what is the difference between swapping int pointer and int array-CodePudding

For educational purposes, I was trying to swap two integer arrays with their memory locations using array pointers but I came across some weird problem and I can't figure out why this is happening,

so when I tried to create an int array and swapped their locations using swap function below, only half of their elements changed but when I switched int fArr[size]; to int *fArr = malloc(sizeof(int) * size); it works perfectly fine. Why this is happening and can casting from &int[] to int ** cause a problem?

int main(void) {
    ...

    int fArr[size];
    int sArr[size];
    initArr(fArr, sArr, size);
    
    ...

    swapArr((int **)&fArr, (int **)&sArr, size);
}

void initArr(int fArr[], int sArr[], size_t size) {
    srand(time(NULL));
    for (size_t i = 0; i < size; i  ) {
        fArr[i] = rand() % 1000;
        sArr[i] = rand() % 1000;
    }
}

void swapArr(int **fArr, int **sArr, size_t size) {
    int *temp = *fArr;
    *fArr = *sArr;
    *sArr = temp;
}

and output of this complete code is

Array : 0x7ffcdb611cf0
521, 210, 426, 263

Array : 0x7ffcdb611ce0
553, 314, 17, 16

------- After Swap -------

Array : 0x7ffcdb611cf0
553, 314, 426, 263

Array : 0x7ffcdb611ce0
521, 210, 17, 16

CodePudding user response：

The function swapArr swaps the values of 2 pointers. You pass the addresses of int arrays, inappropriately cast as pointers to int *. The code has undefined behavior. What is happening is the first 2 entries of the arrays are swapped because int * on your architecture happens to have the size of 2 int: for example int * on a 64-bit CPU has 8 bytes while int is a 32-bit type occupying 4 bytes.

Note that the arguments to initArrays are really int * despite the declaration syntax void initArr(int fArr[], int sArr[], size_t size). When you pass an array as a function argument (and most other expression contexts) it decays as a pointer to its first element.

To swap the array contents, you need to use a loop:

void swapArr(int *fArr, int *sArr, size_t size) {
    for (size_t i = 0; i < size; i  ) {
        int temp = fArr[i];
        fArr[i] = sArr[i];
        sArr[i] = temp;
    }
}