# Vim documentation: channel

main help file

    *channel.txt*      For Vim version 8.1.  Last change: 2018 Apr 18

    		  VIM REFERENCE MANUAL	  by Bram Moolenaar

    		      Inter-process communication		*channel*

    Vim uses channels to communicate with other processes.

    A channel uses a socket or pipes.			*socket-interface*
    Jobs can be used to start processes and communicate with them.
    The Netbeans interface also uses a channel. |netbeans|

    1. Overview				|job-channel-overview|
    2. Channel demo				|channel-demo|
    3. Opening a channel			|channel-open|
    4. Using a JSON or JS channel		|channel-use|
    5. Channel commands			|channel-commands|
    6. Using a RAW or NL channel		|channel-raw|
    7. More channel functions		|channel-more|
    8. Starting a job with a channel	|job-start|
    9. Starting a job without a channel	|job-start-nochannel|
    10. Job options				|job-options|
    11. Controlling a job			|job-control|

    {Vi does not have any of these features}
    {only when compiled with the |+channel| feature for channel stuff}
    	You can check this with: `has('channel')`
    {only when compiled with the |+job| feature for job stuff}
    	You can check this with: `has('job')`

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

    1. Overview						*job-channel-overview*

    There are four main types of jobs:
    1. A daemon, serving several Vim instances.
       Vim connects to it with a socket.
    2. One job working with one Vim instance, asynchronously.
       Uses a socket or pipes.
    3. A job performing some work for a short time, asynchronously.
       Uses a socket or pipes.
    4. Running a filter, synchronously.
       Uses pipes.

    For when using sockets See |job-start|, |job-start-nochannel| and
    |channel-open|.  For 2 and 3, one or more jobs using pipes, see |job-start|.
    For 4 use the ":{range}!cmd" command, see |filter|.

    Over the socket and pipes these protocols are available:
    RAW	nothing known, Vim cannot tell where a message ends
    NL	every message ends in a NL (newline) character
    JSON	JSON encoding |json_encode()|
    JS	JavaScript style JSON-like encoding |js_encode()|

    Common combination are:
    - Using a job connected through pipes in NL mode.  E.g., to run a style
      checker and receive errors and warnings.
    - Using a daemon, connecting over a socket in JSON mode.  E.g. to lookup
      cross-references in a database.

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

    2. Channel demo				*channel-demo* *demoserver.py*

    This requires Python.  The demo program can be found in
    $VIMRUNTIME/tools/demoserver.py
    Run it in one terminal.  We will call this T1.

    Run Vim in another terminal.  Connect to the demo server with:
    	let channel = ch_open('localhost:8765')

    In T1 you should see:
    	=== socket opened ===

    You can now send a message to the server:
    	echo ch_evalexpr(channel, 'hello!')

    The message is received in T1 and a response is sent back to Vim.
    You can see the raw messages in T1.  What Vim sends is:
    	[1,"hello!"]
    And the response is:
    	[1,"got it"]
    The number will increase every time you send a message.

    The server can send a command to Vim.  Type this on T1 (literally, including
    the quotes):
    	["ex","echo 'hi there'"]
    And you should see the message in Vim. You can move the cursor a word forward:
    	["normal","w"]

    To handle asynchronous communication a callback needs to be used:
    	func MyHandler(channel, msg)
    	  echo "from the handler: " . a:msg
    	endfunc
    	call ch_sendexpr(channel, 'hello!', {'callback': "MyHandler"})
    Vim will not wait for a response.  Now the server can send the response later
    and MyHandler will be invoked.

    Instead of giving a callback with every send call, it can also be specified
    when opening the channel:
    	call ch_close(channel)
    	let channel = ch_open('localhost:8765', {'callback': "MyHandler"})
    	call ch_sendexpr(channel, 'hello!')

    When trying out channels it's useful to see what is going on.  You can tell
    Vim to write lines in log file:
    	call ch_logfile('channellog', 'w')
    See |ch_logfile()|.

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

    3. Opening a channel					*channel-open*

    To open a channel:
        let channel = ch_open({address} [, {options}])
        if ch_status(channel) == "open"
          " use the channel

    Use |ch_status()| to see if the channel could be opened.

