0x0F. C - Function pointers

• By Alexandre Gautier
• Ongoing project - started 09-29-2021, must end by 09-30-2021 (in about 18 hours) - you're done with 0% of tasks.
• Checker will be released at 09-29-2021 06:00 PM
• QA review fully automated.

Resources

Read or watch:

• Function Pointer in C
• Pointers to functions
• Function Pointers in C / C++
• why pointers to functions?
• Everything you need to know about pointers in C

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 function pointers and how to use them
• What does a function pointer exactly hold
• Where does a function pointer point to in the virtual memory

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 function_pointers.h
• Don't forget to push your header file
• All your header files should be include guarded

Tasks 0. What's my name mandatory

Write a function that prints a name.

Prototype: void print_name(char *name, void (*f)(char *));

julien@ubuntu:~/0x0e. Function pointers$ cat 0-main.c #include <stdio.h> #include "function_pointers.h"

/**

• print_name_as_is - prints a name as is
• @name: name of the person
• Return: Nothing. */ void print_name_as_is(char *name) { printf("Hello, my name is %s\n", name); }

/**

• print_name_uppercase - print a name in uppercase

• @name: name of the person

• Return: Nothing. */ void print_name_uppercase(char *name) { unsigned int i;

  printf("Hello, my uppercase name is "); i = 0; while (name[i]) { if (name[i] >= 'a' && name[i] <= 'z') { putchar(name[i] + 'A' - 'a'); } else { putchar(name[i]); } i++; } }

/**

• main - check the code
• Return: Always 0. */ int main(void) { print_name("Bob", print_name_as_is); print_name("Bob Dylan", print_name_uppercase); printf("\n"); return (0); } julien@ubuntu:/0x0e. Function pointers$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 0-main.c 0-print_name.c -o a julien@ubuntu:/0x0e. Function pointers$ ./a Hello, my name is Bob Hello, my uppercase name is BOB DYLAN julien@ubuntu:~/0x0e. Function pointers$

Repo:

GitHub repository: alx-low_level_programming
Directory: 0x0F-function_pointers
File: 0-print_name.c

1. If you spend too much time thinking about a thing, you'll never get it done mandatory

Write a function that executes a function given as a parameter on each element of an array.

Prototype: void array_iterator(int *array, size_t size, void (*action)(int));
where size is the size of the array
and action is a pointer to the function you need to use

julien@ubuntu:~/0x0e. Function pointers$ cat 1-main.c #include <stdio.h> #include "function_pointers.h"

/**

• print_elem - prints an integer
• @elem: the integer to print
• Return: Nothing. */ void print_elem(int elem) { printf("%d\n", elem); }

/**

• print_elem_hex - prints an integer, in hexadecimal
• @elem: the integer to print
• Return: Nothing. */ void print_elem_hex(int elem) { printf("0x%x\n", elem); }

/**

• main - check the code

• Return: Always 0. */ int main(void) { int array[5] = {0, 98, 402, 1024, 4096};

  array_iterator(array, 5, &print_elem); array_iterator(array, 5, &print_elem_hex); return (0); } julien@ubuntu:/0x0e. Function pointers$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 1-main.c 1-array_iterator.c -o b julien@ubuntu:/0x0e. Function pointers$ ./b 0 98 402 1024 4096 0x0 0x62 0x192 0x400 0x1000 julien@ubuntu:~//0x0e. Function pointers$

Repo:

GitHub repository: alx-low_level_programming
Directory: 0x0F-function_pointers
File: 1-array_iterator.c

2. To hell with circumstances; I create opportunities mandatory

Write a function that searches for an integer.

