select详解

素颜马尾好姑娘i 2024-02-18 23:58 38阅读 0赞

inux关于并发网络分为Apache模型（Process per Connection (进程连接) ） 和TPC ， 还有select模型，以及poll模型（一般是Epoll模型）

Select模型极其作用：这文章讲述的很好，没必要重述已有的东西，就直接给链接

[ http://blog.csdn.net/turkeyzhou/article/details/8609360][http_blog.csdn.net_turkeyzhou_article_details_8609360]

我的理解：

1 /* According to POSIX.1-2001 */
     2 #include <sys/select.h>
     3 
     4 /* According to earlier standards */
     5 #include <sys/time.h>
     6 #include <sys/types.h>
     7 #include <unistd.h>
     8 
     9 int select(int nfds, fd_set *readfds, fd_set *writefds,
    10 fd_set *exceptfds, struct timeval *timeout);
    11 
    12 void FD_CLR(int fd, fd_set *set);
    13 int FD_ISSET(int fd, fd_set *set);
    14 void FD_SET(int fd, fd_set *set);
    15 void FD_ZERO(fd_set *set);

对于

int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);
            第一个参数 nfds：  第n个文件id的编号   （linux下，一切皆文件）  需要注意的是： nfds = fd+1 (fd 为 FD_SET中的fd)
            第二个参数：  fd_set *readfds   读取文件编号，如果不需要读取的话 可以设置为NULL
            第三 ，四个参数：  同上
            第五个参数：为一个定义超时的结构体

1        struct timeval {
    2                time_t         tv_sec;     /* seconds */
    3                suseconds_t    tv_usec;    /* microseconds */
    4            };

该结构用来设定多少时间为超时 ,比如

struct timeval ss ;
       ss.tv_sec  =3;
       ss.tv_usec =0; 
    //表示设定为3秒后为超时，select将会返回0

对于下面这几个函数：

1 void FD\_CLR(int fd, fd\_set \*set);

用来清除fd的fd\_set ,比如fd为5 ，则表示set集中所有设定等于5的fd\_set 都将被清除

1 int FD\_ISSET(int fd, fd\_set \*set);

判断是否set 与fd是否绑定，如果没有绑定，则返回false，如果绑定了则返回True

void FD_SET(int fd, fd_set *set);

将fd值 和set绑定

void FD_ZERO(fd_set *set);

将set集全部清除

简单的例子：

判断是否有数据输入，有则直接打印出来

![ContractedBlock.gif][]

makefile文件：

![复制代码][copycode.gif]

1 .SUFFIXES: .o.c
     2 CC =gcc
     3 SRC = Se_keyboard.c
     4 OBJ = $(SRC: .c =.o)
     5 BIN = Se_keyboard
     6 
     7 
     8 .PHONY: start
     9 start:  $(OBJ)
    10         $(CC) -o $(BIN) $(OBJ)
    11 .o.c: $(SRC)
    12         $(CC) -g -Wall $@ -c  $<
    13 .PHONY: clean
    14 clean:
    15         rm -f $(OBJ)

![复制代码][copycode.gif]

![041727505482526.png][]

虽然知道这么多，但是还是觉得Select并没有什么作用。

Select一般是和Socket搭配使用，相当于线程池的效果，但是线程池有缺点，详情看这儿：

[ http://blog.csdn.net/tianmohust/article/details/6677985][http_blog.csdn.net_tianmohust_article_details_6677985]

[ http://blog.csdn.net/xifeijian/article/details/17385831][http_blog.csdn.net_xifeijian_article_details_17385831]

关于Select的原理图：

首先来看下一对一的socket的c/s模式

![041749008455310.png][]

看完这个之后，我们现在可以来看看这个图：

![041806564391111.png][]

服务器创建一个socket并bind绑定一个本机地址和设定一个端口，然后进入listen监听状态。采用select模型而非传统apache模型（ppc）或者tpc模型 。 不过Select模型就是有这样一个特点

