curl PUT JSON body having issues processing array's

Question

I'm developing a library that communicates with a REST API. The method I wrote for PUT calls has worked up until this point.

void Command::put(const std::string url, const std::string body)
{

  CURLcode ret;
  struct curl_slist *slist1;
  slist1 = NULL;
  // slist1 = curl_slist_append(slist1, "Content-Type: multipart/form-data;");
  slist1 = curl_slist_append(slist1, "Content-Type: application/json;");

  curl = curl_easy_init();
  if(curl)
  {
    curl_easy_setopt(curl, CURLOPT_BUFFERSIZE, 102400L);
    curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
    curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 1L);
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, body.c_str());
    curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE_LARGE, (curl_off_t)12);
    curl_easy_setopt(curl, CURLOPT_HTTPHEADER, slist1);
    curl_easy_setopt(curl, CURLOPT_USERAGENT, "curl/7.79.1");
    curl_easy_setopt(curl, CURLOPT_MAXREDIRS, 50L);
    curl_easy_setopt(curl, CURLOPT_SSH_KNOWNHOSTS, "/Users/$USER/.ssh/known_hosts");
    curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "PUT");
    curl_easy_setopt(curl, CURLOPT_FTP_SKIP_PASV_IP, 1L);
    curl_easy_setopt(curl, CURLOPT_TCP_KEEPALIVE, 1L);

    ret = curl_easy_perform(curl);

    curl_easy_cleanup(curl);
    curl = NULL;
    curl_slist_free_all(slist1);
    slist1 = NULL;
  }
  curl_easy_cleanup(curl);
  curl = nullptr;
  curl_slist_free_all(slist1);
  slist1 = NULL;
}

However, it seems to not work with arrays. Below are three debug put calls and their output. The first two work. The third does not.

  // *** DEBUG ***
  commandWrapper->put(
    "http://192.168.1.7/api/x1CAh16Nr8NcKwgkUMVbyZX3YKFQrEaMAj5pFz0Z/lights/6/state", "{\"on\":true}");

  commandWrapper->put(
    "http://192.168.1.7/api/x1CAh16Nr8NcKwgkUMVbyZX3YKFQrEaMAj5pFz0Z/lights/6/state", "{\"bri\":123}");

  commandWrapper->put(
    "http://192.168.1.7/api/x1CAh16Nr8NcKwgkUMVbyZX3YKFQrEaMAj5pFz0Z/lights/6/state", "{\"xy\":[0.04,0.98]}");

[{"success":{"/lights/6/state/on":true}}]
[{"success":{"/lights/6/state/bri":123}}]
[{"error":{"type":2,"address":"/lights/6/state","description":"body contains invalid json"}}]2022-04-02T02:56:47.220Z - Living Room 1 - Status: 0

I'm a little lost on how to proceed. I verified that this command does work on the command line.

Answer 1

You need to change this line:

curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE_LARGE, (curl_off_t)12);

To this instead:

curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE_LARGE, (curl_off_t)body.size());

Or, simply omit CURLOPT_POSTFIELDSIZE_LARGE completely, since body.c_str() is a pointer to a null-terminated string in this situation. Curl can determinant the size for you.

The JSON {xy:[0.04,0.98]} is 16 characters, longer than the 12 characters you are claiming, so you are truncating the JSON during transmission, which is why the server thinks it is invalid. Both {"on":true} and {"bri":123} are 11 characters (12 if you count the null terminator), so they are not truncated.