C++ 我可以延长接受服务器上的TCP握手持续时间吗？

这个问题源于

下面的代码生成一个侦听TCP连接的线程。主线程连接到服务器正在侦听的地址

#include <iostream>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <fcntl.h>
#include <unistd.h>
#include <cerrno>
#include <cstring>
#include <pthread.h>
#include <semaphore.h>

class SafeSocket
{
public:

  /** Ctor.
   * Creates a nonblocking socket at the specified IP in the AF_INET family and
   * at a dynamic port.
   */
  SafeSocket( const std::string& ip )
  {
    in_addr_t host_ip = inet_network( ip.c_str() );
    if ( ( socket_ = socket( AF_INET, SOCK_STREAM, 0 ) ) < 0 )
    {
      std::cout << "socket() failed: " << errno << " " << strerror( errno )
                << std::endl;
      socket_ = -1;
    }
    sockaddr_in si;
    memset( &si, 0, sizeof( si ) );
    si.sin_family = AF_INET;
    si.sin_port = 0; // Dynamic port
    si.sin_addr.s_addr = htonl( host_ip );
    if ( bind( socket_, (sockaddr*)&si, sizeof si ) )
    {
      std::cout << "bind() failed: " << errno << " " << strerror( errno )
                << std::endl;
      close( socket_ );
      socket_ = -1;
    }
    // Make the socket do nonblocking connect().
    int flags = fcntl( socket_, F_GETFL, 0 );
    fcntl( socket_, F_SETFL, flags | O_NONBLOCK );
  }

  ~SafeSocket()
  {
    if ( socket_ >= 0 )
    {
      shutdown( socket_, SHUT_RDWR );
      close( socket_ );
    }
  }

  operator int() const
  {
    return socket_;
  }

private:

  int socket_;
};

int connectToClient( const SafeSocket& sock, const std::string& clientIp,
                     const int clientPort )
{
  struct sockaddr_in clientAddr;
  memset( &clientAddr, 0, sizeof clientAddr );
  inet_pton( AF_INET, clientIp.c_str(), &clientAddr.sin_addr );
  clientAddr.sin_family = AF_INET;
  clientAddr.sin_port = htons( clientPort );
  return connect( sock, (sockaddr*)&clientAddr, sizeof clientAddr );
}

std::string serverIp( "127.0.0.200" );
int serverPort = 9099; // Random, hopefully unused.
sem_t listenSem;

/** Entry point to pthread.
 */
void* acceptConnection( void* arg )
{
  int listenSock = socket( PF_INET, SOCK_STREAM, 0 );
  if ( listenSock < 0 )
  {
    std::cout << "socket() failed: " << errno << " " << strerror( errno )
              << std::endl;
    return NULL;
  }

  sockaddr_in si;
  si.sin_family = AF_INET;
  inet_aton( serverIp.c_str(), &si.sin_addr );
  si.sin_port = htons( serverPort );

  int optval = 1;
  setsockopt( listenSock, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof optval);

  int result = bind( listenSock, (sockaddr*)&si, sizeof si );
  if ( result )
  {
    std::cout << "bind() failed: " << errno << " " << strerror( errno )
              << std::endl;
    close( listenSock );
    return NULL;
  }

  std::cout << "listening on socket " << listenSock << std::endl;
  if ( listen( listenSock, 3 ) )
  {
    std::cout << "listen() failed: " << errno << " " << strerror( errno )
              << std::endl;
    close( listenSock );
    return NULL;
  }

  sem_post( &listenSem );

  fd_set readfds;
  FD_ZERO( &readfds );
  FD_SET( listenSock, &readfds );
  struct timeval ts = { 5, 0 };
  if ( -1 != select( listenSock + 1, &readfds, NULL, NULL, &ts ) )
  {
    if ( FD_ISSET( listenSock, &readfds ) )
    {
      sockaddr_in peerSi;
      socklen_t peerAddrLen = sizeof peerSi;
      memset( &peerSi, 0, peerAddrLen );
      sleep( 3 );
      int acceptSock = accept( listenSock, (sockaddr*)&peerSi, &peerAddrLen );
      if ( acceptSock > 0 )
      {
        std::cout << "accepted connection on socket " << acceptSock
                  << std::endl;
        close( acceptSock );
      }
    }
    else
    {
      std::cout << "did not receive a connection to accept." << std::endl;
    }
  }
  close( listenSock );
  return NULL;
}

int main( int argc, char* argv[] )
{
  sem_init( &listenSem, 0, 0 );

