// test.cpp : 定義控制臺應用程序的入口點。//#include "stdafx.h"#include <stdio.h>#include "winsock2.h"#include <stdlib.h>#pragma comment(lib,"ws2_32.lib")int main(){	WSADATA wsa;	if (WSAStartup(MAKEWord(2, 2), &wsa) != NO_ERROR)	{		printf("startup failed/n");	}	SOCKET listensocket;	sockaddr_in server;	int retval, optlen;	if ((listensocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) == INVALID_SOCKET)	{		printf("socket error/n");	}	hostent *phost;	u_short port = 9901;	server.sin_family = AF_INET;	server.sin_port = htons(port);	server.sin_addr.S_un.S_addr = htonl(INADDR_ANY);	if (bind(listensocket, (sockaddr*)&server, sizeof(sockaddr)) == SOCKET_ERROR)	{		printf("%d",GetLastError());		printf("bind error/n");	}	optlen = sizeof(int);	getsockopt(listensocket, SOL_SOCKET, SO_ACCEPTCONN, (char*)&retval, &optlen);	printf("%ld/n", retval);	if (listen(listensocket, 3) == SOCKET_ERROR)	{		printf("listen error/n");	}	getsockopt(listensocket, SOL_SOCKET, SO_ACCEPTCONN, (char*)&retval, &optlen);	printf("%ld/n", retval);	return 0;}執行結果如下:監聽之前為0，監聽之后為12.setsockopt(SOCKET s,int level,int optname, const char* optval, int* optlen)五個參數的意義和getsockopt的意義很相近，只是第3個參數在這里表示要被設置的選項，第四個參數表示要設置的值，需要強制轉換為字符指針。當我們需要實現UDP廣播的時候，需要將socket設置為允許廣播，即
bool setopt = true;retval = setsockopt(s, SOL_SOCKET, SO_BROADCAST, (char*)&setopt, sizeof(bool));然后將目的IP地址設為INADDR_BOARDCAST就行了。完整代碼如下：
// test.cpp : 定義控制臺應用程序的入口點。//#include "stdafx.h"#include <stdio.h>#include "winsock2.h"#include <stdlib.h>#pragma comment(lib,"ws2_32.lib")int main(){	WSADATA wsa;	int retval = WSAStartup(MAKEWORD(2, 2), &wsa);	if (retval == SOCKET_ERROR)	{		printf("start up failed/n");	}	SOCKET s;	s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);	if (s == INVALID_SOCKET)	{		printf("socket failed/n");	}	bool setopt = true;	retval = setsockopt(s, SOL_SOCKET, SO_BROADCAST, (char*)&setopt, sizeof(bool));	if (retval == SOCKET_ERROR)	{		printf("setsocketopt failed/n");	}	sockaddr_in fromaddr;	fromaddr.sin_family = AF_INET;	fromaddr.sin_port = htons(9000);	fromaddr.sin_addr.S_un.S_addr = htonl(INADDR_ANY);	retval = bind(s, (sockaddr*)&fromaddr, sizeof(sockaddr));	if (retval == SOCKET_ERROR)	{		printf("bind failed/n");	}	sockaddr_in toaddr;	toaddr.sin_family = AF_INET;	toaddr.sin_port = htons(9001);	toaddr.sin_addr.S_un.S_addr = htonl(INADDR_BROADCAST);	char buf[] = { "this is a boardcast" };	retval = sendto(s, buf, sizeof(buf), 0, (sockaddr*)&toaddr, sizeof(toaddr));	if (retval == SOCKET_ERROR)	{		printf("sendto failed/n");	}	WSACleanup();	closesocket(s);	return 0;}