Why it is giving error "segmentation fault"?

Question

I want to input two strings at different memory locations but after taking the first input it shows an error "segmentation fault(core dumped"). I am not getting what's wrong with this code.

#include <stdio.h>
#include<iostream>
using namespace std;

int main()
{
    char *str;
    int i;
    scanf("%s",str+0);
    scanf("%s",str+1);
    return 0;
}

But when I only take one input it works fine.

#include <stdio.h>
#include<iostream>
using namespace std;

int main()
{
    char *str;
    int i;
    scanf("%s",str+0);

    return 0;
}

Why?

Why do you have `#include` and `using namespace std;`? They are used in C++, not C. — Spikatrix, May 01 '15 at 10:06
"*But when i take only one input it works fine.*" Both the code exhibits Undefined Behavior. And UB means anything can happen. It might work, might not, or do something else. Avoid UB at all costs. Don't rely on that behavior. — Spikatrix, May 01 '15 at 10:13
@ArunA.S , Why? It is certain that both the code is wrong as mentioned in my previous comment. — Spikatrix, May 01 '15 at 10:14
@CoolGuy , that is correct, but I was just wondering what the OP was intending to do with those two `scanf()`'s in his first code with `%s` — Arun A S, May 01 '15 at 10:16
naresh@ubuntu:~$ g++ c_concept.cpp naresh@ubuntu:~$ ./a.out naresh naresh@ubuntu:~$ — Naresh, May 01 '15 at 10:22

Arun A S · Accepted Answer · 2015-05-01T10:22:40.720

6

Because you do not allocate any memory to str before using it in your scanf().

You need to allocate some memory with malloc()

Both your codes exhibit Undefined Behavior as you try to access unsafe memory.

edited May 01 '15 at 10:22

answered May 01 '15 at 10:04

Arun A S

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Why it is giving error "segmentation fault"?

1 Answers1