CODEWITHSUNDEEP
cpointerslinked-listsegmentation-fault
Kyu Bi
Kyu Bi

Reputation: 31

Linked list segmentation fault in C

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?

Upvotes: 2

Views: 188

Answers (2)

Jonathan Leffler
Jonathan Leffler

Reputation: 753475

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.

Upvotes: 0

Paul Beckingham
Paul Beckingham

Reputation: 14895

This line:

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

is dereferencing

dizi[k].next

and I suspect that has a junk value.

Upvotes: 3

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