path: root/lib/ssh/doc/src/ssh_channel.xml

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  <header>
    <copyright>
      <year>2009</year>
      <year>2016</year>
      <holder>Ericsson AB, All Rights Reserved</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

      The Initial Developer of the Original Code is Ericsson AB.
    </legalnotice>
    <title>ssh_channel</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>ssh_channel</module>
  <modulesummary>-behaviour(ssh_channel).
  </modulesummary>
  <description>
    <p>SSH services (clients and servers) are implemented as channels
    that are multiplexed over an SSH connection and communicates over
    the <url href="http://www.ietf.org/rfc/rfc4254.txt"> SSH
    Connection Protocol</url>. This module provides a callback API
    that takes care of generic channel aspects for clients, such as flow control
    and close messages. It lets the callback functions take care of
    the service (application) specific parts. This behavior also ensures
    that the channel process honors the principal of an OTP-process so
    that it can be part of a supervisor tree. This is a requirement of
    channel processes implementing a subsystem that will be added to
    the <c>ssh</c> applications supervisor tree.
    </p>

    <note><p>When implementing a <c>ssh</c> subsystem for daemons, use
    <seealso marker="ssh_daemon_channel">-behaviour(ssh_daemon_channel)</seealso> instead.
    </p>
    </note>

    <dont>
      <p>Functions in this module are not supposed to be called outside a module implementing this
      behaviour!
      </p>
    </dont>

  </description>

  <funcs>
    <func>
      <name>call(ChannelRef, Msg) -></name>
      <name>call(ChannelRef, Msg, Timeout) -> Reply | {error, Reason}</name>
      <fsummary>Makes a synchronous call to a channel.</fsummary>
      <type>
	<v>ChannelRef = pid() </v>
	<d>As returned by <seealso marker = "#start_link-4">start_link/4</seealso></d>
	<v>Msg = term()</v>
	<v>Timeout = timeout()</v>
	<v>Reply = term()</v>
	<v>Reason = closed | timeout</v>

      </type>
      <desc>
	<p>Makes a synchronous call to the channel process by sending
	a message and waiting until a reply arrives, or a time-out
	occurs. The channel calls <seealso marker =
	"#Module:handle_call-3">Module:handle_call/3</seealso>
	to handle the message. If the channel process does not exist,
	<c>{error, closed}</c> is returned.
	</p>
      </desc>
    </func>
    
    <func>
      <name>cast(ChannelRef, Msg) -> ok </name>
      <fsummary>Sends an asynchronous message to the channel
	ChannelRef and returns ok.</fsummary>
      <type>
	<v>ChannelRef = pid()</v>
	<d>As returned by <seealso marker = "#start_link-4">start_link/4</seealso></d>
	<v>Msg = term()</v>
      </type>
      <desc>
	<p>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 calls
	<seealso marker = "#Module:handle_cast-2">Module:handle_cast/2</seealso>
	 to handle the message.
	</p>
      </desc>
    </func>

    <func>
      <name>enter_loop(State) -> _ </name>
      <fsummary>Makes an existing process an ssh_channel process.</fsummary>
      <type>
	<v>State = term()</v>
	<d>as returned by <seealso marker = "#init-1">init/1</seealso></d>
      </type>
      <desc>
	<p>Makes an existing process an <c>ssh_channel</c>
	process. Does not return, instead the calling process
	enters the <c>ssh_channel</c> process receive loop and become an
	<c>ssh_channel process</c>. The process must have been started using
	one of the start functions in <c>proc_lib</c>, see the <seealso
	marker="stdlib:proc_lib">proc_lib(3)</seealso> manual page in STDLIB.
	The user is responsible for any initialization of the process
	and must call <seealso marker = "#init-1">init/1</seealso>.
	</p>
      </desc>
    </func>
    
