这个小案例主要是演示了服务端客户端通信，时间获取和设定的函数均是从网络上查询的，代码可以顺利编译运行，具体请查看代码和注释。

服务端代码

#include <string.h>
#include <netinet/in.h>
#include <stdio.h>
#include <unistd.h>
#include <strings.h>
#include <arpa/inet.h>
#include <ctype.h>
#include <time.h>
#include "wrap.h"

#define MAXLINE 80
#define SRV_PORT 8000

void datetime(char* _datetime)
{
	// 取当前时间到结构体中
	time_t t = time(NULL);
	struct tm* stime = localtime(&t);
	// 格式化时间内容
	strftime(_datetime, 20, "%F %H:%M:%S", stime); 
}

int main(int argc, char* argv[])
{
	socklen_t cnt_len;
	struct sockaddr_in srv_addr, cnt_addr;

	int sock, conn;
	int n, i;
	pid_t pid;

	char buf[MAXLINE];
	char str[INET_ADDRSTRLEN];

	sock = Socket(AF_INET, SOCK_STREAM, 0);
	bzero(&srv_addr, sizeof(srv_addr));

	srv_addr.sin_family = AF_INET;
	srv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
	srv_addr.sin_port = htons(SRV_PORT);

	Bind(sock, (struct sockaddr*)&srv_addr, sizeof(srv_addr));
	Listen(sock, 20);

	printf("Accepting connections ...\n");
	while (1)
	{
		cnt_len = sizeof(cnt_addr);
		conn = Accept(sock, (struct sockaddr*)&cnt_addr, &cnt_len);	
		printf("received from %s at PORT %d\n", 
				inet_ntop(AF_INET, &cnt_addr.sin_addr, str, sizeof(str)),
				ntohs(cnt_addr.sin_port));

		pid = fork();
		if (pid == 0)
		{
			Close(sock);

			char time[20];
			datetime(time);
			Write(conn, time, sizeof(time));

			Close(conn);
			return 0;
		}
		else if (pid > 0)
		{
			Close(conn);
		}
		else
		{
			perr_exit("fork");
		}
	}

	Close(sock);
	return 0;
}

客户端代码

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <time.h>
#include <linux/rtc.h>

#define MAXLINE 80
#define SRV_PORT 8000

int SetSystemTime(char *dt)
{
	struct rtc_time tm;
	struct tm _tm;
	struct timeval tv;
	time_t timep;
	sscanf(dt, "%d-%d-%d %d:%d:%d", &tm.tm_year,
			&tm.tm_mon, &tm.tm_mday,&tm.tm_hour,
			&tm.tm_min, &tm.tm_sec);
	_tm.tm_sec = tm.tm_sec;
	_tm.tm_min = tm.tm_min;
	_tm.tm_hour = tm.tm_hour;
	_tm.tm_mday = tm.tm_mday;
	_tm.tm_mon = tm.tm_mon - 1;
	_tm.tm_year = tm.tm_year - 1900;

	timep = mktime(&_tm);
	tv.tv_sec = timep;
	tv.tv_usec = 0;
	if(settimeofday (&tv, (struct timezone *) 0) < 0)
	{
		printf("set system datatime error : ");
		fflush(stdout);
		perror("");
		return -1;
	}
	else
	{
		printf("set local datetime success : %s\n", dt);
	}
	return 0;
}

int main(int argc, char* argv[])
{
	struct sockaddr_in srv_addr;
	int sock, conn;

	sock = Socket(AF_INET, SOCK_STREAM, 0);

	bzero(&srv_addr, sizeof(srv_addr));
	srv_addr.sin_family = AF_INET;
	inet_pton(AF_INET, "127.0.0.1", &srv_addr.sin_addr);
	srv_addr.sin_port = htons(SRV_PORT);

	int len;
	char buf[MAXLINE];

	conn = Connect(sock, (struct sockaddr*)&srv_addr, sizeof(srv_addr));
	len = Read(sock, buf, sizeof(buf));
	if (len != 0)
	{
		//char time[20];
		//strncpy(time, buf, sizeof(time));
		SetSystemTime(buf);
	}

	close(sock);
	return 0;
}

公共头文件

/* wrap.h */
#ifndef __WRAP_H__
#define __WRAP_H__

void perr_exit(const char* s);
int Accept(int fd, struct sockaddr* sa, socklen_t* salenptr);
void Bind(int fd, const struct sockaddr* sa, socklen_t salen);
void Connect(int fd, const struct sockaddr* sa, socklen_t salen);
void Listen(int fd, int backlog);
int Socket(int family, int type, int protocol);
ssize_t Read(int fd, void* ptr, size_t nbytes);
ssize_t Write(int fd, const void* ptr, size_t nbytes);
void Close(int fd);
ssize_t Readn(int fd, void* vptr, size_t n);
ssize_t Writen(int fd, const void* vptr, size_t n);
static ssize_t my_read(int fd, char* ptr);
ssize_t Readline(int fd, void* vptr, size_t maxlen);

