c++ program crashes linked list?

Question

                if(tmpPtr->number<tmpPtr->next_number->number)
                {
                    int tmpV1=tmpPtr->next_number->number;
                    int tmpV2=tmpPtr->number;
                    tmpPtr->next_number->number=tmpV2;
                    tmpV2=tmpPtr->number=tmpV1;
                }

This is what I have tried so far, this is supposed to be sorting the linked list as member are being added each time. But when the compiler crashes when I try to put in the second node. The break point is the if statement, if(tmpPtr->number<tmpPtr->next_number->number). I tried really hard to figure out what the problem was, but couldnt.

I'm guessing tmpPtr->next_number is NULL or uninitialized. — David Brown
– David Brown, Commented Mar 12, 2013 at 23:23
@DavidBrown I am guessing it is not properly linked. Any suggestions on how to fix it? — user2155612
– user2155612, Commented Mar 12, 2013 at 23:27
@ThomasMatthews the break point was if(tmpPtr->number<tmpPtr->next_number->number) — user2155612
– user2155612, Commented Mar 12, 2013 at 23:27

dwalter · Accepted Answer · 2013-03-13 00:11:55Z

2

Your problem is that on the second run tmpPtr points to your first element which has a next_number value of NULL. So as soon as you try to dereference it, it will basically reduce itself to a NULL pointer which leads to a SIGSEGV.

after the first run

n->number = input
n->next_number = NULL
h = n
t = n
counter2 = 1

so starting with the second input

n->number
n->next_number = NULL
tmpPtr = h // which is the previous n and therefor h->next_number = NULL
tmpPtr->next_number == NULL // this is your problem since you do not check if next_number is a valid pointer

UPDATE: if uploaded a (hackish) version of a solution at https://gist.github.com/sahne/c36e835e7c7dbb855076

edited Mar 13, 2013 at 0:11

answered Mar 12, 2013 at 23:25

dwalter

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8 Comments

dwalter Over a year ago

you need to check if tmpPtr->next_number is not NULL before trying to access any of its elements.

dwalter Over a year ago

btw. you also need to set tmpPtr to the next element within the loop or you will get a endless loop.

user2155612 Over a year ago

but isnt h pointing to the head of the list, i.e the first element? So how is it null in the first run?

dwalter Over a year ago

h->next_number is NULL after the first run, since it points to the same memory as n and you set n->next_number = NULL

user2155612 Over a year ago

Sorry to be a pain, would you mind submitting the correct code?

|

EHuhtala · Accepted Answer · 2013-03-12 23:59:49Z

1

for the second add, h->next_number is NULL, so on the first iteration of the inner while loop, you dereference NULL (alias of h->next_number->number).

Edit
When you're inserting the 2nd item:
head == tail, so head->next == NULL.
you start the inner loop:
head->number == first inserted item.
head->next == NULL.
head->next->number == dereferenced NULL.

edited Mar 12, 2013 at 23:59

answered Mar 12, 2013 at 23:25

EHuhtala

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