    {address} has the form "hostname:port".  E.g., "localhost:8765".

    {options} is a dictionary with optional entries:	*channel-open-options*

    "mode" can be:						*channel-mode*
    	"json" - Use JSON, see below; most convenient way. Default.
    	"js"   - Use JS (JavaScript) encoding, more efficient than JSON.
    	"nl"   - Use messages that end in a NL character
    	"raw"  - Use raw messages

    						*channel-callback* *E921*
    "callback"	A function that is called when a message is received that is
    		not handled otherwise.  It gets two arguments: the channel
    		and the received message. Example:
    	func Handle(channel, msg)
    	  echo 'Received: ' . a:msg
    	endfunc
    	let channel = ch_open("localhost:8765", {"callback": "Handle"})

    		When "mode" is "json" or "js" the "msg" argument is the body
    		of the received message, converted to Vim types.
    		When "mode" is "nl" the "msg" argument is one message,
    		excluding the NL.
    		When "mode" is "raw" the "msg" argument is the whole message
    		as a string.

    		For all callbacks: Use |function()| to bind it to arguments
    		and/or a Dictionary.  Or use the form "dict.function" to bind
    		the Dictionary.

    		Callbacks are only called at a "safe" moment, usually when Vim
    		is waiting for the user to type a character.  Vim does not use
    		multi-threading.

    							*close_cb*
    "close_cb"	A function that is called when the channel gets closed, other
    		than by calling ch_close().  It should be defined like this:
    	func MyCloseHandler(channel)
     		Vim will invoke callbacks that handle data before invoking
    		close_cb, thus when this function is called no more data will
    		be passed to the callbacks.

    							*channel-drop*
    "drop"		Specifies when to drop messages:
    		    "auto"	When there is no callback to handle a message.
    				The "close_cb" is also considered for this.
    		    "never"	All messages will be kept.

    							*waittime*
    "waittime"	The time to wait for the connection to be made in
    		milliseconds.  A negative number waits forever.

    		The default is zero, don't wait, which is useful if a local
    		server is supposed to be running already.  On Unix Vim
    		actually uses a 1 msec timeout, that is required on many
    		systems.  Use a larger value for a remote server, e.g.  10
    		msec at least.

    							*channel-timeout*
    "timeout"	The time to wait for a request when blocking, E.g. when using
    		ch_evalexpr().  In milliseconds.  The default is 2000 (2
    		seconds).

    When "mode" is "json" or "js" the "callback" is optional.  When omitted it is
    only possible to receive a message after sending one.

    To change the channel options after opening it use |ch_setoptions()|.  The
    arguments are similar to what is passed to |ch_open()|, but "waittime" cannot
    be given, since that only applies to opening the channel.

    For example, the handler can be added or changed:
        call ch_setoptions(channel, {'callback': callback})
    When "callback" is empty (zero or an empty string) the handler is removed.

    After a callback has been invoked Vim will update the screen and put the
    cursor back where it belongs.  Thus the callback should not need to do
    `:redraw`.

    The timeout can be changed:
        call ch_setoptions(channel, {'timeout': msec})

    							  *channel-close* *E906*
    Once done with the channel, disconnect it like this:
        call ch_close(channel)
    When a socket is used this will close the socket for both directions.  When
    pipes are used (stdin/stdout/stderr) they are all closed.  This might not be
    what you want!  Stopping the job with job_stop() might be better.
    All readahead is discarded, callbacks will no longer be invoked.

    Note that a channel is closed in three stages:
      - The I/O ends, log message: "Closing channel". There can still be queued
        messages to read or callbacks to invoke.
      - The readahead is cleared, log message: "Clearing channel".  Some variables
        may still reference the channel.
      - The channel is freed, log message: "Freeing channel".

    When the channel can't be opened you will get an error message.  There is a
    difference between MS-Windows and Unix: On Unix when the port doesn't exist
    ch_open() fails quickly.  On MS-Windows "waittime" applies.

    *E898* *E901* *E902*

    If there is an error reading or writing a channel it will be closed.

