HTTP POST with Basic Auth and Fields (libcurl c++) - c++

Question

I am trying to make an HTTP POST using libcurl. The HTTP POST needs to perform a Basic Authentication, include a field and send a file in multi part. What I am using at the moment is the following:
FILE* fo = nullptr;
std::string fullErrPath(errPath);
fullErrPath.append("debug.txt");
fo = fopen(fullErrPath.c_str(), "wb");
CURL *curl;
CURLcode res;
struct curl_httppost* formpost = nullptr;
struct curl_httppost* lastptr = nullptr;
struct curl_slist* headerlist = nullptr;
static const char buf[] = "Expect:";
const char* imgPath = "/path/to/image.jpg";
curl_global_init(CURL_GLOBAL_ALL);
curl_formadd(&formpost,
&lastptr,
CURLFORM_COPYNAME, "jpgdata",
CURLFORM_FILE, imgPath,
CURLFORM_CONTENTTYPE, "image/jpeg",
CURLFORM_END);
curl_formadd(&formpost,
&lastptr,
CURLFORM_COPYNAME, "submit",
CURLFORM_COPYCONTENTS, "send"
CURLFORM_END);
curl = curl_easy_init();
if (curl) {
curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "POST");
curl_easy_setopt(curl, CURLOPT_URL, urlupload);
headerlist = curl_slist_append(headerlist, buf);
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headerlist);
//curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "field=1");
curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);
curl_easy_setopt(curl, CURLOPT_HTTPPOST, formpost);
curl_easy_setopt(curl, CURLOPT_HTTPAUTH, (long)CURLAUTH_BASIC);
curl_easy_setopt(curl, CURLOPT_USERPWD, credentials);
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, false);
curl_easy_setopt(curl, CURLOPT_STDERR, fo);
curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
// Response information
int httpCode(0);
static std::string httpData;
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, callback);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, &httpData);
res = curl_easy_perform(curl);
curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &httpCode);
curl_easy_cleanup(curl);
curl_formfree(formpost);
curl_slist_free_all(headerlist);
fclose(fo);
}
I left on purpose this part "//curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "field=1");" commented because I found online that it does not make sense to use it with CURLOPT_HTTPPOST. This is the post. But code above does not work.
First, the http response code I get from the server is 400 and by checking "debug.txt", in fact, there is no data. I see only the header.
Second, I need to include the POST field "field=1", but I do not know how to do it without using the command "curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "field=1");".
Thanks.

Answer 1

First, you want to set CURLOPT_USERPWD for the Basic auth.
Then, you need to decide which format of the data you want to POST. Do you want it to be multipart formpost or "regular" - you cannot mix them as that's just how HTTP works. The curl_formadd() you're doing so far is multipart formpost, and CURLOPT_POSTFIELDS is the regular one ("application/x-www-form-urlencoded").
So you say you want to add a "field=1" to the post, and to do that and keep this a multipart formpost you can insert the following snippet after your second call to curl_formadd. It adds another "part" to the post with the name and contents you asked for:
curl_formadd(&formpost,
&lastptr,
CURLFORM_COPYNAME, "field",
CURLFORM_COPYCONTENTS, "1"
CURLFORM_END);