如何用libcurl实现ftp断点续传

如题，在libcurl官网上找了一个上传函数，但测试无法实现
int upload(CURL *curlhandle, const char * remotepath, const char * localpath, long timeout, long tries)
{
    FILE *f;
    long uploaded_len = 0;
    CURLcode r = CURLE_GOT_NOTHING;
    int c;
    f = fopen(localpath, "rb");
    if (f == NULL) {
        perror(NULL);
        return 0;
    }
    curl_easy_setopt(curlhandle, CURLOPT_UPLOAD, 1L);
    curl_easy_setopt(curlhandle, CURLOPT_URL, remotepath);
    curl_easy_setopt(curlhandle, CURLOPT_USERPWD, "spider:spider");   
    if (timeout)
        curl_easy_setopt(curlhandle, CURLOPT_FTP_RESPONSE_TIMEOUT, timeout);
    curl_easy_setopt(curlhandle, CURLOPT_HEADERFUNCTION, getcontentlengthfunc);
    curl_easy_setopt(curlhandle, CURLOPT_HEADERDATA, &uploaded_len);
    curl_easy_setopt(curlhandle, CURLOPT_WRITEFUNCTION, discardfunc);
    curl_easy_setopt(curlhandle, CURLOPT_READFUNCTION, readfunc);
    curl_easy_setopt(curlhandle, CURLOPT_READDATA, f);
    curl_easy_setopt(curlhandle, CURLOPT_FTPPORT, "-"); /* disable passive mode */
    curl_easy_setopt(curlhandle, CURLOPT_FTP_CREATE_MISSING_DIRS, 1L);
    curl_easy_setopt(curlhandle, CURLOPT_VERBOSE, 1L);
    for (c = 0; (r != CURLE_OK) && (c < tries); c++) {
        /* are we resuming? */
        if (c) { /* yes */
            /* determine the length of the file already written */
            /*
            * With NOBODY and NOHEADER, libcurl will issue a SIZE
            * command, but the only way to retrieve the result is
            * to parse the returned Content-Length header. Thus,
            * getcontentlengthfunc(). We need discardfunc() above
            * because HEADER will dump the headers to stdout
            * without it.
            */
            curl_easy_setopt(curlhandle, CURLOPT_NOBODY, 1L);
            curl_easy_setopt(curlhandle, CURLOPT_HEADER, 1L);
            r = curl_easy_perform(curlhandle);
            if (r != CURLE_OK)
                continue;
            curl_easy_setopt(curlhandle, CURLOPT_NOBODY, 0L);
            curl_easy_setopt(curlhandle, CURLOPT_HEADER, 0L);
            fseek(f, uploaded_len, SEEK_SET);
            curl_easy_setopt(curlhandle, CURLOPT_APPEND, 1L);
        }
        else { /* no */
            curl_easy_setopt(curlhandle, CURLOPT_APPEND, 0L);
        }
        r = curl_easy_perform(curlhandle);
    }
    fclose(f);
    if (r == CURLE_OK)
        return 1;
    else {
        fprintf(stderr, "%s\n", curl_easy_strerror(r));
        return 0;
    }
}
有没有人用过libcurl实现ftp断点续传功能的，望指教。