Socket I/O模型之重叠I/O(overlapped I/O)

Winsock2的发布使得Socket I/O有了和文件I/O统一的接口。我们可以通过使用Win32文件操纵函数ReadFile和WriteFile来进行Socket I/O。伴随而来的，用于普通文件I/O的重叠I/O模型和完成端口模型对Socket I/O也适用了。这些模型的优点是可以达到更佳的系统性能，但是实现较为复杂，里面涉及较多的C语言技巧。例如我们在完成端口模型中会经常用到所谓的“尾随数据”。

一、用事件通知方式实现的重叠I/O 模型

C++代码


// write by larry
// 2009-8-20
// This is a server using overlapped IO(event notify).
#include "stdafx.h"
#include <WINSOCK2.H>
#include <stdio.h>
#pragma comment(lib, "ws2_32.lib")
#define PORT 5150
#define MSGSIZE 1024
typedef struct
{
WSAOVERLAPPED overlap;
WSABUF Buffer;
char szMessage[MSGSIZE];
DWORD NumberOfBytesRecvd;
DWORD Flags;
} PER_IO_OPERATION_DATA, *LPPER_IO_OPERATION_DATA;
int g_iTotalConn = 0;
SOCKET g_CliSocketArr[MAXIMUM_WAIT_OBJECTS];
WSAEVENT g_CliEventArr[MAXIMUM_WAIT_OBJECTS];
LPPER_IO_OPERATION_DATA g_pPerIoDataArr[MAXIMUM_WAIT_OBJECTS];
DWORD WINAPI WorkerThread(LPVOID lpParam);
void Cleanup(int index);

int main(int argc, char* argv[])
{
WSADATA wsaData;
SOCKET sListen, sClient;
SOCKADDR_IN local, client;
DWORD dwThreadId;
int iAddrSize = sizeof(SOCKADDR_IN);
// Initialize windows socket library
WSAStartup(0x0202, &wsaData);
// Create listening socket
sListen = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
// Bind
local.sin_family = AF_INET;
local.sin_addr.S_un.S_addr = htonl(INADDR_ANY);
local.sin_port = htons(PORT);
bind(sListen, (sockaddr*)&local, sizeof(SOCKADDR_IN));
// Listen
listen(sListen, 3);
// Create worker thread
CreateThread(NULL, 0, WorkerThread, NULL, 0, &dwThreadId);
while (TRUE)
{
// Accept a connection
sClient = accept(sListen, (sockaddr*)&client, &iAddrSize);
printf("Accepted client:%s:%d\n", inet_ntoa(client.sin_addr), ntohs(client.sin_port));
g_CliSocketArr[g_iTotalConn] = sClient;
// Associate a PER_IO_OPERATION_DATA structure
g_pPerIoDataArr[g_iTotalConn] = (LPPER_IO_OPERATION_DATA)HeapAlloc(
GetProcessHeap(),
HEAP_ZERO_MEMORY,
sizeof(PER_IO_OPERATION_DATA));
g_pPerIoDataArr[g_iTotalConn]->Buffer.len = MSGSIZE;
g_pPerIoDataArr[g_iTotalConn]->Buffer.buf = g_pPerIoDataArr[g_iTotalConn]->szMessage;
g_CliEventArr[g_iTotalConn] = g_pPerIoDataArr[g_iTotalConn]->overlap.hEvent = WSACreateEvent();
// Launch an asynchronous operation
WSARecv(g_CliSocketArr[g_iTotalConn],
&g_pPerIoDataArr[g_iTotalConn]->Buffer,
1,
&g_pPerIoDataArr[g_iTotalConn]->NumberOfBytesRecvd,
&g_pPerIoDataArr[g_iTotalConn]->Flags,
&g_pPerIoDataArr[g_iTotalConn]->overlap,
NULL);
g_iTotalConn++;
}

