NOTE: These gozmq bindings are in maintenance mode. Only critical bugs will be fixed. Henceforth I would suggest using @pebbe's actively maintained bindings for zmq2, zmq3 and zmq4.

Go (golang) Bindings for 0mq (zmq, zeromq)

This package implements Go (golang) bindings for the 0mq C API.

GoZMQ does not support zero-copy.

A full list of examples is included in the zguide.

Note that this is not the same as this implementation or this implementation.

Upgrading

GoZMQ has made some public changes that will break old code. Fortunately, we've also written a tool based on go fix that will upgrade your code for you! Here's how to run it over your source (after making a backup of course):

go get github.com/alecthomas/gozmq/gozmqfix
cd $YOUR_SOURCE_DIR
gozmqfix .

Installing

GoZMQ currently supports ZMQ 2.1.x, 2.2.x, 3.x and 4.x. Following are instructions on how to compile against these versions.

For ZeroMQ 2.2.x install with:

go get github.com/alecthomas/gozmq

For 2.1.x install with:

go get -tags zmq_2_1 github.com/alecthomas/gozmq

For 3.x install with:

go get -tags zmq_3_x github.com/alecthomas/gozmq

For 4.x install with:

go get -tags zmq_4_x github.com/alecthomas/gozmq

Troubleshooting

Go can't find ZMQ

If the go tool can't find zmq and you know it is installed, you may need to override the C compiler/linker flags.

eg. If you installed zmq into /opt/zmq you might try:

CGO_CFLAGS=-I/opt/zmq/include CGO_LDFLAGS=-L/opt/zmq/lib
go get github.com/alecthomas/gozmq

Mismatch in version of ZMQ

If you get errors like this with 'go get' or 'go build':

1: error: 'ZMQ_FOO' undeclared (first use in this function)

There are two possibilities:

Your version of zmq is very old. In this case you will need to download and build zmq yourself.
You are building gozmq against the wrong version of zmq. See the installation instructions for details on how to target the correct version.

Differences from the C API

The API implemented by this package does not attempt to expose zmq_msg_t at all. Instead, Recv() and Send() both operate on byte slices, allocating and freeing the memory automatically. Currently this requires copying to/from C malloced memory, but a future implementation may be able to avoid this to a certain extent.

All major features are supported: contexts, sockets, devices, and polls.

Example

Here are direct translations of some of the examples from this blog post.

A simple echo server:

package main

import zmq "github.com/alecthomas/gozmq"

func main() {
  context, _ := zmq.NewContext()
  socket, _ := context.NewSocket(zmq.REP)
  socket.Bind("tcp://127.0.0.1:5000")
  socket.Bind("tcp://127.0.0.1:6000")

  for {
    msg, _ := socket.Recv(0)
    println("Got", string(msg))
    socket.Send(msg, 0)
  }
}

A simple client for the above server:

package main

import "fmt"
import zmq "github.com/alecthomas/gozmq"

func main() {
  context, _ := zmq.NewContext()
  socket, _ := context.NewSocket(zmq.REQ)
  socket.Connect("tcp://127.0.0.1:5000")
  socket.Connect("tcp://127.0.0.1:6000")

  for i := 0; i < 10; i   {
    msg := fmt.Sprintf("msg %d", i)
    socket.Send([]byte(msg), 0)
    println("Sending", msg)
    socket.Recv(0)
  }
}

Caveats

Zero-copy

GoZMQ does not support zero-copy.

GoZMQ does not attempt to expose zmq_msg_t at all. Instead, Recv() and Send() both operate on byte slices, allocating and freeing the memory automatically. Currently this requires copying to/from C malloced memory, but a future implementation may be able to avoid this to a certain extent.

Memory management

It's not entirely clear from the 0mq documentation how memory for zmq_msg_t and packet data is managed once 0mq takes ownership. After digging into the source a little, this package operates under the following (educated) assumptions:

References to zmq_msg_t structures are not held by the C API beyond the duration of any function call.
Packet data is reference counted internally by the C API. The count is incremented when a packet is queued for delivery to a destination (the inference being that for delivery to N destinations, the reference count will be incremented N times) and decremented once the packet has either been delivered or errored.

