type netFD struct {
    pfd poll.FD
    // immutable until Close
    family      int
    sotype      int
    isConnected bool // handshake completed or use of association with peer
    net         string
    laddr       Addr
    raddr       Addr
}

type FD struct {
    // Lock sysfd and serialize access to Read and Write methods.
    fdmu fdMutex
    // System file descriptor. Immutable until Close.
    Sysfd syscall.Handle
    // Read operation.
    rop operation
    // Write operation.
    wop operation
    // I/O poller.
    pd pollDesc
    // Used to implement pread/pwrite.
    l sync.Mutex
    // For console I/O.
    lastbits       []byte   // first few bytes of the last incomplete rune in last write
    readuint16     []uint16 // buffer to hold uint16s obtained with ReadConsole
    readbyte       []byte   // buffer to hold decoding of readuint16 from utf16 to utf8
    readbyteOffset int      // readbyte[readOffset:] is yet to be consumed with file.Read
    // Semaphore signaled when file is closed.
    csema uint32
    skipSyncNotif bool
    // Whether this is a streaming descriptor, as opposed to a
    // packet-based descriptor like a UDP socket.
    IsStream bool
    // Whether a zero byte read indicates EOF. This is false for a
    // message based socket connection.
    ZeroReadIsEOF bool
    // Whether this is a file rather than a network socket.
    isFile bool
    // The kind of this file.
    kind fileKind
}

type pollDesc struct {
    runtimeCtx uintptr
}

func (pd *pollDesc) init(fd *FD) error {
    serverInit.Do(runtime_pollServerInit)
    ctx, errno := runtime_pollOpen(uintptr(fd.Sysfd))
    if errno != 0 {
        if ctx != 0 {
            runtime_pollUnblock(ctx)
            runtime_pollClose(ctx)
        }
        return errnoErr(syscall.Errno(errno))
    }
    pd.runtimeCtx = ctx
    return nil
}

//go:linkname poll_runtime_pollOpen internal/poll.runtime_pollOpen
func poll_runtime_pollOpen(fd uintptr) (*pollDesc, int) {
    pd := pollcache.alloc()
    lock(&pd.lock)
    if pd.wg != 0 && pd.wg != pdReady {
        throw("runtime: blocked write on free polldesc")
    }
    if pd.rg != 0 && pd.rg != pdReady {
        throw("runtime: blocked read on free polldesc")
    }
    pd.fd = fd
    pd.closing = false
    pd.everr = false
    pd.rseq++
    pd.rg = 0
    pd.rd = 0
    pd.wseq++
    pd.wg = 0
    pd.wd = 0
    unlock(&pd.lock)
    var errno int32
    errno = netpollopen(fd, pd)
    return pd, int(errno)
}

func (c *pollCache) alloc() *pollDesc {
    lock(&c.lock)
    if c.first == nil {
        const pdSize = unsafe.Sizeof(pollDesc{})
        n := pollBlockSize / pdSize
        if n == 0 {
            n = 1
        }
        // Must be in non-GC memory because can be referenced
        // only from epoll/kqueue internals.
        mem := persistentalloc(n*pdSize, 0, &memstats.other_sys)
        for i := uintptr(0); i < n; i++ {
            pd := (*pollDesc)(add(mem, i*pdSize))
            pd.link = c.first
            c.first = pd
        }
    }
    pd := c.first
    c.first = pd.link
    unlock(&c.lock)
    return pd
}

type pollDesc struct {
    link *pollDesc // in pollcache, protected by pollcache.lock
    // The lock protects pollOpen, pollSetDeadline, pollUnblock and deadlineimpl operations.
    // This fully covers seq, rt and wt variables. fd is constant throughout the PollDesc lifetime.
    // pollReset, pollWait, pollWaitCanceled and runtime·netpollready (IO readiness notification)
    // proceed w/o taking the lock. So closing, everr, rg, rd, wg and wd are manipulated
    // in a lock-free way by all operations.
    // NOTE(dvyukov): the following code uses uintptr to store *g (rg/wg),
    // that will blow up when GC starts moving objects.
    lock    mutex // protects the following fields
    fd      uintptr
    closing bool
    everr   bool    // marks event scanning error happened
    user    uint32  // user settable cookie
    rseq    uintptr // protects from stale read timers
    rg      uintptr // pdReady, pdWait, G waiting for read or nil
    rt      timer   // read deadline timer (set if rt.f != nil)
    rd      int64   // read deadline
    wseq    uintptr // protects from stale write timers
    wg      uintptr // pdReady, pdWait, G waiting for write or nil
    wt      timer   // write deadline timer
    wd      int64   // write deadline
}

