curl_formfree man page
curl_formfree — free a previously build multipart/formdata HTTP POST chain
void curl_formfree(struct curl_httppost *form);
curl_formfree() is used to clean up data previously built/appended with curl_formadd(3). This must be called when the data has been used, which typically means after curl_easy_perform(3) has been called.
The pointer to free is the same pointer you passed to the CURLOPT_HTTPPOST(3) option, which is the firstitem pointer from the curl_formadd(3) invoke(s).
form is the pointer as returned from a previous call to curl_formadd(3) and may be NULL.