Segmentation-Fault in C--Linked List - c

Question

I understand that a set fault happens when you try to access memory out of the scope of your program, but I can't figure out where mine is happening, or why. I have a list defined:
*ListType list = NULL; ,
and then I add data to the List calling the add function in main:
addNodeToList(&list, song);
void addNodeToList(ListType **list, SongType *song)
{
printf("Starting Add");
NodeType *newNode = malloc(sizeof(NodeType));
NodeType *currNode;
currNode = (*list)->head;
newNode->data = song;
newNode->next = NULL;
if(currNode == NULL) {
printf("List is Empty");
(*list)->tail = newNode;
(*list)->head = newNode;
}
else {
(*list)->tail->next = newNode;
(*list)->tail = newNode;
}
}
The song I'm passing is properly initialized, and I can access it's elements, so I know that isn't causing the seg fault. Any help would be very appreciated!
typedef struct Song {
char title[MAX_STR];
char artist[MAX_STR];
char album[MAX_STR];
char duration[MAX_STR];
} SongType;
typedef struct Node {
struct Node *next;
SongType *data;
} NodeType;
typedef struct List {
NodeType *head;
NodeType *tail;
} ListType;

Answer 1

in main if you're declaring
ListType* list = NULL;
then in your function you're calling
(*list)->head
dereferencing (*list) is fine, but as soon as you do ->head it's trying to dereference that original NULL assignment. You need to allocate some space for list first.

Answer 2

It appears that your calling code contains:
ListType *list = NULL;
and you call your function like this:
addNodeToList(&list, song);
and in your addNodeToList() function you have:
NodeType *currNode;
currNode = (*list)->head;
which means you are dereferencing a null pointer (*list), which crashes your code. At minimum, check whether *list == NULL before setting currNode, but you'll need rethink the code to handle the case where list is a null pointer.
This code compiles, and runs. It avoids the problem by allocating the list when necessary:
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
enum { MAX_STR = 64 };
typedef struct Song
{
char title[MAX_STR];
char artist[MAX_STR];
char album[MAX_STR];
char duration[MAX_STR];
} SongType;
typedef struct Node
{
struct Node *next;
SongType *data;
} NodeType;
typedef struct List
{
NodeType *head;
NodeType *tail;
} ListType;
void addNodeToList(ListType **list, SongType *song);
void addNodeToList(ListType **list, SongType *song)
{
printf("Starting Add\n");
assert(list != NULL);
if (*list == NULL)
{
*list = malloc(sizeof(**list));
assert(*list != 0); // Too lazy for production
(*list)->head = 0;
(*list)->tail = 0;
printf("List created\n");
}
NodeType *newNode = malloc(sizeof(NodeType));
newNode->data = song;
newNode->next = NULL;
printf("Node created\n");
NodeType *currNode = (*list)->head;
if (currNode == NULL)
{
printf("List is Empty\n");
(*list)->tail = newNode;
(*list)->head = newNode;
}
else
{
(*list)->tail->next = newNode;
(*list)->tail = newNode;
}
printf("Node added - all done\n");
}
int main(void)
{
ListType *list = NULL;
SongType data = { "Title", "Artist", "Album", "2m 30s" };
SongType *song = &data;
printf("Add song once\n");
addNodeToList(&list, song);
printf("Add song again\n");
addNodeToList(&list, song);
return 0;
}
Example run:
Add song once
Starting Add
List created
Node created
List is Empty
Node added - all done
Add song again
Starting Add
Node created
Node added - all done
The code leaks like a sieve; no memory is freed.

Answer 3

The problem deals with this statement
ListType *list = NULL;
You may not call addNodeToList for a list initialized such a way. Otherwise there will be undefined behaviour.
You should declare the list the following way
ListType list = { NULL, NULL };
Also the function addNodeToList can be declared simpler
void addNodeToList(ListType *list, SongType *song);
^^^^^^^^^^^^^^
You can call it like
addNodeToList( &list, song );
For example
void addNodeToList( ListType *list, SongType *song )
{
printf("Starting Add");
NodeType *newNode = malloc( sizeof( NodeType ) );
if ( newNode != NULL )
{
newNode->data = song;
newNode->next = NULL;
if ( list->tail == NULL )
{
printf( "List is Empty\n" );
list->head = list->tail = newNode;
}
else
{
list->tail->next = newNode;
list->tail = newNode;
}
}
}
Or if you want to have a pointer to list then you should write
ListType *list = malloc( sizeof( ListType ) );
list->head = list-tail = NULL;;
and then call the function the definition of which I showed the following way
addNodeToList( list, song );