    *E630* *E631*

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

    4. Using a JSON or JS channel					*channel-use*

    If mode is JSON then a message can be sent synchronously like this:
        let response = ch_evalexpr(channel, {expr})
    This awaits a response from the other side.

    When mode is JS this works the same, except that the messages use
    JavaScript encoding.  See |js_encode()| for the difference.

    To send a message, without handling a response or letting the channel callback
    handle the response:
        call ch_sendexpr(channel, {expr})

    To send a message and letting the response handled by a specific function,
    asynchronously:
        call ch_sendexpr(channel, {expr}, {'callback': Handler})

    Vim will match the response with the request using the message ID.  Once the
    response is received the callback will be invoked.  Further responses with the
    same ID will be ignored.  If your server sends back multiple responses you
    need to send them with ID zero, they will be passed to the channel callback.

    The {expr} is converted to JSON and wrapped in an array.  An example of the
    message that the receiver will get when {expr} is the string "hello":
    	[12,"hello"]

    The format of the JSON sent is:
        [{number},{expr}]

    In which {number} is different every time.  It must be used in the response
    (if any):

        [{number},{response}]

    This way Vim knows which sent message matches with which received message and
    can call the right handler.  Also when the messages arrive out of order.

    A newline character is terminating the JSON text.  This can be used to
    separate the read text.  For example, in Python:
    	splitidx = read_text.find('\n')
    	message = read_text[:splitidx]
    	rest = read_text[splitidx + 1:]

    The sender must always send valid JSON to Vim.  Vim can check for the end of
    the message by parsing the JSON.  It will only accept the message if the end
    was received.  A newline after the message is optional.

    When the process wants to send a message to Vim without first receiving a
    message, it must use the number zero:
        [0,{response}]

    Then channel handler will then get {response} converted to Vim types.  If the
    channel does not have a handler the message is dropped.

    It is also possible to use ch_sendraw() and ch_evalraw() on a JSON or JS
    channel.  The caller is then completely responsible for correct encoding and
    decoding.

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

    5. Channel commands					*channel-commands*

    With a JSON channel the process can send commands to Vim that will be
    handled by Vim internally, it does not require a handler for the channel.

    Possible commands are:				*E903* *E904* *E905*
        ["redraw", {forced}]
        ["ex",     {Ex command}]
        ["normal", {Normal mode command}]
        ["expr",   {expression}, {number}]
        ["expr",   {expression}]
        ["call",   {func name}, {argument list}, {number}]
        ["call",   {func name}, {argument list}]

    With all of these: Be careful what these commands do!  You can easily
    interfere with what the user is doing.  To avoid trouble use |mode()| to check
    that the editor is in the expected state.  E.g., to send keys that must be
    inserted as text, not executed as a command:
        ["ex","if mode() == 'i' | call feedkeys('ClassName') | endif"]

    Errors in these commands are normally not reported to avoid them messing up
    the display.  If you do want to see them, set the 'verbose' option to 3 or
    higher.

    Command "redraw"

    The other commands do not update the screen, so that you can send a sequence
    of commands without the cursor moving around.  You must end with the "redraw"
    command to show any changed text and show the cursor where it belongs.

    The argument is normally an empty string:
    	["redraw", ""]
    To first clear the screen pass "force":
    	["redraw", "force"]

    Command "ex"

    The "ex" command is executed as any Ex command.  There is no response for
    completion or error.  You could use functions in an |autoload| script:
    	["ex","call myscript#MyFunc(arg)"]

    You can also use "call |feedkeys()|" to insert any key sequence.

    When there is an error a message is written to the channel log, if it exists,
    and v:errmsg is set to the error.

    Command "normal"

    The "normal" command is executed like with ":normal!", commands are not
    mapped.  Example to open the folds under the cursor:
    	["normal" "zO"]

    Command "expr"  with response

    The "expr" command can be used to get the result of an expression.  For
    example, to get the number of lines in the current buffer:
    	["expr","line('$')", -2]

    It will send back the result of the expression:
    	[-2, "last line"]
    The format is:
    	[{number}, {result}]

    Here {number} is the same as what was in the request.  Use a negative number
    to avoid confusion with message that Vim sends.  Use a different number on
    every request to be able to match the request with the response.

