ssh_channel
-behaviour(ssh_channel).
SSH services (clients and servers) are implemented as channels
that are multiplexed over an SSH connection and communicates over
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
Functions
call(ChannelRef, Msg) ->
call(ChannelRef, Msg, Timeout) -> Reply | {error, Reason}
ChannelRef = pid()
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
ChannelRef = pid()
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) -> _
State = term()
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(3) 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}
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) -> _
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}
SshConnection = ssh_connection_ref()
ChannelId = ssh_channel_id()
ChannelCb = atom()
CbInitArgs = [term()]
ChannelRef = pid()
init
function in the callback module.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)
.
Functions
Module:code_change(OldVsn, State, Extra) -> {ok, NewState}
OldVsn = term()
State = term()
Extra = term()
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.{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}
Args = term()
State = term()
Reason = term()
ssh_channel:start_link/4
.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
Msg = term()
From = opaque()
State = term()
Result = {reply, Reply, NewState} | {reply, Reply, NewState, timeout()} | {noreply, NewState} | {noreply , NewState, timeout()} | {stop, Reason, Reply, NewState} | {stop, Reason, NewState}
Reply = term()
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
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}
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}
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) -> _
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.