Prototype: int int_index(int *array, int size, int (*cmp)(int));
where size is the number of elements in the array array
cmp is a pointer to the function to be used to compare values
int_index returns the index of the first element for which the cmp function does not return 0
If no element matches, return -1
If size <= 0, return -1

julien@ubuntu:~/0x0e. Function pointers$ cat 2-main.c #include <stdio.h> #include "function_pointers.h"

/**

• is_98 - check if a number is equal to 98
• @elem: the integer to check
• Return: 0 if false, something else otherwise. */ int is_98(int elem) { return (98 == elem); }

/**

• is_strictly_positive - check if a number is greater than 0
• @elem: the integer to check
• Return: 0 if false, something else otherwise. */ int is_strictly_positive(int elem) { return (elem > 0); }

/**

• abs_is_98 - check if the absolute value of a number is 98
• @elem: the integer to check
• Return: 0 if false, something else otherwise. */ int abs_is_98(int elem) { return (elem == 98 || -elem == 98); }

/**

• main - check the code

• Return: Always 0. */ int main(void) { int array[20] = {0, -98, 98, 402, 1024, 4096, -1024, -98, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 98}; int index;

  index = int_index(array, 20, is_98); printf("%d\n", index); index = int_index(array, 20, abs_is_98); printf("%d\n", index); index = int_index(array, 20, is_strictly_positive); printf("%d\n", index); return (0); } julien@ubuntu:/0x0e. Function pointers$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 2-main.c 2-int_index.c -o c julien@ubuntu:/0x0e. Function pointers$ ./c 2 1 2 julien@ubuntu:~/0x0e. Function pointers$

Repo:

GitHub repository: alx-low_level_programming
Directory: 0x0F-function_pointers
File: 2-int_index.c

3. A goal is not always meant to be reached, it often serves simply as something to aim at mandatory

Write a program that performs simple operations.

You are allowed to use the standard library
Usage: calc num1 operator num2
You can assume num1 and num2 are integers, so use the atoi function to convert them from the string input to int
operator is one of the following:
    +: addition
    -: subtraction
    *: multiplication
    /: division
    %: modulo
The program prints the result of the operation, followed by a new line
You can assume that the result of all operations can be stored in an int
if the number of arguments is wrong, print Error, followed by a new line, and exit with the status 98
if the operator is none of the above, print Error, followed by a new line, and exit with the status 99
if the user tries to divide (/ or %) by 0, print Error, followed by a new line, and exit with the status 100

This task requires that you create four different files.

3-calc.h

This file should contain all the function prototypes and data structures used by the program. You can use this structure:

/**

• struct op - Struct op
• @op: The operator
• @f: The function associated */ typedef struct op { char *op; int (*f)(int a, int b); } op_t;

3-op_functions.c

This file should contain the 5 following functions (not more):

op_add: returns the sum of a and b. Prototype: int op_add(int a, int b);
op_sub: returns the difference of a and b. Prototype: int op_sub(int a, int b);
op_mul: returns the product of a and b. Prototype: int op_mul(int a, int b);
op_div: returns the result of the division of a by b. Prototype: int op_div(int a, int b);
op_mod: returns the remainder of the division of a by b. Prototype: int op_mod(int a, int b);

3-get_op_func.c

This file should contain the function that selects the correct function to perform the operation asked by the user. You’re not allowed to declare any other function.

Prototype: int (*get_op_func(char *s))(int, int);
where s is the operator passed as argument to the program
This function returns a pointer to the function that corresponds to the operator given as a parameter. Example: get_op_func("+") should return a pointer to the function op_add
You are not allowed to use switch statements
You are not allowed to use for or do ... while loops
You are not allowed to use goto
You are not allowed to use else
You are not allowed to use more than one if statement in your code
You are not allowed to use more than one while loop in your code
If s does not match any of the 5 expected operators (+, -, *, /, %), return NULL
You are only allowed to declare these two variables in this function:

op_t ops[] = {
    {"+", op_add},
    {"-", op_sub},
    {"*", op_mul},
    {"/", op_div},
    {"%", op_mod},
    {NULL, NULL}
};
int i;

3-main.c

This file should contain your main function only.