    {result} is the result of the evaluation and is JSON encoded.  If the
    evaluation fails or the result can't be encoded in JSON it is the string
    "ERROR".

    Command "expr" without a response

    This command is similar to "expr" above, but does not send back any response.
    Example:
    	["expr","setline('$', ['one', 'two', 'three'])"]
    There is no third argument in the request.

    Command "call"

    This is similar to "expr", but instead of passing the whole expression as a
    string this passes the name of a function and a list of arguments.  This
    avoids the conversion of the arguments to a string and escaping and
    concatenating them.  Example:
    	["call", "line", ["$"], -2]

    Leave out the fourth argument if no response is to be sent:
    	["call", "setline", ["$", ["one", "two", "three"]]]

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

    6. Using a RAW or NL channel				*channel-raw*

    If mode is RAW or NL then a message can be sent like this:
        let response = ch_evalraw(channel, {string})

    The {string} is sent as-is.  The response will be what can be read from the
    channel right away.  Since Vim doesn't know how to recognize the end of the
    message you need to take care of it yourself.  The timeout applies for reading
    the first byte, after that it will not wait for anything more.

    If mode is "nl" you can send a message in a similar way.  You are expected
    to put in the NL after each message.  Thus you can also send several messages
    ending in a NL at once.  The response will be the text up to and including the
    first NL.  This can also be just the NL for an empty response.
    If no NL was read before the channel timeout an empty string is returned.

    To send a message, without expecting a response:
        call ch_sendraw(channel, {string})
    The process can send back a response, the channel handler will be called with
    it.

    To send a message and letting the response handled by a specific function,
    asynchronously:
        call ch_sendraw(channel, {string}, {'callback': 'MyHandler'})

    This {string} can also be JSON, use |json_encode()| to create it and
    |json_decode()| to handle a received JSON message.

    It is not possible to use |ch_evalexpr()| or |ch_sendexpr()| on a raw channel.

    A String in Vim cannot contain NUL bytes.  To send or receive NUL bytes read
    or write from a buffer.  See |in_io-buffer| and |out_io-buffer|.

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

    7. More channel functions				*channel-more*

    To obtain the status of a channel: ch_status(channel).  The possible results
    are:
    	"fail"		Failed to open the channel.
    	"open"		The channel can be used.
    	"buffered"	The channel was closed but there is data to read.
    	"closed"	The channel was closed.

    To obtain the job associated with a channel: ch_getjob(channel)

    To read one message from a channel:
    	let output = ch_read(channel)
    This uses the channel timeout.  To read without a timeout, just get any
    message that is available:
    	let output = ch_read(channel, {'timeout': 0})
    When no message was available then the result is v:none for a JSON or JS mode
    channels, an empty string for a RAW or NL channel.  You can use |ch_canread()|
    to check if there is something to read.

    Note that when there is no callback, messages are dropped.  To avoid that add
    a close callback to the channel.

    To read all output from a RAW channel that is available:
    	let output = ch_readraw(channel)
    To read the error output:
    	let output = ch_readraw(channel, {"part": "err"})

    ch_read() and ch_readraw() use the channel timeout.  When there is nothing to
    read within that time an empty string is returned.  To specify a different
    timeout in msec use the "timeout" option:
    	{"timeout": 123}
    To read from the error output use the "part" option:
    	{"part": "err"}
    To read a message with a specific ID, on a JS or JSON channel:
    	{"id": 99}
    When no ID is specified or the ID is -1, the first message is returned. This
    overrules any callback waiting for this message.

    For a RAW channel this returns whatever is available, since Vim does not know
    where a message ends.
    For a NL channel this returns one message.
    For a JS or JSON channel this returns one decoded message.
    This includes any sequence number.

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

    8. Starting a job with a channel			*job-start* *job*

    To start a job and open a channel for stdin/stdout/stderr:
        let job = job_start(command, {options})

    You can get the channel with:
        let channel = job_getchannel(job)

    The channel will use NL mode.  If you want another mode it's best to specify
    this in {options}.  When changing the mode later some text may have already
    been received and not parsed correctly.