Usage

const (
  // NewSocket types
  PAIR   = SocketType(C.ZMQ_PAIR)
  PUB    = SocketType(C.ZMQ_PUB)
  SUB    = SocketType(C.ZMQ_SUB)
  REQ    = SocketType(C.ZMQ_REQ)
  REP    = SocketType(C.ZMQ_REP)
  DEALER = SocketType(C.ZMQ_DEALER)
  ROUTER = SocketType(C.ZMQ_ROUTER)
  PULL   = SocketType(C.ZMQ_PULL)
  PUSH   = SocketType(C.ZMQ_PUSH)
  XPUB   = SocketType(C.ZMQ_XPUB)
  XSUB   = SocketType(C.ZMQ_XSUB)

  // Deprecated aliases
  XREQ       = DEALER
  XREP       = ROUTER
  UPSTREAM   = PULL
  DOWNSTREAM = PUSH

  // NewSocket options
  AFFINITY          = UInt64SocketOption(C.ZMQ_AFFINITY)
  IDENTITY          = StringSocketOption(C.ZMQ_IDENTITY)
  SUBSCRIBE         = StringSocketOption(C.ZMQ_SUBSCRIBE)
  UNSUBSCRIBE       = StringSocketOption(C.ZMQ_UNSUBSCRIBE)
  RATE              = Int64SocketOption(C.ZMQ_RATE)
  RECOVERY_IVL      = Int64SocketOption(C.ZMQ_RECOVERY_IVL)
  SNDBUF            = UInt64SocketOption(C.ZMQ_SNDBUF)
  RCVBUF            = UInt64SocketOption(C.ZMQ_RCVBUF)
  FD                = Int64SocketOption(C.ZMQ_FD)
  EVENTS            = UInt64SocketOption(C.ZMQ_EVENTS)
  TYPE              = UInt64SocketOption(C.ZMQ_TYPE)
  LINGER            = IntSocketOption(C.ZMQ_LINGER)
  RECONNECT_IVL     = IntSocketOption(C.ZMQ_RECONNECT_IVL)
  RECONNECT_IVL_MAX = IntSocketOption(C.ZMQ_RECONNECT_IVL_MAX)
  BACKLOG           = IntSocketOption(C.ZMQ_BACKLOG)

  // Send/recv options
  SNDMORE = SendRecvOption(C.ZMQ_SNDMORE)
)

const (
  POLLIN  = PollEvents(C.ZMQ_POLLIN)
  POLLOUT = PollEvents(C.ZMQ_POLLOUT)
  POLLERR = PollEvents(C.ZMQ_POLLERR)
)

const (
  STREAMER  = DeviceType(C.ZMQ_STREAMER)
  FORWARDER = DeviceType(C.ZMQ_FORWARDER)
  QUEUE     = DeviceType(C.ZMQ_QUEUE)
)

const (
  RCVTIMEO = IntSocketOption(C.ZMQ_RCVTIMEO)
  SNDTIMEO = IntSocketOption(C.ZMQ_SNDTIMEO)
)

const (
  RCVMORE           = UInt64SocketOption(C.ZMQ_RCVMORE)
  RECOVERY_IVL_MSEC = Int64SocketOption(C.ZMQ_RECOVERY_IVL_MSEC)
  SWAP              = Int64SocketOption(C.ZMQ_SWAP)
  MCAST_LOOP        = Int64SocketOption(C.ZMQ_MCAST_LOOP)
  HWM               = UInt64SocketOption(C.ZMQ_HWM)
  NOBLOCK           = SendRecvOption(C.ZMQ_NOBLOCK)

  // Forwards-compatible aliases:
  DONTWAIT = NOBLOCK
)

const (
  RCVMORE = IntSocketOption(C.ZMQ_RCVMORE)
  SNDHWM  = IntSocketOption(C.ZMQ_SNDHWM)
  RCVHWM  = IntSocketOption(C.ZMQ_RCVHWM)

