libcurl not uploading the entier file - c++

Question

I'm trying to upload a file to an http server. I'm getting the 200 OK from the server, but the code below is only transmitting 4 bytes.
size_t myclass::read_callback(void *ptr, size_t size, size_t nmemb, void *userp)
{
handler->read(buffer, buffer_size); // buffer_size is 100000
size_t res = handler->gcount();
if( res == 0 )
return 0;
ptr = buffer; // buffer is array of char, defined in myclass
size = res;
nmemb = sizeof(char);
return 1;
}
void myclass::upload_function(const std::string& url)
{
CURL *curl;
CURLcode res;
std::ifstream if_file;
if_file.open("/path_to_file", std::ios::binary);
handler = &if_file; // handler is defined in myclass
/* In windows, this will init the winsock stuff */
res = curl_global_init(CURL_GLOBAL_DEFAULT);
/* Check for errors */
if(res != CURLE_OK) {
// failure
return;
}
curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_URL, "hostname");
curl_easy_setopt(curl, CURLOPT_POST, 1L);
curl_easy_setopt(curl, CURLOPT_READFUNCTION, myclass::read_callback);
curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
struct curl_slist *chunk = NULL;
chunk = curl_slist_append(chunk, "Transfer-Encoding: chunked");
chunk = curl_slist_append(chunk, "Content-Type: application/x-mpegURL");
res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, chunk);
/* Perform the request, res will get the return code */
res = curl_easy_perform(curl);
/* Check for errors */
if(res != CURLE_OK) {
// failed
}
else
{
double speed_upload, total_time;
curl_easy_getinfo(curl, CURLINFO_SPEED_UPLOAD, &speed_upload);
curl_easy_getinfo(curl, CURLINFO_TOTAL_TIME, &total_time);
fprintf(stderr, "Speed: %0.3f b/sec during %.3f seconds\n",
speed_upload, total_time);
}
/* always cleanup */
curl_easy_cleanup(curl);
}
curl_global_cleanup();
if_file.close();
}

Answer 1

The callback doesn't seem to copy data to the buffer. It just assigns the local pointer, quite without any effect.
The callback looks like to be a C++ method that can't be used like that as a callback in a C API that doesn't know about C++ objects...