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0x0F-function_pointers

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0x0F. C - Function pointers

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Tasks

  • All *-main.c files will be located in the main directory, apart from challenge 3 which is required in this directory for the checker to check for it.

  1. What's my name : A function that prints a name.

    • Prototype: void print_name(char *name, void (*f)(char *));
    • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/0-main.c 0-print_name.c -o a

  2. If you spend too much time thinking about a thing, you'll never get it done : 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.
    • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/1-main.c 1-array_iterator.c -o b

  3. To hell with circumstances; I create opportunities : 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
    • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/2-main.c 2-int_index.c -o c

  4. A goal is not always meant to be reached, it often serves simply as something to aim at : A function 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 results 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_functions.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 use 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 allowed to use a maximum of 3 if statements.

    • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 3-main.c 3-op_functions.c 3-get_op_func.c -o calc

  5. Most hackers are young because young people tend to be adaptable. As long as you remain adaptable, you can always be a good hacker : A program that prints 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: 
         [...]
 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>
 [...]

 # --------------------------------------------------------------- #

 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
    • 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 of 1
    • If the number of bytes is negative, print Error, followed by a new line, and exit with the status of 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.
    • Compile the code this way: gcc -std=gnu89 100-main_opcodes.c -o main1
    • Run code this way: ./main1 21
    • Test case 1: objdump -d -j.text -M intel main1
      • Then note the starting address of <main>
    • Test case 2: ./main1 21 | udcli -64 -x -o 4005f6
      • 4005f6 is the starting address of <main>
    • 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.