Variables Get Changed after First Call - Undefined Behavior in C?

Question

I am creating a Linked List Queue of a struct type. The problem is when I enQueue an object and print the Linked List it displays fine. Enqueueing another item and THEN printing gives me garble. I used GDB and found the variables are getting changed to a bunch of garble and I anticipate that this is due to undefined behavior but I'm having trouble figuring out what I need to do to FIX this problem.

Here's where the variables are defined by the user

printf("First name of employee?\n");
char firstName[MAX_LENGTH];
scanf(" %s", &firstName);
printf("Last name?\n");
char lastName[MAX_LENGTH];
scanf(" %s", &lastName);

if(head->next == NULL) //if there is currently no employee in the list
    head->next = hireEmployee(head, lastName, firstName, employeeCount);
else
{
    Employee *tmp;
    head->next = tmp;
    while(tmp->next != NULL)
    {
        tmp = tmp->next;
    }

    hireEmployee(tmp, lastName, firstName, employeeCount);
}

Here is part of the enQueue operation.

Employee *new = malloc(sizeof(Employee));
strcpy(new->lastName, lastName);
strcpy(new->firstName, firstName);

And finally here is my print method.

Employee *tmp;
tmp = head->next;
if(head->next == NULL)
    printf("Nothing in this list.");
else
{

    printf("%s, %s\nEmployee Number: %i\n", tmp->lastName, tmp->firstName, tmp->employeeNumber);
    while(tmp->next != NULL)
    {
        printf("%s, %s\nEmployee Number: %i\n", tmp->lastName, tmp->firstName, tmp->employeeNumber);
        tmp = tmp->next;
    }
}

What do I need to be doing to fix this? I have ideas of where the undefined behavior may be occurring but NO idea what I should be doing instead.

z242 · Accepted Answer · 2013-10-26 02:06:34Z

2

This part is a problem:

Employee *tmp;

At this point tmp is a pointer whose contents are undefined.

head->next = tmp;

Now you just stored the undefined contents into head-> next.

while(tmp->next != NULL)

And now you just dereferenced your undefined pointer.

answered Oct 26, 2013 at 2:06

z242

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