Linux Select

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在Linux中，我們可以使用select函數實現I/O端口的復用，傳遞給 select函數的參數會告訴內核：

? ? ? ?我們所關心的文件描述符
? ? ? ?對每個描述符，我們所關心的狀態。(我們是要想從一個文件描述符中讀或者寫，還是關注一個描述符中是否出現異常)
? ? ? ?我們要等待多長時間。(我們可以等待無限長的時間，等待固定的一段時間，或者根本就不等待)
? ?從 select函數返回后，內核告訴我們一下信息：
? ? ? ?對我們的要求已經做好準備的描述符的個數
? ? ? ?對于三種條件哪些描述符已經做好準備.(讀，寫，異常)

select
——用于IO多路復用
（1）函數原型

＃include<sys/time.h> ＃include<sys/types.h> ＃include<unistd.h> int select(int n,fd_set * readfds,fd_set * writefds,fd_set * exceptfds,struct timeval * timeout);

（2）參數
? ? ? ? n：最大的文件描述詞加1；
? ? ? ? readfds、writefds 和exceptfds：稱為描述詞組，是用來回傳該描述詞的讀，寫或例外的狀況；
? ? ? ? timeout：用來設置select()的等待時間。

struct timeval {time_t tv_sec;time_t tv_usec; };

（3）返回值
? ? ? ? 如果參數timeout設為NULL則表示select（）沒有timeout。
執行成功則返回文件描述詞狀態已改變的個數，如果返回0代表在描述詞狀態改變前已超過timeout時間，當有錯誤發生時則返回-1，錯誤原因存于errno，此時參數readfds，writefds，exceptfds和timeout的值變成不可預測。
EBADF 文件描述詞為無效的或該文件已關閉
EINTR 此調用被信號所中斷
EINVAL 參數n 為負值。

ENOMEM 核心內存不足

常見的程序片段:fs_set readset； FD_ZERO(&readset); FD_SET(fd,&readset); select(fd+1,&readset,NULL,NULL,NULL); if(FD_ISSET(fd,readset){……} #ifndef _SELECT_H_ #define _SELECT_H_#include "wrap.h" #include "client_list.h" #include "server_queue.h"#define SERVER_PORT 6780 #define MAXLINE 100 #define OPEN_MAX 65535 #define TCP_FRAME_SIZE 1200typedef struct {int sockfd; // server socketint port; // server portstruct sockaddr_in addr; // server addrint maxi; // select maxint maxfd;int aggregate[FD_SETSIZE];// select aggregationfd_set allset;server_queue_t send_queue; // server send data queue to clientserver_queue_t recv_queue; // server recv data queue from clientpthread_t send_thread;pthread_t recv_thread;client_t *client; // client list -- save all client info } server_t;/* recv and send queue frame */ typedef struct {int sockfd; // client socketuint16_t length;char data[TCP_FRAME_SIZE]; } __packed tcp_frame_t;//========================================================== server_t *SocketInit(void);#endif /* _SELECT_H_ */ #include "select.h" #include "debug.h"static server_t *socket_init(void) {int opt = 1, i;server_t *current;current = (server_t *)malloc(sizeof(server_t));current->port = SERVER_PORT;current->sockfd = Socket(AF_INET, SOCK_STREAM, 0);// SOL_SOCKET: port can same, ip notSetsockopt(current->sockfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));current->addr.sin_family = AF_INET;current->addr.sin_port = htons(current->port);current->addr.sin_addr.s_addr = INADDR_ANY;Bind(current->sockfd, (struct sockaddr *)&current->addr, sizeof(current->addr));Listen(current->sockfd, MAXLINE);current->maxi = -1;current->maxfd = current->sockfd;for(i = 0; i < FD_SETSIZE; ++i){current->aggregate[i] = -1;}FD_ZERO(current->allset);FD_SET(current->sockfd , current->allset);ServerQueueInit(&current->send_queue, TCP_FRAME_SIZE);ServerQueueInit(&current->recv_queue, TCP_FRAME_SIZE);return current; }static void socket_accept(server_t *arg, fd_set *rset) {server_t *current = arg;struct sockaddr_in addr;int len = sizeof(struct sockaddr_in), i;int new_fd = Accept(current->sockfd, (struct sockaddr *)&addr, &len);debug("new connection client_fd ( %d ) %s: %d\n", new_fd, inet_ntoa(addr.sin_addr), ntohs(addr.sin_port));for(i = 0; i < FD_SETSIZE; ++i){if(current->aggregate[i] < 0){// add new_fd to aggregatecurrent->aggregate[i] = new_fd;break;}}if(FD_SETSIZE == i){printf("too many connects\n");Close(new_fd);return;}FD_SET(new_fd , current->allset);if(new_fd > current->maxfd){current->maxfd = new_fd;}if(i > current->maxi){current->maxi = i;}/* add client node */client_t *node = (client_t *)malloc(sizeof(client_t));node->sockfd = new_fd;memcpy(&node->addr, &addr, sizeof(struct sockaddr_in));ClientAdd(node); }static void socket_recv(server_t *arg, fd_set *rset, int ret) {server_t *current = arg;int i, sockfd, length = 0;tcp_frame_t write;for(i = 0; i <= current->maxi; ++i){if((sockfd = current->aggregate[i]) < 0){continue;}if(FD_ISSET(sockfd , rset)){length = recv(sockfd, write.data, TCP_FRAME_SIZE, 0);if(0 == length){/* delete client node, close connect socket */debug("client[%d] close\n", sockfd);ClientDel(sockfd);Close(sockfd);FD_CLR(sockfd , current->allset);current->aggregate[i] = -1;continue;}else if(length > 0){write.sockfd = sockfd;write.length = length;server_debug(write.data, write.length);// add data to recv_queue, pop in other,if(ServerQueueWrite(&current->recv_queue, (uint8_t *)&write, sizeof(tcp_frame_t)) == 0){debug("push failure...queue full...\n");}}if(--ret <= 0){break;}}} }static void *server_recv_thread(void *arg) {server_t *current = (server_t *)arg;fd_set rset;struct timeval timeout;while(1){rset = current->allset;timeout.tv_sec = 0;timeout.tv_usec = 200;int ret = Select(current->maxfd + 1 , &rset, NULL , NULL , &timeout);if(0 == ret){continue;}if(FD_ISSET(current->sockfd, &rset)){socket_accept(current, &rset); // a new connect come}if(--ret < 0){continue;}socket_recv(current, &rset, ret); // a exsit connect send data to us}Close(current->sockfd);return NULL; }static void *server_send_thread(void *arg) {server_t *current = (server_t *)arg;tcp_frame_t *read = NULL;while(1){//read = (tcp_frame_t*)ServerQueueRead(&current->send_queue, sizeof(tcp_frame_t));if(read != NULL){//server_debug(read->data, read->length);}usleep(100);}return NULL; }server_t * SocketInit(void) {server_t *current = socket_init();debug("create thread...\r\n");pthread_create(&current->send_thread, NULL, server_send_thread, current);pthread_create(&current->recv_thread, NULL, server_recv_thread, current);return current; }

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