《Windows API巡礼》---listen、accept和connect

listen函数是用来在socket服务器端将套接字置入监听模式并准备接受客户端连接请求：
int listen(
  __in  SOCKET s, //一个已绑定但尚未连接的套接字
  __in  int backlog //指定正在等待连接的最大队列长度。如果backlog为3，而有4个客户机
                  //同时发出连接请求，则前3个放在等待队列中，最后一个将被忽略
);
返回值：
成功时，返回0；
失败时，返回SOCKET_ERROR，调用WSAGetLastError函数查看进一步错误信息。
 
accept函数用于响应客户端的连接请求：
SOCKET accept(
  __in     SOCKET s, //处于监听模式的套接字
  __out    struct sockaddr *addr, //一个有效的sockaddr_in结构的地址，
//函数成功返回时，包含发出连接请求的客户机的IP地址信息
  __inout  int *addrlen //sockaddr_in结构的长度
);
返回值：
成功时，返回一个新的socket，对应于已经接受的那个客户端连接。对于该客户端所有的后续操作，都使用这个新的套接字；原来的套接字则仍处于监听模式，继续接受其他客户端的连接请求。
失败时，返回INCVALID_SOCKET，调用WSAGetLastError函数查看进一步错误信息。
 
listen和accept的实例代码如下：
#include <winsock2.h>
#include <stdio.h>
#include <windows.h>
#pragma comment(lib, "wininet.lib")
 
int main(void) {
 
  //----------------------
  // Initialize Winsock.
  WSADATA wsaData;
  int iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
  if (iResult != NO_ERROR) {
    printf("Error at WSAStartup()/n");
    return 1;
  }
 
  //----------------------
  // Create a SOCKET for listening for
  // incoming connection requests.
  SOCKET ListenSocket;
  ListenSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
  if (ListenSocket == INVALID_SOCKET) {
    printf("Error at socket(): %ld/n", WSAGetLastError());
    WSACleanup();
    return 1;
  }
 
  //----------------------
  // The sockaddr_in structure specifies the address family,
  // IP address, and port for the socket that is being bound.
  sockaddr_in service;
  service.sin_family = AF_INET;
  service.sin_addr.s_addr = inet_addr("127.0.0.1");
  service.sin_port = htons(27015);
 
  if (bind( ListenSocket, 
    (SOCKADDR*) &service, 
    sizeof(service)) == SOCKET_ERROR) {
    printf("bind() failed./n");
    closesocket(ListenSocket);
    WSACleanup();
    return 1;
  }
 
  //----------------------
  // Listen for incoming connection requests.
  // on the created socket
  if (listen( ListenSocket, 1 ) == SOCKET_ERROR) {
    printf("Error listening on socket./n");
    closesocket(ListenSocket);
    WSACleanup();
    return 1;
  }
 
  //----------------------
  // Create a SOCKET for accepting incoming requests.
  SOCKET AcceptSocket;
  printf("Waiting for client to connect.../n");
 
  //----------------------
  // Accept the connection.
  AcceptSocket = accept( ListenSocket, NULL, NULL );
  if (AcceptSocket == INVALID_SOCKET) {
    printf("accept failed: %d/n", WSAGetLastError());
    closesocket(ListenSocket);
    WSACleanup();
    return 1;
  } else 
    printf("Client connected./n");
  
  // No longer need server socket
  closesocket(ListenSocket);
 
  WSACleanup();
  return 0;
}
 
connect函数用于客户端与网络中的服务器建立连接：
int connect(
  __in  SOCKET s, //进行连接的套接字
  __in  const struct sockaddr *name, //要连接服务器的套接字地址结构
  __in  int namelen //name的长度
);
返回值：
成功时，返回0；
失败时，返回SOCKET_ERROR，调用WSAGetLastError函数查看进一步错误信息。
connect的实例代码如下：
#include <stdio.h>
#include "winsock2.h"
#include <windows.h>
#pragma comment(lib, "wininet.lib")
 
void main() {
  //----------------------
  // Initialize Winsock
  WSADATA wsaData;
  int iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
  if (iResult != NO_ERROR)
    printf("Error at WSAStartup()/n");
 
  //----------------------
  // Create a SOCKET for connecting to server
  SOCKET ConnectSocket;
  ConnectSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
  if (ConnectSocket == INVALID_SOCKET) {
    printf("Error at socket(): %ld/n", WSAGetLastError());
    WSACleanup();
    return;
  }
 
  //----------------------
  // The sockaddr_in structure specifies the address family,
  // IP address, and port of the server to be connected to.
  sockaddr_in clientService; 
  clientService.sin_family = AF_INET;
  clientService.sin_addr.s_addr = inet_addr( "127.0.0.1" );
  clientService.sin_port = htons( 27015 );
 
  //----------------------
  // Connect to server.
  if ( connect( ConnectSocket, (SOCKADDR*) &clientService, sizeof(clientService) ) == SOCKET_ERROR) {
    printf( "Failed to connect./n" );
    WSACleanup();
    return;
  }
 
  printf("Connected to server./n");
  WSACleanup();
  return;
}