linxu select 返回值

hjs1122

于 2011-03-16 11:44:00 发布

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分类专栏： linux C开发文章标签： struct descriptor socket buffer include null

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本文探讨了在网络编程中使用select函数时遇到的问题，并详细解释了一个常见错误的原因及解决方法。作者通过具体代码示例展示了如何正确初始化文件描述符集以避免后续调用失败。

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#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;