Unable to send dynamic mail body on CURL

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I am new to CURL and I am trying to send a dynamic message body. But I receive only an empty subject and message body when I attempt to use CURL lib in visual c++.
Please help.

What I have tried:

C++

<pre>    size_t Process::processmail(void *ptr, size_t size, size_t nmemb, void *userp)
    {
	std::string a, b, c, d;
	std::string arr[MAXSPACE];
	std::string search = ",";
	int spacePos;
	int currPos = 0;
	int k = 0;
	int prevPos = 0;
	std::string &statistics = *(static_cast<std::string*>(userp));
	do
	{
		spacePos = statistics.find(search, currPos);
		if (spacePos >= 0)
		{

			currPos = spacePos;
			arr[k] = statistics.substr(prevPos, currPos - prevPos);
			currPos++;
			prevPos = currPos;
			k++;
		}
	} while (spacePos >= 0);

	arr[k] = statistics.substr(prevPos, statistics.length());

	for (int i = 0; i < k; i++)
	{
		std::cout << arr[i] << std::endl;
	}
	a = arr[3];
	b = arr[2];
	c = arr[1];
	d = arr[0];
	const char *data;
	time_t now = time(0);
	char* dt = ctime(&now);
	const char *payload_text[] = {
		"Date: ", dt, "\r\n",
		"To: ",mail.c_str(), "\r\n",
		"From: " FROM "\r\n",
		"Subject: Analysis\r\n",
		"\r\n",
		"This is just a test message.\r\n",
		"\r\n",
		"A:", a.c_str(), "\r\n",
		"B:", b.c_str(), "\r\n",
		"C:", c.c_str(), "\r\n",
		NULL
	};

		if ((size == 0) || (nmemb == 0) || ((size*nmemb) < 1)) {
		return 0;
	}

	data = payload_text[lines_read];

	if (data) {
		size_t len = strlen(data);
		memcpy(ptr, data, len);
		lines_read++;
		return len;
	}
	return 0;
    }

     void Process::sendmail(std::string mail)
    {
	CURL *curl;
	CURLcode res = CURLE_OK;
	struct curl_slist *recipients = NULL;
	curl = curl_easy_init();
	if (curl) {
	
		curl_easy_setopt(curl, CURLOPT_USERNAME, "User Name");
		curl_easy_setopt(curl, CURLOPT_PASSWORD, "Password");
		curl_easy_setopt(curl, CURLOPT_URL, "smtp.hushmail.com: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, mail.c_str());
		curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients);
		curl_easy_setopt(curl, CURLOPT_READFUNCTION, &Process::processmail);
		curl_easy_setopt(curl, CURLOPT_READDATA, lines_read);
		curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);

		
		curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);

		
		res = curl_easy_perform(curl);

		
		if (res != CURLE_OK)
			fprintf(stderr, "curl_easy_perform() failed: %s\n",
			curl_easy_strerror(res));

		
		curl_slist_free_all(recipients);

		curl_easy_cleanup(curl);
	 }
    }

Posted 10-Jan-17 15:27pm

Member 12890589

Updated 11-Jan-17 0:14am

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Jochen Arndt · Accepted Answer · 2017-01-11T00:14:00

So you have adopted the example from libcurl example - smtp-mail.c[^] and modified it.

One of your modifications is related to the mail content which is created by the callback function. So the problem is obviously located there. A candidate would be the lines_read variable. But you did not show the definition of that variable and where it is initialised with which value.

Callback functions must be static and support therefore usually an pointer argument to allow access to non-static parts of your application. And so it is here. But you did not use this mechanism (removed the corresponding handling provided by the example code). So setting the CURLOPT_READDATA option is useless in your code and also wrong (the argument must be a pointer but you are passing lines_read which is probably an int).

With C++ classes, usually a pointer to the class intance is passed to the callback function. Then you have access to the class members from within the static callback function. Adopting the example code for your case might be done in this way:

C++

class Process
{
    // ...
protected:
    int lines_read;
    static size_t processmail(void *ptr, size_t size, size_t nmemb, void *userp);
};

size_t Process::processmail(void *ptr, size_t size, size_t nmemb, void *userp)
{
    // Must pass 'this' pointer when setting CURLOPT_READDATA
    Process *self = reinterpret_cast<Process*>(userp);
    // ...
    const char *data = payload_text[self->lines_read];
    size_t len = data ? strlen(data) : 0;
    // Check for sufficient buffer size
    // This is missing in the libcurl example
    if (len > nmemb * size)
        len = 0;
    if (len)
    {
        memcpy(ptr, data, len);
        self->lines_read++;
    }
    return len;
}

void Process::sendmail(std::string mail)
{
    lines_read = 0;
    // ...
    curl_easy_setopt(curl, CURLOPT_READDATA, this);
    // ...
}

[EDIT]
Answering below comment and extending solution about an additional bug I did not recognised initially.

The line

std::string &statistics = *(static_cast<std::string*>(userp));

is not working with the above solution and wasn't working with the initial code. The casting must be according to the type passed with CURLOPT_READDATA.

When using my solution passing the class pointer it should be something like:

std::string &statistics = self->statistics;

when statistics is a Process class member of type string.

[/EDIT]