Increment integer pointer in a loop in C - c

Question

My original code to display nodes in a queue in order from lowest to greatest:
void display (queue *q) {
node *ptr = q->front;
int i = 0;
int size = q->size;
while (i <= size) {
while (ptr->id != i) {
ptr = ptr->next;
}
if (i == 0) {
printf("%d ", ptr->id);
printf("%d\n", ptr->running);
}
else {
printf("%d ", ptr->id);
}
i++;
ptr = q->front;
}
}
Had kept producing Segmentation Fault (Core Dumped) errors. I have malloc the two variables being compared and this error has been fixed.
void display (queue *q) {
node *ptr = malloc(10);
ptr = q->front;
int *i = NULL;
i = malloc(sizeof(int));
*i = 0;
int size = q->size;
while(*i <= size){
while (ptr->id != *i) {
ptr = ptr->next;
}
if (*i == 0) {
printf("%d %d\n", ptr->id, ptr->running);
}
else {
printf("%d %d %d %d\n", ptr->id, ptr->running, ptr->ready, ptr->blocked);
}
i = i + 1 * (sizeof(char));
ptr = q->front;
}
}
However now this doesn't produce the output that I want. I want to increment the i pointer so that it can be the same as my original code.
This has been immensely frustrating, any help would be greatly appreciated!

Answer 1

If I read your first code listing correctly, there's at least one important thing here you need to think about. You seem to have a linked list here, and you're iterating over that list using ptr = ptr->next. This means you need to know when to stop. One common way of setting up a linked list is that the last item in the list has a next value of NULL. Then process the nodes one at a time, and once you are done with one node, you you check whether the next value is NULL: if it isn't you can move on that that next node, if it is NULL you stop.
Here you're not doing checks like this, so you need another way to ensure that you know when to stop. What you seem to be doing is taking the value of q->size and using that to inform you how many items there are in the linked list. So the first thing to think about is how confident you are that that value is correct. For example, if the code building the list puts only two items into the list, bet sets size to three, you'll end up falling off the end of the list, and a segmentation fault is not unlikely.
But there's something even more important than that. Even if you're getting the correct number of items in the list from q->size, you're comparing your loop variable i to size like this:
int i = 0;
while (i <= size) {
⋮
}
This is going to loop with i having the values [ 0, 1, … size ], which is actually size + 1 times. If you want to loop exactly size times, you want a test like i < size rather than i <= size. This is a common mistake, often called an off-by-one error.
Unfortunately, your second listing complicates things, rather than making them better. Go back to your first one and see if you can fix the things I've mentioned here.