Set the absolute path with a variable on Linux using libcurl in C++

How can i set the absolute path in libcurl Linux, using a variable?

Here's an example code:

string absolute_path = "/home/user_name";
CURL *curl;
FILE *fp;
CURLcode res;
const char *url = "http://google.com";
const char outfilename[FILENAME_MAX] = absolute_path;
curl = curl_easy_init();
if (curl)
{
    fp = fopen(outfilename,"wb");
    curl_easy_setopt(curl, CURLOPT_URL, url);
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, NULL);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
    res = curl_easy_perform(curl);
    curl_easy_cleanup(curl);
    fclose(fp);
}

But the compiler returns this error:

error: array must be initialized with a brace-enclosed initializer

Does anyone know how to fix it? Thank you for your attention!