C++ libcurl seg fault in write callback function - c++

Question

I'm trying to get something done quick and dirty. I saw another SO question and tried to reuse the code. I'm hitting a couple rest services (not multithreaded) that return json and when the CURLOPT_WRITEFUNCTION is called it throws a seg fault. I'm still trying to grasp all the c++ concepts so it's been pretty difficult diagnosing.
Here's what I see
static std::string *DownloadedResponse;
static size_t writer(char *data, size_t size, size_t nmemb, std::string *buffer_in)
{
cout << "In writer callback" << endl;
// Is there anything in the buffer?
if (buffer_in != NULL)
{
cout << "Buffer not null" << endl;
// Append the data to the buffer
buffer_in->append(data, size * nmemb);
cout <<" Buffer appended, seting response" << endl;
DownloadedResponse = buffer_in;
cout << "Set downloadedResponse" << endl;
return size * nmemb;
}
return 0;
}
std::string downloadJSON(std::string URL)
{
CURL *curl;
CURLcode res;
struct curl_slist *headers=NULL; // init to NULL is important
std::ostringstream oss;
curl_slist_append(headers, "Accept: application/json");
curl_slist_append( headers, "Content-Type: application/json");
curl_slist_append( headers, "charsets: utf-8");
curl = curl_easy_init();
if (curl)
{
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
curl_easy_setopt(curl, CURLOPT_URL, URL.c_str());
curl_easy_setopt(curl, CURLOPT_HTTPGET,1);
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
curl_easy_setopt(curl,CURLOPT_WRITEFUNCTION,writer); // I comment this to display response in stdout.
cout << "calling easy_perform" << endl;
res = curl_easy_perform(curl);
cout << "call made.." << endl;
if (CURLE_OK == res)
{
char *ct;
res = curl_easy_getinfo(curl, CURLINFO_CONTENT_TYPE, &ct);
if((CURLE_OK == res) && ct)
{
cout << "returning downloaded resposne" << endl;
return *DownloadedResponse;
}
}
else
{
cout << "CURLCode: " << res << endl;
}
}
cout << "Returning null" << endl;
return NULL;
}
Output
$ ./test-rest
calling easy_perform
In writer callback
Buffer not nullSegmentation fault (core dumped)
How am I improperly using the string in the writer callback function?

Answer 1

You forgot to pass in a string pointer/reference with CURLOPT_WRITEDATA.