C undefined behavior - Singly Linked List - c

Question

I've implemented a Singly Linked List, and I'm noticing really strange behavior but can't pinpoint the exact reason why it's happening. I've tried using gdb to figure out what the problem is, and it looks like whenever I compute the size of a list, that's when things start to go wrong. This is the program I'm using to test out my implementation, and following is the actual implementation.
#include <stdio.h>
#include "singlylinked.h"
slist initialize(void); /* initializes test singly linked list */
slist initialize(){
int i, a[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
slist l = sl_alloc();
int a_size = sizeof(a)/sizeof(a[0]);
for(i=0;i<a_size;i++){
sl_add(l, (void *)&a[i]);
}
return l;
}
int main(){
slist l = initialize();
int i, size = sl_size(l);
for(i = 0; i < size; i++){
printf("%d ", *(int *)sl_get(l,i));
}
fputc('\n',stdout);
return 0;
}
And now the actual implementation, I'm only going to post the methods used in the testing:
/* allocates an empty slist */
slist sl_alloc(){
return (slist) calloc(1, sizeof(struct node));
}
/* adds val to linked list */
void sl_add(slist l, void *val){
node *current, *new;
new = calloc(1, sizeof(struct node));
new->content = val;
new->next = NULL;
if((current = *l)){
while(current->next){
current = current->next;
}
current->next = new;
} else {
*l = new;
}
}
/* returns ith value in singly linked list l */
void *sl_get(slist l, int i){
node *current;
int j; /* counter */
void *result = NULL;
if((current = *l)){
int size = sl_size(l);
if(i < size){
for(j = i; j > 0; j--){
current = current->next;
}
result = current->content;
}
}
return result;
}
/* returns the size of the singly linked list */
int sl_size(slist l){
int size = 0;
node *current;
if((current = *l)){
do {
size++;
current = current->next;
} while (current);
}
return size;
}
And now this is how I define slist and struct node:
typedef struct node **slist;
typedef struct node {
void *content;
struct node *next;
} node;
EDIT: The strange behavior is this: When I try to print things out, it prints out the wrong values in the list. When I ran the program using gdb, this starts to happen after the first call to sl_size.

Answer 1

The problem is your initialisation of the list.
You add the 10 elements of the array a[] to the list that you are creating in initialize(). The only problem is that you store in your list nodes pointers to the data in the array a[]. Unfortunately this array is local to the function ! As soon as you return from initialize(), this array is no longer valid, and the pointer point to no valid place anymore. In consequence, the numbers that you expect to point to will be replaced by "garbage" value.