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<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>2009</year>
<year>2009</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>Ingela Anderton Andin</prepared>
<responsible></responsible>
<docno></docno>
<approved></approved>
<checked></checked>
<date></date>
<rev></rev>
</header>
<module>ssh_channel</module>
<modulesummary>Generic Ssh Channel Behavior
</modulesummary>
<description>
<p>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.
</p>
</description>
<section>
<title>COMMON 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 a 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 RFC 4254 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
<c>CallbackModule:handle_call/3</c> 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
<c>CallbackModule:handle_cast/2</c> to handle the message.
</p>
</desc>
</func>
<func>
<name>enter_loop(State) -> _ </name>
<fsummary> Makes an existing process into a ssh_channel process. </fsummary>
<type>
<v> State = term() - as returned by ssh_channel:init/1</v>
</type>
<desc>
<p> Makes an existing process into a <c>ssh_channel</c>
process. Does not return, instead the calling process will
enter the <c>ssh_channel</c> process receive loop and become a
<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 ssh_channel:init/1.
</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>
</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 behavior.</item>
<tag><c><![CDATA[{init_args(), list()}]]></c></tag>
<item> The list of arguments to the callback modules
init function.</item>
<tag><c><![CDATA[{cm, connection_ref()}]]></c></tag>
<item> Reference to the ssh connection.</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, it is
only needed if for some reason the channel process needs
to be started with help of <c>proc_lib</c> instead 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
<c>CallbackModule:handle_call/3</c>.</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
<c>ssh_channel:call/[2,3].</c></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 ssh_connection:session_channel/[2,4]</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 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.
</p>
</desc>
</func>
</funcs>
<section>
<title>CALLBACK FUNCTIONS</title>
<p>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
<seealso marker="ssh">ssh:daemon/[2, 3]</seealso>. The
handle_call/3, handle_cast/2 code_change/3 and enter_loop/1
functions are only relevant when implementing a client side
channel.</p>
</section>
<section>
<marker id="cb_timeouts"></marker>
<title> CALLBACK TIMEOUTS</title>
<p> If an integer timeout value is provided in a return value of
one of the callback functions, a timeout will occur unless a
message is received within <c>Timeout</c> milliseconds. A timeout
is represented by the atom <c>timeout</c> which should be handled
by the <seealso marker="#handle_msg">handle_msg/2</seealso>
callback function. The atom infinity can be used to wait
indefinitely, this is the default value. </p>
</section>
<funcs>
<func>
<name>CallbackModule:code_change(OldVsn, State, Extra) -> {ok,
NewState}</name>
<fsummary> Converts process state when code is changed.</fsummary>
<type>
<v> Converts process state when code is changed.</v>
</type>
<desc>
<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. Any new
connection will benefit from a server side upgrade but
already started connections on the server side will not be
affected.
</p>
<note><p>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 simultaneously.</p></note>
<p>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.</p>
<p><c>State</c> is the internal state of the channel.</p>
<p><c>Extra</c> is passed as-is from the <c>{advanced,Extra}</c>
part of the update instruction.</p>
<p>The function should return the updated internal state.</p>
</desc>
</func>
<func>
<name>CallbackModule: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>Timeout = timeout() </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>CallbackModule: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>Timeout = timeout() </v>
<v>NewState = term() - a possible updated version of State</v>
<v>Reason = term()</v>
</type>
<desc>
<p>Handles messages sent by calling
<c>ssh_channel:call/[2,3]</c>
</p>
<p>For more detailed information on timeouts see the section
<seealso marker="#cb_timeouts">CALLBACK TIMEOUTS</seealso>. </p>
</desc>
</func>
<func>
<name>CallbackModule: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() - a possible updated version of State</v>
<v>Timeout = timeout() </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>
<marker id="handle_msg"></marker>
</desc>
</func>
<func>
<name>CallbackModule: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>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 ssh_channel:init/1 function returns successfully.
This is especially 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 particular problem just ignore it by immediately
returning {ok, State}.
