How to initialize my pointer to NULL

Question

Learning C, I'm trying to implement a little program. I have two structs like this:

typedef struct QueueNode_ QueueNode;
typedef struct TQueue_ TQueue;

struct QueueNode_ {
    QueueNode* next;
    const Task task;
};

struct TQueue_ {
    QueueNode* first;
    QueueNode* last;
};

Next I defined a method to initialize my queue:

void initializeQueue(TQueue* queue){
    queue = malloc(sizeof(TQueue)); //check if malloc returns NULL!
    queue->first = NULL;
    printf("%d", queue->first == NULL);
    queue->last = NULL;
}

And the main:

int main() {
    puts("TQueue example:");

    TQueue q;
    initializeQueue(&q);
    printf("%d", q.first == NULL);
}

I though that the above would print 11 but it prints 10 meaning that my pointer first is not set to null. I'm pretty sure that I miss something basic...

Why the above prints 10 and how to initialize properly the pointer of first to NULL in my initializeQueue method?

Thanks!

rodrigo · Accepted Answer · 2014-04-29 12:46:33Z

5

The problem is that in C, arguments are always passed by value, unless explicitly passed as a pointer, so generally speaking you should not assign the arguments directly: it is confusing.

void initializeQueue(TQueue* queue) {
    queue = malloc(sizeof(TQueue)); //check if malloc returns NULL!

See? Upon entering the function, queue points to the queue from main, but then you allocate a new struct and make queue point to it. So the queue from main is never initialized!

The solution, remove the malloc:

void initializeQueue(TQueue* queue){
    queue->first = NULL;
    printf("%d", queue->first == NULL);
    queue->last = NULL;
}

Or if you prefer to go fully dynamic, to not take the queue as an argument but return a newly allocated one:

TQueue *initializeQueue(){
    TQueue* queue = malloc(sizeof(TQueue)); //check if malloc returns NULL!
    queue->first = NULL;
    printf("%d", queue->first == NULL);
    queue->last = NULL;
    return queue;
}

And modify the main function accordingly.

edited Apr 29, 2014 at 12:46

answered Apr 29, 2014 at 12:45

rodrigo

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1 Comment

user2336315 Over a year ago

Yes I had the first version working dynamically, but the teacher requires to use the signature void initializeQueue(TQueue* queue) and I forgot to remove the malloc call... Thanks for the answer!

Some programmer dude · Accepted Answer · 2014-04-29 12:43:29Z

3

This line is the fault:

queue = malloc(sizeof(TQueue)); //check if malloc returns NULL!

You have already allocated memory for the structure in the main function, so you don't need to do it again.

In fact, what the line is doing is allocating memory, assigning it to the pointer (and remember that arguments are passed by value so it overwrites the local copy) and you change only that structure in the function, not the one passed in by the call.

answered Apr 29, 2014 at 12:43

Some programmer dude

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1 Comment

user2336315 Over a year ago

Arrg. Stupid of me.. Thanks :)

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