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0x14. C - Bit manipulation

  • By Julien Barbier

Resources

Read or watch:

Learning Objectives

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

General

  • Look for the right source of information without too much help
  • How to manipulate bits and use bitwise operators

Requirements

General

  • 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
  • The only C standard library functions allowed are malloc, free and exit. Any use of functions like printf, puts, calloc, realloc 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
  • All your header files should be include guarded

Tasks

0. 0

mandatory

Write a function that converts a binary number to an unsigned int.

  • Prototype: unsigned int binary_to_uint(const char *b);
  • where b is pointing to a string of 0 and 1 chars
  • Return: the converted number, or 0 if
    • there is one or more chars in the string b that is not 0 or 1
    • b is NULL
julien@ubuntu:~/0x14. Binary$ cat 0-main.c
#include <stdio.h>
#include "main.h"

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    unsigned int n;

    n = binary_to_uint("1");
    printf("%u\n", n);
    n = binary_to_uint("101");
    printf("%u\n", n);
    n = binary_to_uint("1e01");
    printf("%u\n", n);
    n = binary_to_uint("1100010");
    printf("%u\n", n);
    n = binary_to_uint("0000000000000000000110010010");
    printf("%u\n", n);
    return (0);
}
julien@ubuntu:~/0x14. Binary$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 0-main.c 0-binary_to_uint.c -o a
julien@ubuntu:~/0x14. Binary$ ./a
1
5
0
98
402
julien@ubuntu:~/0x14. Binary$

Repo:

  • GitHub repository: alx-low_level_programming
  • Directory: 0x14-bit_manipulation
  • File: 0-binary_to_uint.c

1. 1

mandatory

Write a function that prints the binary representation of a number.

  • Prototype: void print_binary(unsigned long int n);
  • Format: see example
  • You are not allowed to use arrays
  • You are not allowed to use malloc
  • You are not allowed to use the % or / operators
julien@ubuntu:~/0x14. Binary$ cat 1-main.c
#include <stdio.h>
#include "main.h"

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    print_binary(0);
    printf("\n");
    print_binary(1);
    printf("\n");
    print_binary(98);
    printf("\n");
    print_binary(1024);
    printf("\n");
    print_binary((1 << 10) + 1);
    printf("\n");
    return (0);
}
julien@ubuntu:~/0x14. Binary$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 1-main.c 1-print_binary.c _putchar.c -o b
julien@ubuntu:~/0x14. Binary$ ./b
0
1
1100010
10000000000
10000000001
julien@ubuntu:~/0x14. Binary$

Repo:

  • GitHub repository: alx-low_level_programming
  • Directory: 0x14-bit_manipulation
  • File: 1-print_binary.c

2. 10

mandatory

Write a function that returns the value of a bit at a given index.

  • Prototype: int get_bit(unsigned long int n, unsigned int index);
  • where index is the index, starting from 0 of the bit you want to get
  • Returns: the value of the bit at index index or -1 if an error occured
julien@ubuntu:~/0x14. Binary$ cat 2-main.c
#include <stdio.h>
#include "main.h"

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    int n;

    n = get_bit(1024, 10);
    printf("%d\n", n);
    n = get_bit(98, 1);
    printf("%d\n", n);
    n = get_bit(1024, 0);
    printf("%d\n", n);
    return (0);
}
julien@ubuntu:~/0x14. Binary$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 2-main.c 2-get_bit.c -o c
julien@ubuntu:~/0x14. Binary$ ./c
1
1
0
julien@ubuntu:~/0x14. Binary$

Repo:

  • GitHub repository: alx-low_level_programming
  • Directory: 0x14-bit_manipulation
  • File: 2-get_bit.c

3. 11

mandatory

Write a function that sets the value of a bit to 1 at a given index.

