Why is it incorrect to work with variables and pointers in C++?

Question

Programming

Good day. Tell me what to do - there is an incomprehensible work with variables and pointers in VS C++ 15
In the main part of the program in the DO loop, the content of the _data variable changes to some kind of dregs after its first use.
prntscr.com/7bnfmk - everything is OK here. We go further, I press "continue".
prntscr.com/7bng9h - some heresy returns to the screen.
In theory, the string that I entered in the console should be returned to me. Implementation in _setString method
PS I'm a first day in C++ so don't kick

#include <conio.h> 
#include <stdio.h>
#include <string.h>
#include <windows.h>
#include <iostream>

using namespace std;

/* Перегрузка функция копирования */
char *strcat_s(char *_string, char _buffer[100]) {
  _string = _buffer;
  return _string;
}

/* Функция ввода строки */
char *setString(char *_string = (char *)malloc(1)) {

  char _buffer[100];
  int i, _lenght = 0;
  _string = '\0';

  cout << "Введите строку:\n";

  do
  {
    i = scanf_s("%99[^\n]", _buffer, 100);

    if (i < 0) {
      free(_string);
      _string = NULL;
      continue;
    }

    if (i == 0)
      scanf_s("%*c");
    else {
      _lenght += strlen(_buffer);
      _string = (char *)realloc(_string, _lenght + 1);
      _string = strcat_s (_string, _buffer);
    }

  } while (i > 0);

  return _string;

}

char *removeSpaces(char *_string) {
  /* Функция удаления пробелов из строки */
  /* Функция удаления лишних пробелов после знаков */
  /* Функция удаления лишних пробелов перед знаками */
  return _string;
}


int main() {

  char *_data, *_result;
  
  // Не забудем поменять настройки консоли при запуске программы - шрифт Lucida Console
  SetConsoleCP(1251);
  SetConsoleOutputCP(1251);
  
  do {
    _data = setString();
    //if (strlen(_data) < 1)
      cout << _data << "\n";
      //continue;
    _result = removeSpaces(_data);
    cout << _data << "\n";
    cout << _result << "\n";

    cout << "Преобразованная строка:\n";

    //for (int i = 0; _result[i] != '\0'; i++) {
      //printf("%c", _result[i]);
    //}
    cout << "\n\n===========\n\n";

  } while (_data[0] != '\0');

  system("pause");
  return 0;

}

Answer 1

As I wrote above, I compiled this case under gcc.
UPD:
The problem was solved by replacing the incomprehensible strcat_s with the standard strncpy(_string, _buffer, _lenght);
UPD2:
The whole essence of the error can be understood here valera.asf.ru/cpp/book/c08.shtml
Specifically, the item " 8.3.1. Automatic objects "

After returning to main(), mainResult points to a region of memory not allocated to any object. ( In this example, this area may still contain the correct value, since we did not call any other functions after trouble() and its activation record is probably not overwritten yet .)

Answer 2

Why is it incorrect to work with variables and pointers in C++? - because the one who writes on it does not know how to use them.