Question

I have a function that I pass an array into and an int into from my main function. I am doing operations to the array inside this new function, let’s call it foo. In foo, I initialize another array with 52 cells all with 0. I do operations on the array that I passed from main, and transfer that data to the newly initialized array. I want to return the new array back to the main function. But of course, I can’t return data structures like arrays. So I instead return an int pointer that points to this array. Inside the int main, I pass the pointer to have it point to various cells in the array. When I print the results of what the pointer is pointing to, it should either be pointing to 0 or an integer greater than 0. But instead, I get inconsistent results. For some reason, some of the values that SHOULD be 0, prints out garbage data. I’ve been trying to spot the bug for some time, but I just wanted a second hand look at it. Here is just the GENERAL idea for the code for this portion anyways…

int main(){
    int *retPtr;
    char input[] = "abaecedg";
    retPtr = foo(input, size);
    for(i=0; i<52; i++){
        // error displayed here
        printf("%d\n", *(retPr + i));
    }
}

int foo(char input[], int size)
{
    int arr[52] = {0};      // should initialize all 52 cells with 0.
    int i=0, value;         // looking for non-zero results in the end.        
    int *ptr = &arr[0];                        
    for(i=0; i<size; i++){
        if(arr[i] > 64 && arr[i] < 91){
            value = input[i] - 65;
            arr[value]++;
        }
    }
    return ptr;
}

Hopefully this makes sense of what I’m trying to do. In the foo function, I am trying to find the frequency of certain alphabets. I know this might be a bit cryptic, but the code is quite long with comments and everything so I wanted to make it as succinct as possible. Is there any possible reason why I’m getting correct values for some (numbers > 0, 0) and garbage values in the other?

Answer 1

The reason you get garbage back is that the array created in foo is allocated in foos stack frame, and you then return a pointer into that frame. That frame is discarded when foo returns.

You should allocate the array on the heap (using malloc and friends) if you want it to remain after foo returns. Don’t forget to free() it when you’re done with the array.

int main(){
    char input[] = "abaecedg";
    int retPtr[] = foo(input, size); //An array and a pointer is the same thing
    ...
    free(retPtr);
}

int *foo(char input[], int size)
{
    int arr[] = calloc(52*sizeof(int); // should initialize all 52 cells with 0.
    ...
    arr[value]++;
    ...
    return arr;
}

---

Another way is to let foo take an array as a parameter and work with that, in this way:

int main(){
    int ret[52] = {0};
    ...
    foo(input, size, ret);
    ...
}

void foo(char input[], int size, int *arr)
{
    ...
    arr[value]++;
    ...
    return; //Don't return anything, you have changed the array in-place
}

The reason this works is because an array is the exact same thing as a pointer, so you are really passing the array by reference into foo. arr will be pointing to the same place as ret, into the stack frame of main.