closesocket(sListen);
WSACleanup();
return 0;
}
DWORD WINAPI WorkerThread(LPVOID lpParam)
{
int ret, index;
DWORD cbTransferred;
while (TRUE)
{
ret = WSAWaitForMultipleEvents(g_iTotalConn, g_CliEventArr, FALSE, 1000, FALSE);
if (ret == WSA_WAIT_FAILED || ret == WSA_WAIT_TIMEOUT)
{
continue;
}
index = ret - WSA_WAIT_EVENT_0;
WSAResetEvent(g_CliEventArr[index]);
WSAGetOverlappedResult(g_CliSocketArr[index],
&g_pPerIoDataArr[index]->overlap,
&cbTransferred,
TRUE,
&g_pPerIoDataArr[g_iTotalConn]->Flags);
if (cbTransferred == 0)
{
// The connection was closed by client
Cleanup(index);
}
else
{
// g_pPerIoDataArr[index]->szMessage contains the recvived data
g_pPerIoDataArr[index]->szMessage[cbTransferred] = '\0';
send(g_CliSocketArr[index], g_pPerIoDataArr[index]->szMessage, cbTransferred, 0);
// Launch another asynchronous operation
WSARecv(g_CliSocketArr[index],
&g_pPerIoDataArr[index]->Buffer,
1,
&g_pPerIoDataArr[index]->NumberOfBytesRecvd,
&g_pPerIoDataArr[index]->Flags,
&g_pPerIoDataArr[index]->overlap,
NULL);
}
}

return 0;
}
void Cleanup(int index)
{
closesocket(g_CliSocketArr[index]);
WSACloseEvent(g_CliEventArr[index]);
HeapFree(GetProcessHeap(), 0, g_pPerIoDataArr[index]);
if (index < g_iTotalConn-1)
{
g_CliSocketArr[index] = g_CliSocketArr[g_iTotalConn-1];
g_CliEventArr[index] = g_CliEventArr[g_iTotalConn-1];
g_pPerIoDataArr[index] = g_pPerIoDataArr[g_iTotalConn-1];
}

g_pPerIoDataArr[--g_iTotalConn] = NULL;
}

这个模型与上述其他模型不同的是它使用Winsock2提供的异步I/O函数WSARecv。在调用WSARecv时，指定一个 WSAOVERLAPPED 结构，这个调用不是阻塞的，也就是说，它会立刻返回。一旦有数据到达的时候，被指定的WSAOVERLAPPED结构中的hEvent被 Signaled。由于下面这个语句

g_CliEventArr[g_iTotalConn] = g_pPerIODataArr[g_iTotalConn]->overlap.hEvent；

使得与该套接字相关联的WSAEVENT对象也被Signaled，所以WSAWaitForMultipleEvents的调用操作成功返回。我们现在应该做的就是用与调用WSARecv相同的WSAOVERLAPPED结构为参数调用WSAGetOverlappedResult，从而得到本次I/O 传送的字节数等相关信息。在取得接收的数据后，把数据原封不动的发送到客户端，然后重新激活一个WSARecv异步操作。

二、 用完成例程方式实现的重叠I/O 模型

C++代码


// write by larry
// 2009-8-20
// This is a server using overlapped IO(completion routine).
#include "stdafx.h"
#include <WINSOCK2.H>
#include <stdio.h>
#pragma comment(lib, "ws2_32.lib")
#define PORT 5150
#define MSGSIZE 1024
typedef struct
{
WSAOVERLAPPED overlap;
WSABUF Buffer;
char szMessage[MSGSIZE];
DWORD NumberOfBytesRecvd;
DWORD Flags;
SOCKET sClient;
} PER_IO_OPERATION_DATA, *LPPER_IO_OPERATION_DATA;
int g_iTotalConn = 0;
SOCKET g_CliSocketArr[MAXIMUM_WAIT_OBJECTS];
WSAEVENT g_CliEventArr[MAXIMUM_WAIT_OBJECTS];
LPPER_IO_OPERATION_DATA g_pPerIoDataArr[MAXIMUM_WAIT_OBJECTS];
DWORD WINAPI WorkerThread(LPVOID lpParam);
void CALLBACK CompletionRoutine(DWORD dwError, DWORD cbTransferred, LPWSAOVERLAPPED lpOverlapped, DWORD dwFlags);
SOCKET g_sNewClientConnection;
BOOL g_bNewConnectionArrived = FALSE;

