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<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<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>
</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 via
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 and 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 SSH applications supervisor tree.
</p>
<note> <p>When implementing a SSH subsystem use the
<c>-behaviour(ssh_daemon_channel).</c> instead of <c>-behaviour(ssh_channel).</c>
as the only relevant callback functions for subsystems are
init/1, handle_ssh_msg/2, handle_msg/2 and terminate/2, so the ssh_daemon_channel
behaviour is limited version of the ssh_channel behaviour.
</p> </note>
</description>
<section>
<title>DATA TYPES </title>
<p>Type definitions that are used more than once in this module
and/or abstractions to indicate the intended use of the data
type:</p>
<p><c>boolean() = true | false </c></p>
<p><c>string() = list of ASCII characters</c></p>
<p><c>timeout() = infinity | integer() - in milliseconds.</c></p>
<p><c>ssh_connection_ref() - opaque to the user returned by
ssh:connect/3 or sent to an SSH channel process</c></p>
<p><c>ssh_channel_id() = integer() </c></p>
<p><c>ssh_data_type_code() = 1 ("stderr") | 0 ("normal") are
currently valid values see <url href="http://www.ietf.org/rfc/rfc4254.txt">RFC 4254 </url> section 5.2.</c></p>
</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 start_link/4 </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 timeout
occurs. The channel will call <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 start_link/4 </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 will call
<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 will
enter 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 proc_lib, see <seealso
marker="stdlib:proc_lib">proc_lib(3)</seealso>. The
user is responsible for any initialization of the process
and needs to 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 a ssh_channel 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>The module that implements the channel behaviour.</item>
<tag><c><![CDATA[{init_args(), list()}]]></c></tag>
<item> The list of arguments to the callback module's
init function.</item>
<tag><c><![CDATA[{cm, connection_ref()}]]></c></tag>
<item> Reference to the ssh connection as returned by <seealso
marker="ssh#connect-3">ssh:connect/3</seealso></item>
<tag><c><![CDATA[{channel_id, channel_id()}]]></c></tag>
<item> Id of the SSH channel.</item>
</taglist>
<note><p>This function is normally not called by the
user. The user only needs to call if for some reason 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>Send 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 explicitly 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 will be 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 processes that handles a SSH channel. </fsummary>
<type>
<v>SshConnection = ssh_connection_ref()</v>
<v>ChannelId = ssh_channel_id() </v>
<d> As returned by cannot be defined in the return value of
<seealso marker ="ssh_connection#session_channel/2">ssh_connection:session_channel/[2,4]</seealso></d>
<v>ChannelCb = atom()</v>
<d> The name of the module implementing the service specific parts
of the channel.</d>
<v>CbInitArgs = [term()]</v>
<d>Argument list for the init function in the callback module. </d>
<v>ChannelRef = pid()</v>
</type>
<desc>
<p>Starts a processes that handles an SSH channel. It will be
called internally by the SSH daemon or explicitly by the SSH
client implementations. The behavior will set the
<c>trap_exit</c> flag to true.
</p>
</desc>
</func>
</funcs>
<section>
<marker id="cb_timeouts"></marker>
<title> CALLBACK TIMEOUTS</title>
<p>The timeout values that may be returned by the callback functions
has the same semantics as in a <seealso marker="stdlib:gen_server">gen_server</seealso>
If the timeout occurs <seealso marker="#Module:handle_msg-2">handle_msg/2</seealso>
will be 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>The 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
should update its internal state during a release
upgrade/downgrade, i.e. when the instruction
<c>{update,Module,Change,...}</c> where
<c>Change={advanced,Extra}</c> is given in the <c>appup</c>
file. See <seealso marker="doc/design_principles:release_handling#instr">OTP
Design Principles</seealso> for more information.
</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 SSH application and
hence added to its supervisor tree. It could be possible to
upgrade the subsystem channels, when upgrading the user
application, if the callback functions can handle two
versions of the state, but this function can not 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 ssh_channel:start_link/4.</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 timeouts see the 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>ssh_channel:call/[2,3]</c></fsummary>
<type>
<v>Msg = term()</v>
<v>From = opaque to the user should 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 timeouts see the 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>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 timeouts see the 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> Handle 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>Handle other messages than ssh connection protocol, call or
cast messages sent to the channel.
</p>
<p> Possible erlang 'EXIT'-messages should be handled by this
function and all channels should handle the following message.</p>
<taglist>
<tag><c><![CDATA[{ssh_channel_up, ssh_channel_id(),
ssh_connection_ref()}]]></c></tag>
<item>This is the first messages that will be received by
the channel, 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 just ignore it by
immediately returning {ok, State}.
</item>
</taglist>
</desc>
</func>
<func>
<name>Module:handle_ssh_msg(Msg, State) -> {ok, State} | {stop,
ChannelId, State}</name>
<fsummary> Handles ssh connection protocol messages. </fsummary>
<type>
<v>Msg = <seealso marker="ssh_connection"> ssh_connection:event() </seealso> </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.
</p>
<p> The following message is completely taken care of by the
SSH channel behavior</p>
<taglist>
<tag><c><![CDATA[{closed, ssh_channel_id()}]]></c></tag>
<item> The channel behavior will send a close message to the
other side if such a message has not already been sent and
then terminate the channel with reason normal.</item>
</taglist>
</desc>
</func>
<func>
<name>Module:terminate(Reason, State) -> _</name>
<fsummary> </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> will be called if it has not been called earlier.
This function should do 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>
</erlref>
