4

根据“man select”信息： 

     "On  success,  select() and pselect() return the number of file descrip‐
   tors contained in the three returned  descriptor  sets 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."

Select 会唤醒，因为： 

       1)read/write availability       
       2)select error                 
       3)descriptoris closed.

但是，如果没有可用数据并且 select 仍在超时范围内，我们如何从另一个线程中唤醒 select()？

[更新]
伪代码

          // Thread blocks on Select
          void *SocketReadThread(void *param){
               ...
               while(!(ReadThread*)param->ExitThread()) {
                   struct timeval timeout;
                   timeout.tv_sec = 60; //one minute
                   timeout.tv_usec = 0;

                   fd_set rds;
                   FD_ZERO(&rds);
                   FD_SET(sockfd, &rds)'

                   //actually, the first parameter of select() is 
                    //ignored on windows, though on linux this parameter
                   //should be (maximum socket value + 1)
                   int ret = select(sockfd + 1, &rds, NULL, NULL, &timeout );
                   //handle the result
                   //might break from here

               }
               return NULL;
          }

          //main Thread
          int main(){
                //create the SocketReadThread
                ReaderThread* rthread = new ReaderThread;
                pthread_create(&pthreadid, NULL, SocketReaderThread, 
                          NULL, (void*)rthread);

                 // do lots of things here
                 ............................

                //now main thread wants to exit SocketReaderThread
                //it sets the internal state of ReadThread as true
                rthread->SetExitFlag(true);
                //but how to wake up select ??????????????????
                //if SocketReaderThread currently blocks on select
          }

[更新]
1）@trojanfoe 提供了一种方法来实现这一点，他的方法将套接字数据（可能是脏数据或退出消息数据）写入唤醒选择。我将在那里进行测试并更新结果。
2）还有一点要提一下，关闭套接字并不能保证唤醒选择函数调用，请参阅这篇文章。

[UPDATE2]
经过多次测试，以下是关于唤醒 select 的一些事实：
1）如果 select 监视的套接字被另一个应用程序关闭，则 select() 调用将立即唤醒。此后，读取或写入套接字将返回 0 且 errno = 0
2) 如果 select 监视的套接字被同一应用程序的另一个线程关闭，则 select() 将在超时之前唤醒没有要读取或写入的数据。选择超时后，进行读/写操作会导致错误 errno = EBADF（因为在超时期间套接字已被另一个线程关闭）

4

1 回答 1

7

我使用基于以下的事件对象pipe()：

IOEvent.h：

#pragma once

class IoEvent {
protected:
    int m_pipe[2];
    bool m_ownsFDs;

public:
    IoEvent();              // Creates a user event
    IoEvent(int fd);        // Create a file event

    IoEvent(const IoEvent &other);

    virtual ~IoEvent();

    /**
     * Set the event to signalled state.
     */
    void set();

    /**
     * Reset the event from signalled state.
     */
    void reset();

    inline int fd() const {
        return m_pipe[0];
    }
};

IoEvent.cpp：

#include "IoEvent.h"
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <poll.h>

using namespace std;

IoEvent::IoEvent() : 
    m_ownsFDs(true) {
    if (pipe(m_pipe) < 0)
        throw MyException("Failed to create pipe: %s (%d)", strerror(errno), errno);

    if (fcntl(m_pipe[0], F_SETFL, O_NONBLOCK) < 0)
        throw MyException("Failed to set pipe non-blocking mode: %s (%d)", strerror(errno), errno);
}

IoEvent::IoEvent(int fd) : 
    m_ownsFDs(false) {
    m_pipe[0] = fd;
    m_pipe[1] = -1;
}

IoEvent::IoEvent(const IoEvent &other) {
    m_pipe[0] = other.m_pipe[0];
    m_pipe[1] = other.m_pipe[1];
    m_ownsFDs = false;
}

IoEvent::~IoEvent() {
    if (m_pipe[0] >= 0) {
        if (m_ownsFDs)
            close(m_pipe[0]);

        m_pipe[0] = -1;
    }

    if (m_pipe[1] >= 0) {
        if (m_ownsFDs)
            close(m_pipe[1]);

        m_pipe[1] = -1;
    }
}

void IoEvent::set() {
    if (m_ownsFDs)
        write(m_pipe[1], "x", 1);
}

void IoEvent::reset() {
    if (m_ownsFDs) {
        uint8_t buf;

        while (read(m_pipe[0], &buf, 1) == 1)
            ;
    }
}

您可以放弃该m_ownsFDs成员；我什至不确定我是否会再使用它。

于 2013-07-02T13:07:05.923 回答