0x07-pointers_arrays_strings

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

Learning Objectives

• At the end of this project, you are expected to be able to explain to anyone, without the help of Google:

General

• What are pointers to pointers and how to use them
• What are multidimensional arrays and how to use them
• What are the most common C standard library functions to manipulate strings

Copyright - Plagiarism

• You are tasked to come up with solutions for the tasks below yourself to meet with the above learning objectives.
• You will not be able to meet the objectives of this or any following project by copying and pasting someone else’s work.
• You are not allowed to publish any content of this project.
• Any form of plagiarism is strictly forbidden and will result in removal from the program.

Requirements

• Allowed editors: vi, vim, emacs
• All your files will be compiled on Ubuntu 20.04 LTS using gcc, using the options -Wall -Werror -Wextra -pedantic -std=gnu89
• All your files should end with a new line
• A README.md file, at the root of the folder of the project is mandatory
• Your code should use the Betty style. It will be checked using betty-style.pl and betty-doc.pl
• You are not allowed to use global variables
• No more than 5 functions per file
• You are not allowed to use the standard library. Any use of functions like printf, puts, etc… is forbidden
• You are allowed to use _putchar
• You don’t have to push _putchar.c, we will use our file. If you do it won’t be taken into account
• In the following examples, the main.c files are shown as examples. You can use them to test your functions, but you don’t have to push them to your repo (if you do we won’t take them into account). We will use our own main.c files at compilation. Our main.c files might be different from the one shown in the examples
• The prototypes of all your functions and the prototype of the function _putchar should be included in your header file called main.h
• Don’t forget to push your header file

Tasks

0-memset.c

Write a function that fills memory with a constant byte.

• Prototype: char *_memset(char *s, char b, unsigned int n);
• The _memset() function fills the first n bytes of the memory area pointed to by s with the constant byte b
• Returns a pointer to the memory area s

FYI: The standard library provides a similar function: memset. Run man memset to learn more.

1-memcpy.c

Write a function that copies memory area.

• Prototype: char *_memcpy(char *dest, char *src, unsigned int n);
• The _memcpy() function copies n bytes from memory area src to memory area dest
• Returns a pointer to dest

FYI: The standard library provides a similar function: memcpy. Run man memcpy to learn more.

2-strchr.c

Write a function that locates a character in a string.

• Prototype: char *_strchr(char *s, char c);
• Returns a pointer to the first occurrence of the character c in the string s, or NULL if the character is not found

FYI: The standard library provides a similar function: strchr. Run man strchr to learn more.

3-strspn.c

Write a function that gets the length of a prefix substring.

• Prototype: unsigned int _strspn(char *s, char *accept);
• Returns the number of bytes in the initial segment of s which consist only of bytes from accept

FYI: The standard library provides a similar function: strspn. Run man strspn to learn more.

4-strpbrk.c

Write a function that searches a string for any of a set of bytes.

• Prototype: char *_strpbrk(char *s, char *accept);
• The _strpbrk() function locates the first occurrence in the string s of any of the bytes in the string accept
• Returns a pointer to the byte in s that matches one of the bytes in accept, or NULL if no such byte is found

FYI: The standard library provides a similar function: strpbrk. Run man strpbrk to learn more.

5-strstr.c

Write a function that locates a substring.

• Prototype: char *_strstr(char *haystack, char *needle);
• The _strstr() function finds the first occurrence of the substring needle in the string haystack. The terminating null bytes (\0) are not compared
• Returns a pointer to the beginning of the located substring, or NULL if the substring is not found.

FYI: The standard library provides a similar function: strstr. Run man strstr to learn more.

7-print_chessboard.c

Write a function that prints the chessboard.

• Prototype: void print_chessboard(char (*a)[8]);

8-print_diagsums.c

Write a function that prints the sum of the two diagonals of a square matrix of integers.

• Prototype: void print_diagsums(int *a, int size);
• Format: see example
• You are allowed to use the standard library

Note that in the following example we are casting an int[][] into an int*. This is not something you should do. The goal here is to make sure you understand how an array of array is stored in memory.

100-set_string.c

Write a function that sets the value of a pointer to a char.

• Prototype: void set_string(char **s, char *to);

101-crackme_password

Create a file that contains the password for the crackme2 executable.

• Your file should contain the exact password, no new line, no extra space
• ltrace, ldd, gdb and objdump can help
• You may need to install the openssl library to run the crakme2 program: sudo apt install libssl-dev
• Edit the source list sudo nano /etc/apt/sources.list to add the following line: deb http://security.ubuntu.com/ubuntu xenial-security main Then sudo apt update and sudo apt install libssl1.0.0

Find resources to solve this task here: Crackme_Open