emacs/doc/lispref/processes.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990--1995, 1998--1999, 2001--2023 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Processes
@chapter Processes
@cindex child process
@cindex parent process
@cindex subprocess
@cindex process

  In the terminology of operating systems, a @dfn{process} is a space in
which a program can execute.  Emacs runs in a process.  Emacs Lisp
programs can invoke other programs in processes of their own.  These are
called @dfn{subprocesses} or @dfn{child processes} of the Emacs process,
which is their @dfn{parent process}.

  A subprocess of Emacs may be @dfn{synchronous} or @dfn{asynchronous},
depending on how it is created.  When you create a synchronous
subprocess, the Lisp program waits for the subprocess to terminate
before continuing execution.  When you create an asynchronous
subprocess, it can run in parallel with the Lisp program.  This kind of
subprocess is represented within Emacs by a Lisp object which is also
called a ``process''.  Lisp programs can use this object to communicate
with the subprocess or to control it.  For example, you can send
signals, obtain status information, receive output from the process, or
send input to it.

  In addition to processes that run programs, Lisp programs can open
connections of several types to devices or processes running on the
same machine or on other machines.  The supported connection types
are: TCP and UDP network connections, serial port connections, and
pipe connections.  Each such connection is also represented by a
process object.

@defun processp object
This function returns @code{t} if @var{object} represents an Emacs
process object, @code{nil} otherwise.  The process object can
represent a subprocess running a program or a connection of any
supported type.
@end defun

  In addition to subprocesses of the current Emacs session, you can
also access other processes running on your machine.  @xref{System
Processes}.

@menu
* Subprocess Creation::      Functions that start subprocesses.
* Shell Arguments::          Quoting an argument to pass it to a shell.
* Synchronous Processes::    Details of using synchronous subprocesses.
* Asynchronous Processes::   Starting up an asynchronous subprocess.
* Deleting Processes::       Eliminating an asynchronous subprocess.
* Process Information::      Accessing run-status and other attributes.
* Input to Processes::       Sending input to an asynchronous subprocess.
* Signals to Processes::     Stopping, continuing or interrupting
                               an asynchronous subprocess.
* Output from Processes::    Collecting output from an asynchronous subprocess.
* Sentinels::                Sentinels run when process run-status changes.
* Query Before Exit::        Whether to query if exiting will kill a process.
* System Processes::         Accessing other processes running on your system.
* Transaction Queues::       Transaction-based communication with subprocesses.
* Network::                  Opening network connections.
* Network Servers::          Network servers let Emacs accept net connections.
* Datagrams::                UDP network connections.
* Low-Level Network::        Lower-level but more general function
                               to create connections and servers.
* Misc Network::             Additional relevant functions for net connections.
* Serial Ports::             Communicating with serial ports.
* Byte Packing::             Using bindat to pack and unpack binary data.
@end menu

@node Subprocess Creation
@section Functions that Create Subprocesses
@cindex create subprocess
@cindex process creation

  There are three primitives that create a new subprocess in which to run
a program.  One of them, @code{make-process}, creates an asynchronous
process and returns a process object (@pxref{Asynchronous Processes}).
The other two, @code{call-process} and @code{call-process-region},
create a synchronous process and do not return a process object
(@pxref{Synchronous Processes}).  There are various higher-level
functions that make use of these primitives to run particular types of
process.

  Synchronous and asynchronous processes are explained in the following
sections.  Since the three functions are all called in a similar
fashion, their common arguments are described here.

@cindex execute program
@cindex @env{PATH} environment variable
@cindex @env{HOME} environment variable
  In all cases, the functions specify the program to be run.  An error
is signaled if the file is not found or cannot be executed.  If the
file name is relative, the variable @code{exec-path} contains a list
of directories to search.  Emacs initializes @code{exec-path} when it
starts up, based on the value of the environment variable @env{PATH}.
The standard file name constructs, @samp{~}, @samp{.}, and @samp{..},
are interpreted as usual in @code{exec-path}, but environment variable
substitutions (@samp{$HOME}, etc.)@: are not recognized; use
@code{substitute-in-file-name} to perform them (@pxref{File Name
Expansion}).  @code{nil} in this list refers to
@code{default-directory}.

