服务器代码：

#include <unp.h>

void
str_echo(int sockfd)
{
ssize_t n;
char buf[MAXLINE];

again:
while ((n = read(sockfd, buf, MAXLINE)) > 0)
Writen(sockfd, buf, n);

if (n < 0 && errno == EINTR)
goto again;
else if (n < 0)
err_sys("str_echo: read error");
}

void sig_chld(int singo)
{
pid_t pid;
int stat;

#if 0
pid = wait(&stat);
printf("child %d terminated\n", pid);
#else
while ((pid = waitpid(-1, &stat, WNOHANG)) > 0 ) {
printf("child %d terminated\n", pid);
}
#endif

return;
}

int
main(int argc, char **argv)
{
int listenfd, connfd;
pid_t childpid;
socklen_t clilen;
struct sockaddr_in cliaddr, servaddr;

listenfd = Socket(AF_INET, SOCK_STREAM, 0);

bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(6001);

Bind(listenfd, (SA *)&servaddr, sizeof(servaddr));

Listen(listenfd, LISTENQ);
Signal(SIGCHLD, sig_chld);

for ( ; ; ) {
clilen = sizeof(cliaddr);
if ((connfd = Accept(listenfd, (SA*)&cliaddr, &clilen)) < 0) {
if (errno == EINTR)
continue;
else
err_sys("accept error");
}

if ((childpid = fork()) == 0) {
Close(listenfd);
str_echo(connfd);
exit(0);
}
Close(connfd);
}
}

客户端代码：

#include "unp.h"

void
str_cli(FILE *fp, int sockfd)
{
char sendline[MAXLINE], recvline[MAXLINE];

while (Fgets(sendline, MAXLINE, fp)) {
Writen (sockfd, sendline, strlen(sendline));

if (Readline(sockfd, recvline, MAXLINE) == 0 )
err_quit("str_cli:serv terminated prematurely");

Fputs(recvline, stdout);
}
}

int
main(int argc, char **argv)
{
int i;
int sockfd[5];
int ret = 0;
struct sockaddr_in servaddr;

if (argc != 2) {
err_quit("usage:tcpcli <IPaddress>");
}

for (i = 0; i < 5; i++) {
sockfd[i] = Socket(AF_INET, SOCK_STREAM, 0);

bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(6001);
Inet_pton(AF_INET, argv[1], &servaddr.sin_addr);

Connect(sockfd[i], (SA *)&servaddr, sizeof(servaddr));
}

str_cli(stdin, sockfd[0]);

exit(0);
}

wait的测试结果如下：

 waitpid的测试结果如下：