Question

根据“男人选择”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."

选择将wakup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
          }

[UPDATE]
1）@trojanfoe提供了一种实现此目的的方法，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。我将进行测试并在那里更新结果。
2）另外要提一下，关闭套接字并不能保证唤醒select函数调用，请参阅 this post 。

[UPDATE2]
经过多次测试后，这里有一些关于唤醒选择的事实：
1）如果select选择的套接字被另一个应用程序关闭，则选择（）调用会立刻醒来。此后，读取或写入套接字将返回0，其中errno = 0
2）如果由同一应用程序的另一个线程关闭了由select监视的套接字，如果没有要读取或写入的数据，则select（）将在超时之前不会唤醒。选择超时后，使用errno = EBADF进行读/写操作会导致错误（因为在超时期间套接字已被另一个线程关闭）

Answer 1

我使用基于pipe()的事件对象：

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)
            ;
    }
}

你可以抛弃m_ownsFDs成员;我甚至不确定我是否会再使用它了。

如何在另一个线程的超时内唤醒select（）

1 个答案: