cURL write_callback does not pass userdata argument - c++

Question

I am trying to collect some data from a URL. If I do not define any CURLOPT_WRITEFUNCTION and CURLOPT_WRITEDATA I can obviously see the output on console. Then I tried to write that data to memory by copiying the example code, however userdata argument of my callback function returned NULL and I got following exception on line:
char* ptr = (char*)realloc(mem->memory, mem->size + realsize + 1);
Exception thrown: read access violation.
mem was nullptr.
Am I doing something wrong?
Here is my code:
struct MemoryStruct {
char* memory;
size_t size;
};
//-----------------
// Curl's callback
//-----------------
size_t CurlWrapper::curl_cb(char* data, size_t size, size_t nmemb, void* response)
{
size_t realsize = size * nmemb;
std::cout << "CALLBACK CALLED" << std::endl;
MemoryStruct* mem = (struct MemoryStruct*)response;
char* ptr = (char*)realloc(mem->memory, mem->size + realsize + 1);
if (!ptr) {
/* out of memory! */
printf("not enough memory (realloc returned NULL)\n");
return 0;
}
mem->memory = ptr;
memcpy(&(mem->memory[mem->size]), data, realsize);
mem->size += realsize;
mem->memory[mem->size] = 0;
return realsize;
}
//--------------------
// Do the curl
//--------------------
void CurlWrapper::curl_api(
const std::string& url,
std::string& str_result)
{
MemoryStruct chunk;
if (curl) {
curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
curl_easy_setopt(curl, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_V4);
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, &CurlWrapper::curl_cb);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, (void*)&chunk);
// TODO: enable ssh certificate
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, false); // true
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, false); // 2
curl_easy_setopt(curl, CURLOPT_ACCEPT_ENCODING, "zlib");
auto res = curl_easy_perform(curl);
/* Check for errors */
if (res != CURLE_OK) {
// nothing
std::cout << "curl_easy_perform() failed: " << curl_easy_strerror(res) << std::endl;
}
}
}
libcurl version: 7.82.0

Answer 1

Since libcurl is a C library, it does not know anything about C++ member functions or objects. You can overcome this "limitation" with relative ease using for example a static member function that is passed a pointer to the class.
See this example (from the everything curl book).
// f is the pointer to your object.
static size_t YourClass::func(void *buffer, size_t sz, size_t n, void *f)
{
// Call non-static member function.
static_cast<YourClass*>(f)->nonStaticFunction();
}
// This is how you pass pointer to the static function:
curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass::func);
curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);