如何扩展内存映射文件的长度？

int fd = open("file.txt", O_RDWR);
if (fd == -1) {
    perror("open");
    exit(EXIT_FAILURE);
}

void *addr = mmap(NULL, length, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (addr == MAP_FAILED) {
    perror("mmap");
    exit(EXIT_FAILURE);
}

off_t new_length = 2 * length; // 假设我们要将文件长度翻倍
int ret = ftruncate(fd, new_length);
if (ret == -1) {
    perror("ftruncate");
    exit(EXIT_FAILURE);
}

void *new_addr = mremap(addr, length, new_length, MREMAP_MAYMOVE);
if (new_addr == MAP_FAILED) {
    perror("mremap");
    exit(EXIT_FAILURE);
}

// 更新内存映射的长度
length = new_length;

ret = munmap(new_addr, length);
if (ret == -1) {
    perror("munmap");
    exit(EXIT_FAILURE);
}

close(fd);