Setting a pointer to null segfault

Question

I have a struct called node as follows:

struct node {
    int data;
}

stored in some structure:

struct structure {
  struct node *pointer;
}

I'm trying to set pointer to NULL as follows:

struct structure *elements;
elements->pointer = NULL;

Why does this segfault? Does it actually attempt to dereference the pointer before setting it to null?

When I switch elements from a pointer to the actual struct and do the following:

struct structure elements;
elements.pointer = NULL;

It stops segfaulting and works. Why doesn't setting a pointer to null work?

wow, this question was resolved quickly

Hans Z
– Hans Z

2012-07-02 14:29:50 +00:00
Commented Jul 2, 2012 at 14:29 — Hans Z
– Hans Z, Commented Jul 2, 2012 at 14:29

ouah · Accepted Answer · 2012-07-02 14:27:15Z

7

struct structure *elements;
elements->pointer = NULL;

elements pointer points to nowhere. Dereferencing an invalid pointer (elements pointer) is undefined behavior.

You need to initialize elements pointer to a valid object, like:

struct structure my_struct;
struct structure *elements = &my_struct;
elements->pointer = NULL;

answered Jul 2, 2012 at 14:27

ouah

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Comments

RedX · Accepted Answer · 2012-07-02 14:27:56Z

4

You need to initialize the pointer

struct structure *elements = malloc(sizeof(struct structure));

If you don't do this it will point to a arbitrary memory location.

answered Jul 2, 2012 at 14:27

RedX

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Comments

Coffee on Mars · Accepted Answer · 2012-07-02 14:54:16Z

2

The invalid pointer you're derefrencing, thus the segfault, is not elements->pointer, but elements itself. Since it has not been set (e.g.: by a malloc), it could point to any location in memory.

edited Jul 2, 2012 at 14:54

answered Jul 2, 2012 at 14:28

Coffee on Mars

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1 Comment

Graham Borland Over a year ago

He's dereferencing an uninitialized pointer. It may or may not be NULL.

Hans Z · Accepted Answer · 2012-07-02 14:27:20Z

1

you didn't initialize *elements.

*elements right now points to nothing, so elements->pointer is dereferencing nothing, which gives you your segfault.

answered Jul 2, 2012 at 14:27

Hans Z

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2 Comments

Graham Borland Over a year ago

He's dereferencing an uninitialized pointer. It may or may not be null.

Hans Z Over a year ago

Stop downvoting everyone on a technicality, @Graham, and I've editted my answer.

Graham Borland · Accepted Answer · 2012-07-02 14:27:20Z

0

elements hasn't been initialized to point to anything yet.

answered Jul 2, 2012 at 14:27

Graham Borland

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