Find the missing number in an array using XOR in C Solution, output & explanation

#include <stdio.h>

int missing(const int *a, int len) {
    int n = len + 1, x = 0;
    for (int i = 1; i <= n; i++)
        x ^= i;                 /* XOR of 1..n */
    for (int i = 0; i < len; i++)
        x ^= a[i];              /* cancels every number that is present */
    return x;
}

int main(void) {
    int a[] = {1, 2, 4, 6, 3, 7, 8};
    printf("missing number = %d\n", missing(a, 7));
    return 0;
}

missing number = 5