关于curl下载文件的问题

Cocos2d-x
#1 
我用curl下载文件，CURLcode res = curl_easy_perform(curl);这一行代码返回的是res ＝ CURLE_OK， 
但是下载到的文件和服务器上的文件并不一样有可能是什么原因呢，请指教，谢谢了！
我的代码调用如下：


//设置curl的下载选项
curl_easy_setopt(curl, CURLOPT_URL, srcUrl.c_str());
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, fileWriteFunc);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, fDesc.fp);
curl_easy_setopt(curl, CURLOPT_NOPROGRESS, false);
curl_easy_setopt(curl, CURLOPT_PROGRESSFUNCTION, downloadProgressFunc);
curl_easy_setopt(curl, CURLOPT_PROGRESSDATA, &data);
curl_easy_setopt(curl, CURLOPT_FAILONERROR, true);
if (_connectionTimeout) {
    curl_easy_setopt(curl, CURLOPT_CONNECTTIMEOUT, _connectionTimeout);
}
curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1L);
curl_easy_setopt(curl, CURLOPT_LOW_SPEED_LIMIT, LOW_SPEED_LIMIT);
curl_easy_setopt(curl, CURLOPT_LOW_SPEED_TIME, LOW_SPEED_TIME);

//启动curl下载
CURLcode res = curl_easy_perform(curl);
if (res != CURLE_OK) {
    _fileUtils->removeFile(data.path + data.name + TEMP_EXT);
    std::string msg = cocos2d::StringUtils::format("Unable to download file: %s", curl_easy_strerror(res));
    this->notifyError(msg, customId, res);
}
fclose(fDesc.fp);
curl_easy_cleanup(curl);

if (res == CURLE_OK) {
    _fileUtils->renameFile(data.path, data.name + TEMP_EXT, data.name);
    cocos2d::Director::getInstance()->getScheduler()->performFunctionInCocosThread(=]{
        if (!ptr.expired()) {
            std::shared_ptr<Downloader> downloader = ptr.lock();
            auto successCB = downloader->getSuccessCallback();
            if (successCB != nullptr) {
                successCB(data.url, data.path + data.name, data.customId);
            }
        }
    });
}