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ssh_channel(3erl)		     Erlang Module Definition			ssh_channel(3erl)

       ssh_channel - Generic Ssh Channel Behavior

       Ssh  services  are implemented as channels that are multiplexed over an ssh connection and
       communicates via the ssh connection protocol. This module provides  a  callback	API  that
       takes care of generic channel aspects such as flow control and close messages and lets the
       callback functions take care of the service specific parts.

       Type definitions that are used more than once in this module and/or abstractions to  indi-
       cate the intended use of the data type:

       boolean() = true | false

       string() = list of ASCII characters

       timeout() = infinity | integer() - in milliseconds.

       ssh_connection_ref() - opaque to the user returned by ssh:connect/3 or sent to a ssh chan-
       nel process

       ssh_channel_id() = integer()

       ssh_data_type_code() = 1 ("stderr") | 0 ("normal") are currently valid values see RFC 4254
       section 5.2.

       call(ChannelRef, Msg) ->
       call(ChannelRef, Msg, Timeout) -> Reply | {error, Reason}

	      Types  ChannelRef = pid()
		     As returned by start_link/4
		     Msg = term()
		     Timeout = timeout()
		     Reply = term()
		     Reason = closed | timeout

	      Makes  a	synchronous  call to the channel process by sending a message and waiting
	      until a reply arrives or a timeout  occurs.  The	channel  will  call  CallbackMod-
	      ule:handle_call/3  to  handle  the  message.  If the channel process does not exist
	      {error, closed} is returned.

       cast(ChannelRef, Msg) -> ok

	      Types  ChannelRef = pid()
		     As returned by start_link/4
		     Msg = term()

	      Sends an asynchronous message to the channel process and	returns  ok  immediately,
	      ignoring	if  the  destination  node or channel process does not exist. The channel
	      will call CallbackModule:handle_cast/2 to handle the message.

       enter_loop(State) -> _

	      Types  State = term() - as returned by ssh_channel:init/1

	      Makes an existing process into a ssh_channel process. Does not return, instead  the
	      calling  process	will  enter  the  ssh_channel  process	receive loop and become a
	      ssh_channel process. The process must have been started  using  one  of  the  start
	      functions  in  proc_lib,	see proc_lib(3erl) . The user is responsible for any ini-
	      tialization of the process and needs to call ssh_channel:init/1.

       init(Options) -> {ok, State} | {ok, State, Timeout} | {stop, Reason}

	      Types  Options = [{Option, Value}]

	      The following options must be present:

		{channel_cb, atom()} :
		  The module that implements the channel behavior.

		{init_args(), list()} :
		  The list of arguments to the callback modules init function.

		{cm, connection_ref()} :
		  Reference to the ssh connection.

		{channel_id, channel_id()} :
		  Id of the ssh channel.

       This function is normally not called by the user, it is only needed if for some reason the
       channel	process  needs	to  be	started  with  help of proc_lib instead calling ssh_chan-
       nel:start/4 or ssh_channel:start_link/4

       reply(Client, Reply) -> _

	      Types  Client - opaque to the user, see explanation below
		     Reply = term()

	      This function can be used by a channel to explicitly send a reply to a client  that
	      called call/[2,3] when the reply cannot be defined in the return value of Callback-
	      Module:handle_call/3 .

	      Client must be the From argument provided to the callback function handle_call/3	.
	      Reply  is  an  arbitrary term, which will be given back to the client as the return
	      value of ssh_channel:call/[2,3].

       start(SshConnection, ChannelId, ChannelCb, CbInitArgs) ->
       start_link(SshConnection, ChannelId, ChannelCb, CbInitArgs) -> {ok, ChannelRef} |  {error,

	      Types  SshConnection = ssh_connection_ref()
		     ChannelId = ssh_channel_id()
		     As returned by ssh_connection:session_channel/[2,4]
		     ChannelCb = atom()
		     The  name of the module implementing the service specific parts of the chan-
		     CbInitArgs = [term()]
		     Argument list for the init function in the callback module.
		     ChannelRef = pid()

	      Starts a processes that handles a ssh channel. Will be called internally by the ssh
	      daemon  or  explicitly  by  the ssh client implementations. A channel process traps
	      exit signals by default.

       The functions init/1, terminate/2, handle_ssh_msg/2 and	handle_msg/2  are  the	functions
       that  are  required to provide the implementation for a server side channel, such as a ssh
       subsystem channel that can be plugged into the erlang ssh daemon see ssh:daemon/[2,  3]	.
       The  handle_call/3,  handle_cast/2 code_change/3 and enter_loop/1 functions are only rele-
       vant when implementing a client side channel.

