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### 36.4 Creating an Asynchronous Process

In this section, we describe how to create an asynchronous process. After an asynchronous process is created, it runs in parallel with Emacs, and Emacs can communicate with it using the functions described in the following sections (see Input to Processes, and see Output from Processes). Note that process communication is only partially asynchronous: Emacs sends data to the process only when certain functions are called, and Emacs accepts data from the process only while waiting for input or for a time delay.

An asynchronous process is controlled either via a pty (pseudo-terminal) or a pipe. The choice of pty or pipe is made when creating the process, by default based on the value of the variable `process-connection-type` (see below). If available, ptys are usually preferable for processes visible to the user, as in Shell mode, because they allow for job control (`C-c`, `C-z`, etc.) between the process and its children, and because interactive programs treat ptys as terminal devices, whereas pipes don’t support these features. However, for subprocesses used by Lisp programs for internal purposes, it is often better to use a pipe, because pipes are more efficient, and because they are immune to stray character injections that ptys introduce for large (around 500 byte) messages. Also, the total number of ptys is limited on many systems and it is good not to waste them.

Function: make-process &rest args

  This function is the basic low-level primitive for starting asynchronous subprocesses. It returns a process object representing the subprocess. Compared to the more high-level `start-process`, described below, it takes keyword arguments, is more flexible, and allows to specify process filters and sentinels in a single call.

  The arguments args are a list of keyword/argument pairs. Omitting a keyword is always equivalent to specifying it with value `nil`. Here are the meaningful keywords:

  :name name

    Use the string name as the process name; if a process with this name already exists, then name is modified (by appending ‘`<1>`’, etc.) to be unique.

  :buffer buffer

    Use buffer as the process buffer. If the value is `nil`, the subprocess is not associated with any buffer.

  :command command

    Use command as the command line of the process. The value should be a list starting with the program’s executable file name, followed by strings to give to the program as its arguments. If the first element of the list is `nil`, Emacs opens a new pseudoterminal (pty) and associates its input and output with buffer, without actually running any program; the rest of the list elements are ignored in that case.

  :coding coding

    If coding is a symbol, it specifies the coding system to be used for both reading and writing of data from and to the connection. If coding is a cons cell `(decoding . encoding)`, then decoding will be used for reading and encoding for writing. The coding system used for encoding the data written to the program is also used for encoding the command-line arguments (but not the program itself, whose file name is encoded as any other file name; see file-name-coding-system).

    If coding is `nil`, the default rules for finding the coding system will apply. See Default Coding Systems.

  :connection-type TYPE

    Initialize the type of device used to communicate with the subprocess. Possible values are `pty` to use a pty, `pipe` to use a pipe, or `nil` to use the default derived from the value of the `process-connection-type` variable. This parameter and the value of `process-connection-type` are ignored if a non-`nil` value is specified for the `:stderr` parameter; in that case, the type will always be `pipe`.

  :noquery query-flag

    Initialize the process query flag to query-flag. See Query Before Exit.

  :stop stopped

    If stopped is non-`nil`, start the process in the stopped state.

  :filter filter

    Initialize the process filter to filter. If not specified, a default filter will be provided, which can be overridden later. See Filter Functions.

  :sentinel sentinel

    Initialize the process sentinel to sentinel. If not specified, a default sentinel will be used, which can be overridden later. See Sentinels.

  :stderr stderr

    Associate stderr with the standard error of the process. A non-`nil` value should be either a buffer or a pipe process created with `make-pipe-process`, described below.

  The original argument list, modified with the actual connection information, is available via the `process-contact` function.

Function: make-pipe-process &rest args

  This function creates a bidirectional pipe which can be attached to a child process. This is useful with the `:stderr` keyword of `make-process`. The function returns a process object.

  The arguments args are a list of keyword/argument pairs. Omitting a keyword is always equivalent to specifying it with value `nil`.

  Here are the meaningful keywords:

  :name name

    Use the string name as the process name. As with `make-process`, it is modified if necessary to make it unique.

  :buffer buffer

    Use buffer as the process buffer.

  :coding coding

    If coding is a symbol, it specifies the coding system to be used for both reading and writing of data from and to the connection. If coding is a cons cell `(decoding . encoding)`, then decoding will be used for reading and encoding for writing.

    If coding is `nil`, the default rules for finding the coding system will apply. See Default Coding Systems.

  :noquery query-flag

    Initialize the process query flag to query-flag. See Query Before Exit.

