One of many ways to do compile-time assertions in C

static_assert.c

#define STATIC_ASSERT(cond) ((void)sizeof(char[1 - 2 * !(cond)]))

struct S {
    int x, y;
    short z;
};

int main(void) {
    STATIC_ASSERT(1 + 2 == 3);
    STATIC_ASSERT(sizeof(float) <= sizeof(double));
    STATIC_ASSERT(sizeof(struct S) == 3 * sizeof(int));

    return 0;
}