if (2 != argc)
    {
        fprintf(stderr, "Usage: %s \n",
  argv[0]);
        exit(1);
    }

    myPort = atoi(argv[1]);

    udpSocket = socket(PF_INET, SOCK_DGRAM, /* PF_INET和SOCK_DGRAM组合代表了UDP */
  IPPROTO_UDP);
    if (-1 == udpSocket)
    {
        perror("socket()");
        exit(1);
    }

    memset(&serverName, 0, sizeof(serverName));
    memset(&clientName, 0, sizeof(clientName));

    serverName.sin_family = AF_INET;
    serverName.sin_addr.s_addr = htonl(INADDR_ANY);
    serverName.sin_port = htons(myPort);

    status = bind(udpSocket, (struct sockaddr *)
  &serverName, sizeof(serverName));
    if (-1 == status)
    {
        perror("bind()");
        exit(1);
    }

    for (;;)
    {
/* ssize_t  recvfrom(int s, void *buf, size_t len, int flags, struct sock-
 *     addr *from, socklen_t *fromlen);
 * /    
        size = recvfrom(udpSocket, mesg,
  MAX_MESG_SIZE, 0,
  (struct sockaddr *) &clientName,
  &clientLength);
        if (size == -1)
        {
            perror("recvfrom()");
            exit(1);
        }
/* ssize_t  sendto(int  s,  const  void *msg, size_t len, int flags, const
 *     struct sockaddr *to, socklen_t tolen);
 */
        status = sendto(udpSocket, mesg, size, 0,
            (struct sockaddr *) &clientName,
  clientLength);
        if (status != size)
        {
            fprintf(stderr,
  "sendto(): short write.\n");
            exit(1);
        }
    }

    /* never reached */
    return 0;
}

将TCP的客户端改写成UDP的客户端将留作一个练习

4、/etc/services文件
    为了连接到一个服务端，你必须首先知道其监听的地址和端口。许多常见的服务(FTP,TELNET等等)的信息都列在了一个文本文

件/etc/services中。getservbyname()函数可以用名称询问一个服务的详细情况包括它的端口号(注意！它已经是网络字节序了)，它的原型在

/usr/include：
struct servent *getservbyname(const char *name, const char *proto); 默认的proto是"tcp"

struct servent
{
 char *s_name; /* official service name */
 char **s_aliases; /* alias list */
 int s_port; /* port number, network
    * byte-order--so do not
     * use host-to-network macros */
 char *s_proto; /* protocol to use */
};

5、总结
    这篇文章介绍了Linux下使用C和BSD Socket API进行网络编程。总的来说，用这套API来写代码将是相当耗费劳力的，特别是与其他一些

技术相比。在今后的文章中，我会将BSD Socket API与另外两个可以选择的Linux下的方法进行比较：远程过程调用(RPCs,Remote Procedure

Calls)和CORBA(Common Object Request Broker Achitecture,公用对象请求代理[调度]程序体系结构)。RPCs在Ed Petron的文章"Remote

Procedure Calls"(Linux Journal Issue #42，1997年十月)中有介绍。

相关资源：
UNIX Network Programming, W. Richard Steves, Prentice Hall, 1990.

An Introductory 4.4BSD Interprocess Communication Tutorial, Stuart Sechrest, University of California, Berkeley. 1986.

Available via anonymous FTP at: ftp://ftp.NetBSD.ORG/pub/NetBSD/misc/lite2-docs/psd/20.ipctut.ps.gz.

An Advanced 4.4BSD Interprocess Communication Tutorial, Samuel J. Leffler, Robert S. Fabry, William N. Joy, Phil Lapsley,

University of California, Berkeley. 1986 Available via anonymous FTP at: ftp://ftp.NetBSD.ORG/pub/NetBSD/misc/lite2-

docs/psd/21.ipc.ps.gz.

Java and the Client-Server, Joe Novosel, Linux Journal, Issue 33, January 1997.

RFC 793: Transmission Control Protocol, J. Postel (ed.), September 1981. Available via HTTP at

http://www.internic.net/rfc/rfc793.txt.

RFC 1337: TIME-WAIT Assassination Hazards in TCP, R. Braden, May 1992. Available via HTTP at

http://www.internic.net/rfc/rfc1337.txt.

Programming UNIX Sockets in C FAQ, Vic Metcalfe, Andrew Gierth and other contributers, February 1997. Available via HTTP at

http://kipper.york.ac.uk/~vic/sock-faq/html/unix-socket-faq.html

作者简介：
Ivan Griffin is a research postgraduate student in the ECE department at the University of Limerick, Ireland. His interests

include C++/Java, WWW, ATM, the UL Computer Society (http://www.csn.ul.ie/) and, of course, Linux

(http://www.trc.ul.ie/~griffini/linux.html). His e-mail address is ivan.griffin@ul.ie.

Dr. John Nelson is a senior lecturer in Computer Engineering at the University of Limerick. His interests include mobile

communications, intelligent networks, Software Engineering and VLSI design. His e-mail address is john.nelson@ul.ie.

所有的程序都可以在这里获得： ftp://ftp.ssc.com/pub/lj/listings/issue46/2333.tgz

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