  Executing a program can also try adding suffixes to the specified
name:

@defopt exec-suffixes
This variable is a list of suffixes (strings) to try adding to the
specified program file name.  The list should include @code{""} if you
want the name to be tried exactly as specified.  The default value is
system-dependent.
@end defopt

  @strong{Please note:} The argument @var{program} contains only the
name of the program file; it may not contain any command-line
arguments.  You must use a separate argument, @var{args}, to provide
those, as described below.

  Each of the subprocess-creating functions has a @var{buffer-or-name}
argument that specifies where the output from the program will go.  It
should be a buffer or a buffer name; if it is a buffer name, that will
create the buffer if it does not already exist.  It can also be
@code{nil}, which says to discard the output, unless a custom filter
function handles it.  (@xref{Filter Functions}, and @ref{Read and
Print}.)  Normally, you should avoid having multiple processes send
output to the same buffer because their output would be intermixed
randomly.  For synchronous processes, you can send the output to a
file instead of a buffer (and the corresponding argument is therefore
more appropriately called @var{destination}).  By default, both
standard output and standard error streams go to the same destination,
but all the 3 primitives allow optionally to direct the standard error
stream to a different destination.

@cindex program arguments
  All three of the subprocess-creating functions allow to specify
command-line arguments for the process to run. For @code{call-process}
and @code{call-process-region}, these come in the form of a
@code{&rest} argument, @var{args}.  For @code{make-process}, both the
program to run and its command-line arguments are specified as a list
of strings.  The command-line arguments must all be strings, and they
are supplied to the program as separate argument strings.  Wildcard
characters and other shell constructs have no special meanings in
these strings, since the strings are passed directly to the specified
program.

@cindex environment variables, subprocesses
  The subprocess inherits its environment from Emacs, but you can
specify overrides for it with @code{process-environment}.  @xref{System
Environment}.  The subprocess gets its current directory from the
value of @code{default-directory}.

@defvar exec-directory
@pindex movemail
The value of this variable is a string, the name of a directory that
contains programs that come with GNU Emacs and are intended for Emacs
to invoke.  The program @code{movemail} is an example of such a program;
Rmail uses it to fetch new mail from an inbox.
@end defvar

@defopt exec-path
The value of this variable is a list of directories to search for
programs to run in subprocesses.  Each element is either the name of a
directory (i.e., a string), or @code{nil}, which stands for the default
directory (which is the value of @code{default-directory}).
@xref{Locating Files, executable-find}, for the details of this search.
@cindex program directories

The value of @code{exec-path} is used by @code{call-process} and
@code{start-process} when the @var{program} argument is not an absolute
file name.

Generally, you should not modify @code{exec-path} directly.  Instead,
ensure that your @env{PATH} environment variable is set appropriately
before starting Emacs.  Trying to modify @code{exec-path}
independently of @env{PATH} can lead to confusing results.
@end defopt

@defun exec-path
This function is an extension of the variable @code{exec-path}.  If
@code{default-directory} indicates a remote directory, this function
returns a list of directories used for searching programs on the
respective remote host.  In case of a local @code{default-directory},
the function returns just the value of the variable @code{exec-path}.
@end defun

@node Shell Arguments
@section Shell Arguments
@cindex arguments for shell commands
@cindex shell command arguments

  Lisp programs sometimes need to run a shell and give it a command
that contains file names that were specified by the user.  These
programs ought to be able to support any valid file name.  But the shell
gives special treatment to certain characters, and if these characters
occur in the file name, they will confuse the shell.  To handle these
characters, use the function @code{shell-quote-argument}:

@defun shell-quote-argument argument &optional posix
This function returns a string that represents, in shell syntax,
an argument whose actual contents are @var{argument}.  It should
work reliably to concatenate the return value into a shell command
and then pass it to a shell for execution.

Precisely what this function does depends on your operating system.  The
function is designed to work with the syntax of your system's standard
shell; if you use an unusual shell, you will need to redefine this
function.  @xref{Security Considerations}.