    <func>
      <name>init(Options) -> {ok, State} |  {ok, State, Timeout} | {stop, Reason} </name>
      <fsummary>Initiates an <c>ssh_channel</c> process.</fsummary>
      <type>
	<v>Options = [{Option, Value}]</v>
	<v>State = term()</v>
	<v>Timeout = timeout()</v>
	<v>Reason = term()</v>
      </type>
      <desc>
	<p>
	  The following options must be present:
	</p>
	  <taglist>
	  <tag><c>{channel_cb, atom()}</c></tag>
	  <item><p>The module that implements the channel behaviour.</p></item>
	  
	  <tag><c>{init_args(), list()}</c></tag>
	  <item><p>The list of arguments to the <c>init</c> function of the callback module.</p></item>

	  <tag><c>{cm, ssh:connection_ref()}</c></tag>
	  <item><p>Reference to the <c>ssh</c> connection as returned by
	  <seealso marker="ssh#connect-3">ssh:connect/3</seealso>.
	  </p></item>
	  
	  <tag><c>{channel_id, ssh:channel_id()}</c></tag>
	  <item><p>Id of the <c>ssh</c> channel as returned by
	  <seealso marker="ssh_connection#session_channel/2">ssh_connection:session_channel/2,4</seealso>.
	  </p></item>
	  
	  </taglist>

	  <note><p>This function is normally not called by the
	  user. The user only needs to call if the
	  channel process needs to be started with help of
	  <c>proc_lib</c> instead of calling
	  <c>start/4</c> or
	  <c>start_link/4</c>.</p>
	  </note>
      </desc>
    </func>

    <func>
      <name>reply(Client, Reply) -> _</name>
      <fsummary>Sends a reply to a client.</fsummary>
      <type>
        <v>Client = opaque()</v>
        <v>Reply = term()</v>
      </type>
      <desc>
        <p>This function can be used by a channel to send a
        reply to a client that called <c>call/[2,3]</c> when the reply
        cannot be defined in the return value of
	<seealso marker ="#Module:handle_call-3">Module:handle_call/3</seealso>.</p>
        <p><c>Client</c> must be the <c>From</c> argument provided to
          the callback function <c>handle_call/3</c>.
	  <c>Reply</c> is an arbitrary term,
	  which is given back to the client as the return value of
          <seealso marker="#call-2">call/[2,3].</seealso></p>
      </desc>
    </func>

    <func>
      <name>start(SshConnection, ChannelId, ChannelCb, CbInitArgs) -> </name>
      <name>start_link(SshConnection, ChannelId, ChannelCb, CbInitArgs) ->
      {ok, ChannelRef} | {error, Reason}</name>
      <fsummary>Starts a process that handles an SSH channel.</fsummary>
      <type>
	<v>SshConnection = ssh:connection_ref()</v>
	<d>As returned by <seealso marker="ssh#connect-3">ssh:connect/3</seealso></d>

	<v>ChannelId = <seealso marker="ssh#type-channel_id">ssh:channel_id()</seealso></v>
	<d>As returned by
	<seealso marker ="ssh_connection#session_channel/2">
	ssh_connection:session_channel/[2,4]</seealso>.</d>

	<v>ChannelCb = atom()</v>
	<d>Name of the module implementing the service-specific parts
	of the channel.</d>

	<v>CbInitArgs = [term()]</v>
	<d>Argument list for the <c>init</c> function in the callback module.</d>

	<v>ChannelRef = pid()</v>
      </type>
      <desc>
	<p>Starts a process that handles an SSH channel. It is
	called internally, by the <c>ssh</c> daemon, or explicitly by the <c>ssh</c>
	client implementations. The behavior sets the
	<c>trap_exit</c> flag to <c>true</c>.
	</p>
      </desc>
    </func>
    
