What is the difference between Array[n] ; and Array[ ]={ };

Question

#include <stdio.h>

int main() {
    int n, i, j, k, l;
    int temp;

    printf("Enter how many element on the array : ");
    scanf("%d", &n);
    int arr1[100] = {};
    for (i = 0; i < n; i++) {
        scanf("%d", &arr1[i]);
    }
    for (j = 0; j < n; j++) {
        for (k = j + 1; k < n; k++) {
            if (arr1[j] > arr1[k]) {
                temp = arr1[j];
                arr1[j] = arr1[k];
                arr1[k] = temp;
            }
        }
    }
    for (i = 0; i < n; i++) {
        printf("%d \t", arr1[i]);
    }
}

My code for sorting an array in ascending order works properly. And it doesn't have any error but when I am changed the array size then the code doesn't work properly and has an error called stack smashing detected. What causes this problem?

#include <stdio.h>

int main() {
    int n, i, j, k, l;
    int temp;

    printf("Enter how many element on the array : ");
    scanf("%d", &n);
    int arr1[] = {};
    for (i = 0; i < n; i++) {
        scanf("%d", &arr1[i]);
    }
    for (j = 0; j < n; j++) {
        for (k = j + 1; k < n; k++) {
            if (arr1[j] > arr1[k]) {
                temp = arr1[j];
                arr1[j] = arr1[k];
                arr1[k] = temp;
            }
        }
    }
    for (i = 0; i < n; i++) {
        printf("%d \t", arr1[i]);
    }
}

Answer 1

Neither int arr1[100] = {}; nor int arr1[] = {}; is valid C code.

The program compiles because your compiler implements GNU extensions that allow empty initializers and zero length arrays.

The reason your program no longer works when you remove the length 100 is the array becomes too short for the elements you try and store into it.

You probably meant to write int arr1[n] = {}; which does not compile because VLAs (variable sized arrays) cannot have an initializer.

Here is a modified version:

#include <stdio.h>

int main() {
    int n, i, j, k, l;

    printf("Enter how many element on the array : ");
    if (scanf("%d", &n) != 1 || n <= 0) {
        fprintf(stderr, "invalid size\n");
        return 1;
    }
    int arr1[n];
    for (i = 0; i < n; i++) {
        if (scanf("%d", &arr1[i]) != 1) {
            fprintf(stderr, "invalid input\n");
            return 1;
        }
    }
    for (j = 0; j < n; j++) {
        for (k = j + 1; k < n; k++) {
            if (arr1[j] > arr1[k]) {
                int temp = arr1[j];
                arr1[j] = arr1[k];
                arr1[k] = temp;
            }
        }
    }
    for (i = 0; i < n; i++) {
        printf("%d%c", arr1[i], "\t\n"[i == n - 1]);
    }
    return 0;
}