Thread: Why did the value of my void pointer change?

I have a the following in a header file.

Code:

struct SortedList
{
	void * data;		 
	struct SortedList * next; 		
	struct SortedList * previous;
	int (*compareFunc)(void *, void *);
	void (*destructor)(void *);
};
typedef struct SortedList* SortedListPtr;

SortedListPtr SLCreate(CompareFuncT cf, DestructFuncT df);

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In a .c file, I implemented the SLCreate function.

Code:

SortedListPtr SLCreate(CompareFuncT cf, DestructFuncT df)
{
	struct SortedList item; 		
	item.data = NULL;				
	item.next = (struct SortedList *) malloc(sizeof(struct SortedList));
	item.previous = (struct SortedList *) malloc(sizeof(struct SortedList));
	item.compareFunc = cf;
	item.destructor = df;
	SortedListPtr ptr = (struct SortedList *) malloc(sizeof(struct SortedList));	
	ptr = &item;
	return ptr;
};

In main.c, I try to do the following:

Code:

SortedListPtr list = SLCreate(&compareInts, &destroy);

...... A bunch other code that does not alter list or it's contents at all. 



struct SortedList item = (*list);
void * data = item.data;

if (data != NULL)	
{
	printf("Why did data become not null???\n");
}

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So my problem is how come my variable data became not null anymore when I haven't altered it at all and how can I fix this??

'item' is a localized variable in function SLCreate. You can not expect its contents to remain after you exit the function as it is on the stack only. You are attempting to return its address - but contents are likely corrupted upon exit.