E.g and explaination:

• Pre-condition: Server is running and can reciever CURL command with json format message, libcurl and jsoncpp lib installed and configured in makefile.
• Curl command line. use POST command to request data
  • 　　curl -X POST http://xx.xx.xx.xx:port/rest/xxx -H 'Content-Type: application/json' -H 'fieldname: value' -d'{"yyy" :{"fieldname1" :{"Value" : "999"},"fieldname2" :{"Value" : "777"}}'

C++ code to send request to get access token：

#define RESPONSE_BODY_SIZE 4096 int getData() {     CURLcode res;     CURL * curl;     JSONCPP_STRING errs;     Json::Value root, res_output;     Json::CharReaderBuilder reader_builder;     bool json_res;     char response_body[RESPONSE_BODY_SIZE] = {'\0'};     memset(response_body, 0 , sizeof(response_body));     struct curl_slist *headers=NULL;     try     {                 string auth_str =  "Authorization: Basic " + _auth_header;         headers = curl_slist_append(headers, "Content-Type: application/x-www-form-urlencoded");         headers = curl_slist_append(headers, auth_str.c_str());         string URL = "http://xxxx/...;                  curl = curl_easy_init();         if(curl == NULL)         {             curl_slist_free_all(headers);             return FAILURE;         }         if (//curl_easy_setopt(curl, CURLOPT_VERBOSE, 1) != CURLE_OK ||             //curl_easy_setopt(curl, CURLOPT_DEBUGFUNCTION, OnDebug) != CURLE_OK ||             curl_easy_setopt(curl, CURLOPT_URL, URL.c_str()) != CURLE_OK ||             curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers) != CURLE_OK ||             //curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0)!= CURLE_OK ||             //curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 0)!= CURLE_OK ||             curl_easy_setopt(curl, CURLOPT_NOBODY, 1) != CURLE_OK ||             curl_easy_setopt(curl, CURLOPT_POST, 1)  != CURLE_OK ||             curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1)  != CURLE_OK ||             curl_easy_setopt(curl, CURLOPT_TIMEOUT_MS, 180000) != CURLE_OK ||             curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, OnWriteData) != CURLE_OK ||             curl_easy_setopt(curl, CURLOPT_WRITEDATA, response_body) != CURLE_OK )         {             GFRLOG_ERROR("Couldn't set cURL options");             if (curl)                 {                     curl_slist_free_all(headers);                     curl_easy_cleanup(curl);                 }             return -1;            }         else         {             GFRLOG_DEBUG("curl setopt done");         }                 res = curl_easy_perform(curl);         if (CURLE_OK == res)         {             /* response E.g: 　　　　　　 {             "access_token": "3776be6d-1394-40a3-bbbb-6e54c1ba8594",             "token_type": "bearer",             "expires_in": 43199,             "scope": "read write trust"             }*/             char *content_type;                     res = curl_easy_getinfo(curl, CURLINFO_CONTENT_TYPE, &content_type);             if((CURLE_OK == res) && content_type)             {                 std::unique_ptr<Json::CharReader> const jsonReader(reader_builder.newCharReader());                 json_res = jsonReader->parse(response_body, response_body + strlen(response_body), &root, &errs);                                 if (!json_res || !errs.empty()) {                     GFRLOG_DEBUG("parse response Json err: " << errs);                     if (curl)                     {                         curl_slist_free_all(headers);                         curl_easy_cleanup(curl);                     }                     return FAILURE;                 }                 else                 {                      _access_token =  root["access_token"].asString();                     _expires_in =  atoi(root["expires_in"].asString().c_str()); 　　　　　　　　 }             }         }         else{             GFRLOG_ERROR("Failed to get response from " << URL << " error msg: "<< curl_easy_strerror(res));              if (curl)                 {                     curl_slist_free_all(headers);                     curl_easy_cleanup(curl);                 }              return FAILURE;         }     }     catch(const Json::LogicError &e)     {         GFRLOG_ERROR("Parse json string error!");         if (curl)             {                 curl_slist_free_all(headers);                 curl_easy_cleanup(curl);             }          return FAILURE;     }     if (curl)     {         curl_slist_free_all(headers);         curl_easy_cleanup(curl);     }      return  SUCCESS; }   static int OnDebug(CURL *, curl_infotype itype, char * pData, size_t size, void *)  {      if(itype == CURLINFO_TEXT)     {          printf("[TEXT]%s\n", pData);      }      else if(itype == CURLINFO_HEADER_IN)      {          printf("[HEADER_IN]%s\n", pData);      }      else if(itype == CURLINFO_HEADER_OUT)     {          printf("[HEADER_OUT]%s\n", pData);      }      else if(itype == CURLINFO_DATA_IN)      {          printf("[DATA_IN]%s\n", pData);      }      else if(itype == CURLINFO_DATA_OUT)      {          printf("[DATA_OUT]%s\n", pData);      }      return 0;  }   static size_t OnWriteData(void* buffer, size_t size, size_t nmemb, void* lpVoid)  {        char* response_body = (char*)lpVoid;     uint32_t response_body_len = strlen(response_body);     uint32_t len = size * nmemb;     if (len > RESPONSE_BODY_SIZE - response_body_len - 1)     {         len = RESPONSE_BODY_SIZE - response_body_len - 1;     }     memcpy(response_body + response_body_len, buffer, len);     return size*nmemb; }    /* below functions should call at the beginnig or and in the end Do not call it in each thread!*/ int curl_global_initiate() {     if (curl_global_init(CURL_GLOBAL_ALL) != CURLE_OK )     {         GFRLOG_BYPASS("curl_global_init failed");         return FAILURE;     }     else{         GFRLOG_BYPASS("curl_global_init done");         return SUCCESS;     } } int curl_global_clean() {     GFRLOG_BYPASS("curl_global_cleanup done");     curl_global_cleanup(); }