c++下使用邮槽实现进程间通信

　　Windows API提供了邮槽和命名管道两种机制来实现进程间通信，在这里使用C++实现邮槽。

　　邮槽是Windows提供的一种进程间单向通信的机制，进程中的一方只能读取(或写入)数据，而另一方只能写入(或读取)数据。这种进程间的通信可以发生在本地或者网络之中。而在使用邮槽之前，服务器端必须先创建邮槽，创建的函数原型如下：

 HANDLE WINAPI CreateMailslot(
   _In_     LPCTSTR               lpName,
   _In_     DWORD                 nMaxMessageSize,
   _In_     DWORD                 lReadTimeout,
   _In_opt_ LPSECURITY_ATTRIBUTES lpSecurityAttributes
 );

　　其中参数lpName表示邮槽的名称。邮槽名称的格式为"\\.\mailslot\YourMailslotName"，其中YourMailslotName由用户指定。需要注意的是，在实际编码中反斜杠需要转义；参数nMaxMessageSize表示发送的消息大小的最大值，若设置为0则表示大小为任意值。（实际上邮槽能传输的数据非常小，一般400KB，若数据过大，邮槽可能无法正常工作）；参数lReadTimeout表示读取操作的超时时间；参数lpSecurityAttributes表示邮槽的安全属性，置为NULL表示使用默认的安全属性。

　　客户端在使用邮槽前必须先打开邮槽，通过函数CreateFile()实现，函数原型如下：

 HANDLE WINAPI CreateFile(
   _In_     LPCTSTR               lpFileName,
   _In_     DWORD                 dwDesiredAccess,
   _In_     DWORD                 dwShareMode,
   _In_opt_ LPSECURITY_ATTRIBUTES lpSecurityAttributes,
   _In_     DWORD                 dwCreationDisposition,
   _In_     DWORD                 dwFlagsAndAttributes,
   _In_opt_ HANDLE                hTemplateFile
 );

　　参数的具体设置方法可参考MSDN给出的解释：

　　https://msdn.microsoft.com/en-us/library/windows/desktop/aa363858(v=vs.85).aspx

　　需要注意的是，指定要打开的邮槽时，若程序是在不同主机上运行的，邮槽名称中的点号"."需要改成对方主机的名称。

　　在实际的编程过程中，对邮槽的操作与文件一样，都是通过调用函数ReadFile()和WriteFile()实现的，函数原型如下：

 BOOL WINAPI ReadFile(
   _In_        HANDLE       hFile,
   _Out_       LPVOID       lpBuffer,
   _In_        DWORD        nNumberOfBytesToRead,
   _Out_opt_   LPDWORD      lpNumberOfBytesRead,
   _Inout_opt_ LPOVERLAPPED lpOverlapped
 );

 BOOL WINAPI WriteFile(
   _In_        HANDLE       hFile,
   _In_        LPCVOID      lpBuffer,
   _In_        DWORD        nNumberOfBytesToWrite,
   _Out_opt_   LPDWORD      lpNumberOfBytesWritten,
   _Inout_opt_ LPOVERLAPPED lpOverlapped
 );

　　其中参数lpNumberOfBytesWritten是一个指向DWORD类型的指针，表示实际读取/写入的字节数。

　　最终实现的代码如下，实现面向对象的方法实现：

　　服务器端：

 //header.h
 #ifndef HEADER_H
 #define HEADER_H

 #include <windows.h>

 #define BUFFER_SIZE 1024

 class MailServer
 {
 public:
     MailServer();
     MailServer(const MailServer &) = delete;
     MailServer & operator=(const MailServer &) = delete;
     ~MailServer();
     void ReadMail();
 private:
     HANDLE h_mail;
     char buffer[BUFFER_SIZE];
     DWORD exact_read_num; //指向实际读取的字节数的指针
 };

 #endif

 //definition.cpp
 #include "header.h"
 #include <iostream>

 MailServer::MailServer()
 {
     //邮槽的命名格式为"\\.\mailslot\YourMailslotName",反斜杠需要转义,采用非阻塞式读取方法
     h_mail = ::CreateMailslot("\\\\.\\mailslot\\MyMailSlot", , , nullptr);
     if (h_mail == INVALID_HANDLE_VALUE)
     {
         std::cerr << "Failed to create a mailslot!\n";
         ::system("pause");
         exit();
     }
     else
     {
         std::cout << "Mailslot created successfully..." << std::endl;
     }
 }

 MailServer::~MailServer()
 {
     ::CloseHandle(h_mail);
     std::cout << "Mailslot closed..." << std::endl;
 }

 void MailServer::ReadMail()
 {
     std::cout << "Reading mail from mailslot..." << std::endl;
     while (true)
     {
         if (::ReadFile(h_mail, buffer, BUFFER_SIZE, &exact_read_num, nullptr))
         {
             std::cout << "New mail: " << buffer << std::endl;
         }
     }
 }

 //server.cpp
 #include "header.h"

 int main()
 {
     MailServer mail_svr;
     mail_svr.ReadMail();
     system("pause");
     return ;
 }

　　客户端：

 //header.h
 #ifndef HEADER_H
 #define HEADER_H

 #include "windows.h"

 #define BUFFER_SIZE 1024

 class MailClient
 {
 public:
     MailClient();
     MailClient(const MailClient &) = delete;
     MailClient & operator=(const MailClient &) = delete;
     ~MailClient();
     void SendMail();
 private:
     HANDLE h_mail;
     char buffer[BUFFER_SIZE];
     DWORD exact_write_num;
 };

 #endif

 //definition.cpp
 #include "header.h"
 #include <iostream>

 MailClient::MailClient()
 {
     h_mail = ::CreateFile("\\\\.\\mailslot\\MyMailSlot", GENERIC_WRITE, FILE_SHARE_READ, nullptr, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr);
     if (h_mail == INVALID_HANDLE_VALUE)
     {
         std::cerr << "Failed to create a mailslot!\n";
         system("pause");
         exit();
     }
     else
     {
         std::cout << "Mailslot created successfully..." << std::endl;
     }
 }

 MailClient::~MailClient()
 {
     ::CloseHandle(h_mail);
     std::cout << "Mailslot closed..." << std::endl;
 }

 void MailClient::SendMail()
 {
     while (true)
     {
         std::cout << "Please write a mail: " << std::flush;
         std::cin.getline(buffer, BUFFER_SIZE);
         if (strcmp(buffer, "exit") == )
         {
             std::cout << "User requests to close the mailslot..." << std::endl;
             break;
         }
         else
         {
             if (::WriteFile(h_mail, buffer, BUFFER_SIZE, &exact_write_num, nullptr))
             {
                 std::cout << "Mail sent successfully..." << std::endl;
             }
             else
             {
                 std::cerr << "Failed to send the mail...\n";
                 system("pause");
                 exit();
             }
         }
     }
 }

 #include "header.h"

 int main()
 {
     MailClient mail_clt;
     mail_clt.SendMail();
     system("pause");
     return ;
 }