@example
;; @r{This example shows the behavior on GNU and Unix systems.}
(shell-quote-argument "foo > bar")
     @result{} "foo\\ \\>\\ bar"

;; @r{This example shows the behavior on MS-DOS and MS-Windows.}
(shell-quote-argument "foo > bar")
     @result{} "\"foo > bar\""
@end example

Here's an example of using @code{shell-quote-argument} to construct
a shell command:

@example
(concat "diff -u "
        (shell-quote-argument oldfile)
        " "
        (shell-quote-argument newfile))
@end example

If the optional @var{posix} argument is non-@code{nil}, @var{argument}
is quoted according to POSIX shell quoting rules, regardless of the
system’s shell.  This is useful when your shell could run on a remote
host, which requires a POSIX shell in general.

@example
(shell-quote-argument "foo > bar" (file-remote-p default-directory))
@end example
@end defun

@cindex quoting and unquoting command-line arguments
@cindex minibuffer input, and command-line arguments
@cindex @code{call-process}, command-line arguments from minibuffer
@cindex @code{start-process}, command-line arguments from minibuffer
  The following two functions are useful for combining a list of
individual command-line argument strings into a single string, and
taking a string apart into a list of individual command-line
arguments.  These functions are mainly intended for converting user
input in the minibuffer, a Lisp string, into a list of string
arguments to be passed to @code{make-process}, @code{call-process} or
@code{start-process}, or for converting such lists of arguments into a
single Lisp string to be presented in the minibuffer or echo area.
Note that if a shell is involved (e.g., if using
@code{call-process-shell-command}), arguments should still be
protected by @code{shell-quote-argument};
@code{combine-and-quote-strings} is @emph{not} intended to protect
special characters from shell evaluation.

@defun split-string-shell-command string
This function splits @var{string} into substrings, respecting double
and single quotes, as well as backslash quoting.

@smallexample
(split-string-shell-command "ls /tmp/'foo bar'")
     @result{} ("ls" "/tmp/foo bar")
@end smallexample
@end defun

@defun split-string-and-unquote string &optional separators
This function splits @var{string} into substrings at matches for the
regular expression @var{separators}, like @code{split-string} does
(@pxref{Creating Strings}); in addition, it removes quoting from the
substrings.  It then makes a list of the substrings and returns it.

If @var{separators} is omitted or @code{nil}, it defaults to
@code{"\\s-+"}, which is a regular expression that matches one or more
characters with whitespace syntax (@pxref{Syntax Class Table}).

This function supports two types of quoting: enclosing a whole string
in double quotes @code{"@dots{}"}, and quoting individual characters
with a backslash escape @samp{\}.  The latter is also used in Lisp
strings, so this function can handle those as well.
@end defun

@defun combine-and-quote-strings list-of-strings &optional separator
This function concatenates @var{list-of-strings} into a single string,
quoting each string as necessary.  It also sticks the @var{separator}
string between each pair of strings; if @var{separator} is omitted or
@code{nil}, it defaults to @code{" "}.  The return value is the
resulting string.

The strings in @var{list-of-strings} that need quoting are those that
include @var{separator} as their substring.  Quoting a string encloses
it in double quotes @code{"@dots{}"}.  In the simplest case, if you
are consing a command from the individual command-line arguments,
every argument that includes embedded blanks will be quoted.
@end defun

@node Synchronous Processes
@section Creating a Synchronous Process
@cindex synchronous subprocess

  After a @dfn{synchronous process} is created, Emacs waits for the
process to terminate before continuing.  Starting Dired on GNU or
Unix@footnote{On other systems, Emacs uses a Lisp emulation of
@code{ls}; see @ref{Contents of Directories}.} is an example of this: it
runs @code{ls} in a synchronous process, then modifies the output
slightly.  Because the process is synchronous, the entire directory
listing arrives in the buffer before Emacs tries to do anything with it.