#endif

#include <stdlib.h>
#include <errno.h>
#include <sys/socket.h>
#include <stdio.h>
#include <unistd.h>
#include "wrap.h"

void perr_exit(const char* s)
{
	perror(s);
	exit(1);
}

int Accept(int fd, struct sockaddr* sa, socklen_t* salenptr)
{
	int n;

again:
	if ( (n = accept(fd, sa, salenptr)) < 0 )
	{
		if ((errno == ECONNABORTED) || (errno == EINTR))
		{
			goto again;
		}
		else
		{
			perr_exit("accept error");
		}
	}
	return n;
}

void Bind(int fd, const struct sockaddr* sa, socklen_t salen)
{
	if ( bind(fd, sa, salen) < 0 )
	{
		perr_exit("bind error");
	}
}

void Connect(int fd, const struct sockaddr* sa, socklen_t salen)
{
	if ( connect(fd, sa, salen) < 0 )
	{
		perr_exit("connect error");
	}
}

void Listen(int fd, int backlog)
{
	if ( listen(fd, backlog) < 0 )
	{
		perr_exit("listen error");
	}
}

int Socket(int family, int type, int protocol)
{
	int n = socket(family, type, protocol);
	if ( n < 0 )
	{
		perr_exit("socket error");
	}
	return n;
}

ssize_t Read(int fd, void* ptr, size_t nbytes)
{
	ssize_t n;

again:
	if ( (n = read(fd, ptr, nbytes)) == -1)
	{
		if (errno == EINTR)
		{
			goto again;
		}
		else
		{
			return -1;
		}
	}
	return n;
}

ssize_t Write(int fd, const void* ptr, size_t nbytes)
{
	ssize_t n;

again:
	if ( (n = write(fd, ptr, nbytes)) == -1)
	{
		if (errno == EINTR)
		{
			goto again;
		}
		else
		{
			return -1;
		}
	}
	return n;
}

void Close(int fd)
{
	if (close(fd) == -1)
	{
		perr_exit("close error");
	}
}

ssize_t Readn(int fd, void* vptr, size_t n)
{
	size_t nleft;
	ssize_t nread;
	char* ptr;

	ptr = vptr;
	nleft = n;
	while (nleft > 0)
	{
		if ( (nread = read(fd, ptr, nleft)) < 0 )
		{
			if (errno == EINTR)
			{
				nread = 0;
			}
			else
			{
				return -1;
			}
		}
		else if (nread == 0)
		{
			break;
		}
		
		nleft -= nread;
		ptr += nread;
	}
}

ssize_t Writen(int fd, const void* vptr, size_t n)
{
	size_t nleft;
	ssize_t nwritten;
	const char* ptr;

	ptr = vptr;
	nleft = n;
	while (nleft > 0)
	{
		if ( (nwritten = write(fd, ptr, nleft)) <= 0)
		{
			if (nwritten < 0 && errno == EINTR)
			{
				nwritten = 0;
			}
			else
			{
				return -1;
			}
		}

		nleft -= nwritten;
		ptr += nwritten;
	}

	return n;
}

static ssize_t my_read(int fd, char* ptr)
{
	static int read_cnt;
	static char* read_ptr;
	static char read_buf[100];

	if (read_cnt <= 0)
	{
again:
		if ( (read_cnt = read(fd, read_buf, sizeof(read_buf))) < 0 )
		{
			if (errno == EINTR)
			{
				goto again;
			}
			return -1;
		}
		else if (read_cnt == 0)
		{
			return 0;
		}

		read_ptr = read_buf;
	}

	read_cnt--;
	*ptr = *read_ptr++;
	return 1;
}

ssize_t Readline(int fd, void* vptr, size_t maxlen)
{
	ssize_t n, rc;
	char c, *ptr;

	ptr = vptr;
	for (n = 1; n < maxlen; n++)
	{
		if ( (rc = my_read(fd, &c)) == 1)
		{
			*ptr++ = c;
			if (c == '\n')
			{
				break;
			}
		}
		else if (rc == 0)
		{
			*ptr = 0;
			return n - 1;
		}
		else
		{
			return -1;
		}

		*ptr = 0;
		return n;
	}
}

编译测试代码

编译客户端：gcc time_client.c wrap.c -o time_client

编译服务端：gcc time_server.c wrap.c -o time_server

运行效果（客户端要设定时间需要使用管理员身份运行）：