You are not allowed to code any other function than main in this file
You are not allowed to directly call op_add, op_sub, op_mul, op_div or op_mod from the main function
You have to use atoi to convert arguments to int
You are not allowed to use any kind of loop
You are allowed to use a maximum of 3 if statements

Compilation and examples

julien@ubuntu:/0x0e. Function pointers$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 3-main.c 3-op_functions.c 3-get_op_func.c -o calc julien@ubuntu:/0x0e. Function pointers$ ./calc 1 + 1 2 julien@ubuntu:/0x0e. Function pointers$ ./calc 97 + 1 98 julien@ubuntu:/0x0e. Function pointers$ ./calc 1024 / 10 102 julien@ubuntu:/0x0e. Function pointers$ ./calc 1024 '*' 98 100352 julien@ubuntu:/0x0e. Function pointers$ ./calc 1024 '*' 98 Error julien@ubuntu:/0x0e. Function pointers$ ./calc 1024 - 98 926 julien@ubuntu:/0x0e. Function pointers$ ./calc 1024 '%' 98 44 julien@ubuntu:~/0x0e. Function pointers$

Repo:

GitHub repository: alx-low_level_programming
Directory: 0x0F-function_pointers
File: 3-main.c, 3-op_functions.c, 3-get_op_func.c, 3-calc.h

4. Most hackers are young because young people tend to be adaptable. As long as you remain adaptable, you can always be a good hacker

#advanced

Score: 0.00% (Checks completed: 0.00%)

Write a program that prints the opcodes of its own main function.

• Usage: ./main number_of_bytes
• Output format:
  • the opcodes should be printed in hexadecimal, lowercase
  • each opcode is two char long
  • listing ends with a new line
  • see example
• You are allowed to use printf and atoi
• You have to use atoi to convert the argument to an int
• If the number of argument is not the correct one, print Error, followed by a new line, and exit with the status 1
• If the number of bytes is negative, print Error, followed by a new line, and exit with the status 2
• You do not have to compile with any flags

Note: if you want to translate your opcodes to assembly instructions, you can use, for instance udcli.

julien@ubuntu:~/0x0e. Function pointers$ gcc -std=gnu89 100-main_opcodes.c -o main
julien@ubuntu:~/0x0e. Function pointers$ ./main 21
55 48 89 e5 48 83 ec 30 89 7d dc 48 89 75 d0 83 7d dc 02 74 14
julien@ubuntu:~/0x0e. Function pointers$ objdump -d -j.text -M intel main
[...]
00000000004005f6 <main>:
  4005f6:   55                      push   rbp
  4005f7:   48 89 e5                mov    rbp,rsp
  4005fa:   48 83 ec 30             sub    rsp,0x30
  4005fe:   89 7d dc                mov    DWORD PTR [rbp-0x24],edi
  400601:   48 89 75 d0             mov    QWORD PTR [rbp-0x30],rsi
  400605:   83 7d dc 02             cmp    DWORD PTR [rbp-0x24],0x2
  400609:   74 14                   je     40061f <main+0x29>
[...]
julien@ubuntu:~/0x0e. Function pointers$ ./main 21 | udcli -64 -x -o 4005f6
00000000004005f6 55               push rbp
00000000004005f7 4889e5           mov rbp, rsp
00000000004005fa 4883ec30         sub rsp, 0x30
00000000004005fe 897ddc           mov [rbp-0x24], edi
0000000000400601 488975d0         mov [rbp-0x30], rsi
0000000000400605 837ddc02         cmp dword [rbp-0x24], 0x2
0000000000400609 7414             jz 0x40061f
julien@ubuntu:~/0x0e. Function pointers$

• Note 0: je is equivalent to jz
• Note 1: depending on how you write your main function, and on which machine you compile your program, the opcodes (and by extension the assembly code) might be different than the above example

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x0F-function_pointers
• File: 100-main_opcodes.c