柳大的Linux游记·基础篇（5）select IO复用机制

• Author: 柳大·Poechant（钟超）
• Blog: blog.CSDN.net/Poechant
• Email: zhongchao.ustc#gmail.com (#->@)
• Date: March 13^th, 2012
• Copyright © 柳大·Poechant

1 基本原理

注：select 原理图，摘自 IBM iSeries 信息中心。

1 数据结构与函数原型

1.1 select

• 函数原型

     int select(      int nfds,      fd_set *readset,      fd_set *writeset,      fd_set* exceptset,      struct timeval *timeout   );

• 头文件

  • select位于：

    #include <sys/select.h>

  • struct timeval位于：

    #include <sys/time.h>

• 返回值

  返回对应位仍然为1的fd的总数。

• 参数

  • nfds：第一个参数是：最大的文件描述符值+1；
  • readset：可读描述符集合；
  • writeset：可写描述符集合；
  • exceptset：异常描述符；
  • timeout：select 的监听时长，如果这短时间内所监听的 socket 没有事件发生。

1.2 fd_set

1.2.1 清空描述符集合

FD_ZERO(fd_set *)

1.2.2 向描述符集合添加指定描述符

FD_SET(int, fd_set *)

1.2.3 从描述符集合删除指定描述符

FD_CLR(int, fd_set *)

1.2.4 检测指定描述符是否在描述符集合中

FD_ISSET(int, fd_set *)

1.2.5 描述符最大数量

#define FD_SETSIZE 1024

1.3 描述符集合

可读描述符集合中可读的描述符，为1，其他为0；可写也类似。异常描述符集合中有异常等待处理的描述符的值为1，其他为0。

1.4 ioctl

• 函数原型：

    int ioctl(int handle, int cmd,[int *argdx, int argcx]);

• 头文件：

    #include <sys/ioctl.h>

• 返回值：

  • 0 - 成功
  • 1 - 失败

2 示例

程序各部分的解释在注释中。

#include <sys/socket.h>#include <string.h>#include <sys/time.h>#include <netinet/in.h>#include <sys/ioctl.h>#include <stdlib.h>#include <errno.h>#include <stdio.h>#include <unistd.h>#define TRUE  1#define FALSE 0int main(int argc, char *argv[]){    int i, len, rc, on = TRUE;    int listen_sd, new_sd = 0, max_sd;    int desc_ready;    char buffer[80];    int close_conn, end_server = FALSE;    struct sockaddr_in server_addr;    struct timeval timeout;    struct fd_set master_set, working_set;    // Listen    listen_sd = socket(AF_INET, SOCK_STREAM, 0);    if (listen_sd < 0)    {        perror("socket() failed");        exit(-1);    }    // Set socket options    rc = setsockopt(listen_sd, SOL_SOCKET, SO_REUSEADDR, (char *) &on, sizeof(on));    if (rc < 0)    {        perror("setsockopt() failed");        close(listen_sd);        exit(-1);    }    // Set IO control    rc = ioctl(listen_sd, FIONBIO, (char *) &on);    if (rc < 0)    {        perror("ioctl() failed");        close(listen_sd);        exit(-1);    }    // Bind    memset(&server_addr, 0, sizeof(server_addr));    server_addr.sin_family = AF_INET;    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);    server_addr.sin_port = htons(atoi(argv[1]));    rc = bind(listen_sd, (struct sockaddr *) &server_addr, sizeof(server_addr));    if (rc < 0)    {        perror("bind() failed\n");        close(listen_sd);        exit(-1);    }    // Listen    rc = listen(listen_sd, 32);    if (rc < 0)    {        perror("listen() failed\n");        close(listen_sd);        exit(-1);    }    // Intialize sd set    FD_ZERO(&master_set);    max_sd = listen_sd;    FD_SET(listen_sd, &master_set);    timeout.tv_sec = 3 * 60;    timeout.tv_usec = 0;    // Start    do    {        // Copy master_set into working_set        memcpy(&working_set, &master_set, sizeof(master_set));        printf("Waiting on select()...\n");        rc = select(max_sd + 1, &working_set, NULL, NULL, &timeout);        if (rc < 0)        {            perror("  select() failed\n");            break;        }        if (rc == 0)        {            printf("  select() timed out. End program.\n");            break;        }        desc_ready = rc; // number of sds ready in working_set        // Check each sd in working_set        for (i = 0; i <= max_sd && desc_ready > 0; ++i)        {            // Check to see if this sd is ready            if (FD_ISSET(i, &working_set))            {                --desc_ready;                // Check to see if this is the listening sd                if (i == listen_sd)                {                    printf("  Listeing socket is readable\n");                    do                    {                        // Accept                        new_sd = accept(listen_sd, NULL, NULL);                        // Nothing to be accepted                        if (new_sd < 0)                        {                            // All have been accepted                            if (errno != EWOULDBLOCK)                            {                                perror("  accept() failed\n");                                end_server = TRUE;                            }                            break;                        }                        // Insert new_sd into master_set                        printf("  New incoming connection - %d\n", new_sd);                        FD_SET(new_sd, &master_set);                        if (new_sd > max_sd)                        {                            max_sd = new_sd;                        }                    }                    while (new_sd != -1);                }                // This is not the listening sd                else                {                    close_conn = FALSE;                    printf("  Descriptor %d is avaliable\n", i);                    do                    {                        rc = recv(i, buffer, sizeof(buffer), 0);                        // Receive data on sd "i", until failure occurs                        if (rc < 0)                        {                            // Normal failure                            if (errno != EWOULDBLOCK)                            {                                perror("  recv() failed\n");                                close_conn = TRUE;                            }                            break;                        }                        // The connection has been closed by the client                        if (rc == 0)                        {                            printf("  Connection closed\n");                            close_conn = TRUE;                            break;                        }                        /* Receiving data succeeded and echo it back                           the to client */                        len = rc;                        printf("  %d bytes received\n", len);                        rc = send(i, buffer, len, 0);                        if (rc < 0)                        {                            perror("  send() failed");                            close_conn = TRUE;                            break;                        }                    }                    while (TRUE);                    // If unknown failure occured                    if (close_conn)                    {                        // Close the sd and remove it from master_set                        close(i);                        FD_CLR(i, &master_set);                        // If this is the max sd                        if (i == max_sd)                        {                            // Find the max sd in master_set now                            while (FD_ISSET(max_sd, &master_set) == FALSE)                            {                                --max_sd;                            }                        } // End of if (i == max_sd)                    } // End of if (close_conn)                }            }        }    }    while (end_server == FALSE);    /* Close each sd in master_set */    for (i = 0; i < max_sd; ++i)    {        if (FD_ISSET(i, &master_set))        {            close(i);        }    }    return 0;}

参考

1. http://publib.boulder.ibm.com/infocenter/iseries/v5r3/index.jsp?topic=%2Frzab6%2Frzab6xnonblock.htm

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