0x0F. C - Function pointers

Resources

Function pointers 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

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

Function Pointers in C

Function pointers are variables that hold the address of a function in memory. They allow you to pass functions as arguments to other functions, or to store functions in arrays or structures.

Here's an example of a function pointer declaration:

int (*myFunc)(int, int);

This declares a function pointer called myFunc that takes two int arguments and returns an int.

To use a function pointer, you need to assign it the address of a function. Here's an example:

int add(int a, int b) {
    return a + b;
}

int (*myFunc)(int, int) = add;

This assigns the address of the add function to myFunc.

You can then call the function using the pointer, like this:

int result = myFunc(4, 5); // result will be 9

Function pointers can also be used with function pointers in arrays or structures. Here's an example of a function pointer array:

int add(int a, int b) {
    return a + b;
}

int subtract(int a, int b) {
    return a - b;
}

int multiply(int a, int b) {
    return a * b;
}

int (*mathFuncs[3])(int, int) = { add, subtract, multiply };

This declares an array called mathFuncs that holds three function pointers to the add, subtract, and multiply functions.

You can then call the functions using the pointers in the array, like this:

int result1 = mathFuncs[0](4, 5); // result1 will be 9
int result2 = mathFuncs[1](4, 5); // result2 will be -1
int result3 = mathFuncs[2](4, 5); // result3 will be 20

Function pointers hold the address of a function in virtual memory. When you call a function through a function pointer, the program looks up the function's address in memory and jumps to that location to execute the function.

What are function pointers and how to use them

Tasks

0. What's my name

#include "function_pointers.h"
#include <stdio.h>

/**
 * print_name_as_is - prints a name as is
 * @name: name to print
 *
 * Return: void
 */
void print_name_as_is(char *name)
{
	printf("Hello, my name is %s\n", name);
}

/**
 * print_name_uppercase - prints a name in uppercase
 * @name: name to print
 *
 * Return: void
 */
void print_name_uppercase(char *name)
{
    int i;

    printf("Hello, my uppercase name is ");
    for (i = 0; name[i]; i++)
    {
        if (name[i] >= 'a' && name[i] <= 'z')
            printf("%c", name[i] - 32);
        else
            printf("%c", name[i]);
    }
    printf("\n");
}

/**
 * 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);
}

Compiled with: gcc -Wall -pedantic -Werror -Wextra 0-main.c 0-print_name.c -o 0-print_name Output:
$ ./0-print_name
Hello, my name is Bob
Hello, my uppercase name is BOB DYLAN

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

#include "function_pointers.h"
#include <stdio.h>

/**
 * print_elem - prints an integer
 * @elem: integer to print
 *
 * Return: void
 */
void print_elem(int elem)
{
    printf("%d\n", elem);
}

/**
 * print_elem_hex - prints an integer in hexadecimal
 * @elem: integer to print
 *
 * Return: void
 */
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);
    printf("\n");
    array_iterator(array, 5, print_elem_hex);
    return (0);
}

Compiled with: gcc -Wall -pedantic -Werror -Wextra 1-main.c 1-array_iterator.c -o 1-array_iterator Output:
$ ./1-array_iterator
0
98
402
1024
4096

0x0
0x62
0x192
0x400
0x1000

2. To hell with circumstances; I create opportunities

#include "function_pointers.h"
#include <stdio.h>

/**
 * is_98 - checks if a number is equal to 98
 * @elem: integer to check
 *
 * Return: 1 if true, 0 otherwise
 */
int is_98(int elem)
{
    return (elem == 98);
}

/**
 * is_strictly_positive - checks if a number is greater than 0
 * @elem: integer to check
 *
 * Return: 1 if true, 0 otherwise
 */
int is_strictly_positive(int elem)
{
    return (elem > 0);
}

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

    index = int_index(array, 5, is_98);
    printf("%d\n", index);
    index = int_index(array, 5, is_strictly_positive);
    printf("%d\n", index);
    index = int_index(array, 5, NULL);
    printf("%d\n", index);
    return (0);
}

Compiled with: gcc -Wall -pedantic -Werror -Wextra 2-main.c 2-int_index.c -o 2-int_index Output:
$ ./2-int_index
1
2
-1

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

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 should contain all the function prototypes and data structures used by the program. You can only 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 should contain the 5 following functions (not more):
- op_add that returns the sum of a and b. (prototype: int op_add(int a, int b);)
- op_sub that returns the difference of a and b. (prototype: int op_sub(int a, int b);)
- op_mul that returns the product of a and b. (prototype: int op_mul(int a, int b);)
- op_div that returns the result of the division of a by b. (prototype: int op_div(int a, int b);)
- op_mod that returns the remainder of the division of a by b. (prototype: int op_mod(int a, int b);)
3-get_op_func.c 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.
- Your 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 else, switch, case, goto, or ternary operator (?:).
- You are not allowed to use for or do ... while loops.
- 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 should contain your main function only.
- You are not allowed to code any other function than main in the file 3-main.c.
- You are not allowed to directly call op_add, op_sub, op_mul, op_div and op_mod from the file 3-main.c.
- You have to use atoi to convert arguments to int.
- You are not allowed to use any kind of loop (for, do ... while, while, etc.).
- You are allowed a maximum of 3 if statements in your code.

Compiled with: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 3-main.c 3-op_functions.c 3-get_op_func.c -o calc

Output:

$ ./calc 1 + 1
2
$ ./calc 1024 '*' 98
100352
$ ./calc 1024 '/' 98
10
$ ./calc 1024 '%' 98
38
$ ./calc 1024 '+' 98
1122
$ ./calc 1024 '-' 98
926
$ ./calc 1024 'a' 98
Error
$ ./calc 1024 '+' 98 98
Error
$

4. Once an idea has taken hold of the brain it's almost impossible to eradicate

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 arguments 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.

Output:

$ ./main 21
55 48 89 e5 48 83 ec 30 89 7d dc 48 89 75 d0 83 7d dc 02 74 14
$ 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>
[...]
$ ./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
[...]
$

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

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README.md

README.md

0x0F. C - Function pointers

Resources

Learning Objectives

Function Pointers in C

What are function pointers and how to use them

Tasks

0. What's my name

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

2. To hell with circumstances; I create opportunities

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

4. Once an idea has taken hold of the brain it's almost impossible to eradicate

Files

README.md

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History

README.md

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0x0F. C - Function pointers

Resources

Learning Objectives

Function Pointers in C

What are function pointers and how to use them

Tasks

0. What's my name

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

2. To hell with circumstances; I create opportunities

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

4. Once an idea has taken hold of the brain it's almost impossible to eradicate