       If an integer timeout value is provided in a return value of one  of  the  callback  func-
       tions,  a  timeout  will occur unless a message is received within Timeout milliseconds. A
       timeout is represented by the atom timeout which should be  handled  by	the  handle_msg/2
       callback function. The atom infinity can be used to wait indefinitely, this is the default

       CallbackModule:code_change(OldVsn, State, Extra) -> {ok, NewState}

	      Types  Converts process state when code is changed.

	      This function is called by a client side channel when it should update its internal
	      state  during  a	release upgrade/downgrade, i.e. when the instruction {update,Mod-
	      ule,Change,...} where Change={advanced,Extra} is given in the appup file.  See  OTP
	      Design  Principles  for  more  information.  Any new connection will benefit from a
	      server side upgrade but already started connections on the server side will not  be

       If there are long lived ssh connections and more than one upgrade in a short time this may
       cause the old connections to fail as only two versions of the code may be loaded  simulta-

       In  the	case  of  an  upgrade,	OldVsn is Vsn , and in the case of a downgrade, OldVsn is
       {down,Vsn} . Vsn is defined by the vsn attribute(s) of the old  version	of  the  callback
       module  Module  . If no such attribute is defined, the version is the checksum of the BEAM

       State is the internal state of the channel.

       Extra is passed as-is from the {advanced,Extra} part of the update instruction.

       The function should return the updated internal state.

       CallbackModule:init(Args) -> {ok, State} | {ok, State, Timeout} | {stop, Reason}

	      Types  Args = term()
		     Last argument to ssh_channel:start_link/4.
		     State = term()
		     Timeout = timeout()
		     Reason = term()

	      Makes necessary initializations and returns the initial channel state if	the  ini-
	      tializations succeed.

	      For more detailed information on timeouts see the section CALLBACK TIMEOUTS .

       CallbackModule:handle_call(Msg, From, State) -> Result

	      Types  Msg = term()
		     From = opaque to the user should be used as argument to ssh_channel:reply/2
		     State = term()
		     Result  =	{reply,  Reply,  NewState}  | {reply, Reply, NewState, Timeout} |
		     {noreply, NewState} | {noreply , NewState, Timeout} | {stop, Reason,  Reply,
		     NewState} | {stop, Reason, NewState}
		     Reply = term() - will be the return value of ssh_channel:call/[2,3]
		     Timeout = timeout()
		     NewState = term() - a possible updated version of State
		     Reason = term()

	      Handles messages sent by calling ssh_channel:call/[2,3]

	      For more detailed information on timeouts see the section CALLBACK TIMEOUTS .

       CallbackModule:handle_cast(Msg, State) -> Result

	      Types  Msg = term()
		     State = term()
		     Result = {noreply, NewState} | {noreply, NewState, Timeout} | {stop, Reason,
		     NewState = term() - a possible updated version of State
		     Timeout = timeout()
		     Reason = term()

	      Handles messages sent by calling ssh_channel:cast/2

	      For more detailed information on timeouts see the section CALLBACK TIMEOUTS .

       CallbackModule:handle_msg(Msg, State) -> {ok, State} | {stop, ChannelId, State}

	      Types  Msg = timeout | term()
		     State = term()

	      Handle other messages than ssh connection protocol, call or cast messages  sent  to
	      the channel.

	      Possible erlang 'EXIT'-messages should be handled by this function and all channels
	      should handle the following message.

		{ssh_channel_up, ssh_channel_id(), ssh_connection_ref()} :
		  This is the first messages that will be received by the  channel,  it  is  sent
		  just before the ssh_channel:init/1 function returns successfully. This is espe-
		  cially useful if the server wants to send a message to the client without first
		  receiving a message from the client. If the message is not useful for your par-
		  ticular problem just ignore it by immediately returning {ok, State}.

       CallbackModule:handle_ssh_msg(Msg, State) -> {ok, State} | {stop, ssh_channel_id(), State}

	      Types  Msg = {ssh_cm, ssh_connection_ref(), SshMsg}
		     SshMsg = tuple() - see message list below
		     State = term()

	      Handles ssh connection protocol messages that may need service specific attention.

	      All channels should handle the following messages. For channels  implementing  sub-
	      systems the handle_ssh_msg-callback will not be called for any other messages.

