How to send a POST request with Kotlin/Native using libcurl? - spring-boot

Question

I'm trying to send a JSON through a POST request with a Kotlin/Native application using libcurl. I'm working on a Windows 11 machine, and the endpoint lies under a Spring Boot (version 2.7.8) backend written with Kotlin and Java 11.
The following is the code I wrote to accomplish this task.
import kotlinx.cinterop.*
import libcurl.*
fun main() {
val curl = curl_easy_init()
curl?.let {
var headers: CValuesRef<curl_slist>? = null
headers = curl_slist_append(headers, "Content-Type: application/json")
setCurl(curl, headers)
val res = curl_easy_perform(curl)
if (res != CURLE_OK) println("curl_easy_perform() failed ${curl_easy_strerror(res)?.toKString()}")
curl_easy_cleanup(curl)
curl_slist_free_all(headers)
} ?: println("curl_easy_init() failed to return curl easy handle")
}
private fun setCurl(curl: COpaquePointer?, headers: CPointer<curl_slist>?) {
val body = "{ JSON object }"
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers)
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, body)
curl_easy_setopt(curl, CURLOPT_URL, "http://localhost:8080/dishes/add/")
}
The JSON string I need to send represents a simple Dish object having three fields: name, type and description.
These are the attempts I made to correctly format the JSON string to initialize the val body and the relative outputs that the Spring Boot endpoint return:
{\"name\":\"Fish\",\"type\":\"Second\",\"description\":\"Fry\"} => HTTP 200;
{\"name\":\"Fish fry\",\"type\":\"Second course\",\"description\":\"Lots of fresh fish to delight the palate with an excellent, fried second course\"} => HTTP 400: JSON parse error: Unexpected character (' ' (code 160)): expected a valid value (JSON String, Number, Array, Object or token 'null', 'true' or 'false');
{\\\"name\\\":\\\"Fish fry\\\",\\\"type\\\":\\\"Second course\\\",\\\"description\\\":\\\"Lots of fresh fish to delight the palate with an excellent, fried second course\\\"} => HTTP 400: JSON parse error: Illegal character ((CTRL-CHAR, code 0)): only regular white space (\r, \n, \t) is allowed between tokens;
"\"{\\\"name\\\":\\\"Fish fry\\\",\\\"type\\\":\\\"Second course\\\",\\\"description\\\":\\\"Lots of fresh fish to delight the palate with an excellent, fried second course\\\"}\"" => HTTP 400: JSON parse error: Unexpected character (' ' (code 160)): expected a valid value (JSON String, Number, Array, Object or token 'null', 'true' or 'false')
From the above tests, I can effectively POST something on the backend only with the first body string, but if I try to send a longer one with even the same format (see the second one), the POST request fails too.
Besides, I also tried to execute a POST request using Postman and the terminal (with the following command: curl -H "Content-Type: application/json" -X POST -d "{\"name\":\"Fish fry\",\"type\":\"Second course\",\"description\":\"Lots of fresh fish to delight the palate with an excellent, fried second course\"}" http://localhost:8080/dishes/add/), and with both, I can correctly send the POSTs. And since using the commands prompt, curl accepts the last body JSON I tried, maybe it could be the right approach to format the string, but I'm not sure.
What am I missing?
Thanks for your precious time!
UPDATE 1:
I just discovered the --libcurl curl parameter, which lets you convert a curl command into libcurl code.
Using this helpful tool, I converted my working cmd POST request
curl -H "Content-Type: application/json" -X POST -d "{\"name\":\"Fish fry\",\"type\":\"Second course\",\"description\":\"Lots of fresh fish to delight the palate with an excellent, fried second course\"}" http://localhost:8080/dishes/add/
Into the following, Kotlin adapted, C snippet:
private fun setCurl(hnd: COpaquePointer?, certPath: String, url: String) {
var headers: CValuesRef<curl_slist>? = null
headers = curl_slist_append(headers, "Content-Type: application/json")
curl_easy_setopt(hnd, CURLOPT_BUFFERSIZE, 102400L)
curl_easy_setopt(hnd, CURLOPT_URL, "http://localhost:8080/dishes/add/")
curl_easy_setopt(hnd, CURLOPT_NOPROGRESS, 1L)
curl_easy_setopt(
hnd,
CURLOPT_POSTFIELDS,
"{\"name\":\"Fish fry\",\"type\":\"Second course\",\"description\":\"Lots of fresh fish to delight the palate with an excellent, fried second course\"}"
)
val postFieldSize: curl_off_t = 138
curl_easy_setopt(hnd, CURLOPT_POSTFIELDSIZE_LARGE, postFieldSize)
curl_easy_setopt(hnd, CURLOPT_HTTPHEADER, headers)
curl_easy_setopt(hnd, CURLOPT_USERAGENT, "curl/7.83.1")
curl_easy_setopt(hnd, CURLOPT_MAXREDIRS, 50L)
curl_easy_setopt(hnd, CURLOPT_CUSTOMREQUEST, "POST")
curl_easy_setopt(hnd, CURLOPT_FTP_SKIP_PASV_IP, 1L)
curl_easy_setopt(hnd, CURLOPT_TCP_KEEPALIVE, 1L)
}
But still, my Kotlin/Native application failed to execute the request with Spring Boot returning the same error: Unexpected character (' ' (code 160)): expected a valid value (JSON String, Number, Array, Object or token 'null', 'true' or 'false').
At this time, I ran out of ideas. Please, let me know about any other solutions.
UPDATE 2:
Since the first val body initialization was the only one to be successful (despite the body string having to be small), I started to do some other tests with that type of formatted JSON, so I found that the POST request is successful if the entire string does not cross the 63 chars of length otherwise, the Spring Boot endpoint fires the error regarding the code 160 unexpected character.
The body string I'm currently using is the following one, which length is exactly 63 chars.
{\"name\":\"Fettuccine Alfredo\",\"type\":\"Main co\",\"description\":\"\"}
I don't know why this situation is happening, and I'm very frustrated.
Every tip of advice is much appreciated.