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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 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 (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 a SSH subsystem use the -behaviour(ssh_daemon_channel). instead of
-behaviour(ssh_channel). as 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 limited version of the ssh_channel behaviour.
DATA TYPES
Type definitions that are used more than once in this module and/or abstractions to
indicate the intended use of the data type:
boolean() = true | false
string() = list of ASCII characters
timeout() = infinity | integer() - in milliseconds.
ssh_connection_ref() - opaque to the user 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 currently 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 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 timeout occurs. The channel will call
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 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
will call 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 will enter 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 proc_lib(3erl). The user is responsible for any
initialization of the process and needs to 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 callback module's init function.
{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
for some reason 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 to the user, see explanation below
Reply = term()
This function can be used by a channel to explicitly 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 will be 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 cannot be defined in the return value of
ssh_connection:session_channel/[2,4]
ChannelCb = atom()
The 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 processes that handles an SSH channel. It will be called internally by the
SSH daemon or explicitly by the SSH client implementations. The behavior will set
the trap_exit flag to true.
CALLBACK TIMEOUTS
The timeout values that may be returned by the callback functions has the same semantics
as in a gen_server If the timeout occurs handle_msg/2 will be 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()
The 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 should update its internal
state during a release upgrade/downgrade, i.e. when the instruction
{update,Module,Change,...} where Change={advanced,Extra} is given in the appup
file. See OTP Design Principles for more information.
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. It could be possible to upgrade the subsystem
channels, when upgrading the user application, if the callback functions can handle
two versions of the state, but this function can not 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 timeouts see the section CALLBACK TIMEOUTS.
Module:handle_call(Msg, From, State) -> Result
Types:
Msg = term()
From = opaque to the user should 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 timeouts see the section CALLBACK TIMEOUTS.
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 timeouts see the section CALLBACK TIMEOUTS.
Module:handle_msg(Msg, State) -> {ok, State} | {stop, ChannelId, State}
Types:
Msg = timeout | term()
ChannelId = ssh_channel_id()
State = term()
Handle other messages than ssh connection protocol, call or cast messages sent to
the channel.
Possible erlang 'EXIT'-messages should be handled by this function and all channels
should handle the following message.
{ssh_channel_up, ssh_channel_id(), ssh_connection_ref()}:
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 it. If the message is not useful for your
particular scenario just 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.
The following message is completely taken care of by the SSH channel behavior
{closed, ssh_channel_id()}:
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.
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 will be called if it has not been
called earlier. This function should do any necessary cleaning up. When it returns,
the channel process terminates with reason Reason. The return value is ignored.