Trouble Iterating over Linked List

[joatmon]

I have a singly-linked list. I am confident that I have the list set up and the data populated. However, when I go to print out the contents of the list, I'm not able to traverse it.

int main(int argc, char *argv[]) {
    
...

    Neighbor *cursor, *root_ptr;
    Neighbor this_neighbor;

    // open the initialization file
    if ((file = fopen(argv[5], "r")) == NULL) {
        fprintf(stderr, "Failed to open file: %s\n", argv[5]);
		time(&starttime);
        exit(-1);
    }
    
    root_ptr = malloc(sizeof(Neighbor));
    root_ptr->next = &this_neighbor; //keep a pointer to the root
    while ( fgets ( line, sizeof line, file ) != NULL ) /* read a line */
    {

        char* p;
        p = strtok(line, ",");
        p++;

        if (*p == *argv[1]) {

            p = strtok(NULL, ",");
            this_neighbor.send_to_using_port = atoi(p);

            p = strtok(NULL, ",");
            memcpy(this_neighbor.host, p, 1);

            p = strtok(NULL, ",");
            this_neighbor.neighbor_receives_on_port = atoi(p);
            
            p = strtok(NULL, ",");
            this_neighbor.link_cost = atoi(p);

            fprintf(stdout, "host: %s\tsend on: %d\t receives on: %d\tcost: %d\n", this_neighbor.host, this_neighbor.send_to_using_port, this_neighbor.neighbor_receives_on_port, this_neighbor.link_cost);

            Neighbor next_neighbor;
            this_neighbor.next = &next_neighbor;
            this_neighbor = next_neighbor;
            
        }
        
        this_neighbor.next = NULL;
    }

    // iterate over the lines in the file as a test
    // THIS IS THE PART THAT ISN'T WORKING
    cursor = malloc(sizeof(Neighbor));
    cursor = root_ptr;
    fprintf(stdout, "here\n");
    fprintf(stdout, "%s\n", root_ptr->host); //<-- BLANK LINE
    i = 1;
    while (cursor != NULL) {
        fprintf(stdout, "iteration number: %d\n", i);
        fprintf(stdout, "%s\n", cursor->host);        
        cursor = cursor->next;
        i++;
    }
    free (cursor);

...
    return 0;
}

My output looks like this:

host: F send on: 6712 receives on: 7045 cost: 2
host: G send on: 6713 receives on: 7046 cost: 2
host: H send on: 6714 receives on: 7047 cost: 2
Send Receive Link
Neighbor Port on Port Cost
-------- ----- -------- ----

iteration number: 1

iteration number: 2
(blank line here)

So you can see that wherever I am trying to output a value from my list (were I am trying to iterate over it), I am getting a blank line. Further, the loop only iterates twice where it should iterate 3 times.

What am I doing wrong? Thanks!

[tabstop]

You should sit down with a piece of paper and graph out what your code for adding a new node is doing. Remember when you hit that brace on line 44 (in the posted code) you need to erase your next_neighbor node, as it will cease to exist at that point (every variable only exists inside the set of braces it is declared in). I suspect you'll find that you have "a big mess" when you are done.

[joatmon]

Thanks. How then do I create a new node that will survive beyond the scope of the current iteration? I'm very confused.

[Salem]

Each new node on your linked list needs to be obtained by calling malloc.

[joatmon]

Thanks. I tried that method many times, but never successfully. I'm giving up. Linked lists hurt my brain. Arrays here I come.