Why Input/Output Error occurs when use ls command in my fuse filesystem?

Why I/O Error causes when I tried ls . command in my fuse filesystem?

My filesystem has a limitation that it only allows mail address type as individual filename and it does not allows sub directory.
Now I want to display a list of file name when use ls . but it does not work.

I understood that it must implement a callback function. (Correspond function is ll_readdir in mycode)

but I have no idea what points are causes the errors.

Update:

Now I use strace command to investigate what system call raise a this error.
According to result of strace, this error caused in getdents64 syscall.

getdents64(3, 0x5611ed000540, 32768)    = -1 EIO (Input/output error)

Code1 (implementation of mm:

struct mutex_map {
    int counter = 2;
    std::mutex _mtx;
    std::unordered_map<int, std::string> _data;
    std::unordered_map<std::string, int> _rev_data;

   public:
    int set_value(const char* value) {
        std::string s = std::string(value);
        std::lock_guard<std::mutex> lock(_mtx);
        counter++;
        _data[counter] = s;
        _rev_data[s] = counter;
        return counter;
    }

    const char* get_value(int key) { return _data[key].c_str(); }
    int get_ino(const char* name) { return _rev_data[std::string(name)]; }
};

static mutex_map mm;

Code2: (sendmailfs_stat)

static int sendmailfs_stat(fuse_ino_t ino, struct stat* stbuf,
                           size_t name_length) {
    uid_t uid = getuid();
    gid_t gid = getgid();
    stbuf->st_ino = ino;
    if (ino == 1) {
        stbuf->st_mode = S_IFDIR | 0755;
        stbuf->st_nlink = 2;
        stbuf->st_uid = uid;
        stbuf->st_mode = S_IFDIR;
    } else {
        stbuf->st_mode = S_IFCHR | 0666;
        stbuf->st_nlink = 1;
        stbuf->st_size = name_length;
        stbuf->st_uid = uid;
        stbuf->st_gid = gid;
    }
    return 0;
}

Code 3: (implementation of readdir callback)

static void ll_readdir(fuse_req_t req, fuse_ino_t ino, size_t size, off_t off,
                       struct fuse_file_info* fi) {
    // printf("size_t=%ld, fh=%ld, ino=%ld\n", size, fi->fh, ino);
    if (ino == 1) {
        off_t o = 0;
        size_t rest = size;
        size_t res;
        char* buf = (char*)calloc(1, size);
        struct stat dotst;
        sendmailfs_stat(ino, &dotst, strlen("."));
        res = fuse_add_direntry(req, buf, rest, ".", &dotst, o);
        rest -= res;
        o++;
        printf("%s\n", "start of loop");
        uint64_t num_contain = 0;
        for (auto& c : mm._data) {
            const char* t = c.second.c_str();
            int ino2 = mm.get_ino(t);
            struct stat st;
            sendmailfs_stat(ino2, &st, strlen(t));
            fuse_entry_param e;
            e.ino = ino2;
            e.attr_timeout = 0;
            sendmailfs_stat(ino2, &e.attr, strlen(t));
            res = fuse_add_direntry_plus(req, buf, rest, t, &e, o);
            o += 1;
            rest -= res;
        }
        fuse_reply_buf(req, buf, size);
    }
}