  </funcs>
  
  <section>
    <title>Callback Functions</title>
    <p>
      The following functions are to be exported from a
      <c>ssh_daemon_channel</c> callback module.
    </p>
    <marker id="cb_timeouts"></marker>
    <section>
      <title>Callback timeouts</title>
      <p>The timeout values that can be returned by the callback functions
      have the same semantics as in a <seealso marker="stdlib:gen_server">gen_server</seealso>.
      If the time-out occurs, <seealso marker="#Module:handle_msg-2">handle_msg/2</seealso>
      is called as <c>handle_msg(timeout, State)</c>.</p>
    </section>
  </section>

  <funcs>
    <func>
      <name>Module:code_change(OldVsn, State, Extra) -> {ok,
      NewState}</name>
      <fsummary>Converts process state when code is changed.</fsummary>
      <type>
	<v>OldVsn = term()</v>
	<d>In the case of an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
	in the case of a downgrade, <c>OldVsn</c> is
	<c>{down,Vsn}</c>. <c>Vsn</c> is defined by the <c>vsn</c>
	attribute(s) of the old version of the callback module
	<c>Module</c>. If no such attribute is defined, the version is
	the checksum of the BEAM file.</d>
	<v>State = term()</v>
	<d>Internal state of the channel.</d>
        <v>Extra = term()</v>
	<d>Passed "as-is" from the <c>{advanced,Extra}</c>
	part of the update instruction.</d>
      </type>
      <desc>
	<p>Converts process state when code is changed.</p>

	<p>This function is called by a client-side channel when it
	is to update its internal state during a release
	upgrade or downgrade, that is, when the instruction
	<c>{update,Module,Change,...}</c>, where
	<c>Change={advanced,Extra}</c>, is given in the <c>appup</c>
	file. For more information, refer to Section 9.11.6
	Release Handling Instructions in the
	<seealso marker="doc/design_principles:release_handling#instr">System Documentation</seealso>.
	</p>

	<note><p>Soft upgrade according to the OTP release concept
	is not straight forward for the server side, as subsystem
	channel processes are spawned by the <c>ssh</c> application and
	hence added to its supervisor tree. The subsystem channels can
	be upgraded when upgrading the user application, if the callback
	functions can handle two versions of the state, but this function
	cannot be used in the normal way.</p>
	</note>
	  
      </desc>
    </func>
    
    <func>
      <name>Module:init(Args) -> {ok, State} | {ok, State, timeout()} |
      {stop, Reason}</name>
      <fsummary>Makes necessary initializations and returns the
      initial channel state if the initializations succeed.</fsummary>
      <type>
	<v>Args = term()</v>
	<d>Last argument to <c>start_link/4</c>.</d>
	<v>State = term()</v>
	<v>Reason = term()</v>
      </type>
      <desc>
	<p>Makes necessary initializations and returns the initial channel
	state if the initializations succeed.
	</p>
	<p>For more detailed information on time-outs, see Section
	<seealso marker="#cb_timeouts">Callback timeouts</seealso>. </p>
      </desc>
    </func>
   
    <func>
      <name>Module:handle_call(Msg, From, State) -> Result</name>
      <fsummary>Handles messages sent by calling
      <c>call/[2,3]</c>.</fsummary>
      <type>
	<v>Msg = term()</v>
	<v>From = opaque()</v>
	<d>Is to be used as argument to
	<seealso marker="#reply-2">reply/2</seealso></d>
	<v>State = term()</v>
	<v>Result = {reply, Reply, NewState} | {reply, Reply, NewState, timeout()}
	| {noreply, NewState} | {noreply , NewState, timeout()}
	| {stop, Reason, Reply, NewState} | {stop, Reason, NewState} </v>
	<v>Reply = term()</v>
	<d>Will be the return value of <seealso marker="#call-2">call/[2,3]</seealso></d>
	<v>NewState = term()</v>
	<v>Reason = term()</v>
      </type>
      <desc>
	<p>Handles messages sent by calling
	<seealso marker="#call-2">call/[2,3]</seealso>
	</p>
	<p>For more detailed information on time-outs,, see Section
	<seealso marker="#cb_timeouts">Callback timeouts</seealso>.</p>
      </desc>
    </func>

