0x14-bit_manipulation

0x14. C - Bit manipulation

Resource

• Number Systems Introduction
• How to add and minus binary numbers
• Binary ..
• Binary Addition and Subtraction With Negative Numbers
• Bitwise Operators in C part 1
• Bitwise Operators in C part 2
• Bitwise Operators in C part 3
• Bitwise Operators in C part 4
• Algorithms: Bit Manipulation

---

• Below are some notes on ~, >> and << bitwise operators.

• NOT operator ~

  • Result is 0 when bit is 1 and 1 when bit is 0

• Right Shift >>

  • leftOperand >> rightOperand
    • leftOperand: whose bits get right shifted
    • rightOperand: Decides number of places to shift the bits
    • Example: 0x02 >> 1 = 0x01
  • When bits are shifted right the leading positions are filled with zeros
  • It is equivalent to division by 2^rightOperand
    • Example: 0x02 >> 1 == 0x02 / 2¹

• Left Shift <<

  • leftOperand << rightOperand
    • leftOperand: whose bits get right shifted
    • rightOperand: Decides number of places to shift the bits
    • Example: 0x01 << 1 = 0x02
  • When bits are shifted left the trailing positions are filled with zeros
  • It is equivalent to multiplication by 2^rightOperand
    • Example: 0x01 << 1 == 0x01 / 2¹

Tasks

• All *-main.c are in the main directory.

---

0. 0 : 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
   • To enable debug mode that will print statements of what is happening add this -D DEBUG at the end of the compile line.
   • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/0-main.c 0-binary_to_uint.c -o a
1. 1 : A function that prints the binary representation of a number.
   • Prototype: void print_binary(unsigned long int n);
   • Format: see below.

      julien@ubuntu:~/0x14. Binary$ ./b 
      0
      1
      1100010
      10000000000
      10000000001
      julien@ubuntu:~/0x14. Binary$

   • You are not allowed to use arrays.
   • You are not allowed to use malloc
   • You are not allowed to use the % or / operators.
   • If you are here for a solution for ALX, checker version.
   • To enable debug mode that will print statements of what is happening add this -D DEBUG at the end of the compile line.
   • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/1-main.c 1-print_binary.c _putchar.c -o b
2. 10 : 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 eror occured.
   • To enable debug mode that will print statements of what is happening add this -D DEBUG at the end of the compile line.
   • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/2-main.c 2-get_bit.c -o c
3. 11 : 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.
   • To enable debug mode that will print statements of what is happening add this -D DEBUG at the end of the compile line.
   • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/3-main.c 3-set_bit.c -o d
4. 100 : 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 occured.
   • To enable debug mode that will print statements of what is happening add this -D DEBUG at the end of the compile line.
   • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/4-main.c 4-clear_bit.c -o e
5. 101 : 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.
   • If you are here for a solution for ALX, checker version.
   • To enable debug mode that will print statements of what is happening add this -D DEBUG at the end of the compile line.
   • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/5-main.c 5-flip_bits.c -o f
6. Endianness : A function that checks the endianness.
   • Prototype: int get_endianness(void);
   • Returns: 0 if big endian, 1 if little endian.
   • Read more about it here.
   • Confirm your architecture version:

      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$

   • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/100-main.c 100-get_endianness.c -o h
7. Crackme3 : Find the password for this program.
   • Get the file this way: curl -fsSL https://raw.githubusercontent.com/holbertonschool/0x13.c/master/crackme3 -o crackme3
   • Will revisit this challenge and explain how to solve it.