C++调用libcurl开源库实现邮件的发送功能流程详解

libcurl是一个免费开源的网络传输库，支持ftp、ftps、tftp，http、https、telnet、ldap、pop3、smtp等多种协议，接下来让我们一起来了解吧

libcurl中封装了支持这些协议的网络通信模块，支持跨平台，支持Windows，Unix，Linux等多个操作系统。libcurl提供了一套统一样式的API接口，我们不用关注各种协议下网络通信的实现细节，只需要调用这些API就能轻松地实现基于这些协议的数据通信。本文将简单地讲述一下使用libcurl实现邮件发送的相关细节。

1、为啥要选择libcurl库去实现邮件的发送

如果我们自己去使用socket套接字去编码，实现连接smtp邮件服务器，并完成和服务器的smtp协议的交互，整个过程走下来会非常地复杂，特别是要处理网络通信过程中的多种异常，整个流程的稳定性和健壮性没有保证。

而libcurl中已经实现了smtp协议的所有流程，我们不需要去关注协议的具体实现细节，我们只要去调用libcurl的API接口就能实现发送邮件的功能。libcurl库的稳定性是毋庸置疑的。

我们可以到官网上下载libcurl开源库最新的源码，直接使用Visual Studio编译出要用的dll库，至于使用Visual Studio如何编译libcurl代码，后面我会写一篇文章去详细介绍。

2、调用libcurl库的API接口实现邮件发送

先调用curl_easy_init接口初始化libcurl库，然后调用curl_easy_setopt（使用CURLOPT_URL选项）设置url请求地址，正是通过该url的前缀确定具体使用哪种协议。比如本例中发送邮件时需要使用smtp协议：

char urlBuf[256] = { 0 };
sprintf( urlBuf, "smtp://%s:%s", m_strServerName.c_str(), m_strPort.c_str() );
curl_easy_setopt(curl, CURLOPT_URL, urlBuf);

设置url时使用的就是smtp前缀，然后带上目标服务器的IP和端口。

在使用相关协议完成数据交互时，可能还要设置一些其他的信息，比如用户名和密码等，都是通过调用curl_easy_setopt设置的：

curl_easy_setopt(curl, CURLOPT_USERNAME, m_strUserName.c_str());  
curl_easy_setopt(curl, CURLOPT_PASSWORD, m_strPassword.c_str());

要发送的数据，则通过CURLOPT_READDATA选项去设置：

std::stringstream stream;  
stream.str(m_strMessage.c_str());  
stream.flush();

/* We're using a callback function to specify the payload (the headers and 
* body of the message). You could just use the CURLOPT_READDATA option to 
* specify a FILE pointer to read from. */  
curl_easy_setopt(curl, CURLOPT_READFUNCTION, &CSmtpSendMail::payload_source);  
curl_easy_setopt(curl, CURLOPT_READDATA, (void *)&stream);  
curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);

最后调用curl_easy_perform或者curl_multi_perform接口发起请求，该接口内部将去连接url中指定的服务器，并完成指定的协议协商与交互，并最终完成与服务器之间的数据通信。

调用libcurl库发送邮件的完整代码如下所示：

CURLcode CSmtpSendMail::SendMail()  
{  
	CreatMessage();  
	bool ret = true;  
	CURL *curl;  
	CURLcode res = CURLE_OK;  
	struct curl_slist *recipients = NULL;  

	curl = curl_easy_init();  
	if (curl) {  
		/* Set username and password */                                        
		curl_easy_setopt(curl, CURLOPT_USERNAME, m_strUserName.c_str());  
		curl_easy_setopt(curl, CURLOPT_PASSWORD, m_strPassword.c_str());  

		char urlBuf[256] = { 0 };
		sprintf( urlBuf, "smtp://%s:%s", m_strServerName.c_str(), m_strPort.c_str() );
		curl_easy_setopt(curl, CURLOPT_URL, urlBuf);  
		/* If you want to connect to a site who isn't using a certificate that is 
		* signed by one of the certs in the CA bundle you have, you can skip the 
		* verification of the server's certificate. This makes the connection 
		* A LOT LESS SECURE. 
		* 
		* If you have a CA cert for the server stored someplace else than in the 
		* default bundle, then the CURLOPT_CAPATH option might come handy for 
		* you. */  
#ifdef SKIP_PEER_VERIFICATION  
		curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L);  
#endif  

		/* If the site you're connecting to uses a different host name that what 
		* they have mentioned in their server certificate's commonName (or 
		* subjectAltName) fields, libcurl will refuse to connect. You can skip 
		* this check, but this will make the connection less secure. */  
#ifdef SKIP_HOSTNAME_VERIFICATION  
		curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 0L);  
#endif  

		/* Note that this option isn't strictly required, omitting it will result 
		* in libcurl sending the MAIL FROM command with empty sender data. All 
		* autoresponses should have an empty reverse-path, and should be directed 
		* to the address in the reverse-path which triggered them. Otherwise, 
		* they could cause an endless loop. See RFC 5321 Section 4.5.5 for more 
		* details. 
		*/  
		//curl_easy_setopt(curl, CURLOPT_MAIL_FROM, FROM);   
		curl_easy_setopt(curl, CURLOPT_MAIL_FROM, m_strSendMail.c_str());  
		/* Add two recipients, in this particular case they correspond to the 
		* To: and Cc: addressees in the header, but they could be any kind of 
		* recipient. */  
		for (size_t i = 0; i < m_vRecvMail.size(); i++) {  

			recipients = curl_slist_append(recipients, m_vRecvMail[i].c_str());  
		}  
		curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients);  

		std::stringstream stream;  
		stream.str(m_strMessage.c_str());  
		stream.flush();  
		/* We're using a callback function to specify the payload (the headers and 
		* body of the message). You could just use the CURLOPT_READDATA option to 
		* specify a FILE pointer to read from. */  

