cURL send POST data-binary - c++

Question

I have cURL command:
curl "https://pelevin.gpt.dobro.ai/generate/" --data-binary '{"prompt" : "text" , "length" : 40}
and i want to do the same thing in C++.
I tried:
string params = "prompt=text&length=40";
string buffer;
CURL* curl;
curl = curl_easy_init();
if (curl) {
curl_easy_setopt(curl, CURLOPT_URL, "https://pelevin.gpt.dobro.ai/generate/");
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_callback);
curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, params.size());
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, params.c_str());
curl_easy_setopt(curl, CURLOPT_VERBOSE, 1);
curl_easy_perform(curl);
}
curl_easy_cleanup(curl);
and got message like this:
{"detail":[{"loc":["body",0],"msg":"Expecting value: line 1 column 1 (char 0)","type":"value_error.jsondecode","ctx":{"msg":"Expecting value","doc":"prompt=text&length=40","pos":0,"lineno":1,"colno":1}}]}
How can the request be made correctly?

Answer 1

When you went from command-line to libcurl, you changed the data you provide. Why?
First it was a JSON object string (which seems to be what the server is expecting). Now you've made it an URL-encoded string.
Just pass the right data to the server, as you did in the first example.
string params = "{\"prompt\" : \"text\" , \"length\" : 40}";
N.B. there are "better" ways to format that in modern C++, if you're up for it:
string params = R"RAW({"prompt" : "text" , "length" : 40})RAW";
BTW, it's weird to use --data-binary with data specified on the command-line. It's intended for use with file input, as it prevents newlines being stripped from the file.