  • Prototype: int set_bit(unsigned long int *n, unsigned int index);
  • where index is the index, starting from 0 of the bit you want to set
  • Returns: 1 if it worked, or -1 if an error occurred
julien@ubuntu:~/0x14. Binary$ cat 3-main.c
#include <stdio.h>
#include "main.h"

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    unsigned long int n;

    n = 1024;
    set_bit(&n, 5);
    printf("%lu\n", n);
    n = 0;
    set_bit(&n, 10);
    printf("%lu\n", n);
    n = 98;
    set_bit(&n, 0);
    printf("%lu\n", n);
    return (0);
}
julien@ubuntu:~/0x14. Binary$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 3-main.c 3-set_bit.c -o d
julien@ubuntu:~/0x14. Binary$ ./d
1056
1024
99
julien@ubuntu:~/0x14. Binary$

Repo:

  • GitHub repository: alx-low_level_programming
  • Directory: 0x14-bit_manipulation
  • File: 3-set_bit.c

4. 100

mandatory

Write a function that sets the value of a bit to 0 at a given index.

  • Prototype: int clear_bit(unsigned long int *n, unsigned int index);
  • where index is the index, starting from 0 of the bit you want to set
  • Returns: 1 if it worked, or -1 if an error occurred
julien@ubuntu:~/0x14. Binary$ cat 4-main.c
#include <stdio.h>
#include "main.h"

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    unsigned long int n;

    n = 1024;
    clear_bit(&n, 10);
    printf("%lu\n", n);
    n = 0;
    clear_bit(&n, 10);
    printf("%lu\n", n);
    n = 98;
    clear_bit(&n, 1);
    printf("%lu\n", n);
    return (0);
}
julien@ubuntu:~/0x14. Binary$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 4-main.c 4-clear_bit.c -o e
julien@ubuntu:~/0x14. Binary$ ./e
0
0
96
julien@ubuntu:~/0x14. Binary$

Repo:

  • GitHub repository: alx-low_level_programming
  • Directory: 0x14-bit_manipulation
  • File: 4-clear_bit.c

5. 101

mandatory

Write a function that returns the number of bits you would need to flip to get from one number to another.

  • Prototype: unsigned int flip_bits(unsigned long int n, unsigned long int m);
  • You are not allowed to use the % or / operators
julien@ubuntu:~/0x14. Binary$ cat 5-main.c
#include <stdio.h>
#include "main.h"

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    unsigned int n;

    n = flip_bits(1024, 1);
    printf("%u\n", n);
    n = flip_bits(402, 98);
    printf("%u\n", n);
    n = flip_bits(1024, 3);
    printf("%u\n", n);
    n = flip_bits(1024, 1025);
    printf("%u\n", n);
    return (0);
}
julien@ubuntu:~/0x14. Binary$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 5-main.c 5-flip_bits.c -o f
julien@ubuntu:~/0x14. Binary$ ./f
2
5
3
1
julien@ubuntu:~/0x14. Binary$

Repo:

  • GitHub repository: alx-low_level_programming
  • Directory: 0x14-bit_manipulation
  • File: 5-flip_bits.c

6. Endianness

#advanced

Write a function that checks the endianness.

  • Prototype: int get_endianness(void);
  • Returns: 0 if big endian, 1 if little endian
julien@ubuntu:~/0x14. Binary$ cat 100-main.c
#include <stdio.h>
#include "main.h"

int main(void)
{
    int n;

    n = get_endianness();
    if (n != 0)
    {
        printf("Little Endian\n");
    }
    else
    {
        printf("Big Endian\n");
    }
    return (0);
}
julien@ubuntu:~/0x14. Binary$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 100-main.c 100-get_endianness.c -o h
julien@ubuntu:~/0x14. Binary$ ./h
Little Endian
julien@ubuntu:~/0x14. Binary$ lscpu | head
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                1
On-line CPU(s) list:   0
Thread(s) per core:    1
Core(s) per socket:    1
Socket(s):             1
NUMA node(s):          1
Vendor ID:             GenuineIntel
julien@ubuntu:~/0x14. Binary$

Repo:

  • GitHub repository: alx-low_level_programming
  • Directory: 0x14-bit_manipulation
  • File: 100-get_endianness.c

7. Crackme3

#advanced

Find the password for this program.

  • Save the password in the file 101-password
  • Your file should contain the exact password, no new line, no extra space
julien@ubuntu:~/0x14. Binary$ ./crackme3 `cat 101-password`
Congratulations!
julien@ubuntu:~/0x14. Binary$

Repo:

  • GitHub repository: alx-low_level_programming
  • Directory: 0x14-bit_manipulation
  • File: 101-password