03
    • If it is equal to e99a18c428cb38d5f260853678922e03 then the program prints Access Granted and exits.

The solution

  • create a environment variable called jennieandjayloveasm= and set it to the password abc123
  • export jennieandjayloveasm=abc123
  • run the program ./101-crackme_password
  • The program should print Access Granted