  :stop stopped

    If stopped is non-`nil`, start the process in the stopped state.

  :filter filter

    Initialize the process filter to filter. If not specified, a default filter will be provided, which can be changed later. See Filter Functions.

  :sentinel sentinel

    Initialize the process sentinel to sentinel. If not specified, a default sentinel will be used, which can be changed later. See Sentinels.

  The original argument list, modified with the actual connection information, is available via the `process-contact` function.

Function: start-process name buffer-or-name program &rest args

  This function is a higher-level wrapper around `make-process`, exposing an interface that is similar to `call-process`. It creates a new asynchronous subprocess and starts the specified program running in it. It returns a process object that stands for the new subprocess in Lisp. The argument name specifies the name for the process object; as with `make-process`, it is modified if necessary to make it unique. The buffer buffer-or-name is the buffer to associate with the process.

  If program is `nil`, Emacs opens a new pseudoterminal (pty) and associates its input and output with buffer-or-name, without creating a subprocess. In that case, the remaining arguments args are ignored.

  The rest of args are strings that specify command line arguments for the subprocess.

  In the example below, the first process is started and runs (rather, sleeps) for 100 seconds (the output buffer ‘`foo`’ is created immediately). Meanwhile, the second process is started, and given the name ‘`my-process<1>`’ for the sake of uniqueness. It inserts the directory listing at the end of the buffer ‘`foo`’, before the first process finishes. Then it finishes, and a message to that effect is inserted in the buffer. Much later, the first process finishes, and another message is inserted in the buffer for it.

      (start-process "my-process" "foo" "sleep" "100")
           ⇒ #<process my-process>

      (start-process "my-process" "foo" "ls" "-l" "/bin")
           ⇒ #<process my-process<1>>

      ---------- Buffer: foo ----------
      total 8336
      -rwxr-xr-x 1 root root 971384 Mar 30 10:14 bash
      -rwxr-xr-x 1 root root 146920 Jul  5  2011 bsd-csh
      …
      -rwxr-xr-x 1 root root 696880 Feb 28 15:55 zsh4

      Process my-process<1> finished

      Process my-process finished
      ---------- Buffer: foo ----------

Function: start-file-process name buffer-or-name program &rest args

  Like `start-process`, this function starts a new asynchronous subprocess running program in it, and returns its process object.

  The difference from `start-process` is that this function may invoke a file handler based on the value of `default-directory`. This handler ought to run program, perhaps on the local host, perhaps on a remote host that corresponds to `default-directory`. In the latter case, the local part of `default-directory` becomes the working directory of the process.

  This function does not try to invoke file name handlers for program or for the rest of args.

  Depending on the implementation of the file handler, it might not be possible to apply `process-filter` or `process-sentinel` to the resulting process object. See Filter Functions, and Sentinels.

  Some file handlers may not support `start-file-process` (for example the function `ange-ftp-hook-function`). In such cases, this function does nothing and returns `nil`.

Function: start-process-shell-command name buffer-or-name command

  This function is like `start-process`, except that it uses a shell to execute the specified command. The argument command is a shell command string. The variable `shell-file-name` specifies which shell to use.

  The point of running a program through the shell, rather than directly with `make-process` or `start-process`, is so that you can employ shell features such as wildcards in the arguments. It follows that if you include any arbitrary user-specified arguments in the command, you should quote them with `shell-quote-argument` first, so that any special shell characters do not have their special shell meanings. See Shell Arguments. Of course, when executing commands based on user input you should also consider the security implications.

Function: start-file-process-shell-command name buffer-or-name command

  This function is like `start-process-shell-command`, but uses `start-file-process` internally. Because of this, command can also be executed on remote hosts, depending on `default-directory`.

Variable: process-connection-type

  This variable controls the type of device used to communicate with asynchronous subprocesses. If it is non-`nil`, then ptys are used, when available. Otherwise, pipes are used.

  The value of `process-connection-type` takes effect when `make-process` or `start-process` is called. So you can specify how to communicate with one subprocess by binding the variable around the call to these functions.

  Note that the value of this variable is ignored when `make-process` is called with a non-`nil` value of the `:stderr` parameter; in that case, Emacs will communicate with the process using pipes.

      (let ((process-connection-type nil))  ; use a pipe
        (start-process …))

  To determine whether a given subprocess actually got a pipe or a pty, use the function `process-tty-name` (see Process Information).

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