How to add dynamic mail body using curl in c++

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C++

cURL

mail

Hello, Friends I am using following code to send a mail but here the body of mail is statically defined but i want to send some variable values like UserNamemail and Passwordmail. Here if i declare UserNamemail="sljfgsg",Passwordmail="lksdhfi" then my mail going successfully but my value for these variables will be change so i can not declare constantly,so please help me how to add my variable in my mail body.

#define FROM    "<sender@example.org>"
#define TO      "<addressee@example.net>"
#define CC      "<info@example.org>"
 
static const char *payload_text[] = {
  "Date: Mon, 29 Nov 2010 21:54:29 +1100\r\n",
  "To: " TO "\r\n",
  "From: " FROM "(Example User)\r\n",
  "Cc: " CC "(Another example User)\r\n",
  "Message-ID: <dcd7cb36-11db-487a-9f3a-e652a9458efd@rfcpedant.example.org>\r\n",
  "Subject: SMTP TLS example message\r\n",
  "\r\n", /* empty line to divide headers from body, see RFC5322 */ 
  "Your request for XXXXX Demo is successfully accepted.\r\n",
  "Your Login Credentials are as follows-.\r\n",
  "\r\n",
  "UserName is - ",UserNamemail,"\r\n",
  "Password is - ",Passwordmail,"\r\n",
  "\r\n",
  "Thanks for making interest in XXXX. \r\n",
  "\r\n",
  "It could be a lot of lines, could be MIME encoded, whatever.\r\n",
  "Check RFC5322.\r\n",
  NULL
};
 
struct upload_status {
  int lines_read;
};
 
static size_t payload_source(void *ptr, size_t size, size_t nmemb, void *userp)
{
  struct upload_status *upload_ctx = (struct upload_status *)userp;
  const char *data;
 
  if((size == 0) || (nmemb == 0) || ((size*nmemb) < 1)) {
    return 0;
  }
 
  data = payload_text[upload_ctx->lines_read];
 
  if(data) {
    size_t len = strlen(data);
    memcpy(ptr, data, len);
    upload_ctx->lines_read++;
 
    return len;
  }
 
  return 0;
}
 
int main(void)
{
  CURL *curl;
  CURLcode res = CURLE_OK;
  struct curl_slist *recipients = NULL;
  struct upload_status upload_ctx;
 
  upload_ctx.lines_read = 0;
 
  curl = curl_easy_init();
  if(curl) {
    /* Set username and password */ 
    curl_easy_setopt(curl, CURLOPT_USERNAME, "user");
    curl_easy_setopt(curl, CURLOPT_PASSWORD, "secret");
 
    curl_easy_setopt(curl, CURLOPT_URL, "smtp://mainserver.example.net:587");
 
   
    curl_easy_setopt(curl, CURLOPT_USE_SSL, (long)CURLUSESSL_ALL);
 
    curl_easy_setopt(curl, CURLOPT_CAINFO, "/path/to/certificate.pem");
 
    curl_easy_setopt(curl, CURLOPT_MAIL_FROM, FROM);
 
    recipients = curl_slist_append(recipients, TO);
    recipients = curl_slist_append(recipients, CC);
    curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients);
 
    
    curl_easy_setopt(curl, CURLOPT_READFUNCTION, payload_source);
    curl_easy_setopt(curl, CURLOPT_READDATA, &upload_ctx);
    curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);
 
   
    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
 
    /* Send the message */ 
    res = curl_easy_perform(curl);
 
    /* Check for errors */ 
    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n",
              curl_easy_strerror(res));
 
    /* Free the list of recipients */ 
    curl_slist_free_all(recipients);
 
    /* Always cleanup */ 
    curl_easy_cleanup(curl);
  }
 
  return (int)res;
}