  While Emacs waits for the synchronous subprocess to terminate, the
user can quit by typing @kbd{C-g}.  The first @kbd{C-g} tries to kill
the subprocess with a @code{SIGINT} signal; but it waits until the
subprocess actually terminates before quitting.  If during that time the
user types another @kbd{C-g}, that kills the subprocess instantly with
@code{SIGKILL} and quits immediately (except on MS-DOS, where killing
other processes doesn't work).  @xref{Quitting}.

  The synchronous subprocess functions return an indication of how the
process terminated.

  The output from a synchronous subprocess is generally decoded using a
coding system, much like text read from a file.  The input sent to a
subprocess by @code{call-process-region} is encoded using a coding
system, much like text written into a file.  @xref{Coding Systems}.

@defun call-process program &optional infile destination display &rest args
This function calls @var{program} and waits for it to finish.

The current working directory of the subprocess is set to the current
buffer's value of @code{default-directory} if that is local (as
determined by @code{unhandled-file-name-directory}), or "~" otherwise.
If you want to run a process in a remote directory use
@code{process-file}.

The standard input for the new process comes from file @var{infile} if
@var{infile} is not @code{nil}, and from the null device otherwise.
The argument @var{destination} says where to put the process output.
Here are the possibilities:

@table @asis
@item a buffer
Insert the output in that buffer, before point.  This includes both the
standard output stream and the standard error stream of the process.

@item a buffer name (a string)
Insert the output in a buffer with that name, before point.

@item @code{t}
Insert the output in the current buffer, before point.

@item @code{nil}
Discard the output.

@item 0
Discard the output, and return @code{nil} immediately without waiting
for the subprocess to finish.

In this case, the process is not truly synchronous, since it can run in
parallel with Emacs; but you can think of it as synchronous in that
Emacs is essentially finished with the subprocess as soon as this
function returns.

MS-DOS doesn't support asynchronous subprocesses, so this option doesn't
work there.

@item @code{(:file @var{file-name})}
Send the output to the file name specified, overwriting it if it
already exists.

@item @code{(@var{real-destination} @var{error-destination})}
Keep the standard output stream separate from the standard error stream;
deal with the ordinary output as specified by @var{real-destination},
and dispose of the error output according to @var{error-destination}.
If @var{error-destination} is @code{nil}, that means to discard the
error output, @code{t} means mix it with the ordinary output, and a
string specifies a file name to redirect error output into.

You can't directly specify a buffer to put the error output in; that is
too difficult to implement.  But you can achieve this result by sending
the error output to a temporary file and then inserting the file into a
buffer when the subprocess finishes.
@end table

If @var{display} is non-@code{nil}, then @code{call-process} redisplays
the buffer as output is inserted.  (However, if the coding system chosen
for decoding output is @code{undecided}, meaning deduce the encoding
from the actual data, then redisplay sometimes cannot continue once
non-@acronym{ASCII} characters are encountered.  There are fundamental
reasons why it is hard to fix this; see @ref{Output from Processes}.)

Otherwise the function @code{call-process} does no redisplay, and the
results become visible on the screen only when Emacs redisplays that
buffer in the normal course of events.

The remaining arguments, @var{args}, are strings that specify command
line arguments for the program.  Each string is passed to
@var{program} as a separate argument.

The value returned by @code{call-process} (unless you told it not to
wait) indicates the reason for process termination.  A number gives the
exit status of the subprocess; 0 means success, and any other value
means failure.  If the process terminated with a signal,
@code{call-process} returns a string describing the signal.  If you
told @code{call-process} not to wait, it returns @code{nil}.

In the examples below, the buffer @samp{foo} is current.