</item>
</taglist>
</desc>
</func>
<func>
<name>CallbackModule:handle_ssh_msg(Msg, State) -> {ok, State} | {stop,
ssh_channel_id(), State}</name>
<fsummary> Handles ssh connection protocol messages. </fsummary>
<type>
<v>Msg = {ssh_cm, ssh_connection_ref(), SshMsg}</v>
<v> SshMsg = tuple() - see message list below</v>
<v>State = term()</v>
</type>
<desc>
<p> Handles ssh connection protocol messages that may need
service specific attention.
</p>
<p> All channels should handle the following messages. For
channels implementing subsystems the handle_ssh_msg-callback
will not be called for any other messages. </p>
<taglist>
<tag><c><![CDATA[{ssh_cm, ssh_connection_ref(), {data, ssh_channel_id(),
ssh_data_type_code(), binary() = Data}}]]></c></tag>
<item> Data has arrived on the channel. When the callback
for this message returns the channel behavior will adjust
the ssh flow control window.</item>
<tag><c><![CDATA[{ssh_cm, ssh_connection_ref(), {eof,
ssh_channel_id()}}]]></c></tag>
<item>Indicteas that the other side will not send any more
data.</item>
<tag><c><![CDATA[{ssh_cm, ssh_connection_ref(), {signal,
ssh_channel_id(), ssh_signal()}} ]]></c></tag>
<item>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.</item>
<tag><c><![CDATA[{ssh_cm, ssh_connection_ref(),
{exit_signal, ssh_channel_id(), string() = exit_signal,
string() = ErrorMsg, string() =
LanguageString}}]]></c></tag>
<item>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</item>
<tag><c><![CDATA[{ssh_cm, ssh_connection_ref(), {exit_status,
ssh_channel_id(), integer() = ExitStatus}}]]></c></tag>
<item> 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.</item>
</taglist>
<p> Channels implementing a shell and command execution on the server side
should also handle the following messages. </p>
<taglist>
<tag><c><![CDATA[{ssh_cm, ssh_connection_ref(), {env, ssh_channel_id(),
boolean() = WantReply, string() = Var, string() = Value}}]]></c></tag>
<item> 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_connection:reply_request/4 with the boolean value of <c>
WantReply</c> as the second argument.
</item>
<tag><c><![CDATA[{ssh_cm, ConnectionRef, {exec, ssh_channel_id(),
boolean() = WantReply, string() = Cmd}}]]></c></tag>
<item> This message will request that the server start the
execution of the given command. Note that before the
callback returns it should call the function
ssh_connection:reply_request/4 with the boolean value of <c>
WantReply</c> as the second argument.</item>
<tag><c><![CDATA[{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}}}]]></c></tag>
<item>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 <c>Opcode</c> in the
<c>TerminalModes</c> list is the mnemonic name, represented
as an lowercase erlang atom, defined in RFC 4254 section 8,
or the opcode if the mnemonic name is not listed in the
RFC. Example <c>OP code: 53, mnemonic name ECHO erlang atom:
echo</c>. Note that before the callback returns it should
call the function ssh_connection:reply_request/4 with the
boolean value of <c> WantReply</c> as the second
argument.</item>
<tag><c><![CDATA[{ssh_cm, ConnectionRef, {shell, boolean() =
WantReply}}]]></c></tag>
<item> 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_connection:reply_request/4 with the value of <c>
WantReply</c> as the second argument.
</item>
<tag><c><![CDATA[ {ssh_cm, ssh_connection_ref(), {window_change,
ssh_channel_id(), integer() = CharWidth, integer() = RowHeight,
integer() = PixWidth, integer() = PixHeight}}]]></c></tag>
<item> When the window (terminal) size changes on the client
side, it MAY send a message to the other side to inform it
of the new dimensions.</item>
</taglist>
<p> The following message is completely taken care of by the
ssh channel behavior</p>
<taglist>
<tag><c><![CDATA[{ssh_cm, ssh_connection_ref(), {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>CallbackModule: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 ssh_connection:close/2
will be called if it has not been called earlier.
This function should be the opposite of <c>CallbackModule:init/1</c>
and 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>
