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

Question

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 username and password so please give me the solution how to add variable in my mail body.

#define FROM "<[email protected]>"
#define TO "<[email protected]>"
#define CC "<[email protected]>"

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: <[email protected]>\r\n",
"Subject: SMTP TLS example message\r\n",
"\r\n", /* empty line to divide headers from body, see RFC5322 */ 
"The body of the message starts here.\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;
}