Problem with fscanf function while using *char

Question

My code looks like this:

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main()
{
    FILE* file;
    file = fopen("file.txt", "w+");
    char *tab;
    char *tab2;
    cout << "Input: " << endl;
    cin >> tab;
    fprintf(file, "%s", tab);
    rewind(file);
    fscanf(file, "%s", tab2);
    printf("%s", tab2);
    fclose(file);
}

So saving to file is working, but while I want to read from file to *tab2 it doesn't work and program instantly crashes. Any help with this?

You are operating on uninitialized pointers, that's _undefined behavior_. — πάντα ῥεῖ, Mar 16 '22 at 17:53
It's frankly a miracle the first half works. `tab` and `tab2` point to, erm, whatever they feel like pointing to. Could be valid memory, could be NULL, could be your operating system. See [Undefined behavior](https://stackoverflow.com/questions/2397984/undefined-unspecified-and-implementation-defined-behavior). — Silvio Mayolo, Mar 16 '22 at 17:53

score 1 · Answer 1 · edited Mar 16 '22 at 18:11

char *tab;
char *tab2;

These lines just declare pointers to unspecified addresses. When you try reading or writing through those pointers, you are invoking undefined behavior. You might get a crash, or you might not. Or you might just corrupt memory.

You need to allocate memory buffers and then make those pointers point to those buffers. But, because you are using C++, not just plain C, just use C++ ways of working with strings, reading files, etc. There are std::string and std::ifstream classes for this purpose.

Problem with fscanf function while using *char

1 Answers1