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NAME
ssh_channel - -behaviour(ssh_channel).
DESCRIPTION
SSH services (clients and servers) are implemented as channels that are multiplexed over
an SSH connection and communicates over the SSH Connection Protocol. 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 ssh applications supervisor
tree.
Note:
When implementing an ssh subsystem, use -behaviour(ssh_daemon_channel) instead of
-behaviour(ssh_channel). The reason is that 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 a limited version of the ssh_channel behaviour.
DATA TYPES
Type definitions that are used more than once in this module, or abstractions to indicate
the intended use of the data type, or both:
boolean() =:
true | false
string() =:
list of ASCII characters
timeout() =:
infinity | integer() in milliseconds
ssh_connection_ref() =:
opaque() -as returned by ssh:connect/3 or sent to an SSH channel process
ssh_channel_id() =:
integer()
ssh_data_type_code() =:
1 ("stderr") | 0 ("normal") are the valid values, see RFC 4254 Section 5.2
EXPORTS
call(ChannelRef, Msg) ->
call(ChannelRef, Msg, Timeout) -> Reply | {error, Reason}
Types:
ChannelRef = pid()
As returned by ssh_channel: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 time-out occurs. The channel calls Module: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 ssh_channel: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
calls Module:handle_cast/2 to handle the message.
enter_loop(State) -> _
Types:
State = term()
as returned by ssh_channel:init/1
Makes an existing process an ssh_channel process. Does not return, instead the
calling process enters the ssh_channel process receive loop and become an
ssh_channel process. The process must have been started using one of the start
functions in proc_lib, see the proc_lib(3erl) manual page in stdlib. The user is
responsible for any initialization of the process and must call ssh_channel:init/1.
init(Options) -> {ok, State} | {ok, State, Timeout} | {stop, Reason}
Types:
Options = [{Option, Value}]
State = term()
Timeout = timeout()
Reason = term()
The following options must be present:
{channel_cb, atom()}:
The module that implements the channel behaviour.
{init_args(), list()}:
The list of arguments to the init function of the callback module.
{cm, connection_ref()}:
Reference to the ssh connection as returned by ssh:connect/3
{channel_id, channel_id()}:
Id of the ssh channel.
Note:
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 proc_lib instead of calling
ssh_channel:start/4 or ssh_channel:start_link/4.
reply(Client, Reply) -> _
Types:
Client = opaque()
Reply = term()
This function can be used by a channel to send a reply to a client that called
call/[2,3] when the reply cannot be defined in the return value of
Module:handle_call/3.
Client must be the From argument provided to the callback function handle_call/3.
Reply is an arbitrary term, which is 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,
Reason}
Types:
SshConnection = ssh_connection_ref()
ChannelId = ssh_channel_id()
As returned by ssh_connection:session_channel/[2,4].
ChannelCb = atom()
Name of the module implementing the service-specific parts of the channel.
CbInitArgs = [term()]
Argument list for the init function in the callback module.
ChannelRef = pid()
Starts a process that handles an SSH channel. It is called internally, by the ssh
daemon, or explicitly by the ssh client implementations. The behavior sets the
trap_exit flag to true.
CALLBACK TIME-OUTS
The time-out values that can be returned by the callback functions have the same semantics
as in a gen_server. If the time-out occurs, handle_msg/2 is called as handle_msg(timeout,
State).
EXPORTS
Module:code_change(OldVsn, State, Extra) -> {ok, NewState}
Types:
OldVsn = term()
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 file.
State = term()
Internal state of the channel.
Extra = term()
Passed "as-is" from the {advanced,Extra} part of the update instruction.
Converts process state when code is changed.
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
{update,Module,Change,...}, where Change={advanced,Extra}, is given in the appup
file. For more information, refer to Section 9.11.6 Release Handling Instructions
in the System Documentation.
Note:
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. 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.
Module:init(Args) -> {ok, State} | {ok, State, timeout()} | {stop, Reason}
Types:
Args = term()
Last argument to ssh_channel:start_link/4.
State = term()
Reason = term()
Makes necessary initializations and returns the initial channel state if the
initializations succeed.
For more detailed information on time-outs, see Section CALLBACK TIME-OUTS.
Module:handle_call(Msg, From, State) -> Result
Types:
Msg = term()
From = opaque()
Is to 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]
NewState = term()
Reason = term()
Handles messages sent by calling ssh_channel:call/[2,3]
For more detailed information on time-outs,, see Section CALLBACK TIME-OUTS.
Module:handle_cast(Msg, State) -> Result
Types:
Msg = term()
State = term()
Result = {noreply, NewState} | {noreply, NewState, timeout()} | {stop, Reason,
NewState}
NewState = term()
Reason = term()
Handles messages sent by calling ssh_channel:cast/2.
For more detailed information on time-outs, see Section CALLBACK TIME-OUTS.
Module:handle_msg(Msg, State) -> {ok, State} | {stop, ChannelId, State}
Types:
Msg = timeout | term()
ChannelId = ssh_channel_id()
State = term()
Handles other messages than SSH Connection Protocol, call, or cast messages sent to
the channel.
Possible Erlang 'EXIT' messages is to be handled by this function and all channels
are to handle the following message.
{ssh_channel_up, ssh_channel_id(), ssh_connection_ref()}:
This is the first message that the channel receives. 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 it. If the message is not useful for your particular scenario,
ignore it by immediately returning {ok, State}.
Module:handle_ssh_msg(Msg, State) -> {ok, State} | {stop, ChannelId, State}
Types:
Msg = ssh_connection:event()
ChannelId = ssh_channel_id()
State = term()
Handles SSH Connection Protocol messages that may need service-specific attention.
For details, see ssh_connection:event().
The following message is taken care of by the ssh_channel behavior.
{closed, ssh_channel_id()}:
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 normal.
Module: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 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 Reason. The return value is ignored.