  // TODO Not documented in the man page...
  //LAST_ENDPOINT       = UInt64SocketOption(C.ZMQ_LAST_ENDPOINT)
  FAIL_UNROUTABLE     = BoolSocketOption(C.ZMQ_FAIL_UNROUTABLE)
  TCP_KEEPALIVE       = IntSocketOption(C.ZMQ_TCP_KEEPALIVE)
  TCP_KEEPALIVE_CNT   = IntSocketOption(C.ZMQ_TCP_KEEPALIVE_CNT)
  TCP_KEEPALIVE_IDLE  = IntSocketOption(C.ZMQ_TCP_KEEPALIVE_IDLE)
  TCP_KEEPALIVE_INTVL = IntSocketOption(C.ZMQ_TCP_KEEPALIVE_INTVL)
  TCP_ACCEPT_FILTER   = StringSocketOption(C.ZMQ_TCP_ACCEPT_FILTER)

  // Message options
  MORE = MessageOption(C.ZMQ_MORE)

  // Send/recv options
  DONTWAIT = SendRecvOption(C.ZMQ_DONTWAIT)

  // Deprecated aliases
  NOBLOCK = DONTWAIT
)

var (
  // Additional ZMQ errors
  ENOTSOCK       error = zmqErrno(C.ENOTSOCK)
  EFSM           error = zmqErrno(C.EFSM)
  EINVAL         error = zmqErrno(C.EINVAL)
  ENOCOMPATPROTO error = zmqErrno(C.ENOCOMPATPROTO)
  ETERM          error = zmqErrno(C.ETERM)
  EMTHREAD       error = zmqErrno(C.EMTHREAD)
)

func Device

func Device(t DeviceType, in, out *Socket) error

run a zmq_device passing messages between in and out

func Poll

func Poll(items []PollItem, timeout time.Duration) (count int, err error)

Poll ZmqSockets and file descriptors for I/O readiness. Timeout is in time.Duration. The smallest possible timeout is time.Millisecond for ZeroMQ version 3 and above, and time.Microsecond for earlier versions.

func Proxy

func Proxy(in, out, capture *Socket) error

run a zmq_proxy with in, out and capture sockets

func Version

func Version() (int, int, int)

void zmq_version (int *major, int *minor, int *patch);

type BoolSocketOption

type BoolSocketOption int

type Context

type Context struct {
}

A context handles socket creation and asynchronous message delivery. * There should generally be one context per application.

func NewContext

func NewContext() (*Context, error)

Create a new context.

func (*Context) Close

func (c *Context) Close()

func (*Context) IOThreads

func (c *Context) IOThreads() (int, error)

Get a context option.

func (*Context) MaxSockets

func (c *Context) MaxSockets() (int, error)

func (*Context) NewSocket

func (c *Context) NewSocket(t SocketType) (*Socket, error)

Create a new socket. void *zmq_socket (void *context, int type);

func (*Context) SetIOThreads

func (c *Context) SetIOThreads(value int) error

Set a context option.

func (*Context) SetMaxSockets

func (c *Context) SetMaxSockets(value int) error

type DeviceType

type DeviceType int

type Int64SocketOption

type Int64SocketOption int

type IntSocketOption

type IntSocketOption int

type MessageOption

type MessageOption int

type PollEvents

type PollEvents C.short

type PollItem

type PollItem struct {
  Socket  *Socket         // socket to poll for events on
  Fd      ZmqOsSocketType // fd to poll for events on as returned from os.File.Fd()
  Events  PollEvents      // event set to poll for
  REvents PollEvents      // events that were present
}

Item to poll for read/write events on, either a *Socket or a file descriptor

type PollItems

type PollItems []PollItem

a set of items to poll for events on

type SendRecvOption

type SendRecvOption int

type Socket

type Socket struct {
}

func (*Socket) Affinity

func (s *Socket) Affinity() (uint64, error)

