string input not printed correctly on the screen-CodePudding

I am trying to read in a string from the user input and then print it on screen. However, when the string is printed on the console, it is kind of gibberish. The funny thing is that it works in Visual Studio and not in CodeBlocks.

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <windows.h>

int main() {
    int i, num_bytes;
    char sentence[] = "";
    std::cout << "Please enter your name: ";
    //fgets(sentence, 100, stdin);
    //scanf("%[^\n]%*c", sentence);
    //scanf("%[^\n]", sentence);
    std::cin >> sentence;
    num_bytes = strlen(sentence);

    LPVOID ptr = VirtualAlloc(NULL, num_bytes, MEM_RESERVE, PAGE_READWRITE);
    ptr = VirtualAlloc(ptr, num_bytes, MEM_COMMIT, PAGE_READWRITE);

    if (ptr) {
        char* char_ptr = static_cast<char*>(ptr);

        for (i = 0; i < num_bytes; i  ) {
            char_ptr[i] = sentence[i];
        }

        std::cout << "Allocated Memory Address: " << (void *)ptr << std::endl;

        std::cout << "Press Enter to print out the characters.\n";
        getchar();

        for (i = 0; i < num_bytes; i  ) {
            std::cout << char_ptr[i];
        }

        std::cout << "\nPress Enter to clear memory." << std::endl;
        getchar();

        VirtualFree(ptr, 0, MEM_RELEASE);
    } else {
        std::cout << "Could not allocate " << num_bytes << " of memory." << std::endl;
    }

    std::cout << "\nPress Enter to continue." << std::endl;
    getchar();
}

CodePudding user response：

You should explicitly specify memory size for string. That code:

char sentence[] = "";

declares sentence with max size is 0 ( 1 zero symbol). Of course, you write more data into not-your memory. Try this:

char sentence[200] = "";

CodePudding user response：

Instead of this:

    char sentence[] = "";
    std::cout << "Please enter your name: ";
    //fgets(sentence, 100, stdin);
    //scanf("%[^\n]%*c", sentence);
    //scanf("%[^\n]", sentence);
    std::cin >> sentence;
    num_bytes = strlen(sentence);

you better write this:

    constexpr std::streamsize BUFFER_SIZE { 100 };
    std::array<char, BUFFER_SIZE> sentence { }; // use the more modern std::array

    std::cout << "Please enter your name: ";
    std::cin.getline( sentence.data( ), BUFFER_SIZE ); // will only read 100 chars
                                                       // including spaces and tabs

    num_bytes = strlen(sentence.data( ));

and the rest is the same as your code. But now the user can enter space characters too and they can't cause a buffer overrun since std::cin.getline will only read a limited number of characters from standard input stream.