    If the command produces a line of output that you want to deal with, specify
    a handler for stdout:
        let job = job_start(command, {"out_cb": "MyHandler"})
    The function will be called with the channel and a message. You would define
    it like this:
        func MyHandler(channel, msg)

    Without the handler you need to read the output with |ch_read()| or
    |ch_readraw()|. You can do this in the close callback, see |read-in-close-cb|.

    Note that if the job exits before you read the output, the output may be lost.
    This depends on the system (on Unix this happens because closing the write end
    of a pipe causes the read end to get EOF).  To avoid this make the job sleep
    for a short while before it exits.

    The handler defined for "out_cb" will not receive stderr.  If you want to
    handle that separately, add an "err_cb" handler:
        let job = job_start(command, {"out_cb": "MyHandler",
    	    \			  "err_cb": "ErrHandler"})

    If you want to handle both stderr and stdout with one handler use the
    "callback" option:
        let job = job_start(command, {"callback": "MyHandler"})

    Depending on the system, starting a job can put Vim in the background, the
    started job gets the focus.  To avoid that, use the `foreground()` function.
    This might not always work when called early, put in the callback handler or
    use a timer to call it after the job has started.

    You can send a message to the command with ch_evalraw().  If the channel is in
    JSON or JS mode you can use ch_evalexpr().

    There are several options you can use, see |job-options|.
    For example, to start a job and write its output in buffer "dummy":
    	let logjob = job_start("tail -f /tmp/log",
    			     \ {'out_io': 'buffer', 'out_name': 'dummy'})
    	sbuf dummy

    Job input from a buffer

    							*in_io-buffer*
    To run a job that reads from a buffer:
    	let job = job_start({command},
    	    \ {'in_io': 'buffer', 'in_name': 'mybuffer'})

    							*E915* *E918*
    The buffer is found by name, similar to |bufnr()|. The buffer must exist and
    be loaded when job_start() is called.

    By default this reads the whole buffer.  This can be changed with the "in_top"
    and "in_bot" options.

    A special mode is when "in_top" is set to zero and "in_bot" is not set: Every
    time a line is added to the buffer, the last-but-one line will be sent to the
    job stdin.  This allows for editing the last line and sending it when pressing
    Enter.

    							*channel-close-in*
    When not using the special mode the pipe or socket will be closed after the
    last line has been written.  This signals the reading end that the input
    finished.  You can also use |ch_close_in()| to close it sooner.

    NUL bytes in the text will be passed to the job (internally Vim stores these
    as NL bytes).

    Reading job output in the close callback

    							*read-in-close-cb*
    If the job can take some time and you don't need intermediate results, you can
    add a close callback and read the output there:

    	func! CloseHandler(channel)
    	  while ch_status(a:channel, {'part': 'out'}) == 'buffered'
    	    echomsg ch_read(a:channel)
    	  endwhile
    	endfunc
    	let job = job_start(command, {'close_cb': 'CloseHandler'})

    You will want to do something more useful than "echomsg".

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

    9. Starting a job without a channel			*job-start-nochannel*

    To start another process without creating a channel:
        let job = job_start(command,
    	\ {"in_io": "null", "out_io": "null", "err_io": "null"})

    This starts {command} in the background, Vim does not wait for it to finish.

    When Vim sees that neither stdin, stdout or stderr are connected, no channel
    will be created.  Often you will want to include redirection in the command to
    avoid it getting stuck.

    There are several options you can use, see |job-options|.

    							*job-start-if-needed*
    To start a job only when connecting to an address does not work, do something
    like this:
    	let channel = ch_open(address, {"waittime": 0})
    	if ch_status(channel) == "fail"
    	  let job = job_start(command)
    	  let channel = ch_open(address, {"waittime": 1000})
    	endif

    Note that the waittime for ch_open() gives the job one second to make the port
    available.