    <func>
      <name>Module:handle_cast(Msg, State) -> Result</name>
      <fsummary>Handles messages sent by calling
      <c>cast/2</c>.</fsummary>
      <type>
	<v>Msg = term()</v>
	<v>State = term()</v>
	<v>Result = {noreply, NewState} | {noreply, NewState, timeout()}
	| {stop, Reason, NewState}</v>
	<v>NewState = term() </v>
	<v>Reason = term()</v>
      </type>
      <desc>
	<p>Handles messages sent by calling
	<c>cast/2</c>.
	</p>
	<p>For more detailed information on time-outs, see Section
	<seealso marker="#cb_timeouts">Callback timeouts</seealso>.</p>
      </desc>
    </func>
 
    <func>
      <name>Module:handle_msg(Msg, State) -> {ok, State} |
      {stop, ChannelId, State}</name>

      <fsummary>Handles other messages than SSH connection protocol,
      call, or cast messages sent to the channel.</fsummary>
      <type>
	<v>Msg = timeout | term()</v>
	<v>ChannelId = <seealso marker="ssh#type-channel_id">ssh:channel_id()</seealso></v>
	<v>State = term() </v>
      </type>
      <desc>
	<p>Handles other messages than SSH Connection Protocol, call, or
	cast messages sent to the channel. 
	</p>

	<p>Possible Erlang 'EXIT' messages is to be handled by this
	function and all channels are to handle the following message.</p>
	
	<taglist>
	  <tag><c>{ssh_channel_up, ssh:channel_id(), ssh:connection_ref()}</c></tag>
	  <item><p>This is the first message that the channel receives.
	  It is sent just before the <seealso
	  marker="#init-1">init/1</seealso> function
	  returns successfully. This is especially useful if the
	  server wants to send a message to the client without first
	  receiving a message from it. If the message is not
	  useful for your particular scenario, ignore it by
	  immediately returning <c>{ok, State}</c>.
	  </p></item>
	</taglist>
      </desc>
    </func>

    <func>
      <name>Module:handle_ssh_msg(Msg, State) -> {ok, State} | {stop,
      ChannelId, State}</name>
      <fsummary>Handles <c>ssh</c> connection protocol messages.</fsummary>
      <type>
	<v>Msg = ssh_connection:event()</v>
	<v>ChannelId = <seealso marker="ssh#type-channel_id">ssh:channel_id()</seealso></v>
	<v>State = term()</v>
      </type>
      <desc>
	<p>Handles SSH Connection Protocol messages that may need
	service-specific attention. For details,
	see <seealso marker="ssh_connection"> ssh_connection:event()</seealso>.
	</p>
	
	<p>The following message is taken care of by the
	<c>ssh_channel</c> behavior.</p>
	
	<taglist>
	  <tag><c>{closed, ssh:channel_id()}</c></tag>
	  <item><p>The channel behavior sends a close message to the
	  other side, if such a message has not already been sent.
	  Then it terminates the channel with reason <c>normal</c>.</p></item>
	</taglist>
      </desc>
    </func>

    <func>
      <name>Module:terminate(Reason, State) -> _</name>
      <fsummary>Does cleaning up before channel process termination.
      </fsummary>
      <type>
	<v>Reason = term()</v>
	<v>State = term()</v>
      </type>
      <desc>
	<p>This function is called by a channel process when it is
	about to terminate. Before this function is called, <seealso
	marker="ssh_connection#close-2"> ssh_connection:close/2
	</seealso> is called, if it has not been called earlier.
	This function does any necessary cleaning
	up. When it returns, the channel process terminates with
	reason <c>Reason</c>. The return value is ignored.
	</p>
      </desc>
    </func>
    
  </funcs>
  
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