@smallexample
@group
(call-process "pwd" nil t)
     @result{} 0

---------- Buffer: foo ----------
/home/lewis/manual
---------- Buffer: foo ----------
@end group

@group
(call-process "grep" nil "bar" nil "lewis" "/etc/passwd")
     @result{} 0

---------- Buffer: bar ----------
lewis:x:1001:1001:Bil Lewis,,,,:/home/lewis:/bin/bash

---------- Buffer: bar ----------
@end group
@end smallexample

Here is an example of the use of @code{call-process}, as used to
be found in the definition of the @code{insert-directory} function:

@smallexample
@group
(call-process insert-directory-program nil t nil switches
              (if full-directory-p
                  (concat (file-name-as-directory file) ".")
                file))
@end group
@end smallexample
@end defun

@defun process-file program &optional infile buffer display &rest args
This function processes files synchronously in a separate process.  It
is similar to @code{call-process}, but may invoke a file name handler
based on the value of the variable @code{default-directory}, which
specifies the current working directory of the subprocess.

The arguments are handled in almost the same way as for
@code{call-process}, with the following differences:

Some file name handlers may not support all combinations and forms of the
arguments @var{infile}, @var{buffer}, and @var{display}.  For example,
some file name handlers might behave as if @var{display} were @code{nil},
regardless of the value actually passed.  As another example, some
file name handlers might not support separating standard output and error
output by way of the @var{buffer} argument.

If a file name handler is invoked, it determines the program to run based
on the first argument @var{program}.  For instance, suppose that a
handler for remote files is invoked.  Then the path that is used for
searching for the program might be different from @code{exec-path}.

The second argument @var{infile} may invoke a file name handler.  The file
name handler could be different from the handler chosen for the
@code{process-file} function itself.  (For example,
@code{default-directory} could be on one remote host, and
@var{infile} on a different remote host.  Or @code{default-directory}
could be non-special, whereas @var{infile} is on a remote host.)

If @var{buffer} is a list of the form @code{(@var{real-destination}
@var{error-destination})}, and @var{error-destination} names a file,
then the same remarks as for @var{infile} apply.

The remaining arguments (@var{args}) will be passed to the process
verbatim.  Emacs is not involved in processing file names that are
present in @var{args}.  To avoid confusion, it may be best to avoid
absolute file names in @var{args}, but rather to specify all file
names as relative to @code{default-directory}.  The function
@code{file-relative-name} is useful for constructing such relative
file names.  Alternatively, you can use @code{file-local-name}
(@pxref{Magic File Names}) to obtain an absolute file name as seen
from the remote host's perspective.
@end defun

@defvar process-file-side-effects
This variable indicates whether a call of @code{process-file} changes
remote files.

By default, this variable is always set to @code{t}, meaning that a
call of @code{process-file} could potentially change any file on a
remote host.  When set to @code{nil}, a file name handler could optimize
its behavior with respect to remote file attribute caching.

You should only ever change this variable with a let-binding; never
with @code{setq}.
@end defvar

@defopt process-file-return-signal-string
This user option indicates whether a call of @code{process-file}
returns a string describing the signal interrupting a remote process.

When a process returns an exit code greater than 128, it is
interpreted as a signal.  @code{process-file} requires to return a
string describing this signal.

Since there are processes violating this rule, returning exit codes
greater than 128 which are not bound to a signal, @code{process-file}
returns always the exit code as natural number for remote processes.
Setting this user option to non-@code{nil} forces @code{process-file} to
interpret such exit codes as signals, and to return a corresponding
string.
@end defopt

@defun call-process-region start end program &optional delete destination display &rest args
This function sends the text from @var{start} to @var{end} as
standard input to a process running @var{program}.  It deletes the text
sent if @var{delete} is non-@code{nil}; this is useful when
@var{destination} is @code{t}, to insert the output in the current
buffer in place of the input.

The arguments @var{destination} and @var{display} control what to do
with the output from the subprocess, and whether to update the display
as it comes in.  For details, see the description of
@code{call-process}, above.  If @var{destination} is the integer 0,
@code{call-process-region} discards the output and returns @code{nil}
immediately, without waiting for the subprocess to finish (this only
works if asynchronous subprocesses are supported; i.e., not on MS-DOS).

The remaining arguments, @var{args}, are strings that specify command
line arguments for the program.

The return value of @code{call-process-region} is just like that of
@code{call-process}: @code{nil} if you told it to return without
waiting; otherwise, a number or string which indicates how the
subprocess terminated.

