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

从最近的研究中，我了解到，如果你想做一个非阻塞套接字连接，你可以将你的套接字设置为非阻塞模式，然后在select()将你的套接字添加到writefds参数中。

那么，为什么在这个简化的示例中，select()提前取消阻止并留下一个状态，说明我的未连接的套接字是可写的，而我甚至没有执行连接，更不用说让对等方接受连接了？

#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!