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

    10. Job options						*job-options*

    The {options} argument in job_start() is a dictionary.  All entries are
    optional.  Some options can be used after the job has started, using
    job_setoptions(job, {options}).  Many options can be used with the channel
    related to the job, using ch_setoptions(channel, {options}).
    See |job_setoptions()| and |ch_setoptions()|.

    						*in_mode* *out_mode* *err_mode*
    "in_mode"		mode specifically for stdin, only when using pipes
    "out_mode"		mode specifically for stdout, only when using pipes
    "err_mode"		mode specifically for stderr, only when using pipes
    			See |channel-mode| for the values.

    			Note: when setting "mode" the part specific mode is
    			overwritten.  Therefore set "mode" first and the part
    			specific mode later.

    			Note: when writing to a file or buffer and when
    			reading from a buffer NL mode is used by default.

    						*job-callback*
    "callback": handler	Callback for something to read on any part of the
    			channel.

    						*job-out_cb* *out_cb*
    "out_cb": handler	Callback for when there is something to read on
    			stdout.  Only for when the channel uses pipes.  When
    			"out_cb" wasn't set the channel callback is used.
    			The two arguments are the channel and the message.

    						*job-err_cb* *err_cb*
    "err_cb": handler	Callback for when there is something to read on
    			stderr.  Only for when the channel uses pipes.  When
    			"err_cb" wasn't set the channel callback is used.
    			The two arguments are the channel and the message.

    						*job-close_cb*
    "close_cb": handler	Callback for when the channel is closed.  Same as
    			"close_cb" on |ch_open()|, see |close_cb|.

    						*job-drop*
    "drop": when		Specifies when to drop messages.  Same as "drop" on
    			|ch_open()|, see |channel-drop|.  For "auto" the
    			exit_cb is not considered.

    						*job-exit_cb*
    "exit_cb": handler	Callback for when the job ends.  The arguments are the
    			job and the exit status.
    			Vim checks up to 10 times per second for jobs that
    			ended.  The check can also be triggered by calling
    			|job_status()|, which may then invoke the exit_cb
    			handler.
    			Note that data can be buffered, callbacks may still be
    			called after the process ends.

    							*job-timeout*
    "timeout": time		The time to wait for a request when blocking, E.g.
    			when using ch_evalexpr().  In milliseconds.  The
    			default is 2000 (2 seconds).

    						*out_timeout* *err_timeout*
    "out_timeout": time	Timeout for stdout.  Only when using pipes.
    "err_timeout": time	Timeout for stderr.  Only when using pipes.
    			Note: when setting "timeout" the part specific mode is
    			overwritten.  Therefore set "timeout" first and the
    			part specific mode later.

    						*job-stoponexit*
    "stoponexit": {signal}	Send {signal} to the job when Vim exits.  See
    			|job_stop()| for possible values.
    "stoponexit": ""	Do not stop the job when Vim exits.
    			The default is "term".

    						*job-term*
    "term": "open"		Start a terminal in a new window and connect the job
    			stdin/stdout/stderr to it.  Similar to using
    			`:terminal`.
    			NOTE: Not implemented yet!

    "channel": {channel}	Use an existing channel instead of creating a new one.
    			The parts of the channel that get used for the new job
    			will be disconnected from what they were used before.
    			If the channel was still used by another job this may
    			cause I/O errors.
    			Existing callbacks and other settings remain.

    "pty": 1		Use a pty (pseudo-tty) instead of a pipe when
    			possible.  This is most useful in combination with a
    			terminal window, see |terminal|.
    			{only on Unix and Unix-like systems}

    				*job-in_io* *in_top* *in_bot* *in_name* *in_buf*
    "in_io": "null"		disconnect stdin (read from /dev/null)
    "in_io": "pipe"		stdin is connected to the channel (default)
    "in_io": "file"		stdin reads from a file
    "in_io": "buffer"	stdin reads from a buffer
    "in_top": number	when using "buffer": first line to send (default: 1)
    "in_bot": number	when using "buffer": last line to send (default: last)
    "in_name": "/path/file"	the name of the file or buffer to read from
    "in_buf": number	the number of the buffer to read from

    				*job-out_io* *out_name* *out_buf*
    "out_io": "null"	disconnect stdout (goes to /dev/null)
    "out_io": "pipe"	stdout is connected to the channel (default)
    "out_io": "file"	stdout writes to a file
    "out_io": "buffer"	stdout appends to a buffer (see below)
    "out_name": "/path/file" the name of the file or buffer to write to
    "out_buf": number	the number of the buffer to write to
    "out_modifiable": 0	when writing to a buffer, 'modifiable' will be off
    			(see below)
    "out_msg": 0		when writing to a new buffer, the first line will be
    			set to "Reading from channel output..."