In the following example, we use @code{call-process-region} to run the
@code{cat} utility, with standard input being the first five characters
in buffer @samp{foo} (the word @samp{input}).  @code{cat} copies its
standard input into its standard output.  Since the argument
@var{destination} is @code{t}, this output is inserted in the current
buffer.

@smallexample
@group
---------- Buffer: foo ----------
input@point{}
---------- Buffer: foo ----------
@end group

@group
(call-process-region 1 6 "cat" nil t)
     @result{} 0

---------- Buffer: foo ----------
inputinput@point{}
---------- Buffer: foo ----------
@end group
@end smallexample

  For example, the @code{shell-command-on-region} command uses
@code{call-shell-region} in a manner similar to this:

@smallexample
@group
(call-shell-region
 start end
 command              ; @r{shell command}
 nil                  ; @r{do not delete region}
 buffer)              ; @r{send output to @code{buffer}}
@end group
@end smallexample
@end defun

@defun call-process-shell-command command &optional infile destination display
This function executes the shell command @var{command} synchronously.
The other arguments are handled as in @code{call-process}.  An old
calling convention allowed passing any number of additional arguments
after @var{display}, which were concatenated to @var{command}; this is
still supported, but strongly discouraged.
@end defun

@defun process-file-shell-command command &optional infile destination display
This function is like @code{call-process-shell-command}, but uses
@code{process-file} internally.  Depending on @code{default-directory},
@var{command} can be executed also on remote hosts.  An old calling
convention allowed passing any number of additional arguments after
@var{display}, which were concatenated to @var{command}; this is still
supported, but strongly discouraged.
@end defun

@defun call-shell-region start end command &optional delete destination
This function sends the text from @var{start} to @var{end} as
standard input to an inferior shell running @var{command}.  This function
is similar than @code{call-process-region}, with process being a shell.
The arguments @code{delete}, @code{destination} and the return value
are like in @code{call-process-region}.
Note that this function doesn't accept additional arguments.
@end defun

@defun shell-command-to-string command
This function executes @var{command} (a string) as a shell command,
then returns the command's output as a string.
@end defun

@c There is also shell-command-on-region, but that is more of a user
@c command, not something to use in programs.

@defun process-lines program &rest args
This function runs @var{program}, waits for it to finish, and returns
its output as a list of strings.  Each string in the list holds a
single line of text output by the program; the end-of-line characters
are stripped from each line.  The arguments beyond @var{program},
@var{args}, are strings that specify command-line arguments with which
to run the program.

If @var{program} exits with a non-zero exit status, this function
signals an error.

This function works by calling @code{call-process}, so program output
is decoded in the same way as for @code{call-process}.
@end defun

@defun process-lines-ignore-status program &rest args
This function is just like @code{process-lines}, but does not signal
an error if @var{program} exits with a non-zero exit status.
@end defun

@node Asynchronous Processes
@section Creating an Asynchronous Process
@cindex asynchronous subprocess

  In this section, we describe how to create an @dfn{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 (@pxref{Input to
Processes}, and @pxref{Output from Processes}).  Note that process
communication is only partially asynchronous: Emacs sends and receives
data to and from a process only when those functions are called.

@cindex pty, when to use for subprocess communications
@cindex pipe, when to use for subprocess communications
  An asynchronous process is controlled either via a @dfn{pty}
(pseudo-terminal) or a @dfn{pipe}.  The choice of pty or pipe is made
when creating the process, by default based on the value of the
variable @code{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 (@kbd{C-c}, @kbd{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 (i.e., no user interaction with the subprocess is
required), where significant amounts of data need to be exchanged
between the subprocess and the Lisp program, it is often better to use
a pipe, because pipes are more efficient.  Also, the total number of
ptys is limited on many systems, and it is good not to waste them
unnecessarily.

@defun 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 @code{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 @var{args} are a list of keyword/argument pairs.
Omitting a keyword is always equivalent to specifying it with value
@code{nil}.  Here are the meaningful keywords:

@table @asis
@item :name @var{name}
Use the string @var{name} as the process name; if a process with this
name already exists, then @var{name} is modified (by appending
@samp{<1>}, etc.)@: to be unique.

