c++ - 为什么非阻塞套接字在 connect() 或 accept() 之前是可写的？

Question

从最近的研究中，我了解到如果你想进行非阻塞套接字连接，你可以将你的套接字设置为非阻塞模式，然后使用 select() 将你的套接字添加到writefds 参数。

那么，为什么在这个简化的示例中，select() 会提早解除阻塞并留下一个状态，说明我未连接的套接字在我什至没有执行连接时是可写的，更不用说有一个对端接受连接？

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

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 main( int argc, char* argv[] )
{
  SafeSocket s( "127.0.0.100" );
  std::cout << "Created socket " << s << std::endl;

  fd_set readFds;
  fd_set writeFds;

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

  FD_SET( s, &writeFds );

  timeval timeout = { 5, 0 };

  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;
  }

  return 0;
}

输出:

>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
3 is writable!

Answer 1

select() 告诉您套接字何时准备好执行下一个操作。在这种情况下，它已准备好调用 connect()。

当然，在全新的套接字上调用select() 是不必要的。大多数应用程序不会这样做。