【Unix/Linux编程实践】编写时间服务器（使用Socket）
一.服务端
相关系统调用：
socket：获得一个套接字，SOCK_STREAM为tcp，最后一个参数protocol指的是内核中网络代码所使用的协议，0代表选择标准的协议。
bind：将套接字绑定到（地址，端口号）上
listen：开启对套接字的监听，参数sockid为接收请求的socket，参数qsize为允许接入连接的数目
accept：从连接请求队列中获得连接信息，创建新的套接字，并返回该套接字的文件描述符。新创建的套接字用于服务器与客户机的通信，而原来的套接字仍然处于监听状态
read/write：传送数据
close：挂断
代码：
/* timeserv.c - a socket-based time of day server */
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <time.h>
#include <strings.h>
#define PORTNUM 13000 /* our time service phone number */
#define HOSTLEN 256
#define oops(msg) { perror(msg) ; exit(1) ; }
int main(int ac, char *av[])
{
struct sockaddr_in saddr; /* build our address here */
struct hostent *hp; /* this is part of our */
char hostname[HOSTLEN]; /* address */
int sock_id,sock_fd; /* line id, file desc */
FILE *sock_fp; /* use socket as stream */
char *ctime(); /* convert secs to string */
time_t thetime; /* the time we report */
/* * Step 1: ask kernel for a socket */
sock_id = socket( PF_INET, SOCK_STREAM, 0 ); /* get a socket */
if ( sock_id == -1 )
oops( "socket" );
/* * Step 2: bind address to socket. Address is host,port */
bzero( (void *)&saddr, sizeof(saddr) ); /* clear out struct */
gethostname( hostname, HOSTLEN ); /* where am I ? */
hp = gethostbyname( hostname ); /* get info about host */
/* fill in host part */
bcopy( (void *)hp->h_addr, (void *)&saddr.sin_addr, hp->h_length);
saddr.sin_port = htons(PORTNUM); /* fill in socket port */
saddr.sin_family = AF_INET ; /* fill in addr family */
if ( bind(sock_id, (struct sockaddr *)&saddr, sizeof(saddr)) != 0 )
oops( "bind" );
/* * Step 3: allow incoming calls with Qsize=1 on socket */
if ( listen(sock_id, 1) != 0 )
oops( "listen" );
/* * main loop: accept(), write(), close() */
while ( 1 ){
sock_fd = accept(sock_id, NULL, NULL); /* wait for call */
printf("Wow! got a call!\n");
if ( sock_fd == -1 )
oops( "accept" ); /* error getting calls */
sock_fp = fdopen(sock_fd,"w"); /* we'll write to the */
if ( sock_fp == NULL ) /* socket as a stream */
oops( "fdopen" ); /* unless we can't */
thetime = time(NULL); /* get time */
/* and convert to strng */
fprintf( sock_fp, "The time here is .." );
fprintf( sock_fp, "%s", ctime(&thetime) );
fclose( sock_fp ); /* release connection */
}
}我们使用fdopen将文件描述符定向到缓存的数据流，便于使用fprintf调用来进行输出。
二.客户端
相关系统调用：
socket：同上
connect：尝试把sockid标识的socket和由serv_addrp所指向的socket地址相连接，sockid是一个合法的文件描述符，可以用来进行读写操作
read/write：同上
close：同上
代码：
/* timeclnt.c - a client for timeserv.c * usage: timeclnt hostname portnumber */
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#define oops(msg) { perror(msg); exit(1); }
main(int ac, char *av[])
{
struct sockaddr_in servadd; /* the number to call */
struct hostent *hp; /* used to get number */
int sock_id, sock_fd; /* the socket and fd */
char message[BUFSIZ]; /* to receive message */
int messlen; /* for message length */
/* * Step 1: Get a socket */
sock_id = socket( AF_INET, SOCK_STREAM, 0 ); /* get a line */
if ( sock_id == -1 )
oops( "socket" ); /* or fail */
/* * Step 2: connect to server * need to build address (host,port) of server first */
bzero( &servadd, sizeof( servadd ) ); /* zero the address */
hp = gethostbyname( av[1] ); /* lookup host's ip # */
if (hp == NULL)
oops(av[1]); /* or die */
bcopy(hp->h_addr, (struct sockaddr *)&servadd.sin_addr, hp->h_length);
servadd.sin_port = htons(atoi(av[2])); /* fill in port number */
servadd.sin_family = AF_INET ; /* fill in socket type */
/* now dial */
if ( connect(sock_id,(struct sockaddr *)&servadd, sizeof(servadd)) !=0)
oops( "connect" );
/* * Step 3: transfer data from server, then hangup */
messlen = read(sock_id, message, BUFSIZ); /* read stuff */
if ( messlen == - 1 )
oops("read") ;
if ( write( 1, message, messlen ) != messlen ) /* and write to */
oops( "write" ); /* stdout */
close( sock_id );
}三.运行
将两个程序运行在不同的电脑上，客户端就可以得到服务器的时间。
服务器运行：
./timeserv &
客户端运行：
#computer1为服务器的名字
./timeclnt computer1 13000