Segmentation fault(core dumped) in linked lists C

Question

I have a problem in this piece of code with linked lists. It gives me the error:

 Segmentation Fault (Core Dumped)

Can anyone see what seems to be the problem? Thanks in advance.

pos stands for position and root for the first element

typedef struct
{
  element *root;
  int size;
} list;

typedef struct _element
{
  char *str;
  struct _element *next;
} element;

int
insert_list (list * lst, const char *value, int pos)
{
  element *new;

  int k;
  new = malloc (sizeof (element));

  for (k = 0; k < pos; k++)
    {
      new = lst->root;
      lst->root = lst->root->next;

      if (k == pos - 1)
        {
          lst->root = NULL;
          new->str = value;
        }
    }

  for (k = 0; k <= lst->size; k++)
    {
      new = lst->root;
      lst->root = lst->root->next;

      if (k == lst->size)
        {
          lst->root = NULL;
          new->str = value;
        }
      if (pos < 0 || pos >= lst->size)
        return -1;
      else
        return pos;
    }
}

Hello, you are right, i forgot to declare the variable, but the problem still maintains, any idea? — Bruno S.
– Bruno S., Commented Mar 14, 2014 at 17:17
The function is called correctly, since it was already given to us, we only need to develop the functions — Bruno S.
– Bruno S., Commented Mar 14, 2014 at 17:18
lst == NULL ? and new = malloc(sizeof(element)); for(k=0;k<pos;k++) { new = lst->root; is weird — willll
– willll, Commented Mar 14, 2014 at 17:18

abligh · Accepted Answer · 2014-03-14 18:41:13Z

Let's see how to debug this.

First of all, compile your code with warnings, and read the warnings:

$ gcc -Wall x.c -o xx.c:6:3: error: unknown type name ‘element’
x.c: In function ‘insert_list’:
x.c:26:11: warning: assignment from incompatible pointer type [enabled by default]
x.c:27:28: error: request for member ‘next’ in something not a structure or union
x.c:32:20: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
x.c:38:11: warning: assignment from incompatible pointer type [enabled by default]
x.c:39:28: error: request for member ‘next’ in something not a structure or union
x.c:44:20: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
x.c:51:1: warning: control reaches end of non-void function [-Wreturn-type]

All of those things are problems, and 2 (the errors) indicate the code you posted would never have compiled

So, you have problems beyond a segmentation fault. The first one is this line:

 lst->root = lst->root->next;

This is because you need to define the element struct before the list struct.

Apart from this, your insert routine is broken:

new = malloc (sizeof (element));

for (k = 0; k < pos; k++)
  {
    new = lst->root;

Here you are overwriting your newly allocated element many times.

On the next line you overwrite the root:

  lst->root = lst->root->next;

I'm afraid I can't even figure out what you are trying to do here. If the goal is to insert an element at position pos in a singly linked list, what you want to do is:

Allocate a new element n.
If pos is zero, make your new element n the root, and make n->next point to the current root, and you are done.
Else iterate pos-1 times along the list, call that x.
Make n->next->next = x->next (if n->next exists)
Make n->next = x

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