Linked list segmentation fault in C

Question

I have problem with linked lists. I have two structures:

struct ekstra
{
    char isim[256];
    int deger;
    struct ekstra *sonra;
};

struct node
{
    char name[256];
    int val;
    struct ekstra *next;    
};

and I have these:

struct ekstra *tmp;
struct node dizi[12];

Somewhere in my code there is

tmp = dizi[k].next;
tmp=tmp->sonra;

and if I do this:

tmp = malloc(sizeof(struct ekstra));

there is no problem.

But if I do this:

dizi[k].next->sonra = malloc(sizeof(struct ekstra));

I get a SegFault. Why is this happening?

You're best off running the code with the help of a debugger. On linux, valgrind is great for pointing out segmentation faults. Also, on Windows, the visual studio debugger is quite useful. — Tom
– Tom, Commented Jun 7, 2012 at 23:25
@GeorgeGaál Please don't ask for the full code, but more complete code that can reproduce the problem. — Drise
– Drise, Commented Jun 7, 2012 at 23:28
@Drise That is to say "the least amount of complete code that can reproduce the problem." I agree with George that you need to ensure that k is within the bounds of the array and also ensure that next is not null? The GNU Debugger would also be useful for troubleshooting this. — Rob Kielty
– Rob Kielty, Commented Jun 7, 2012 at 23:33

Paul Beckingham · Accepted Answer · 2012-06-08 00:04:16Z

3

This line:

dizi[k].next->sonra = malloc(sizeof(struct ekstra));

is dereferencing

dizi[k].next

and I suspect that has a junk value.

answered Jun 8, 2012 at 0:04

Paul Beckingham

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Jonathan Leffler · Accepted Answer · 2012-06-08 00:11:09Z

0

These two lines:

tmp = dizi[k].next;
tmp = tmp->sonra;

probably copy an invalid pointer into tmp. When you assign to tmp with malloc(), the valid pointer from malloc() overwrites the invalid value already in tmp.

When you use:

dizi[k].next->sonra = malloc(sizeof(struct ekstra));

you are referencing the invalid pointer (to evaluate the address where the sonra member is stored), and that is causing the segmentation fault.

If you wrote:

*tmp = 0;

you would probably get a segmentation fault too.

answered Jun 8, 2012 at 0:11

Jonathan Leffler

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