ZMQ_AFFINITY: Retrieve I/O thread affinity.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc7

func (*Socket) Backlog

func (s *Socket) Backlog() (int, error)

ZMQ_BACKLOG: Retrieve maximum length of the queue of outstanding connections.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc18

func (*Socket) Bind

func (s *Socket) Bind(address string) error

Bind the socket to a listening address. int zmq_bind (void *s, const char *addr);

func (*Socket) Close

func (s *Socket) Close() error

Shutdown the socket. int zmq_close (void *s);

func (*Socket) Connect

func (s *Socket) Connect(address string) error

Connect the socket to an address. int zmq_connect (void *s, const char *addr);

func (*Socket) Events

func (s *Socket) Events() (uint64, error)

ZMQ_EVENTS: Retrieve socket event state.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc20

func (*Socket) GetSockOptBool

func (s *Socket) GetSockOptBool(option BoolSocketOption) (value bool, err error)

func (*Socket) GetSockOptInt

func (s *Socket) GetSockOptInt(option IntSocketOption) (value int, err error)

Get an int option from the socket. int zmq_getsockopt (void *s, int option, void *optval, size_t *optvallen);

func (*Socket) GetSockOptInt64

func (s *Socket) GetSockOptInt64(option Int64SocketOption) (value int64, err error)

Get an int64 option from the socket. int zmq_getsockopt (void *s, int option, void *optval, size_t *optvallen);

func (*Socket) GetSockOptString

func (s *Socket) GetSockOptString(option StringSocketOption) (value string, err error)

Get a string option from the socket. int zmq_getsockopt (void *s, int option, void *optval, size_t *optvallen);

func (*Socket) GetSockOptUInt64

func (s *Socket) GetSockOptUInt64(option UInt64SocketOption) (value uint64, err error)

Get a uint64 option from the socket. int zmq_getsockopt (void *s, int option, void *optval, size_t *optvallen);

func (*Socket) HWM

func (s *Socket) HWM() (uint64, error)

ZMQ_HWM: Retrieve high water mark.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc5

func (*Socket) Identity

func (s *Socket) Identity() (string, error)

ZMQ_IDENTITY: Retrieve socket identity.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc8

func (*Socket) Linger

func (s *Socket) Linger() (time.Duration, error)

ZMQ_LINGER: Retrieve linger period for socket shutdown.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc15

func (*Socket) McastLoop

func (s *Socket) McastLoop() (bool, error)

ZMQ_MCAST_LOOP: Control multicast loop-back.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc12

func (*Socket) Rate

func (s *Socket) Rate() (int64, error)

ZMQ_RATE: Retrieve multicast data rate.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc9

func (*Socket) RcvBuf

func (s *Socket) RcvBuf() (uint64, error)

ZMQ_RCVBUF: Retrieve kernel receive buffer size.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc14

func (*Socket) RcvHWM

func (s *Socket) RcvHWM() (int, error)

ZMQ_RCVHWM: Retrieve high water mark for inbound messages.

See: http://api.zeromq.org/3.2:zmq-getsockopt#toc6

func (*Socket) RcvMore

func (s *Socket) RcvMore() (bool, error)

ZMQ_RCVMORE: More message parts to follow.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc4

func (*Socket) RcvTimeout

func (s *Socket) RcvTimeout() (time.Duration, error)

ZMQ_RCVTIMEO: Maximum time before a socket operation returns with EAGAIN.