  SafeSocket s( "127.0.0.100" );
  std::cout << "Created socket " << s << std::endl;

  pthread_t tid;
  pthread_create( &tid, NULL, acceptConnection, NULL );

  timespec listenTimeout;
  clock_gettime( CLOCK_REALTIME, &listenTimeout );
  listenTimeout.tv_sec += 5;
  sem_timedwait( &listenSem, &listenTimeout );

  fd_set readFds;
  fd_set writeFds;

  FD_ZERO( &readFds );
  FD_ZERO( &writeFds );

  FD_SET( s, &writeFds );

  timeval timeout = { 5, 0 };
  int result = connectToClient( s, serverIp, serverPort );
  std::cout << "connectToClient() returned " << result << " "
            << errno << " " << strerror( errno ) << std::endl;

  if ( -1 == select( s+1, &readFds, &writeFds, NULL, &timeout ) )
  {
    std::cout << "select() failed: " << errno << " " << strerror( errno )
              << std::endl;
  }

  if ( FD_ISSET( s, &writeFds ) )
  {
    std::cout << s << " is writable!" << std::endl;
    int result = -1;
    socklen_t result_len = sizeof result;
    getsockopt( s, SOL_SOCKET, SO_ERROR, &result, &result_len );
    std::cout << "result: " << result << " " << strerror( result ) << std::endl;
  }

  pthread_join( tid, NULL );
  return 0;
}

EJP评论说，在这种情况下，

select（）

阻塞“目标TCP正在握手”

---

那么，有没有办法延长这个“TCP握手”过程？也就是说，我想延长

select（）

阻塞等待套接字

s

变为可写的时间量。这能做到吗？您可能会注意到，我试图通过在接受之前添加一个

sleep（）

来实现此目的，但这不起作用。

握手遵循以下阶段：

• 阶段1：连接正在客户端上运行。这从调用connect开始运行，直到至少客户端从服务器获取SYN+ACK数据包为止。我这么说至少是因为客户端内核可能需要一段时间来处理该数据包。我的理解是，套接字在第1阶段结束之前是不可写的

• 阶段2：套接字在客户端上可写；连接完成。服务器尚未从accept返回。这包括服务器等待从客户端接收ACK的时间，以及从接收ACK到服务器应用程序调用

  accept

  之间的任何时间（加上等待调用

  accept

  得到处理）

• 第三阶段：双方都完成

我已经完成了以下几个阶段：我所做的区分旨在使讨论您的问题变得容易，而不是遵循TCP状态机的任何方面

没有有效的方法延长阶段1，当然也没有方法使阶段1包含阶段2。您可以在客户机内核中添加一些延迟，但这是任意的，与网络上发生的任何事情都无关

阶段1和阶段2之间的时间差完全与网络延迟和服务器机器内的处理有关。没有客户可以采取行动的触发器

对于许多应用程序，希望套接字一可写就开始写。如果不是这样，一般接受的解决方案是让您的协议让服务器向客户端发送初始问候语。然后，等待套接字可读并在执行操作之前处理该问候语

>g++ --version
g++ (GCC) 4.8.3 20140911 (Red Hat 4.8.3-7)
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

>g++ -g main.cpp 
>
>./a.out 
Created socket 3                                             // Immediate
listening on socket 4                                        // Immediate
connectToClient() returned -1 115 Operation now in progress  // Immediate
3 is writable!                                               // Immediate
result: 0 Success                                            // Immediate
accepted connection on socket 5                              // Delayed