server2.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <unistd.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <string.h>

#define oops(msg) { perror(msg); exit(1); }

int main(int ac, char *av[])
{
	struct sockaddr_in serv_addr;
	int serv_sock, sock_fd1, sock_fd2, value1, value2;
	char message1[BUFSIZ], message2[BUFSIZ];
	char successMSG[BUFSIZ] = "successfully connected!";

	if(ac != 2)
	{
		fprintf(stderr, "usage: %s port\n", av[0]);
		exit(1);
	}

	serv_sock = socket(PF_INET, SOCK_STREAM, 0);
	if(serv_sock == -1)
		oops("socket");

	memset(&serv_addr, 0, sizeof(serv_addr)); // serv_addr[0]부터 sizeof(serv_addr)만큼 0으로 지정	
	serv_addr.sin_family = AF_INET;
	serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
	serv_addr.sin_port = htons(atoi(av[1]));

	if(bind(serv_sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) != 0)
		oops("bind");

	if(listen(serv_sock, 1) != 0)
		oops("listen");

	sock_fd1 = accept(serv_sock, NULL, NULL); // accept은 blocking
	sock_fd2 = accept(serv_sock, NULL, NULL); // 2개가 들어올 때까지 기다림
	if(sock_fd1 == -1 || sock_fd2 == -1)
		oops("accept");
	printf("%s\n", successMSG);	

	write(sock_fd1, successMSG, strlen(successMSG)+1);
	write(sock_fd2, successMSG, strlen(successMSG)+1);

	while(1)
	{
		value1 = recv(sock_fd1, message1, sizeof(message1), MSG_DONTWAIT);
		value2 = recv(sock_fd2, message2, sizeof(message2), MSG_DONTWAIT);

		if(value1 != -1)
		{
			printf("%d send: %s\n", sock_fd1, message1);
			write(sock_fd2, message1, strlen(message1)+1);
		}
		if(value2 != -1)
		{
			printf("%d send: %s\n", sock_fd2, message2);
			write(sock_fd1, message2, strlen(message2)+1);
		}
	}
	close(sock_fd1);
	close(sock_fd2);
	close(serv_sock);
}