一般我们default默认的SOMAXCONN为128 当然我们可以另外取一个设定（下面我们设定的是2048）作为最大连接数，虽然可以设置更大，但是缺点是，select模型是一个轮询模式，就是每一个都需要遍历一边所有的链接的fd

查看是否在fd\_set集合中，这样，当SOMAXCONN取值非常大时，对于每一个客户端，访问时间都会延迟一点点，这样就是效率不是特别高！

下面是一个简单的多路复用的网络并发Select模型

![复制代码][copycode.gif]

1 #include<stdio.h>
      2 #include<string.h>
      3 #include<stdlib.h>
      4 #include<unistd.h>
      5 #include<netinet/in.h>
      6 #include<netinet/ip.h>
      7 #include<sys/socket.h>
      8 #include<sys/types.h>
      9 #include<sys/time.h>
     10 #include<arpa/inet.h>
     11 #include<sys/select.h>
     12 #include<assert.h>
     13 
     14 #ifndef  EXIT_SUCCESS
     15 #define EXIT_SUCCESS 0
     16 #endif
     17 
     18 #ifndef  EXIT_FAILURE
     19 #define EXIT_FAILURE -1
     20 #endif
     21 
     22 #define Max_connect 2048    //usually is SOMAXCONN =128 ,can be change
     23 
     24 #define maxn 1024
     25 #define Port 5567
     26 #define Ser_addr "192.168.132.128"
     27 
     28 #define ERROR_EXIT( inf ) \
     29   do{    \
     30      perror( inf );  \
     31      printf("it happened in %d \n", __LINE__); \
     32      exit(-1); \
     33   }while(0);
     34 
     35 #define  Waring( inf ) \
     36 do{    \
     37      perror( inf );  \
     38      printf("it happened in %d \n", __LINE__); \
     39   }while(0);
     40 
     41 
     42 int fds[Max_connect];
     43 int cnt = 1;
     44 
     45 void
     46 print (int fd, const char *str)
     47 {
     48   assert (str != NULL);
     49   printf ("the fd is %d \n", fd);
     50   puts (str);
     51 }
     52 
     53 int
     54 main (int argv, char *argc[])
     55 {
     56 
     57   int ser_sfd = -1, i;
     58   struct sockaddr_in ser_addr;
     59   struct sockaddr_in client_addr;
     60   int setfd = 0, optval, maxsockfd;
     61 
     62   char rebuf[maxn], wbuf[maxn];
     63 
     64   // build a socket with ipv4 ans tcp 
     65 
     66   if ((ser_sfd = socket (AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)
     67     ERROR_EXIT ("socket...!");
     68 
     69   // set the type socket
     70   /*
     71      level ={ SOL_SOCKET ,IPPROTO_TCP}
     72      setsockopt is to cancle the jiangsi process
     73    */
     74 
     75   printf ("ser_sfd = %d  \n", ser_sfd);
     76   if (setsockopt (ser_sfd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof (optval))
     77       < 0)
     78     ERROR_EXIT ("setsockopt...!");
     79 
     80   memset (&client_addr, 0, sizeof (client_addr));
     81   memset (&ser_addr, 0, sizeof (ser_addr));
     82   ser_addr.sin_family = AF_INET;
     83   ser_addr.sin_port = htons (Port);
     84   ser_addr.sin_addr.s_addr = htonl (INADDR_ANY);
     85 
     86   if (bind (ser_sfd, (struct sockaddr *) &ser_addr, sizeof (ser_addr)) < 0)
     87     ERROR_EXIT ("bind..!");
     88 
     89   if (listen (ser_sfd, Max_connect) < 0)
     90     ERROR_EXIT ("listen...!");
     91 
     92   //user the select moduel
