String Linked List in C gives Segmentation Fault

Below is a program I'm trying to create for a phone directory. This is the basic skeleton. I plan to add some more functions, once I get through this SegFault error. Please help.

#include <stdio.h>
#include <stdlib.h>

struct node
{
    char* pd;
    struct node* next;
};
struct node* head;

void disp ()
{
    struct node* temp = (struct node*) malloc (sizeof (struct node));
    temp = head;
    while (temp != NULL)
    {
        printf ("%s\n", temp -> pd);
        temp = temp -> next;
    }
    free (temp);
}

void insert (char* s)
{
    if (head == NULL)
    {
        head -> pd = s;
    }
    else
    {
        struct node* temp = (struct node*) malloc (sizeof (struct node));
        while (temp -> next != NULL)
        {
            temp = temp -> next;
        }
        temp -> pd = s;
        free (temp);
    }
}

void del (char* s)
{
    if (head == NULL)
    {
        printf ("No element!");
    }
    else
    {
        struct node* prev = (struct node*) malloc (sizeof (struct node));
        struct node* temp = (struct node*) malloc (sizeof (struct node));
        temp = head;
        while (temp -> pd == s)
        {
            prev = temp;
            temp = temp -> next;
        }
        prev -> next = temp -> next;
        free (temp);
        free (prev);
    }
}

main ()
{
    int c;
    char* str;
    do
    {
        printf ("1: Display\n2. Add Contact\n3. Delete Contact\n0. Exit\n");
        scanf ("%d", &c);
        switch (c)
        {
            case 1:
                disp ();
                break;
            case 2:
                //printf ("lol");
                scanf ("%s", str);
                insert (str);
                break;
            case 3:
                scanf ("%s", str);
                del (str);
                break;
            default:
                printf ("Try Again! :(");
        }
    } while (c != 0);
}

Error: Segmentation Fault (Core Dumped)

I can't insert anything into the list. It doesn't even seem to enter the function.