下圖是典型的UDP客戶端/服務(wù)器通訊過程。
以下是簡單的UDP服務(wù)器和客戶端程序。
/**//* server.c */
#include <stdio.h>
#include <string.h>
#include <netinet/in.h>
#include "wrap.h"
#define MAXLINE 80
#define SERV_PORT 8000
int main(void)
{
struct sockaddr_in servaddr, cliaddr;
socklen_t cliaddr_len;
int sockfd;
char buf[MAXLINE];
char str[INET_ADDRSTRLEN];
int i, n;
sockfd = Socket(AF_INET, SOCK_DGRAM, 0);
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(SERV_PORT);
Bind(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr));
printf("Accepting connections \n");
while (1) {
cliaddr_len = sizeof(cliaddr);
n = recvfrom(sockfd, buf, MAXLINE, 0, (struct sockaddr *)&cliaddr, &cliaddr_len);
if (n == -1)
perr_exit("recvfrom error");
printf("received from %s at PORT %d\n",
inet_ntop(AF_INET, &cliaddr.sin_addr, str, sizeof(str)),
ntohs(cliaddr.sin_port));
for (i = 0; i < n; i++)
buf[i] = toupper(buf[i]);
n = sendto(sockfd, buf, n, 0, (struct sockaddr *)&cliaddr, sizeof(cliaddr));
if (n == -1)
perr_exit("sendto error");
}
}
/**//* client.c */
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <netinet/in.h>
#include "wrap.h"
#define MAXLINE 80
#define SERV_PORT 8000
int main(int argc, char *argv[])
{
struct sockaddr_in servaddr;
int sockfd, n;
char buf[MAXLINE];
char str[INET_ADDRSTRLEN];
socklen_t servaddr_len;
sockfd = Socket(AF_INET, SOCK_DGRAM, 0);
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
inet_pton(AF_INET, "127.0.0.1", &servaddr.sin_addr);
servaddr.sin_port = htons(SERV_PORT);
while (fgets(buf, MAXLINE, stdin) != NULL) {
n = sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&servaddr, sizeof(servaddr));
if (n == -1)
perr_exit("sendto error");
n = recvfrom(sockfd, buf, MAXLINE, 0, NULL, 0);
if (n == -1)
perr_exit("recvfrom error");
Write(STDOUT_FILENO, buf, n);
}
Close(sockfd);
return 0;
}
由于UDP不需要維護(hù)連接，程序邏輯簡單了很多，但是UDP協(xié)議是不可靠的，實(shí)際上有很多保證通訊可靠性的機(jī)制需要在應(yīng)用層實(shí)現(xiàn)。
編譯運(yùn)行server，在兩個(gè)終端里各開一個(gè)client與server交互，看看server是否具有并發(fā)服務(wù)的能力。用Ctrl+C關(guān)閉server，然后再運(yùn)行server，看此時(shí)client還能否和server聯(lián)系上。和前面TCP程序的運(yùn)行結(jié)果相比較，體會(huì)無連接的含義。