Accept

// tcpsock_posix.go
// TCPListener is a TCP network listener.  Clients should typically
// use variables of type Listener instead of assuming TCP.
type TCPListener struct {
	fd *netFD
}

// AcceptTCP accepts the next incoming call and returns the new
// connection.
func (l *TCPListener) AcceptTCP() (*TCPConn, error) {
	if l == nil || l.fd == nil {
		return nil, syscall.EINVAL
	}
	fd, err := l.fd.accept(sockaddrToTCP)
	if err != nil {
		return nil, err
	}
	return newTCPConn(fd), nil
}

// Accept implements the Accept method in the Listener interface; it
// waits for the next call and returns a generic Conn.
func (l *TCPListener) Accept() (Conn, error) {
	c, err := l.AcceptTCP()
	if err != nil {
		return nil, err
	}
	return c, nil
}

func (fd *netFD) accept(toAddr func(syscall.Sockaddr) Addr) (netfd *netFD, err error) {
	if err := fd.readLock(); err != nil {
		return nil, err
	}
	defer fd.readUnlock()

	var s int
	var rsa syscall.Sockaddr
	if err = fd.pd.PrepareRead(); err != nil {
		return nil, &OpError{"accept", fd.net, fd.laddr, err}
	}
	for {
		s, rsa, err = accept(fd.sysfd) ///accept
		if err != nil {
			if err == syscall.EAGAIN {
				if err = fd.pd.WaitRead(); err == nil {
					continue
				}
			} else if err == syscall.ECONNABORTED {
				// This means that a socket on the listen queue was closed
				// before we Accept()ed it; it's a silly error, so try again.
				continue
			}
			return nil, &OpError{"accept", fd.net, fd.laddr, err}
		}
		break
	}

	if netfd, err = newFD(s, fd.family, fd.sotype, fd.net); err != nil {
		closesocket(s)
		return nil, err
	}
	if err = netfd.init(); err != nil {
		fd.Close()
		return nil, err
	}
	lsa, _ := syscall.Getsockname(netfd.sysfd)
	netfd.setAddr(toAddr(lsa), toAddr(rsa))
	return netfd, nil
}

/// net/sock_cloexec.go
// Wrapper around the accept system call that marks the returned file
// descriptor as nonblocking and close-on-exec.
func accept(s int) (int, syscall.Sockaddr, error) {
	ns, sa, err := syscall.Accept4(s, syscall.SOCK_NONBLOCK|syscall.SOCK_CLOEXEC)
	// On Linux the accept4 system call was introduced in 2.6.28
	// kernel and on FreeBSD it was introduced in 10 kernel. If we
	// get an ENOSYS error on both Linux and FreeBSD, or EINVAL
	// error on Linux, fall back to using accept.
	switch err {
	default: // nil and errors other than the ones listed
		return ns, sa, err
	case syscall.ENOSYS: // syscall missing
	case syscall.EINVAL: // some Linux use this instead of ENOSYS
	case syscall.EACCES: // some Linux use this instead of ENOSYS
	case syscall.EFAULT: // some Linux use this instead of ENOSYS
	}

	// See ../syscall/exec_unix.go for description of ForkLock.
	// It is probably okay to hold the lock across syscall.Accept
	// because we have put fd.sysfd into non-blocking mode.
	// However, a call to the File method will put it back into
	// blocking mode. We can't take that risk, so no use of ForkLock here.
	ns, sa, err = syscall.Accept(s)
	if err == nil {
		syscall.CloseOnExec(ns)
	}
	if err != nil {
		return -1, nil, err
	}
	if err = syscall.SetNonblock(ns, true); err != nil {
		syscall.Close(ns)
		return -1, nil, err
	}
	return ns, sa, nil
}