Segmentation Fault when using Linked Lists and Char arrays - c

Question

I'm struggling through my programming homework and need a hand with Linked Lists. Basically we have to create a program that contains certain functions to manage linked lists. Fairly standard. I tried it last week and just couldn't get it. I'm trying to get it working this weekend and was making good progress until I ran into segmentation faults.
struct STUDENT
{
char *FirstName;
char *LastName;
char *PUID;
int age;
struct STUDENT *next;
};
This is the structure I'm trying to use. The first three values are char arrays and the fourth is just a number. Following this I tried to declare a starting and current node globally.
struct STUDENT *head = NULL;
struct STUDENT *curr = NULL;
Following this I have my create node function, which takes a user input and puts it into the list.
void *createListNode()
{
char first[MAXNAME];
char last[MAXNAME];
char ID[MAXID];
char *pfirst;
char *plast;
char *pID;
int tage;
struct STUDENT *temp = (struct STUDENT *) malloc (sizeof(struct STUDENT));
printf("Enter a first name: ");
scanf("%s", first);
pfirst = first;
printf("entered name: %s\n", pfirst);
printf("Enter a last name: ");
scanf("%s", last);
plast = last;
printf("entered name: %s\n", plast);
printf("Enter the Purdue ID: ");
scanf("%s", ID);
pID = ID;
printf("ID: %s\n", pID);
printf("Enter an age: ");
scanf("%d", &tage);
printf("age: %d\n", tage);
temp->FirstName = strdup(first);
printf("first\n");
temp->LastName = strdup(last);
printf("last\n");
temp->PUID = strdup(ID);
printf("id\n");
temp->age = tage;
printf("age\n");
temp->next = NULL;
printf("next\n");
if (curr == NULL)
{
printf("inside if\n");
head->next = temp; //-------SEGMENTATION FAULT---------------------
printf("line 107\n");
head = curr = temp;
printf("line 109\n");
}
else
{
curr = temp;
}
}
Background of what I'm done up to this point:
I was getting segmentation faults when I tried to assign my arrays to my 'temp' node, but by using malloc on it I solved that problem. Using print statements, I've tracked the problem to the indicated line. When I try to run the code, I get a segmentation fault. I tried the same malloc code above on the 'head' and 'curr' nodes, but that gave me:
carlton#carlton-Inspiron-N7010:~/CNIT315$ gcc lab5.c
lab5.c:20:64: error: invalid initializer
struct STUDENT head = (struct STUDENT *) malloc (sizeof(struct STUDENT));
This is where I believe the problem is, but I've been searching and experimenting for several hours and haven't gotten anywhere. And I just realized this is now a very long post. Thanks for reading it through and I appreciate any push in the right direction!

Answer 1

You initialize head to NULL. You can't dereference head before assigning a valid pointer to it. head->next = temp; gives you a segfault because head is a null pointer.
Thus, you need to review this part of the code:
if (curr == NULL)
{
printf("inside if\n");
head->next = temp; //-------SEGMENTATION FAULT---------------------
printf("line 107\n");
head = curr = temp;
printf("line 109\n");
} else
{
curr = temp;
}
Since it seems like you want to insert at the end of the list, I would suggest something like this:
temp->next = NULL;
if (head == NULL) {
curr = head = temp;
} else {
curr->next = temp;
curr = temp;
}
This makes curr always point to the last node, and head always points to the beginning of the list.

Answer 2

Am I wrong or do you set head->next = temp; but never set head to a valid object before?
You either have to initialize head with an 'empty' Student at the beginning or you just have to set head = temp;

Answer 3

Assuming that you've defined the head variable outside the createListNode() function, here's the problem:
Memory is never assigned to it, since you have never called malloc()