Can't find error in string manipulation/pointers

Question

So first off, this is a homework assignment, so please don't write any code for me, just point out where my code is wrong.

The basics of the code is that it's a 'address/balance' book. I've got a struct with my variables, but for some reason, my doubly linked list is getting all messed up and I can't figure out how. With a bit of fancy (not really) debugging, I've figured out that I think the fgets(string, 200, file); line is overwriting my head->name pointer somehow, which seems to throw the rest of the code off. Relevant code snippets here:

Populating the List:

void populate_list(char* filename){
    FILE *file = NULL;
    char* name = NULL;
    char* streetaddress = NULL;
    char* city = NULL;
    char* state = NULL;
    char line[200];
    int zip;
    float balance;

    file = fopen(filename,"r");

    if(file==NULL){
        printf("Invalid input file\n");
        exit(1);
    }
    if(file == 0){
        printf("Invalid file.\n");
        exit(1);
    }

    while(!feof(file)){

        fgets(line, 200, file);

        name = strtok(line, ",");
        streetaddress = strtok(NULL, ",");
        city = strtok(NULL,",");
        state = strtok(NULL,",");
        zip = atoi(strtok(NULL,","));
        balance = atof(strtok(NULL,","));

        strip_spaces(name);
        strip_spaces(streetaddress);
        strip_spaces(city);
        strip_spaces(state);

        add_node(name, streetaddress, city, state, zip, balance);

    }

    fclose(file);
    return;
}

Then the add_node code:

void add_node(char* name, char* streetaddress, char* city, char* state, int zip, float, balance){
    struct customer* addnode = NULL;

    if(find_duplicate(name)){
        print_filler(1);
        printf("DUPLICATE RECORD: %s\n", name);
        return;
    } else {
        addnode = (struct customer *) malloc(sizeof(struct customer));
        addnode->name = name;
        addnode->streetaddress = streetaddress;
        addnode->city = city;
        addnode->state = state;
        addnode->zip = zip;
        addnode->balance = balance;

        if(head == NULL) {
            head = addnode;
            addnode->prev = NULL;
        } else {
            tail->next = addnode;
            addnode->prev = tail;
        }

        tail = addnode;
        addnode->next = NULL;
    }

    print_list();
    return;
}

The bug seems to be happening after the first time add_node is called and happens when fgets() goes off a second time. It's overwriting head->name with the entire fgets() line for some reason.

There other random irritating bugs I haven't pinned down yet, but this one I think may be the source of the others.

Full code is here, if it helps: http://pastebin.com/k0pqyvT0

My guess is it's something to do with the head->name pointer being overwritten by fgets(), but I can't for the life of me figure out what exactly is doing this, any help/advice would be appreciated.

Edit: Solution was to implement strdup() and duplicate the strings when they're passed to add_node(). Thanks for all the answers.

Answer 1

1. The variable "string" is a pointer to nothing. You need to actually allocate a buffer into which you read the data.
2. strtok returns a pointer to the original string, which then changes. You need to allocate a new buffer and copy the string into it if you wish to keep it around in your customer structure.