Segmentation fault - finding the root of a list

Question

I am getting a segmentation fault when running the following code. I know there is something fundamental I'm misunderstanding about pointers here, but I can't seem to figure it out.

#include <iostream>
using namespace std;

struct intptr{
  intptr* a;
  int value;
};

void printint(intptr q){
  if (q.a == &q){
    cout<<q.value<<endl;
    return;
  }
  else
    printint(*(q.a));
}

int main(){
  intptr y,z;
  z.value = 1;
  y.value = 2;
  *(y.a) = z;
  *(z.a) = z;
  printint(x);
}

I've also tried the following, but it never recognizes that q.a = &q is true.

#include <iostream>
using namespace std;

struct intptr{
  intptr* a;
  int value;
};

void printint(intptr q){
  if (q.a == &q){
    cout<<q.value<<endl;
    return;
  }
  else
    cout<<"not finished"<<endl;
    printint(*(q.a));
}

int main(){
  intptr y,z;
  z.value = 1;
  y.value = 2;
  y.a = &z;
  z.a = &z;
  printint(y);
}

In your first example, you declare the pointer `intptr* a` in the struct but never give it a valid address (i.e., never allocate an address for it). So it points to an undefined location, and `*(q.a) = z` will crash. In both cases, `q` is a parameter on the stack of the function, so its address won't match the pointer variable, `a`. — lurker, Feb 26 '14 at 02:57
changing the function declaration to accept a parameter passed by reference solved the problem. -- void printint(intptr &q) — Matt Pennington, Feb 26 '14 at 03:01

score 3 · Answer 1 · edited May 23 '17 at 12:24

3

Here is your problem:

  *(y.a) = z;
  *(z.a) = z;

You are dereferencing a null pointer. The a variable was never initialized and points to nothing. If you are going to be using pointers, I strongly recommend following the rule of 3.

edited May 23 '17 at 12:24

Community

1
1

answered Feb 26 '14 at 02:57

yizzlez

8,757
4
29
44

score 0 · Answer 2 · answered Feb 26 '14 at 05:05

To find the issue and avoid similar issues in future, it is recommended to initialized pointers to NULL and later check for NULL before operating them. I recommend you to add a constructor to your struct and initialize pointer a:

struct intptr
{
   intptr* a;
   int value;

   intptr()
   : a(NULL)
   { }
};

score 0 · Answer 3 · answered Feb 26 '14 at 05:13

0

The problem has been pointed by @awesomeyi

Try the following codes:

  y.a = &z;
  z.a = &y;

answered Feb 26 '14 at 05:13

Steve

1,448
11
18

Segmentation fault - finding the root of a list

3 Answers3