Vim documentation: channel

main help file
*channel.txt*    Nvim


		 NVIM REFERENCE MANUAL    by Thiago de Arruda



Nvim asynchronous IO					*channel*

				      Type |gO| to see the table of contents.

==============================================================================

1. Introduction						    *channel-intro*

Channels are nvim's way of communicating with external processes.

There are several ways to open a channel:

  1. Through stdin/stdout when `nvim` is started with `--headless`, and a startup
     script or --cmd  command opens the stdio channel using |stdioopen()|.

  2. Through stdin, stdout and stderr of a process spawned by |jobstart()|.

  3. Through the PTY master end of a PTY opened with
     `jobstart(..., {'pty': v:true})` or |termopen()|.

  4. By connecting to a TCP/IP socket or named pipe with |sockconnect()|.

  5. By another process connecting to a socket listened to by nvim. This only
     supports RPC channels, see |rpc-connecting|.

Channels support multiple modes or protocols. In the most basic
mode of operation, raw bytes are read and written to the channel.
The |rpc| protocol, based on the msgpack-rpc standard, enables nvim and the
process at the other end to send remote calls and events to each other.
Additionally, the builtin |terminal-emulator|, is implemented on top of PTY
channels.

==============================================================================

2. Reading and writing raw bytes			      *channel-bytes*

By default, channels opened by vimscript functions will operate with raw
bytes. Additionally, for a job channel using rpc, bytes can still be
read over its stderr. Similarily, only bytes can be written to nvim's own stderr.


						    *channel-callback* *buffered*

				*E5210* *on_stdout* *on_stderr* *on_stdin* *on_data*
A callback function `on_{stream}` will be invoked with data read from the
channel.  By default, the callback will be invoked immediately when data is
available, to facilitate interactive communication. The same callback will
then be invoked with empty data, to indicate that the stream reached EOF.
Alternatively the `{stream}_buffered` option can be set to invoke the callback
only when the underlying stream reaches EOF, and will then be passed in
complete output.  This is helpful when only the complete output is useful, and
not partial data. Futhermore if `{stream}_buffered` is set but not a callback,
the data is saved in the options dict, with the stream name as key. For this
to work a new options dict must be used for each opened channel. If a script
uses a global `s:job_opts` dict, it can be copied with |copy()| before supplying
it to |jobstart()|. If a dict is reused, so that the dict key already is
occupied, error `E5210` will be raised.

- The arguments passed to the callback function are:

  0: The channel id
  1: the raw data read from the channel, formatted as a YXXYreadfile()|-style
     list. If EOF occured, a single empty string `['']` will be passed in.
     Note that the items in this list do not directly correspond to actual
     lines in the output. See |channel-lines|
  2: Stream name as a string, like `"stdout"`. This is to allow multiple
  on_{event} handlers to be implemented by the same function. The available
  events depend on how the channel was opened and in what mode/protocol.


							      *channel-lines*
  Note:
	stream event handlers may receive partial (incomplete) lines. For a given
	invocation of on_stdout etc, `a:data` is not guaranteed to end
	with a newline.
	  - `abcdefg` may arrive as `['abc']`, `['defg']`.
	  - `abc\nefg` may arrive as `['abc', '']`, `['efg']` or `['abc']`,
	    `['','efg']`, or even `['ab']`, `['c','efg']`.
	
	If you only are interested in complete output when the process exits,
	use buffered mode. Otherwise, an easy way to deal with this:
	initialize a list as `['']`, then append to it as follows:
	  let s:chunks = ['']
	  func! s:on_event(job_id, data, event) dict
	    let s:chunks[-1] .= a:data[0]
	    call extend(s:chunks, a:data[1:])
	  endf
 

Additionally, if the callbacks are Dictionary functions, |self| can be used to
refer to the options dictionary containing the callbacks. |Partial|s can also be
used as callbacks.

Data can be sent to the channel using the |chansend()| function. Here is a
simple example, echoing some data through a cat-process:

    function! s:OnEvent(id, data, event) dict
      let str = join(a:data, "\n")
      echomsg str
    endfunction
    let id = jobstart(['cat'], {'on_stdout': function('s:OnEvent') } )
    call chansend(id, "hello!")
 

Here is a example of setting a buffer to the result of grep, but only after
all data has been processed:

    function! s:OnEvent(id, data, event) dict
      call nvim_buf_set_lines(2, 0, -1, v:true, a:data)
    endfunction
    let id = jobstart(['grep', '^[0-9]'], { 'on_stdout': function('s:OnEvent'),
					  \ 'stdout_buffered':v:true } )

    call chansend(id, "stuff\n10 PRINT \"NVIM\"\nxx")
    " no output is received, buffer is empty

    call chansend(id, "xx\n20 GOTO 10\nzz\n")
    call chanclose(id, 'stdin')
    " now buffer has result
 
For additional examples with jobs, see |job-control|.


							      *channel-pty*
A special case is PTY channels opened by  `jobstart(..., {'pty': v:true})` .
No preprocessing of ANSI escape sequences is done, these will be sent raw to
the callback. However, change of PTY size can be signaled to the slave using
|jobresize()|. See also |terminal-emulator|.

==============================================================================

3. Communicating using msgpack-rpc			      *channel-rpc*

When channels are opened with the `rpc` option set to true, the channel can be
used for remote method calls in both directions, see |msgpack-rpc|. Note that
rpc channels are implicitly trusted and the process at the other end can
invoke any |api| function!

==============================================================================

4. Using the stdio channel				    *channel-stdio*

When invoked normally, nvim will use stdin and stdout to interact with the
user over the terminal interface (TUI). However when invoked with
`--headless`, the TUI is not started and stdin and stdout can be used as a
channel. To open the stdio channel |stdioopen()| must be called during
|startup|, as there later will be no way of invoking a command.  As a
convenience, the stdio channel will always have channel id 1.

Here is an example:

    func! OnEvent(id, data, event)
      if a:data == [""]
	quit
      end
      call chansend(a:id, map(a:data, {i,v -> toupper(v)}))
    endfunc

    call stdioopen({'on_stdin': 'OnEvent'})
 
Put this in `uppercase.vim` and invoke nvim with

    nvim --headless --cmd "source uppercase.vim"
 

								*--embed*
An common use case is another program embedding nvim and communicating with it
over rpc. Therefore, the option `--embed` exists as a shorthand for
`nvim --headless --cmd "call stdioopen({'rpc': v:true})"`

Nvim's stderr is implicitly open as a write-only bytes channel. It will
always have channel id 2, however to be explicit |v:stderr| can be used.

==============================================================================
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