See: http://api.zeromq.org/2.2:zmq-getsockopt#toc6

func (*Socket) ReconnectIvl

func (s *Socket) ReconnectIvl() (time.Duration, error)

ZMQ_RECONNECT_IVL: Retrieve reconnection interval.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc16

func (*Socket) ReconnectIvlMax

func (s *Socket) ReconnectIvlMax() (time.Duration, error)

ZMQ_RECONNECT_IVL_MAX: Retrieve maximum reconnection interval.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc17

func (*Socket) RecoveryIvl

func (s *Socket) RecoveryIvl() (time.Duration, error)

ZMQ_RECOVERY_IVL_MSEC: Get multicast recovery interval in milliseconds.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc11

func (*Socket) Recv

func (s *Socket) Recv(flags SendRecvOption) (data []byte, err error)

Receive a message from the socket. int zmq_recv (void *s, zmq_msg_t *msg, int flags);

func (*Socket) RecvMultipart

func (s *Socket) RecvMultipart(flags SendRecvOption) (parts [][]byte, err error)

Receive a multipart message.

func (*Socket) Send

func (s *Socket) Send(data []byte, flags SendRecvOption) error

Send a message to the socket. int zmq_send (void *s, zmq_msg_t *msg, int flags);

func (*Socket) SendMultipart

func (s *Socket) SendMultipart(parts [][]byte, flags SendRecvOption) (err error)

Send a multipart message.

func (*Socket) SetAffinity

func (s *Socket) SetAffinity(value uint64) error

ZMQ_AFFINITY: Set I/O thread affinity.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc5

func (*Socket) SetBacklog

func (s *Socket) SetBacklog(value int) error

ZMQ_BACKLOG: Set maximum length of the queue of outstanding connections.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc18

func (*Socket) SetHWM

func (s *Socket) SetHWM(value uint64) error

ZMQ_HWM: Set high water mark.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc3

func (*Socket) SetIdentity

func (s *Socket) SetIdentity(value string) error

ZMQ_IDENTITY: Set socket identity.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc6

func (*Socket) SetLinger

func (s *Socket) SetLinger(value time.Duration) error

ZMQ_LINGER: Set linger period for socket shutdown.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc15

func (*Socket) SetMcastLoop

func (s *Socket) SetMcastLoop(value bool) error

ZMQ_MCAST_LOOP: Control multicast loop-back.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc12

func (*Socket) SetRate

func (s *Socket) SetRate(value int64) error

ZMQ_RATE: Set multicast data rate.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc9

func (*Socket) SetRcvBuf

func (s *Socket) SetRcvBuf(value uint64) error

ZMQ_RCVBUF: Set kernel receive buffer size.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc14

func (*Socket) SetRcvHWM

func (s *Socket) SetRcvHWM(value int) error

ZMQ_RCVHWM: Set high water mark for inbound messages.

See: http://api.zeromq.org/3.2:zmq-setsockopt#toc4

func (*Socket) SetRcvTimeout

func (s *Socket) SetRcvTimeout(value time.Duration) error

ZMQ_RCVTIMEO: Maximum time before a recv operation returns with EAGAIN.

See: http://api.zeromq.org/2.2:zmq-setsockopt#toc9

func (*Socket) SetReconnectIvl

func (s *Socket) SetReconnectIvl(value time.Duration) error

ZMQ_RECONNECT_IVL: Set reconnection interval.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc16

func (*Socket) SetReconnectIvlMax

func (s *Socket) SetReconnectIvlMax(value time.Duration) error

ZMQ_RECONNECT_IVL_MAX: Set maximum reconnection interval.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc17

func (*Socket) SetRecoveryIvl

func (s *Socket) SetRecoveryIvl(value time.Duration) error

ZMQ_RECOVERY_IVL_MSEC: Set multicast recovery interval in milliseconds.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc11

func (*Socket) SetSndBuf

func (s *Socket) SetSndBuf(value uint64) error

ZMQ_SNDBUF: Set kernel transmit buffer size.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc13

func (*Socket) SetSndHWM

func (s *Socket) SetSndHWM(value int) error

ZMQ_SNDHWM: Set high water mark for outbound messages.

See: http://api.zeromq.org/3.2:zmq-setsockopt#toc3

func (*Socket) SetSndTimeout

func (s *Socket) SetSndTimeout(value time.Duration) error

ZMQ_SNDTIMEO: Maximum time before a send operation returns with EAGAIN.

