udpcli.c

这个是linux下

	#include <stdio.h> 
	#include <stdlib.h>
	#include <unistd.h>
        #include <strings.h>
	#include <string.h>
	#include <sys/types.h> 
	#include <sys/socket.h> 
	#include <netinet/in.h> 
	#include <netdb.h>        /* netbd.h is needed for struct hostent =) */ 

	#define PORT 1234   /* Open Port on Remote Host */ 
	#define MAXDATASIZE 100   /* Max number of bytes of data */ 

	int main(int argc, char *argv[]) 
	{ 
	int fd, numbytes;   /* files descriptors */ 
	char recvbuf[MAXDATASIZE];  /* buf will store received text */ 
        char sendbuf[MAXDATASIZE];
	struct hostent *he;         /* structure that will get information about remote host */ 
	struct sockaddr_in server,client;  /* server's address information */ 
	
	if (argc !=2) {  /* this is used because our program will need two argument (IP address and a message */ 
	printf("Usage: %s <IP Address> \n",argv[0]); 
	exit(1); 
	} 

	if ((he=gethostbyname(argv[1]))==NULL){ /* calls gethostbyname() */ 
	printf("gethostbyname() error\n"); 
	exit(1); 
	} 

	if ((fd=socket(AF_INET, SOCK_DGRAM, 0))==-1){  /* calls socket() */ 
	printf("socket() error\n"); 
	exit(1); 
	} 

	bzero(&server,sizeof(server));
	server.sin_family = AF_INET; 
	server.sin_port = htons(PORT); /* htons() is needed again */ 
	server.sin_addr = *((struct in_addr *)he->h_addr);  /*he->h_addr passes "*he"'s info to "h_addr" */ 
	
        socklen_t len;
         len=sizeof(struct sockaddr_in);
	while (1) {
		printf("input message:");
         	fgets(sendbuf,40,stdin);
	sendto(fd,sendbuf,strlen(sendbuf),0,(struct sockaddr *)&server,len);
	if ((numbytes=recvfrom(fd,recvbuf,MAXDATASIZE,0,(struct sockaddr *)&server,&len)) == -1){  /* calls recvfrom() */ 
	printf("recvfrom() error\n"); 
	exit(1); 
	} 
	recvbuf[numbytes]='\0';
	printf("Server Return Reverse Message: %s\n",recvbuf); /* it prints server's welcome message */

	}

	close(fd);   /* close fd */ 
	}