		// 注意回调函数必须设置为static
		curl_easy_setopt(curl, CURLOPT_READFUNCTION, &CSmtpSendMail::payload_source);  
		curl_easy_setopt(curl, CURLOPT_READDATA, (void *)&stream);  
		curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);  

		/* Since the traffic will be encrypted, it is very useful to turn on debug 
		* information within libcurl to see what is happening during the 
		* transfer */  
		curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);  

		curl_multi_perform()
		/* Send the message */  
		res = curl_easy_perform(curl);  
		CURLINFO info = CURLINFO_NONE;  
		curl_easy_getinfo(curl, info);  
		/* Check for errors */  

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

			char achErrInfo[512] = {0};
			sprintf( achErrInfo, "curl_easy_perform() failed, error info: %s\n\n", curl_easy_strerror(res) );
			::MessageBoxA( NULL, achErrInfo, "Tip", MB_OK);
			ret = false;  

			m_strErrDesription = achErrInfo;

			/*				Sleep( 100 );
			res = curl_easy_perform(curl); */ 
		}  
		else
		{
			m_strErrDesription = "";
		}

		/* Free the list of recipients */  
		curl_slist_free_all(recipients);  

		/* Always cleanup */  
		curl_easy_cleanup(curl);  
	}  
	else
	{
		res = CURLE_FAILED_INIT;
		char achErrInfo[512] = {0};
		sprintf( achErrInfo, "curl_easy_init() failed, error info: %s\n\n", curl_easy_strerror(res) );
		m_strErrDesription = achErrInfo;
  }
	return res;  
}

3、构造待发送的邮件内容

libcurl负责和smtp邮件服务器建链，完成smtp简单邮件协议的协商与交互，但要发送的邮件内容则需要我们自己去根据协议的规范去构建。那邮件发送的内容的数据格式到底是什么样子的呢？其实很简单，找一个支持发送邮件的软件，发送邮件时抓一下包，就能抓出对应的格式，比如：

C++调用libcurl开源库实现邮件的发送功能流程详解

按照上面的格式构建就可以了，相关代码如下：

void CSmtpSendMail::CreatMessage()  
{  
	//m_strMessage = "Date: 13 Nov 2021 12:52:14 +0800";
	m_strMessage = "From: ";  
	m_strMessage += m_strSendMail;  
	m_strMessage += "\r\nReply-To: ";  
	m_strMessage += m_strSendMail;  
	m_strMessage += "\r\nTo: ";  
	for (size_t i = 0; i < m_vRecvMail.size(); i++)  
	{  
		if (i > 0) {  
			m_strMessage += ",";  
		}  
		m_strMessage += m_vRecvMail[i];  
	}  
	m_strMessage += "\r\n";  
	m_strMessage += m_strSubject;  
	m_strMessage += "\r\nX-Mailer: The Bat! (v3.02) Professional";  
	m_strMessage += "\r\nMime-Version: 1.0";  
	m_strMessage += "\r\nContent-Type: multipart/mixed;";  
	m_strMessage += "boundary=\"simple boundary\"";  //__MESSAGE__ID__54yg6f6h6y456345
	//m_strMessage += "\r\nThis is a multi-part message in MIME format.";  
	m_strMessage += "\r\n\r\n--simple boundary";  
	//正文  
	m_strMessage += "\r\nContent-Type: text/html;";  
	m_strMessage += "charset=";  
	//m_strMessage += "\"";  
	m_strMessage += m_strCharset;  
	//m_strMessage += "\"";  
	m_strMessage += "\r\nContent-Transfer-Encoding: 7bit";  
	m_strMessage += "\r\n";  
	m_strMessage += m_strContent;  

	//附件  
	std::string filename = "";  
	std::string filetype = "";  
	for (size_t i = 0; i < m_vAttachMent.size(); i++)  
	{  
		m_strMessage += "\r\n--simple boundary";  
		GetFileName(m_vAttachMent[i], filename);  
		GetFileType(m_vAttachMent[i], filetype);  
		SetContentType(filetype);  
		SetFileName(filename);  

		m_strMessage += "\r\nContent-Type: ";  
		m_strMessage += m_strContentType;  
		m_strMessage += "\tname=";  
		m_strMessage += "\"";  
		m_strMessage += m_strFileName;  
		m_strMessage += "\"";  
		m_strMessage += "\r\nContent-Disposition:attachment;filename=";  
		m_strMessage += "\"";  
		m_strMessage += m_strFileName;  
		m_strMessage += "\"";  
		m_strMessage += "\r\nContent-Transfer-Encoding:base64";   
		m_strMessage += "\r\n\r\n";  


		FILE *pt = NULL;  
		if ((pt = fopen(m_vAttachMent[i].c_str(), "rb")) == NULL) {  

			std::cerr << "打开文件失败: " << m_vAttachMent[i] <<std::endl;  
			continue;  
		}  
		fseek(pt, 0, SEEK_END);  
		int len = ftell(pt);  
		fseek(pt, 0, SEEK_SET);  
		int rlen = 0;  
		char buf[55];  
		for (size_t i = 0; i < len / 54 + 1; i++)  
		{  
			memset(buf, 0, 55);  
			rlen = fread(buf, sizeof(char), 54, pt);  
			m_strMessage += base64_encode((const unsigned char*)buf, rlen);  
			m_strMessage += "\r\n";  
		}  

		fclose(pt);  
		pt = NULL;  
	}  

	m_strMessage += "\r\n--simple boundary--\r\n";  
}