libcurl以get方式请求服务器端文件

static size_t callbackfunction(void *ptr, size_t size, size_t nmemb, void* userdata)
{
    FILE* stream = (FILE*)userdata;
    if (!stream) {
        printf("!!! No stream\n");
        return 0;
    }

size_t written = fwrite((FILE*)ptr, size, nmemb, stream);
    return written;

}

unsigned char* HttpsRequest::requestOnlineImage(const char* url, int& resultLen)
{
    FILE* fp = fopen("D:\\out.png", "wb");
    if (!fp) {

printf("!!! Failed to create file on the disk\n");
        return false;
    }

unsigned char* result = NULL;
    CURL* mCurl = curl_easy_init();
    if (!mCurl) {
        return NULL;

}
    //struct curl_slist* mHeaders = NULL;

//mHeaders = curl_slist_append(mHeaders, "Cache-Control:max-age=0");
    //mHeaders = curl_slist_append(mHeaders, "Connection:keep-alive");
    //mHeaders = curl_slist_append(mHeaders, "*/*");
    //mHeaders = curl_slist_append(mHeaders, "Host:cpi.cn-hangzhou.oss-pub.aliyun-inc.com");
    // mHeaders = curl_slist_append(mHeaders, "User-Agent:com.yunos.map/pc");
    //mHeaders = curl_slist_append(mHeaders, "Content-type:image/png");
    //mHeaders = curl_slist_append(mHeaders, "Response-type:application/octet-stream");
    //mHeaders = curl_slist_append(mHeaders, "Accept-Encoding:gzip, deflate");

struct MemoryStruct chunk;
    chunk.memory = (unsigned char*)malloc(1);
    chunk.size = 0;
    curl_easy_setopt(mCurl, CURLOPT_URL, url);
    //curl_easy_setopt(mCurl, CURLOPT_HTTPHEADER, mHeaders);
    curl_easy_setopt(mCurl, CURLOPT_WRITEFUNCTION, callbackfunction);
    curl_easy_setopt(mCurl, CURLOPT_WRITEDATA, fp);
    CURLcode mRes = curl_easy_perform(mCurl);

if (mRes != CURLE_OK){
        fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(mRes));

}
    else {
        printf("%s\n", chunk.memory);
        if (chunk.size > 0) {
            result = (unsigned char*)malloc(chunk.size);
            memcpy(result, chunk.memory, chunk.size);
            resultLen = chunk.size;
        }
    }
    curl_slist_free_all(mHeaders);
    free(chunk.memory); 
    curl_easy_cleanup(mCurl);
    mCurl = NULL;
    return result;

}