Magic square checker

magicsq.c

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

#define SQUARE_SIZE 4

bool all_same(int *values, size_t num)
{
    int v = values[0];
    for(size_t i = 1; i < num; i++)
        if(values[i] != v)
            return false;
    return true;
}

bool is_square(int data[][SQUARE_SIZE])
{
    struct {
        int rows[SQUARE_SIZE];
        int cols[SQUARE_SIZE];
        int diag[2];
    } sums = {{0}, {0}, {0}};

    for(size_t r = 0; r < SQUARE_SIZE; r++) {
        for(size_t c = 0; c < SQUARE_SIZE; c++) {
            int val = data[r][c];
            sums.rows[r] += val;
            sums.cols[c] += val;
            if(r == c)
                sums.diag[0] += val;
            else if(r == SQUARE_SIZE - 1 - c)
                sums.diag[1] += val;
        }
    }
    return all_same((int *)&sums, SQUARE_SIZE * 2 + 2);
}
            
int main(void)
{
    int data[SQUARE_SIZE][SQUARE_SIZE];
    
    for(size_t r = 0; r < SQUARE_SIZE; r++) {
        for(size_t c = 0; c < SQUARE_SIZE; c++) {
            printf("Enter value for row %zu, col %zu: ", r + 1, c + 1);
            if(scanf("%d", &data[r][c]) != 1) {
                puts("invalid input");
                exit(1);
            }
        }
    }
    
    if(is_square(data))
        puts("\n\nThis is a magic square.");
    else
        puts("\n\nNot a magic square.");
}