Linux网络编程基础之二－－UDP

 

    我们在Linux网络编程基础之一里说了流socket和数据报socket的联系和区别。我们在这里简单说一下基于UDP协议的网络编程。
    两个常用的函数
   int recvfrom(int sockfd,void *buf,int len,unsigned int flags,struct sockaddr * from int *fromlen)
   int sendto(int sockfd,const void *msg,int len,unsigned int flags,struct sockaddr *to int tolen)
其中sockfd,buf,len的意义和read,write一样,分别表示套接字描述符,发送或接收的缓冲区及大小.
recvfrom 负责从 sockfd接收数据,如果from不是NULL,那么在from里面存储了信息来源的情况,如果对信息的来源不感兴趣,可以将from和fromlen 设置为NULL.sendto负责向to发送信息.此时在to里面存储了收信息方的详细资料.

下面是一个简单的例子（选自Understanding Unix/Linux Programming）：

服务器端（接收数据报，dgrecv.c使用命令行传过来的端口号建立socket，然后进入循环，接收和打印从客户端发来的数据报）：
/************************************************************************
 * dgrecv.c  - datagram receiver
 *           usage: dgrecv portnum
 *          action: listens at the specfied port and reports messages
 */

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

#define oops(m,x)  { perror(m);exit(x);}
/*辅助函数make_dgram_server和get_internet_address将在后面给出的文件dgram.c中定义*/
int  make_dgram_server_socket(int);
int  get_internet_address(char *, int,  int *, struct sockaddr_in *);
void say_who_called(struct sockaddr_in *);

int main(int ac, char *av[])
{
 int port;   /* use this port  */
 int sock;   /* for this socket  */
 char buf[BUFSIZ];  /* to receive data here  */
 size_t msglen;   /* store its length here */
 struct  sockaddr_in   saddr; /* put sender's address here */
 socklen_t saddrlen;  /* and its length here  */
 
 if ( ac == 1 || (port = atoi(av[1])) <= 0 ){
  fprintf(stderr,"usage: dgrecv portnumber\n");
  exit(1);
 }

    /*  get a socket and assign it a port number */

 if( (sock = make_dgram_server_socket(port)) == -1 )
  oops("cannot make socket",2);

    /* receive messaages on that socket */

 saddrlen = sizeof(saddr);
 while( (msglen = recvfrom(sock,buf,BUFSIZ,0,
    (struct sockaddr *) &saddr,&saddrlen))>0 ) {
  buf[msglen] = '\0';
  printf("dgrecv: got a message: %s\n", buf);
  say_who_called(&saddr);
 }
 return 0;
}
void say_who_called(struct sockaddr_in *addrp)
{
 char host[BUFSIZ];
 int port;

 get_internet_address(host,BUFSIZ,&port,addrp);
 printf("  from: %s:%d\n", host, port);
}
客户端（创建一个socket，然后用它发送消息到以命令行参数传入的特定的主机和端口号）：
/*********************************************************************
 * dgsend.c  - datagram sender
 *           usage: dgsend hostname portnum "message"
 *          action: sends message to hostname:portnum
 */

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

#define oops(m,x)  { perror(m);exit(x);}

int make_dgram_client_socket();
int make_internet_address(char *,int, struct sockaddr_in *);

int main(int ac, char *av[])
{
 int sock;   /* use this socket to send */
 char *msg;   /* send this messag  */
 struct  sockaddr_in   saddr; /* put sender's address here */

 if ( ac != 4 ){
  fprintf(stderr,"usage: dgsend host port 'message'\n");
  exit(1);
 }
 msg = av[3];

    /* get a datagram socket */

 if( (sock = make_dgram_client_socket()) == -1 )
  oops("cannot make socket",2);

    /* combine hostname and portnumber of destination into an address */

 if ( make_internet_address(av[1], atoi(av[2]), &saddr) == -1 )
  oops("make addr",4);

    /* send a string through the socket to that address */

 if ( sendto(sock, msg, strlen(msg), 0,
    (struct sockaddr *)&saddr,sizeof(saddr)) == -1)
  oops("sendto failed", 3);
 return 0;
}

辅助函数
/***************************************************************
 * dgram.c
 * support functions for datagram based programs
 */

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

#define   HOSTLEN  256

int make_internet_address();

int make_dgram_server_socket(int portnum)
{
 struct  sockaddr_in   saddr;   /* build our address here */
 char hostname[HOSTLEN];     /* address           */
 int sock_id;        /* the socket             */

 sock_id = socket(PF_INET, SOCK_DGRAM, 0);  /* get a socket */
 if ( sock_id == -1 ) return -1;

 /** build address and bind it to socket **/

 gethostname(hostname, HOSTLEN);         /* where am I ?         */
 make_internet_address(hostname, portnum, &saddr);

 if ( bind(sock_id, (struct sockaddr *)&saddr, sizeof(saddr)) != 0 )
        return -1;

 return sock_id;
}
int make_dgram_client_socket()
{
 return socket(PF_INET, SOCK_DGRAM, 0);
}

int make_internet_address(char *hostname, int port, struct sockaddr_in *addrp)
/*
 * constructor for an Internet socket address, uses hostname and port
 *   (host,port) -> *addrp
 */
{
 struct hostent *hp;

 bzero((void *)addrp, sizeof(struct sockaddr_in));
 hp = gethostbyname(hostname);
 if ( hp == NULL ) return -1;
 bcopy((void *)hp->h_addr, (void *)&addrp->sin_addr, hp->h_length);
 addrp->sin_port = htons(port);
 addrp->sin_family = AF_INET;
 return 0;
}

int get_internet_address(char *host, int len, int *portp, struct sockaddr_in *addrp)
/*
 * extracts host and port from an internet socket address
 *   *addrp -> (host,port)
 */
{
 strncpy(host, inet_ntoa(addrp->sin_addr), len );
 *portp = ntohs(addrp->sin_port);
 return 0;
}

Makefile如下：
all:dgrecv.exe dgsend.exe
dgrecv.exe:
 gcc dgrecv.c dgram.c -o dgrecv.exe
dgsend.exe:
 gcc dgsend.c dgram.c -o dgsend.exe
运行结果：
$./dgrecv.exe 8901&
$./dgsend.exe ComputerName 8901 "Yeah,I am on fire"
dgrecv:got a message: Yeah,I am on fire
  from:192.168.0.1:1018

总结：
服务器：socket -> (填充结构) -> bind   -> recvfrom
客户端：socket -> (填充结构) -> sendto