test_join("/root", "/", NULL, "/root/");
test_join(NULL, "/a/b", "/c", "/a/b/c");
test_join(NULL, "a/b", "c", "a/b/c");
test_join(NULL, "/a/b", "c", "/a/b/c");
test_join("/root", "/", NULL, "/root/");
test_join(NULL, "/a/b", "/c", "/a/b/c");
test_join(NULL, "a/b", "c", "a/b/c");
test_join(NULL, "/a/b", "c", "/a/b/c");