libcurl post request multiple lines - c

Question

I am currently working on an application that needs to communicate between two web servers. In order to do that I am using libcurl in c. I am perfectly ok with making GET requests, but the POST ones I'm finding a bit more tricky.
For instance with curl in this case I'd do:
curl --location --request POST '%URL%' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'grant_type=client_credentials' \
--data-urlencode 'scope=%scope%' \
--data-urlencode 'client_id=%client_id%' \
--data-urlencode 'client_secret=%client_secret%'
Reading the libcurl documentation I understand I need to curl_easy_setopt(curl, CURLOPT_POST, 1L); to let libcurl know I'm posting.
The only problem I have is how exactly do I make the different lines?
The fact that the CURLOPT_POSTFIELDS is in fact called "fields" rather than "field" makes me think it should support multiple fields natively, so I instinctively think
curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, (long) strlen(first_line));
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, first_line);
curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, (long) strlen(second_line));
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, second_line);
curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, (long) strlen(third_line));
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, third_line);
...and so on.
But that doesn't work and the documentation goes on saying
To make multipart/formdata posts, check out the CURLOPT_MIMEPOST option combined with curl_mime_init.
Which, since I know very little about, looks sort of scary especially looking at the example under this page.
Can anybody help me with the request I need to make or at least explain the MIME thing a little simpler?
From the mime page on curl.se I get the feeling that I should already know the things I don't know and the research I have done hasn't really shed any more light.

Answer 1

Since the server is expecting the data in application/x-www-form-urlencoded format, MIME doesn't apply, so ignore that.
Despite its name, CURLOPT_POSTFIELDS is not treated in a plural manner. It cannot be used in multiple calls to curl_easy_setopt() to concatenate multiple pieces of data into a single POST body. It expects the entire POST body to be specified in a single call, as per documented in CURLOPT_POSTFIELDS explained:
Pass a char * as parameter, pointing to the full data to send in an HTTP POST operation. You must make sure that the data is formatted the way you want the server to receive it. libcurl will not convert or encode it for you in any way. For example, the web server may assume that this data is URL encoded.
curl.exe concatenates multiple --data... parameters together into a single POST body:
If any of these options is used more than once on the same command line, the data pieces specified will be merged with a separating &-symbol. Thus, using '-d name=daniel -d skill=lousy' would generate a post chunk that looks like 'name=daniel&skill=lousy'.
So, you will have to do the same in your own code, eg:
// build up this string however you need to...
const char *post_body = "grant_type=client_credentials"
"&scope=<url-encoded value>"
"&client_id=<url-encoded value>"
"&client_secret=<url-encoded value>";
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, post_body);
// no need for CURLOPT_POSTFIELDSIZE in this case,
// libcurl will handle that internally...