table of contents
other versions
- jessie 1:17.3-dfsg-4+deb8u2
- jessie-backports 1:19.2.1+dfsg-2+deb9u1~bpo8+1
- stretch 1:19.2.1+dfsg-2+deb9u2
ssh_channel(3erl) | Erlang Module Definition | ssh_channel(3erl) |
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) ->
Types:
ChannelRef = pid()
Timeout = timeout()
Reply = term()
Reason = closed | timeout
As returned by ssh_channel:start_link/4
Msg = term()
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:
reply(Client, Reply) -> _
- {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.
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) ->
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.
Module:init(Args) -> {ok, State} | {ok, State, timeout()} | {stop,
Reason}
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.
Types:
Args = term()
Reason = term()
Last argument to ssh_channel:start_link/4.
State = 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()
Result = {reply, Reply, NewState} | {reply, Reply, NewState, timeout()} |
{noreply, NewState} | {noreply , NewState, timeout()} | {stop, Reason, Reply,
NewState} | {stop, Reason, NewState}
Reply = term()
Reason = term()
Is to be used as argument to
ssh_channel:reply/2
State = term()
Will be the return value of
ssh_channel:call/[2,3]
NewState = 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.
Module:handle_ssh_msg(Msg, State) -> {ok, State} | {stop, ChannelId,
State}
- {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}.
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.
Module:terminate(Reason, State) -> _
- {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.
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.
ssh 4.4 | Ericsson AB |