     93   memset (fds, 0, sizeof (fds));
     94   fd_set fdset, wfd;
     95   maxsockfd = ser_sfd;        //max socket fd
     96 
     97   struct timeval tout;
     98 
     99   tout.tv_sec = 15;
    100   tout.tv_usec = 0;
    101 
    102 
    103   while (1)
    104     {
    105 
    106       FD_ZERO (&fdset);        //clear  
    107       //FD_ZERO (&wfd);
    108 
    109       FD_SET (ser_sfd, &fdset);    //bind
    110       //FD_SET (ser_sfd, &wfd);
    111 
    112       struct timeval tout;
    113 
    114       tout.tv_sec = 15;
    115       tout.tv_usec = 0;
    116 
    117 
    118       for (i = 0; i < cnt; i++)
    119     {
    120       if (fds[i] != 0)
    121         FD_SET (fds[i], &fdset);
    122     }
    123 
    124       int tag = select (maxsockfd + 1, &fdset, NULL, NULL, &tout);
    125 
    126       if (tag == 0)
    127     {
    128       Waring ("select wait timeout !");
    129       continue;
    130     }
    131       else if (tag == -1)
    132     ERROR_EXIT ("Error select ...!");
    133 
    134       //lunxun select
    135       for (i = 0; i < cnt; i++)
    136     {
    137 
    138       if (FD_ISSET (fds[i], &fdset))
    139         {
    140 
    141           int len = recv (fds[i], rebuf, sizeof (rebuf), 0);
    142           if (len <= 0)
    143         {
    144 
    145           printf ("%d: \n", fds[i]);
    146           close (fds[i]);
    147           FD_CLR (fds[i], &fdset);
    148           Waring ("client is closed !");
    149           continue;
    150         }
    151 
    152           printf ("the client_ip : %s\n",
    153               inet_ntoa (client_addr.sin_addr));
    154 
    155           print (fds[i], rebuf);
    156 
    157           send (fds[i], rebuf, sizeof (rebuf), 0);    //hui she
    158           memset (rebuf, 0, sizeof (rebuf));
    159         }
    160     }
    161       //if have a new connect happened
    162       //  memset(&client_addr , 0 ,sizeof(client_addr)); 
    163       if (FD_ISSET (ser_sfd, &fdset))
    164     {
    165       // memset(&client_addr , 0 ,sizeof(client_addr));
    166       int acplen = sizeof (client_addr);
    167       int acp = accept (ser_sfd, (struct sockaddr *) &client_addr,
    168                 &acplen);
    169 
    170       printf ("accept return acp=%d \n", acp);
    171 
    172       if (acp < 0)
    173         {
    174 
    175           Waring ("waring accept acp<=0!");
    176           continue;
    177         }
    178       //add to arr
    179       if (cnt < maxn)
    180         fds[cnt++] = acp;
    181       else
    182         {
    183           ERROR_EXIT ("cnt>maxn");
    184         }
    185       if (acp > maxsockfd)
    186         maxsockfd = acp;
    187     }
    188     }
    189 
    190   for (i = 0; i < cnt; i++)
    191     {
    192       close (fds[i]);
    193     }
    194 
    195   return EXIT_SUCCESS;
    196 }