int main(int argc, char* argv[])
{
WSADATA wsaData;
SOCKET sListen;
SOCKADDR_IN local, client;
DWORD dwThreadId;
int iAddrSize = sizeof(SOCKADDR_IN);
// Initialize windows socket library
WSAStartup(0x0202, &wsaData);
// Create listening socket
sListen = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
// Bind
local.sin_family = AF_INET;
local.sin_addr.S_un.S_addr = htonl(INADDR_ANY);
local.sin_port = htons(PORT);
bind(sListen, (sockaddr*)&local, sizeof(SOCKADDR_IN));
// Listen
listen(sListen, 3);
// Create worker thread
CreateThread(NULL, 0, WorkerThread, NULL, 0, &dwThreadId);
while (TRUE)
{
// Accept a connection
g_sNewClientConnection = accept(sListen, (sockaddr*)&client, &iAddrSize);
g_bNewConnectionArrived = TRUE;
printf("Accepted client:%s:%d\n", inet_ntoa(client.sin_addr), ntohs(client.sin_port));
}
return 0;
}
DWORD WINAPI WorkerThread(LPVOID lpParam)
{
LPPER_IO_OPERATION_DATA lpPerIOData = NULL;
while (TRUE)
{
if (g_bNewConnectionArrived)
{
// Launch an asynchronous operation for new arrived connection
lpPerIOData = (LPPER_IO_OPERATION_DATA)HeapAlloc(
GetProcessHeap(),
HEAP_ZERO_MEMORY,
sizeof(PER_IO_OPERATION_DATA));
lpPerIOData->Buffer.len = MSGSIZE;
lpPerIOData->Buffer.buf = lpPerIOData->szMessage;
lpPerIOData->sClient = g_sNewClientConnection;
WSARecv(lpPerIOData->sClient,
&lpPerIOData->Buffer,
1,
&lpPerIOData->NumberOfBytesRecvd,
&lpPerIOData->Flags,
&lpPerIOData->overlap,
CompletionRoutine);
g_bNewConnectionArrived = FALSE;
}
SleepEx(1000, TRUE);
}
return 0;
}
void CALLBACK CompletionRoutine(DWORD dwError, DWORD cbTransferred, LPWSAOVERLAPPED lpOverlapped, DWORD dwFlags)
{
LPPER_IO_OPERATION_DATA lpPerIOData = (LPPER_IO_OPERATION_DATA)lpOverlapped;
if (dwError != 0 || cbTransferred == 0)
{
// Connection was closed by client
closesocket(lpPerIOData->sClient);
HeapFree(GetProcessHeap(), 0, lpPerIOData);
}
else
{
lpPerIOData->szMessage[cbTransferred] = '\0';
send(lpPerIOData->sClient, lpPerIOData->szMessage, cbTransferred, 0);
// Launch another asynchronous operation
memset(&lpPerIOData->overlap, 0, sizeof(WSAOVERLAPPED));
lpPerIOData->Buffer.len = MSGSIZE;
lpPerIOData->Buffer.buf = lpPerIOData->szMessage;
WSARecv(lpPerIOData->sClient,
&lpPerIOData->Buffer,
1,
&lpPerIOData->NumberOfBytesRecvd,
&lpPerIOData->Flags,
&lpPerIOData->overlap,
CompletionRoutine);
}
}

用完成例程来实现重叠I/O比用事件通知简单得多。在这个模型中，主线程只用不停的接受连接即可；辅助线程判断有没有新的客户端连接被建立，如果有，就为那个客户端套接字激活一个异步的WSARecv操作，然后调用SleepEx使线程处于一种可警告的等待状态，以使得I/O完成后 CompletionROUTINE可以被内核调用。如果辅助线程不调用SleepEx，则内核在完成一次I/O操作后，无法调用完成例程（因为完成例程的运行应该和当初激活WSARecv异步操作的代码在同一个线程之内）。

完成例程内的实现代码比较简单，它取出接收到的数据，然后将数据原封不动的发送给客户端，最后重新激活另一个WSARecv异步操作。注意，在这里用到了 “尾随数据”。我们在调用WSARecv的时候，参数lpOverlapped实际上指向一个比它大得多的结构 PER_IO_OPERATION_DATA，这个结构除了WSAOVERLAPPED以外，还被我们附加了缓冲区的结构信息，另外还包括客户端套接字等重要的信息。这样，在完成例程中通过参数lpOverlapped拿到的不仅仅是WSAOVERLAPPED结构，还有后边尾随的包含客户端套接字和接收数据缓冲区等重要信息。这样的C语言技巧在我后面介绍完成端口的时候还会使用到。