C realloc usage

Question

I'm trying to dynamically increase memory of an int array, However I'm having issues getting it to work. It isn't expanding and adding more elements to the array, I'm not sure what I'm doing wrong. Please help!

int* fibs = NULL;

void genFibs(){
    int i = 1,curSize = 0,curNum = 0;
    int flag = 1;
    while(flag){
        if(curSize != 0 &&curSize != 1){
            curNum = fibs[curSize-2]+fibs[curSize-1];
        }else if(curSize-1 == 1){
            curNum = fibs[curSize-1]+fibs[curSize-1];
        }else{
            curNum = 1;
        }
        if(curNum<=10){
            curSize++;
            fibs = (int*)realloc(fibs,curSize*sizeof(int));
            fibs[curSize-1] = curSize;
        }else{
            flag = 0;
        }
    }
  }
}

void printFibs(){
    int size = sizeof(fibs)/sizeof(int);
    int i = 0;
    for(i = 0;i<size;i++){
        printf("%d is: %d\n",i,fibs[i]);
    }
}

have you allocated memory for `fibs` using `malloc()` or `calloc()` before `realloc()`? And what error exactly? — Grijesh Chauhan, Jun 06 '13 at 09:26
no this function im simply wanting to generate the fibs array. — William McCarty, Jun 06 '13 at 09:27
@Xaqq theres more code, logic stuff, just showing the important realloc part — William McCarty, Jun 06 '13 at 09:28
Also, what is going wrong? Can you tell us more? Because this looks fine — Xaqq, Jun 06 '13 at 09:28
i try printing it after it generates it, it only prints out one element — William McCarty, Jun 06 '13 at 09:29
then show the code you use for printing, aside from the infinite loop the reallocation code is fine — Joe, Jun 06 '13 at 09:30
@nos refer to my edit, the i added more code to avoid an infinite loop. Like i said before, its simply not adding more elements to my array when trying to realloc it. So when I go to print it, it only prints one element. — William McCarty, Jun 06 '13 at 09:36

Xaqq · Accepted Answer · 2013-06-06T12:05:11.097

5

Well, your print code is wrong. sizeof(fibs) will always be evaluated as sizeof(int *) because its just a pointer.

You have to find a way to pass the size to your print function. It could be, for example the first value of your array. But this is implementation detail and it's up to you.

EDIT: Changed sizeof(void *) to sizeof(int *) because pointer size may vary, as pointed out by pmg.

edited Jun 06 '13 at 12:05

answered Jun 06 '13 at 09:35

Xaqq

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Ok, I was going off this post "http://stackoverflow.com/questions/37538/how-do-i-determine-the-size-of-my-array-in-c" Whats the difference between these two??? my array is technically the same is it not?? – William McCarty Jun 06 '13 at 09:40
1

Just a FYI: `void*` and `int*` do not necessarily have the same size (and alignment requirements and padding and ...). – pmg Jun 06 '13 at 09:53
@pmg oO didn't know about that. Alignment requirements and padding can make sense, but the size. Do you have any links explaining why and when? I thought all pointer had the same size. – Xaqq Jun 06 '13 at 09:54
@Xaqq: They don't have to. They just have to be convertible to and from a `void *`. – Hasturkun Jun 06 '13 at 10:19
@Xaqq: see [6.2.5/27 in the C99 Standard](http://port70.net/~nsz/c/c99/n1256.html#6.2.5p27). *"Pointers to other types need not have the same representation or alignment requirements."* – pmg Jun 06 '13 at 10:29
Answer edited, feel free to re-edit if it's not accurate enough :) – Xaqq Jun 06 '13 at 12:25

Joe · Answer 2 · 2013-06-06T09:35:35.893

1

sizeof(fibs) is the size of the pointer fibs which is constant, therefore your printing function will always print the same number of elements

edited Jun 06 '13 at 09:35

answered Jun 06 '13 at 09:28

Joe

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C realloc usage

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