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<!DOCTYPE erlref SYSTEM "erlref.dtd">
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<header>
<copyright>
<year>2009</year>
<year>2013</year>
<holder>Ericsson AB, All Rights Reserved</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights 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, 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 an <c>ssh</c> subsystem, use
<c>-behaviour(ssh_daemon_channel)</c> instead of <c>-behaviour(ssh_channel)</c>.
The reason is that the only relevant callback functions for subsystems are
<c>init/1</c>, <c>handle_ssh_msg/2</c>, <c>handle_msg/2</c>, and <c>terminate/2</c>.
So, the <c>ssh_daemon_channel</c> behaviour is a limited version of the
<c>ssh_channel</c> behaviour.
</p></note>
</description>
<section>
<title>DATA TYPES</title>
<p>Type definitions that are used more than once in this module,
or abstractions to indicate the intended use of the data
type, or both:</p>
<taglist>
<tag><c>boolean()</c></tag>
<item><p>= <c>true | false</c></p></item>
<tag><c>string()</c></tag>
<item><p>= list of ASCII characters</p></item>
<tag><c>timeout()</c></tag>
<item><p>= <c>infinity | integer()</c> in milliseconds</p></item>
<tag><c>ssh_connection_ref()</c></tag>
<item><p>Opaque to the user, returned by
<c>ssh:connect/3</c> or sent to an SSH channel process</p></item>
<tag><c>ssh_channel_id()</c></tag>
<item><p>= <c>integer()</c></p></item>
<tag><c>ssh_data_type_code()</c></tag>
<item><p>= <c>1</c> ("stderr") | <c>0</c> ("normal") are
the valid values,
see <url href="http://www.ietf.org/rfc/rfc4254.txt">RFC 4254</url>
Section 5.2</p></item>
</taglist>
</section>
<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 <c>start_link/4</c></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 <c>start_link/4</c></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() - as returned by
<seealso marker = "#init-1">ssh_channel:init/1</seealso></v>
</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 <c>stdlib</c>.
The user is responsible for any initialization of the process
and must call <seealso marker = "#init-1">ssh_channel: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><![CDATA[{channel_cb, atom()}]]></c></tag>
<item><p>The module that implements the channel behaviour.</p></item>
<tag><c><![CDATA[{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><![CDATA[{cm, 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><![CDATA[{channel_id, channel_id()}]]></c></tag>
<item><p>Id of the <c>ssh</c> channel.</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>ssh_channel:start/4</c> or
<c>ssh_channel: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 to the user, see explanation below</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">ssh_channel: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>
<v>ChannelId = ssh_channel_id()</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>
<marker id="cb_timeouts"></marker>
<title>CALLBACK TIME-OUTS</title>
<p>The time-out 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>
<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>ssh_channel: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 TIME-OUTS</seealso>. </p>
</desc>
</func>
<func>
<name>Module:handle_call(Msg, From, State) -> Result</name>
<fsummary>Handles messages sent by calling
<c>ssh_channel:call/[2,3]</c>.</fsummary>
<type>
<v>Msg = term()</v>
<v>From = Opaque to the user, is to be used as argument to
ssh_channel:reply/2</v>
<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() - will be the return value of ssh_channel:call/[2,3]</v>
<v>NewState = term()</v>
<v>Reason = term()</v>
</type>
<desc>
<p>Handles messages sent by calling
<seealso marker="#call-2">ssh_channel:call/[2,3]</seealso>
</p>
<p>For more detailed information on time-outs,, see Section
<seealso marker="#cb_timeouts">CALLBACK TIME-OUTS</seealso>.</p>
</desc>
</func>
<func>
<name>Module:handle_cast(Msg, State) -> Result</name>
<fsummary>Handles messages sent by calling
<c>ssh_channel:cact/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>ssh_channel:cast/2</c>.
</p>
<p>For more detailed information on time-outs, see Section
<seealso marker="#cb_timeouts">CALLBACK TIME-OUTS</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 = ssh_channel_id()</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><![CDATA[{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">ssh_channel: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 = ssh_channel_id()</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><![CDATA[{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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