func netpollopen(fd uintptr, pd *pollDesc) int32 {
    var ev epollevent
    ev.events = _EPOLLIN | _EPOLLOUT | _EPOLLRDHUP | _EPOLLET
    *(**pollDesc)(unsafe.Pointer(&ev.data)) = pd
    return -epollctl(epfd, _EPOLL_CTL_ADD, int32(fd), &ev)
}

unc (lc *ListenConfig) Listen(ctx context.Context, network, address string) (Listener, error) {
    addrs, err := DefaultResolver.resolveAddrList(ctx, "listen", network, address, nil)
    if err != nil {
        return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
    }
    sl := &sysListener{
        ListenConfig: *lc,
        network:      network,
        address:      address,
    }
    var l Listener
    la := addrs.first(isIPv4)
    switch la := la.(type) {
    case *TCPAddr:
        l, err = sl.listenTCP(ctx, la)
    case *UnixAddr:
        l, err = sl.listenUnix(ctx, la)
    default:
        return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: address}}
    }
    if err != nil {
        return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: err} // l is non-nil interface containing nil pointer
    }
    return l, nil
}

func (sl *sysListener) listenTCP(ctx context.Context, laddr *TCPAddr) (*TCPListener, error) {
    fd, err := internetSocket(ctx, sl.network, laddr, nil, syscall.SOCK_STREAM, 0, "listen", sl.ListenConfig.Control)
    if err != nil {
        return nil, err
    }
    return &TCPListener{fd: fd, lc: sl.ListenConfig}, nil
}

func socket(ctx context.Context, net string, family, sotype, proto int, ipv6only bool, laddr, raddr sockaddr, ctrlFn func(string, string, syscall.RawConn) error) (fd *netFD, err error) {
    s, err := sysSocket(family, sotype, proto)
    if err != nil {
        return nil, err
    }
    if err = setDefaultSockopts(s, family, sotype, ipv6only); err != nil {
        poll.CloseFunc(s)
        return nil, err
    }
    if fd, err = newFD(s, family, sotype, net); err != nil {
        poll.CloseFunc(s)
        return nil, err
    }
    if laddr != nil && raddr == nil {
        switch sotype {
        case syscall.SOCK_STREAM, syscall.SOCK_SEQPACKET:
            if err := fd.listenStream(laddr, listenerBacklog(), ctrlFn); err != nil {
                fd.Close()
                return nil, err
            }
            return fd, nil
        case syscall.SOCK_DGRAM:
            if err := fd.listenDatagram(laddr, ctrlFn); err != nil {
                fd.Close()
                return nil, err
            }
            return fd, nil
        }
    }
    if err := fd.dial(ctx, laddr, raddr, ctrlFn); err != nil {
        fd.Close()
        return nil, err
    }
    return fd, nil
}　

func (fd *netFD) listenStream(laddr sockaddr, backlog int, ctrlFn func(string, string, syscall.RawConn) error) error {
    var err error
    if err = setDefaultListenerSockopts(fd.pfd.Sysfd); err != nil {
        return err
    }
    var lsa syscall.Sockaddr
    if lsa, err = laddr.sockaddr(fd.family); err != nil {
        return err
    }
    if ctrlFn != nil {
        c, err := newRawConn(fd)
        if err != nil {
            return err
        }
        if err := ctrlFn(fd.ctrlNetwork(), laddr.String(), c); err != nil {
            return err
        }
    }
    if err = syscall.Bind(fd.pfd.Sysfd, lsa); err != nil {
        return os.NewSyscallError("bind", err)
    }
    if err = listenFunc(fd.pfd.Sysfd, backlog); err != nil {
        return os.NewSyscallError("listen", err)
    }
    if err = fd.init(); err != nil {
        return err
    }
    lsa, _ = syscall.Getsockname(fd.pfd.Sysfd)
    fd.setAddr(fd.addrFunc()(lsa), nil)
    return nil
}

func (fd *netFD) init() error {
    errcall, err := fd.pfd.Init(fd.net, true)
    if errcall != "" {
        err = wrapSyscallError(errcall, err)
    }
    return err
}