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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.
Ericsson AB ssh 4.2.2 ssh_channel(3erl)