linxu select 返回值

#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; }

曾經寫過如上這樣的代碼，執行時怎麼也得不到預想的結果!
後來，仔細看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.
原來第一次select的時候出錯了，導致rd_fd和tm(主要是rd_fd)的值 become undefined,進而導致以後的select調用的失敗！
解決辦法：
在1處（while循環內的開頭），添加如下代碼：
FD_ZERO(&rd_fd);
FD_SET(sockfd, &rd_fd);
tm.tv_sec = 1;
tm.tv_usec = 0;