![复制代码][copycode.gif]

makefile文件：

![复制代码][copycode.gif]

1 .SUFFIXES: .o.c 
     2 CC =gcc
     3 SRC =  server.c
     4 OBJ = $(SRC: .c =.o) 
     5 BIN = Sez_Server
     6 
     7 
     8 .PHONY: start
     9 start:  $(OBJ)
    10     $(CC) -o $(BIN) $(OBJ)
    11 .o.c: $(SRC)
    12     $(CC) -g -Wall $@ -c  $<
    13 .PHONY: clean
    14 clean:
    15     rm -f $(OBJ)

![复制代码][copycode.gif]

客户端：

![复制代码][copycode.gif]

1 #include<stdio.h>
     2 #include<string.h>
     3 #include<stdlib.h>
     4 #include<netinet/in.h>
     5 #include<arpa/inet.h>
     6 #include<sys/socket.h>
     7 #include<sys/types.h>
     8 #include<assert.h>
     9 #include<unistd.h>
    10 #ifndef EXIT_FAILURE
    11 #define EXIT_FAILURE  -1    //exit failure
    12 #endif
    13 #ifndef EXIT_SUCCESS
    14 #define EXIT_SUCCESS   0    // exit sucessful
    15 #endif
    16 
    17 #define Port 5567
    18 #define IPADDR  "192.168.132.128"
    19 #define maxn 1024
    20 
    21 #define ERROR_EXIT( inf ) \
    22  do{  \
    23    perror( inf ); \
    24    printf("it's happened in %d \n",__LINE__); \
    25    }while(0) ;
    26 
    27 
    28  //use the function to connect the server !!
    29 
    30 
    31 int
    32 main (int argv, char *argc[])
    33 {
    34 
    35   int sfd = -1;
    36   char rbuf[maxn], wbuf[maxn];
    37   // int sfd=-1 ; //socket_fd
    38   int confd = -1;        //connect_fd
    39   struct sockaddr_in soaddr;
    40   if ((sfd = socket (AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)
    41     {
    42       ERROR_EXIT ("socket");
    43       return EXIT_FAILURE;
    44     }
    45 
    46   //memset  a struct    
    47   memset (&soaddr, 0, sizeof (soaddr));
    48 
    49   //int the struct sockaddr_in
    50 
    51   soaddr.sin_family = AF_INET;
    52   soaddr.sin_port = htons (Port);
    53   soaddr.sin_addr.s_addr = inet_addr (IPADDR);
    54 
    55   if ((confd =
    56        connect (sfd, (struct sockadrr *) &soaddr, sizeof (soaddr))) < 0)
    57     {
    58       ERROR_EXIT ("connect");
    59       return EXIT_FAILURE;
    60     }
    61 
    62   printf ("connect is sucessful !\n");
    63 
    64   while (fgets (wbuf, sizeof (rbuf), stdin) != NULL)
    65     {
    66       write (sfd, wbuf, strlen (wbuf));
    67       read (sfd, rbuf, sizeof (rbuf));
    68       fputs (rbuf, stdout);
    69     }
    70 
    71   close (sfd);
    72   close (confd);
    73   return EXIT_SUCCESS;
    74 }

![复制代码][copycode.gif]

makefile文件：

1.  ![复制代码][copycode.gif]
    
        1 .SUFFIXES: .o.c 
         2 CC =gcc
         3 SRC = client.c
         4 OBJ = $(SRC: .c =.o) 
         5 BIN = Se_client
         6 
         7 
         8 .PHONY: start
         9 start:  $(OBJ)
        10     $(CC) -o $(BIN) $(OBJ)
        11 .o.c: $(SRC)
        12     $(CC) -g -Wall $@ -c  $<
        13 .PHONY: clean
        14 clean:
        15     rm -f $(OBJ)
    
    ![复制代码][copycode.gif]
    
    效果图：
2.  ![061110445171967.png][]![061110541261831.png][]![061111033612509.png][]

[http_blog.csdn.net_turkeyzhou_article_details_8609360]: http://blog.csdn.net/turkeyzhou/article/details/8609360
[ContractedBlock.gif]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/d429dbc1acbc4b5aa97c27490a9dd9d9.png
[copycode.gif]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/63fb495d54d5461b84b63c4c506c44fa.png
[041727505482526.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/0411e9ecd3df4104aadd1b077ef876dc.png
[http_blog.csdn.net_tianmohust_article_details_6677985]: http://blog.csdn.net/tianmohust/article/details/6677985
[http_blog.csdn.net_xifeijian_article_details_17385831]: http://blog.csdn.net/xifeijian/article/details/17385831
[041749008455310.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/5b357a71e06c4204ac44e8a0a44f9cf1.png
[041806564391111.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/232b78106c7c45cbaf7ef237f86de653.png
[061110445171967.png]: https://images0.cnblogs.com/blog2015/539316/201508/061110445171967.png
[061110541261831.png]: https://images0.cnblogs.com/blog2015/539316/201508/061110541261831.png
[061111033612509.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/ab6e101df3a341ea8e01fa7378d105ca.png