C – error: storage size of ‘a’ isn’t known

Your struct is called struct xyx but a is of type struct xyz. Once you fix that, the output is 100.

#include <stdio.h>

struct xyx {
    int x;
    int y;
    char c;
    char str[20];
    int arr[2];
};

int main(void)
{
    struct xyx a;
    a.x = 100;
    printf("%d\n", a.x);
    return 0;
}