@item :buffer @var{buffer}
Use @var{buffer} as the process buffer.  If the value is @code{nil},
the subprocess is not associated with any buffer.

@item :command @var{command}
Use @var{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 @code{nil}, Emacs opens a new
pseudoterminal (pty) and associates its input and output with
@var{buffer}, without actually running any program; the rest of the
list elements are ignored in that case.

@item :coding @var{coding}
If @var{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 @var{coding} is a cons cell
@w{@code{(@var{decoding} . @var{encoding})}}, then @var{decoding}
will be used for reading and @var{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; @pxref{Encoding and
I/O, file-name-coding-system}).

If @var{coding} is @code{nil}, the default rules for finding the
coding system will apply.  @xref{Default Coding Systems}.

@item :connection-type @var{type}
Initialize the type of device used to communicate with the subprocess.
Possible values are @code{pty} to use a pty, @code{pipe} to use a
pipe, or @code{nil} to use the default derived from the value of the
@code{process-connection-type} variable.  If @var{type} is a cons cell
@w{@code{(@var{input} . @var{output})}}, then @var{input} will be used
for standard input and @var{output} for standard output (and standard
error if @code{:stderr} is @code{nil}).

On systems where ptys are not available (MS-Windows), this parameter
is ignored, and pipes are used unconditionally.

@item :noquery @var{query-flag}
Initialize the process query flag to @var{query-flag}.
@xref{Query Before Exit}.

@item :stop @var{stopped}
If provided, @var{stopped} must be @code{nil}; it is an error to use
any non-@code{nil} value.  The @code{:stop} key is ignored otherwise
and is retained for compatibility with other process types such as
pipe processes.  Asynchronous subprocesses never start in the stopped
state.

@item :filter @var{filter}
Initialize the process filter to @var{filter}.  If not specified, a
default filter will be provided, which can be overridden later.
@xref{Filter Functions}.

@item :sentinel @var{sentinel}
Initialize the process sentinel to @var{sentinel}.  If not specified,
a default sentinel will be used, which can be overridden later.
@xref{Sentinels}.

@item :stderr @var{stderr}
Associate @var{stderr} with the standard error of the process.  A
non-@code{nil} value should be either a buffer or a pipe process
created with @code{make-pipe-process}, described below.  If
@var{stderr} is @code{nil}, standard error is mixed with standard
output, and both are sent to @var{buffer} or @var{filter}.

@cindex standard error process
If @var{stderr} is a buffer, Emacs will create a pipe process, the
@dfn{standard error process}.  This process will have the default
filter (@pxref{Filter Functions}), sentinel (@pxref{Sentinels}), and
coding systems (@pxref{Default Coding Systems}).  On the other hand,
it will use @var{query-flag} as its query-on-exit flag (@pxref{Query
Before Exit}).  It will be associated with the @var{stderr} buffer
(@pxref{Process Buffers}) and send its output (which is the standard
error of the main process) there.  To get the process object for the
standard error process, pass the @var{stderr} buffer to
@code{get-buffer-process}.

If @var{stderr} is a pipe process, Emacs will use it as standard error
process for the new process.

@item :file-handler @var{file-handler}
If @var{file-handler} is non-@code{nil}, then look for a file name
handler for the current buffer's @code{default-directory}, and invoke
that file name handler to make the process.  If there is no such
handler, proceed as if @var{file-handler} were @code{nil}.
@end table

The original argument list, modified with the actual connection
information, is available via the @code{process-contact} function.

The current working directory of the subprocess is set to the current
buffer's value of @code{default-directory} if that is local (as
determined by @code{unhandled-file-name-directory}), or @file{~}
otherwise.  If you want to run a process in a remote directory, pass
@code{:file-handler t} to @code{make-process}.  In that case, the
current working directory is the local name component of
@code{default-directory} (as determined by @code{file-local-name}).