    				*job-err_io* *err_name* *err_buf*
    "err_io": "out"		stderr messages to go to stdout
    "err_io": "null"	disconnect stderr  (goes to /dev/null)
    "err_io": "pipe"	stderr is connected to the channel (default)
    "err_io": "file"	stderr writes to a file
    "err_io": "buffer"	stderr appends to a buffer (see below)
    "err_name": "/path/file" the name of the file or buffer to write to
    "err_buf": number	the number of the buffer to write to
    "err_modifiable": 0	when writing to a buffer, 'modifiable' will be off
    			(see below)
    "err_msg": 0		when writing to a new buffer, the first line will be
    			set to "Reading from channel error..."

    "block_write": number	only for testing: pretend every other write to stdin
    			will block

    "env": dict		environment variables for the new process
    "cwd": "/path/to/dir"	current working directory for the new process;
    			if the directory does not exist an error is given

    Writing to a buffer

    							*out_io-buffer*
    When the out_io or err_io mode is "buffer" and there is a callback, the text
    is appended to the buffer before invoking the callback.

    When a buffer is used both for input and output, the output lines are put
    above the last line, since the last line is what is written to the channel
    input.  Otherwise lines are appended below the last line.

    When using JS or JSON mode with "buffer", only messages with zero or negative
    ID will be added to the buffer, after decoding + encoding.  Messages with a
    positive number will be handled by a callback, commands are handled as usual.

    The name of the buffer from "out_name" or "err_name" is compared the full name
    of existing buffers, also after expanding the name for the current directory.
    E.g., when a buffer was created with ":edit somename" and the buffer name is
    "somename" it will use that buffer.

    If there is no matching buffer a new buffer is created.  Use an empty name to
    always create a new buffer.  |ch_getbufnr()| can then be used to get the
    buffer number.

    For a new buffer 'buftype' is set to "nofile" and 'bufhidden' to "hide".  If
    you prefer other settings, create the buffer first and pass the buffer number.

    					*out_modifiable* *err_modifiable*
    The "out_modifiable" and "err_modifiable" options can be used to set the
    'modifiable' option off, or write to a buffer that has 'modifiable' off.  That
    means that lines will be appended to the buffer, but the user can't easily
    change the buffer.

    					*out_msg* *err_msg*
    The "out_msg" option can be used to specify whether a new buffer will have the
    first line set to "Reading from channel output...".  The default is to add the
    message.  "err_msg" does the same for channel error.

    When an existing buffer is to be written where 'modifiable' is off and the
    "out_modifiable" or "err_modifiable" options is not zero, an error is given
    and the buffer will not be written to.

    When the buffer written to is displayed in a window and the cursor is in the
    first column of the last line, the cursor will be moved to the newly added
    line and the window is scrolled up to show the cursor if needed.

    Undo is synced for every added line.  NUL bytes are accepted (internally Vim
    stores these as NL bytes).

    Writing to a file

    							*E920*
    The file is created with permissions 600 (read-write for the user, not
    accessible for others).  Use |setfperm()| to change this.

    If the file already exists it is truncated.

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

    11. Controlling a job					*job-control*

    To get the status of a job:
    	echo job_status(job)

    To make a job stop running:
    	job_stop(job)

    This is the normal way to end a job. On Unix it sends a SIGTERM to the job.
    It is possible to use other ways to stop the job, or even send arbitrary
    signals.  E.g. to force a job to stop, "kill it":
    	job_stop(job, "kill")

    For more options see |job_stop()|.

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