Der Rest ist normalerweise selbstklärend. Den Chat einfach im Terminal (unter OS X) starten, dann noch ein Terminal Fenster auf machen, auch hier den Chat starten, und schon funktionierts!
Hint:
- kompilieren per "g++ -o test main.c"
- aufruf per "./chat"
- (natürlich ohne die " " )

#include
#include
#include
#include
#include
#include
#include
#include
#include
#include 

#include
#include 

#include 

using namespace std;

void reportError(string str) {
	cout << endl << str << errno << endl;
}

int main(int argc, char* argv[])
{
	// set the variables
	int sock, bytes_recieved, bytes_send;
	int port1 = atoi("5000");
	int port2 = atoi("5001");
	char send_data [1024] , recv_data[1024];       

	struct sockaddr_in myAddr;
	struct sockaddr_in clientAddr;
	socklen_t sin_size;

	fd_set rfds;

	// create the udp socket
	if ((sock = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
		reportError("Cannot create send socket ....");
		exit(1);
	}

	myAddr.sin_family = AF_INET;
	myAddr.sin_port = htons(port1);
	myAddr.sin_addr.s_addr = inet_addr("127.0.0.1");

	if (bind(sock, (struct sockaddr *)&myAddr, sizeof(struct sockaddr))== -1) {
		myAddr.sin_family = AF_INET;
		myAddr.sin_port = htons(port2);
		myAddr.sin_addr.s_addr = inet_addr("127.0.0.1");

		clientAddr.sin_family = AF_INET;
		clientAddr.sin_port = htons(port1);
		clientAddr.sin_addr.s_addr = inet_addr("127.0.0.1");

		if(bind(sock, (struct sockaddr *)&myAddr, sizeof(struct sockaddr))== -1) {
			reportError("Unable to bind socket.");
			exit(1);
		}
	}
	else
	{
		clientAddr.sin_family = AF_INET;
		clientAddr.sin_port = htons(port2);
		clientAddr.sin_addr.s_addr = inet_addr("127.0.0.1");
	}

	cout << "Wait for data, or type something!" << endl;

	// process incoming connections (forever) ....
	while(1){
		sin_size = sizeof(struct sockaddr_in);

		FD_ZERO(&rfds);
		FD_SET(sock, &rfds);
		FD_SET(0, &rfds);

		select(sock + 1, &rfds, NULL, NULL, NULL);

		if(FD_ISSET(sock, &rfds))
		{
			// receive the reply from the client
			bytes_recieved = recvfrom(sock, recv_data, 1024,0, (struct sockaddr*)&myAddr, &sin_size);

			// client wants the leave, so do we
			if (strcmp(recv_data , "q") == 0 || strcmp(recv_data , "Q") == 0) {
				close(sock);
				break;
			}
			else if(bytes_recieved == -1)
			{
				reportError("recvfrom failed.");
				exit(1);
			} else {
				recv_data[bytes_recieved] = '\0';
				cout << " " << recv_data << endl;
			}
		}
		else if(FD_ISSET(0, &rfds))
		{
			gets(send_data);

			if (strcmp(send_data , "q") != 0 && strcmp(send_data , "Q") != 0) {
				bytes_send = sendto(sock, send_data, 1024, 0, (struct sockaddr*)&clientAddr, sizeof(struct sockaddr));

				if(bytes_send == -1)
				{
					reportError("Error sending data.");
					exit(1);
				}
			}
			else
			{
				close(sock);
				break;
			}
		}
	}

	close(sock);

	return 0;
}

Hoffe jemanden bringts was!