error.c:543
static VALUE
get_syserr(n)
    int n;
{
    VALUE error;

    if (!st_lookup(syserr_tbl, n, &error)) {
	char name[8];	/* some Windows' errno have 5 digits. */

	snprintf(name, sizeof(name), "E%03d", n);
	error = set_syserr(n, name);
    }
    return error;
}
