curl: using multipart with post params - c++

Question

If I do
curl_easy_setopt(curl, CURLOPT_HTTPPOST, formpost);
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, post_params);
the server will do not see multipart, but if I coment the second line
curl_easy_setopt(curl, CURLOPT_HTTPPOST, formpost);
//curl_easy_setopt(curl, CURLOPT_POSTFIELDS, post_params);
it'll do.
What's wrong here?
UPD: So, now the question is, whether it's possible to put value to a param, so that that value was in POST parameters array on server-side? I'm trying
headers = curl_slist_append(headers, "Content-Disposition: form-data");
//action=upload
curl_formadd(&formpost,
&lastptr,
CURLFORM_COPYNAME, "action",
CURLFORM_COPYCONTENTS, "upload",
CURLFORM_CONTENTHEADER, headers,
CURLFORM_CONTENTTYPE, "Content-Type: multipart/form-data",
CURLFORM_END);
But that's not working!
Thank you in advance!

Answer 1

I ended up mostly following the example code here: http://curl.haxx.se/libcurl/c/multi-post.html
Here's a sample of the code that ended up working for me based on the curl example link:
...
CURL *curl;
curl_mime *form;
curl = curl_easy_init();
/* Set the headers for the request. */
scoped_curl_slist headers;
headers.list = NULL;
string session_header = "Authorization: Bearer " + token;
headers.list = curl_slist_append(headers.list, session_header.c_str());
headers.list = curl_slist_append(headers.list, "cache-control: no-cache");
headers.list = curl_slist_append(headers.list, "Content-Type: multipart/form-data");
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers.list);
curl_easy_setopt(curl, CURLOPT_URL, "your_url.com");
/* Create the form */
form = curl_mime_init(curl);
/* Fill in the file upload field */
curl_mimepart *field = curl_mime_addpart(form);
curl_mime_name(field, "upload");
curl_mime_filedata(field, "C:/file_name.txt");
/* Fill in the filename field */
field = curl_mime_addpart(form);
curl_mime_name(field, "name");
curl_mime_data(field, "file_name.txt", CURL_ZERO_TERMINATED);
curl_easy_setopt(curl, CURLOPT_MIMEPOST, form);
CURLcode result = curl_easy_perform(curl);
...
curl_mime_free(form);
curl_easy_cleanup(curl);
curl_slist_free_all(headers);