Question

I do not work with C directly much and a basic language nuance just set me back and I would like to understand it better so I can avoid it in the future.

I am using the Queue implementation found at the following link.

http://www.planet-source-code.com/vb/scripts/ShowCode.asp?txtCodeId=9202&lngWId=3

It’s a pretty common queue where values are pushed and popped. (FIFO) But for some reason the last value that I push onto the queue sets all values in the queue with the same value.

I started with the following code.

Queue q;
queue_init(&q);
for (int i=0;i<10;i++) {
    MyData *in_md;
    in_md->mNumber = i;
    queue_push(&q, in_md);

    MyData *h = (MyData *)q.head->data;
    MyData *t = (MyData *)q.tail->data;

    NSLog(@"in_md: %i", in_md->mNumber);
    NSLog(@"h->mNumber: %i", h->mNumber);
    NSLog(@"t->mNumber: %i", t->mNumber);
    if (q.head->link) {
        MyData *l = (MyData *)q.head->link->data;
        NSLog(@"l->mNumber: %i", l->mNumber);
    }
}

But I found that when I popped the values off the number value was 9 for each instance. I thought I had some pointer bug and tried to fix it for a while. I thought that since I declared in_md inside of the block of the for loop it would be a unique instance with a unique memory address. Eventually updated the code to the following…

Queue q;
queue_init(&q);
for (int i=0;i<10;i++) {
    void *data = malloc(sizeof(MyData)); // key change
    MyData *in_md = (MyData *)data;
    in_md->mNumber = i;
    queue_push(&q, data);

    MyData *h = (MyData *)q.head->data;
    MyData *t = (MyData *)q.tail->data;

    NSLog(@"in_md: %i", in_md->mNumber);
    NSLog(@"h->mNumber: %i", h->mNumber);
    NSLog(@"t->mNumber: %i", t->mNumber);
    if (q.head->link) {
        MyData *l = (MyData *)q.head->link->data;
        NSLog(@"l->mNumber: %i", l->mNumber);
    }
}

Now the ensures that a new instance is created because malloc is used and I cast it to a variable I can set the int value. This creates the output appears as I expect.

I assume this is how C is supposed to work but I did not expect it. Normally in other languages like Java, C# and even JavaScript declaring a variable within a block would create a new instance of the variable which is what I expected here. But that is not the case.

What is happening in this case?

Answer 1

In your code:

You created a pointer:

MyData *in_md;

and without reserving memory for that pointer (that’s what the malloc does in your second code block) you assing values to it’s members:

in_md->mNumber = i;

You were basically accessing undefined memory. In your case the value was ‘9’ but you could have gotten an application crash as well.

In your second code block you allocated memory for that pointer and then stored it in the queue – that’s the right way.

Keep in mind that you need to deallocate the memory using the free() method before you finally release the queue.