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"\.mailslotYourMailslotName"，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"\.mailslotYourMailslotName",反斜杠需要转义,采用非阻塞式读取方法
    h_mail = ::CreateMailslot("\\.\mailslot\MyMailSlot", 0, 0, nullptr);
    if (h_mail == INVALID_HANDLE_VALUE)
    {
        std::cerr << "Failed to create a mailslot!
";
        ::system("pause");
        exit(1);
    }
    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 0;
}

　　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!
";
        system("pause");
        exit(1);
    }
    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") == 0)
        {
            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...
";
                system("pause");
                exit(1);
            }
        }
    }
}

#include "header.h"

int main()
{
    MailClient mail_clt;
    mail_clt.SendMail();
    system("pause");
    return 0;
}