See: http://api.zeromq.org/2.2:zmq-setsockopt#toc10

func (*Socket) SetSockOptInt

func (s *Socket) SetSockOptInt(option IntSocketOption, value int) error

Set an int option on the socket. int zmq_setsockopt (void *s, int option, const void *optval, size_t optvallen);

func (*Socket) SetSockOptInt64

func (s *Socket) SetSockOptInt64(option Int64SocketOption, value int64) error

Set an int64 option on the socket. int zmq_setsockopt (void *s, int option, const void *optval, size_t optvallen);

func (*Socket) SetSockOptString

func (s *Socket) SetSockOptString(option StringSocketOption, value string) error

Set a string option on the socket. int zmq_setsockopt (void *s, int option, const void *optval, size_t optvallen);

func (*Socket) SetSockOptStringNil

func (s *Socket) SetSockOptStringNil(option StringSocketOption) error

Set a string option on the socket to nil. int zmq_setsockopt (void *s, int option, const void *optval, size_t optvallen);

func (*Socket) SetSockOptUInt64

func (s *Socket) SetSockOptUInt64(option UInt64SocketOption, value uint64) error

Set a uint64 option on the socket. int zmq_setsockopt (void *s, int option, const void *optval, size_t optvallen);

func (*Socket) SetSubscribe

func (s *Socket) SetSubscribe(value string) error

ZMQ_SUBSCRIBE: Establish message filter.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc7

func (*Socket) SetSwap

func (s *Socket) SetSwap(value int64) error

ZMQ_SWAP: Set disk offload size.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc4

func (*Socket) SetTCPKeepalive

func (s *Socket) SetTCPKeepalive(value int) error

ZMQ_TCP_KEEPALIVE: Override SO_KEEPALIVE socket option.

See: http://api.zeromq.org/3.2:zmq-setsockopt#toc25

func (*Socket) SetTCPKeepaliveCnt

func (s *Socket) SetTCPKeepaliveCnt(value int) error

ZMQ_TCP_KEEPALIVE_CNT: Override TCP_KEEPCNT socket option.

See: http://api.zeromq.org/3.2:zmq-setsockopt#toc27

func (*Socket) SetTCPKeepaliveIdle

func (s *Socket) SetTCPKeepaliveIdle(value int) error

ZMQ_TCP_KEEPALIVE_IDLE: Override TCP_KEEPCNT(or TCP_KEEPALIVE on some OS).

See: http://api.zeromq.org/3.2:zmq-setsockopt#toc26

func (*Socket) SetTCPKeepaliveIntvl

func (s *Socket) SetTCPKeepaliveIntvl(value int) error

ZMQ_TCP_KEEPALIVE_INTVL: Override TCP_KEEPINTVL socket option.

See: http://api.zeromq.org/3.2:zmq-setsockopt#toc28

func (*Socket) SetUnsubscribe

func (s *Socket) SetUnsubscribe(value string) error

ZMQ_UNSUBSCRIBE: Remove message filter.

See: http://api.zeromq.org/2.1:zmq-setsockopt#toc8

func (*Socket) SndBuf

func (s *Socket) SndBuf() (uint64, error)

ZMQ_SNDBUF: Retrieve kernel transmit buffer size.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc13

func (*Socket) SndHWM

func (s *Socket) SndHWM() (int, error)

ZMQ_SNDHWM: Retrieves high water mark for outbound messages.

See: http://api.zeromq.org/3.2:zmq-getsockopt#toc5

func (*Socket) SndTimeout

func (s *Socket) SndTimeout() (time.Duration, error)

ZMQ_SNDTIMEO: Maximum time before a socket operation returns with EAGAIN.

See: http://api.zeromq.org/2.2:zmq-getsockopt#toc7

func (*Socket) Swap

func (s *Socket) Swap() (int64, error)

ZMQ_SWAP: Retrieve disk offload size.

See: http://api.zeromq.org/2.1:zmq-getsockopt#toc6

func (*Socket) TCPKeepalive

func (s *Socket) TCPKeepalive() (int, error)