command "dir" doesn't work with libcurl language C

Question

I'm trying to send a "DIR" command on an ftp server with the fonction below:

void cpyFileInServeur(char *src, char *dest, char *filename, serveur server)
{
    CURL *curl;
    CURLcode res;
    struct curl_slist *header = NULL;
    char *userpwd = (char *)malloc(sizeof(char) * 100);
    sprintf(userpwd, "%s:%s", server.user, server.passwd);
    header = curl_slist_append(header, "dir");
    curl_global_init(CURL_GLOBAL_DEFAULT);
    curl = curl_easy_init();
    if (curl)
    {
        curl_easy_setopt(curl, CURLOPT_URL, server.url);
        curl_easy_setopt(curl, CURLOPT_USERPWD, userpwd);
        curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
        curl_easy_setopt(curl, CURLOPT_QUOTE, header);
        res = curl_easy_perform(curl);
        curl_easy_cleanup(curl);
        if (res != CURLE_OK)
        {
            printf ("Erreur === %d\n", res);
        }
        else
        {
            printf ("Success .... == %d\n", res);
        }
    }
    curl_global_cleanup();
}

Error:

500 DIR not understood

QUOT command failed with 500

result

when I use the terminal, I get the same error. But if I use the command "pass", then it works

image below

result

Thanks for help.