Recursive Pointer to Element function gives Segmentation Fault (11)

Question

My little Program is build this way:

I have an Object Element that is defined as following:

struct element {
    element* next;
} typedef element;

My Program is calling this recursive function...

int dosomething(element* a) {
    if (a != NULL)
        return dosomething(a->next);
    else
    return 0;
}

In the main function:

int main(void) {
    element* a = (element *) malloc(sizeof(element));
    a-> next = (element *) malloc(sizeof(element));

    cout << dosomething(a) << endl;

    return 0;
}

When running this Program, I get:

Segmentation fault: 11

So, a and a->next is defined. a->next->next should be NULL, so the function should just return 0?

Why isn't this working?

`malloc` does not intialise the memory in any way. So `a->next->next` cannot be assumed to be `NULL`. — kaylum, Dec 07 '15 at 00:09
Please do not tag questions as both C++ and C; only tag the language you are using. — Colonel Thirty Two, Dec 07 '15 at 00:25

score 1 · Answer 1 · answered Dec 07 '15 at 00:08

1

You would have to initialize the next-pointer with 0, otherwise it is uninitialized and "a != NULL" will not be true under some circumstances. You could use calloc instead of malloc, which initializes the memory with 0

answered Dec 07 '15 at 00:08

Ctx

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score 1 · Accepted Answer · edited May 23 '17 at 10:33

a->next->next is not NULL. ```malloc`` does not gurantee that the allocated memory region will be initialized to something (i.e. it has the value of what has previously been there).

Also, Do I cast the result of malloc?

int main(void) {
    element* a = (element *) malloc(sizeof(element));
    a-> next = (element *) malloc(sizeof(element));
    a->next->next = NULL;

    cout << dosomething(a) << endl;

    return 0;
}

score 0 · Answer 3 · answered Dec 07 '15 at 00:18

You weren't making the last element NULL - malloc doesn't do this for you. You should use C++ constructs, too. They're very helpful.

#include <iostream>

struct Element {
  Element* next;
};

int do_something(Element* a) {
  if(a) {
    do_something(a->next);
  }
  return 0;
}

int main() {
  Element* last = NULL;
  Element* next = new Element;
  next->next = last;
  Element* first = new Element;
  first->next = next;
  std::cout << do_something(first) << std::endl;
  delete first;
  delete next;
  delete last;
  return 0;
}

ideone

Recursive Pointer to Element function gives Segmentation Fault (11)

3 Answers3