		{ssh_cm,  ssh_connection_ref(),  {data,  ssh_channel_id(),  ssh_data_type_code(),
		binary() = Data}} :
		  Data has arrived on the channel. When the callback for this message returns the
		  channel behavior will adjust the ssh flow control window.

		{ssh_cm, ssh_connection_ref(), {eof, ssh_channel_id()}} :
		  Indicteas that the other side will not send any more data.

		{ssh_cm, ssh_connection_ref(), {signal, ssh_channel_id(), ssh_signal()}} :
		  A  signal  can  be  delivered to the remote process/service using the following
		  message. Some systems may not implement signals,  in	which  case  they  should
		  ignore this message.

		{ssh_cm,   ssh_connection_ref(),   {exit_signal,   ssh_channel_id(),  string()	=
		exit_signal, string() = ErrorMsg, string() = LanguageString}} :
		  A remote execution may terminate violently due to a signal  then  this  message
		  may be received. For details on valid string values see RFC 4254 section 6.10

		{ssh_cm,  ssh_connection_ref(), {exit_status, ssh_channel_id(), integer() = Exit-
		Status}} :
		  When the command running at the other end terminates, the following message can
		  be  sent to return the exit status of the command. A zero 'exit_status' usually
		  means that the command terminated successfully.

	      Channels implementing a shell and command execution on the server side should  also
	      handle the following messages.

		{ssh_cm,  ssh_connection_ref(),  {env,	ssh_channel_id(),  boolean() = WantReply,
		string() = Var, string() = Value}} :
		  Environment variables may be passed to the shell/command to be  started  later.
		  Note	that  before the callback returns it should call the function ssh_connec-
		  tion:reply_request/4 with the boolean value of WantReply as  the  second  argu-

		{ssh_cm,  ConnectionRef, {exec, ssh_channel_id(), boolean() = WantReply, string()
		= Cmd}} :
		  This message will request that the server start the execution of the given com-
		  mand.  Note  that  before  the  callback  returns  it  should call the function
		  ssh_connection:reply_request/4 with the boolean value of WantReply as the  sec-
		  ond argument.

		{ssh_cm,  ssh_connection_ref(),  {pty,	ssh_channel_id(),  boolean() = WantReply,
		{string() = Terminal, integer() = CharWidth, integer() = RowHeight,  integer()	=
		PixelWidth,  integer()	=  PixelHight, [{atom() | integer() = Opcode, integer() =
		Value}] = TerminalModes}}} :
		  A pseudo-terminal has been requested for the session. Terminal is the value  of
		  the  TERM  environment  variable value (e.g., vt100). Zero dimension parameters
		  must be ignored. The character/row dimensions  override  the	pixel  dimensions
		  (when  nonzero). Pixel dimensions refer to the drawable area of the window. The
		  Opcode in the TerminalModes list is the mnemonic name, represented as an lower-
		  case	erlang atom, defined in RFC 4254 section 8, or the opcode if the mnemonic
		  name is not listed in the RFC. Example OP code: 53, mnemonic name  ECHO  erlang
		  atom:  echo . Note that before the callback returns it should call the function
		  ssh_connection:reply_request/4 with the boolean value of WantReply as the  sec-
		  ond argument.

		{ssh_cm, ConnectionRef, {shell, boolean() = WantReply}} :
		  This message will request that the user's default shell be started at the other
		  end. Note that before the callback returns it should call the function ssh_con-
		  nection:reply_request/4 with the value of WantReply as the second argument.

		{ssh_cm,  ssh_connection_ref(),  {window_change,  ssh_channel_id(),  integer()	=
		CharWidth, integer() = RowHeight, integer() = PixWidth, integer() = PixHeight}} :
		  When the window (terminal) size changes on the client side, it MAY send a  mes-
		  sage to the other side to inform it of the new dimensions.

	      The following message is completely taken care of by the ssh channel behavior

		{ssh_cm, ssh_connection_ref(), {closed, ssh_channel_id()}} :
		  The channel behavior will send a close message to the other side if such a mes-
		  sage has not already been sent and then terminate the channel with reason  nor-

       CallbackModule:terminate(Reason, State) -> _

	      Types  Reason = term()
		     State = term()

	      This  function is called by a channel process when it is about to terminate. Before
	      this function is called ssh_connection:close/2 will be called if it  has	not  been
	      called  earlier.	This function should be the opposite of CallbackModule:init/1 and
	      do any necessary cleaning up. When it returns, the channel process terminates  with
	      reason Reason . The return value is ignored.

Ericsson AB				    ssh 2.0.4				ssh_channel(3erl)
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