DevHeads IoT Integration Server•8mo ago

disturbing issue I'm having with LinkedList in C language, when I count how many nodes should be

Hello world!! @Middleware & OS There is this disturbing issue I'm having with LinkedList in C language, when I count how many nodes should be in the list, I always get 1

LL count: 1

LL count: 1

Here is the relevant code to get the last element , to add, and to count

void addLL(LL * head)
{
LL *newNode;
LL *tail = getLastNode(head);

newNode = malloc(sizeof(LL));
if(newNode != DEF_NULL)
{
    newNode->ID=-1;
    newNode->TCB=-1;
    newNode->next = DEF_NULL;

    if(!head) head = newNode;
    else tail->next = newNode;      
}   
}

LL * getLastNode(LL * head)
{
    LL *temp = head;
    while(temp->next != DEF_NULL)
    {
        temp = temp->next;
    }
    return temp;
}

CPU_INT32U countLL(LL * head)
CPU_INT32U countLL(LL * head)
{
    CPU_INT32U elements = 0;
    LL * temp = head;
    while(temp->next != DEF_NULL)
    {
        temp = temp->next;
        elements++;
    }
    return elements;
}

void addLL(LL * head)
{
LL *newNode;
LL *tail = getLastNode(head);

newNode = malloc(sizeof(LL));
if(newNode != DEF_NULL)
{
    newNode->ID=-1;
    newNode->TCB=-1;
    newNode->next = DEF_NULL;

    if(!head) head = newNode;
    else tail->next = newNode;      
}   
}

LL * getLastNode(LL * head)
{
    LL *temp = head;
    while(temp->next != DEF_NULL)
    {
        temp = temp->next;
    }
    return temp;
}

CPU_INT32U countLL(LL * head)
CPU_INT32U countLL(LL * head)
{
    CPU_INT32U elements = 0;
    LL * temp = head;
    while(temp->next != DEF_NULL)
    {
        temp = temp->next;
        elements++;
    }
    return elements;
}

So here is how I call the function :

LL* list;

addLL(list);
int count = countLL(list);
Debug_LOG("LL count: %i", count);

LL* list;

addLL(list);
int count = countLL(list);
Debug_LOG("LL count: %i", count);

Please help me spot where I went wrong?

12 Replies

steffen•8mo ago

First thing I see is that you do not initialize LL* list to NULL. This means it points to a random address in memory. And that is why the if(!head) check does not work as you would expect

Marvee Amasi•8mo ago

@steffen Thank you, that makes a lot of sense! So, if I initialize list to NULL before calling addLL like this:

 LL* list = NULL;
addLL(list);

 LL* list = NULL;
addLL(list);

This is what you mean?

Marvee Amasi•8mo ago

So @steffen I am curious to know how a non-null list before the first addLL call affects the behavior? Does it create an empty list or point to some unintended memory location?

ZacckOsiemo•8mo ago

The latter

ZacckOsiemo•8mo ago

it points to a random location.

ZacckOsiemo•8mo ago

its indeterminate when you declare a value but dont initialize it.

Marvee Amasi•8mo ago

Okay @ZacckOsiemo , Are there any common pitfalls I should be aware of when working with linked lists in C, especially regarding memory management? @steffen @Middleware & OS @ZacckOsiemo

ZacckOsiemo•8mo ago

It applies to variables and structures

ZacckOsiemo•8mo ago

basically uninitialized items will lead to undefined behavior, you don't know what the compiler will do.

ZacckOsiemo•8mo ago

Initialize your variables. and you start from a known point.

Marvee Amasi•8mo ago

Thanks for clarifying @ZacckOsiemo

ZacckOsiemo•8mo ago

No worries glad to have been of service.

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disturbing issue I'm having with LinkedList in C language, when I count how many nodes should be