C++调用libcurl开源库实现邮件的发送功能流程详解
目录
- 1、为啥要选择libcurl库去实现邮件的发送
- 2、调用libcurl库的API接口实现邮件发送
- 3、构造待发送的邮件内容
- 4、开通163发送邮件账号的SMTP服务
- 5、排查接收的邮件内容为空的问题
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:
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"; }
4、开通163发送邮件账号的SMTP服务
上述代码处理好后,运行如下的测试程序:
在上述界面中输入163的smtp服务器地址,使用默认的25端口,并填写发送邮件地址和发送邮件的密码,点击“发送测试邮件”按钮,结果邮件并没有发送成功。
在代码中添加断点调试,发现curl_easy_perform接口返回的错误码为CURLE_LOGIN_DENIED,如下所示:
于是通过CURLE_OK go到错误码定义的头文件中,去查看CURLE_LOGIN_DENIED错误码的含义:
注释中提示可能是发送邮件的用户名或密码错误引起的。用户名和密码填写的应该没问题啊?于是账号到网页上登陆一下163邮箱,可以成功登陆的,说明账号和密码是没问题的。那到底是咋回事呢?
后来想到,是不是要到发送邮件账号中去开启一下smtp服务才可以登陆到163的smtp服务器上?于是到网页上登陆,按下列的操作步骤找到开启当前账号的smtp服务入口:
点击开启按钮,会弹出如下的提示框:
点击继续开启,进入下面的页面:
提示需要扫码发送短信进行验证。于是使用网易邮件大师APP扫描了一下,自动跳转到发送短信的页面,发送验证短信即可。最后弹出如下的授权密码页面:
要将这个授权密码记录下来,登陆smtp服务器时需要使用这个授权密码,而不是账号的密码!
于是在测试页面中输入授权码,邮件就能发送成功了。
5、排查接收的邮件内容为空的问题
邮件是能正常发送出去了,邮件也能正常接收到,但接收到的邮件内容是空的:
这是啥情况?明明发送邮件时有设置邮件内容的,为啥收到的邮件内容是空的呢?
上述代码在几年前测试过,好像没问题的,难道163邮箱系统升级了,不再兼容老的数据格式了?于是想到了海康的视频监控客户端,该客户端可以到海康官网上下载,免费使用,其中系统设置中有个发送邮件的功能:
海康的上述界面中发送测试邮件是没问题的,接收到的邮件也是有内容的。于是赶紧抓一下海康发送邮件的数据包,以tcp.port==25过滤了一下,抓出海康发出去的邮件内容:
又抓取了一下我们软件发出去的邮件内容如下:
于是详细地对比了海康与我们发出去的数据内容,多次尝试修改我们构建邮件数据的代码,比如charset编码格式、boundry类型等,甚至是否会空行。最后经过多次尝试找到了原因,是在具体的邮件内容上面需要人为加上一个空行,我们代码在构造邮件数据时没有加空行,导致接收到的邮件内容是空的!
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