#include <sys/types.h>
#include <sys/socket.h>
#include <string.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#define BUFFER 800
#define SERV_PORT 3333
int main()
{
int sockfd,n;
socklen_t len;
socklen_t src_len;
struct sockaddr_in servaddr, cliaddr;
char msg[BUFFER];
struct timeval tm;
fd_set rd_fd, wr_fd, ex_fd;
sockfd = socket(AF_INET, SOCK_DGRAM, 0); /* create a socket */
/* init servaddr */
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(SERV_PORT);
/* bind address and port to socket */
if(bind(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) == -1)
{
perror("bind error");
exit(1);
}
src_len = sizeof(cliaddr);
FD_ZERO(&rd_fd);
FD_SET(sockfd, &rd_fd);
tm.tv_sec = 0;
tm.tv_usec = 0;
while(1)
{
FD_ZERO(&rd_fd);
FD_SET(sockfd, &rd_fd);
tm.tv_sec = 1;
tm.tv_usec = 0;
if(select(sockfd + 1, &rd_fd, NULL, NULL, &tm) <= 0){
continue;
}
else
{
printf("%d, %d/n", tm.tv_sec, tm.tv_usec);
printf("there is data/n");
if(recvfrom(sockfd, msg, BUFFER, 0, (struct sockaddr *)&cliaddr, &src_len)< 0)
{
perror("receive error!/n");
exit(0);
}
printf("%s/n", msg);
sleep(5);
}
}
return 0;
}

曾经写过如上这样的代码，执行时怎么也得不到预想的结果!
后来，仔细看man select
On success, select() and pselect() return the number of file descriptors contained in the three returned descriptor sets(that  is,  the  total  number  of  bits  that are set in readfds, writefds, exceptfds) which may be zero if the timeout expires before anything interesting happens.  On error, -1 is returned, and errno is set appropriately;  the  sets  and timeout become undefined, so do not rely on their contents after an error. 
原来第一次select的时候出错了，导致rd_fd和tm(主要是rd_fd)的值 become undefined,进而导致以后的select调用的失败！
解决办法：
在1处（while循环内的开头），添加如下代码：
FD_ZERO(&rd_fd);
FD_SET(sockfd, &rd_fd);
tm.tv_sec = 1;
tm.tv_usec = 0;