Depending on the implementation of the file name handler, it might not
be possible to apply @var{filter} or @var{sentinel} to the resulting
process object.  The @code{:stderr} argument cannot be a pipe process,
file name handlers do not support pipe processes for this.  A buffer
as @code{:stderr} argument is accepted, its contents is shown without
the use of pipe processes.  @xref{Filter Functions}, @ref{Sentinels},
and @ref{Accepting Output}.

Some file name handlers may not support @code{make-process}.  In such
cases, this function does nothing and returns @code{nil}.
@end defun

@anchor{Pipe Processes}
@defun make-pipe-process &rest args
This function creates a bidirectional pipe which can be attached to a
child process.  This is useful with the @code{:stderr} keyword of
@code{make-process}.  The function returns a process object.

The arguments @var{args} are a list of keyword/argument pairs.
Omitting a keyword is always equivalent to specifying it with value
@code{nil}.

Here are the meaningful keywords:

@table @asis
@item :name @var{name}
Use the string @var{name} as the process name.  As with
@code{make-process}, it is modified if necessary to make it unique.

@item :buffer @var{buffer}
Use @var{buffer} as the process buffer.

@item :coding @var{coding}
If @var{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 @var{coding} is a cons cell
@w{@code{(@var{decoding} . @var{encoding})}}, then @var{decoding}
will be used for reading and @var{encoding} for writing.

If @var{coding} is @code{nil}, the default rules for finding the
coding system will apply.  @xref{Default Coding Systems}.

@item :noquery @var{query-flag}
Initialize the process query flag to @var{query-flag}.
@xref{Query Before Exit}.

@item :stop @var{stopped}
If @var{stopped} is non-@code{nil}, start the process in the stopped
state.  In the stopped state, a pipe process does not accept incoming
data, but you can send outgoing data.  The stopped state is set by
@code{stop-process} and cleared by @code{continue-process}
(@pxref{Signals to Processes}).

@item :filter @var{filter}
Initialize the process filter to @var{filter}.  If not specified, a
default filter will be provided, which can be changed later.
@xref{Filter Functions}.

@item :sentinel @var{sentinel}
Initialize the process sentinel to @var{sentinel}.  If not specified,
a default sentinel will be used, which can be changed later.
@xref{Sentinels}.
@end table

The original argument list, modified with the actual connection
information, is available via the @code{process-contact} function.
@end defun

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

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

The rest of @var{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 @samp{foo} is created
immediately).  Meanwhile, the second process is started, and
given the name @samp{my-process<1>} for the sake of uniqueness.  It
inserts the directory listing at the end of the buffer @samp{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.

@smallexample
@group
(start-process "my-process" "foo" "sleep" "100")
     @result{} #<process my-process>
@end group

@group
(start-process "my-process" "foo" "ls" "-l" "/bin")
     @result{} #<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
@dots{}
-rwxr-xr-x 1 root root 696880 Feb 28 15:55 zsh4

Process my-process<1> finished

Process my-process finished
---------- Buffer: foo ----------
@end group
@end smallexample
@end defun

@defun start-file-process name buffer-or-name program &rest args
Like @code{start-process}, this function starts a new asynchronous
subprocess running @var{program} in it, and returns its process
object.

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

This function does not try to invoke file name handlers for
@var{program} or for the rest of @var{args}.  For that reason, if
@var{program} or any of @var{args} use the remote-file syntax
(@pxref{Magic File Names}), they must be converted either to file
names relative to @code{default-directory}, or to names that identify
the files locally on the remote host, by running them through
@code{file-local-name}.

Depending on the implementation of the file name handler, it might not be
possible to apply @code{process-filter} or @code{process-sentinel} to
the resulting process object.  @xref{Filter Functions}, and @ref{Sentinels}.

@c FIXME  Can we find a better example (i.e., a more modern function
@c that is actually documented).
Some file name handlers may not support @code{start-file-process} (for
example the function @code{ange-ftp-hook-function}).  In such cases,
this function does nothing and returns @code{nil}.
@end defun

@defun start-process-shell-command name buffer-or-name command
This function is like @code{start-process}, except that it uses a
shell to execute the specified @var{command}.  The argument
@var{command} is a shell command string.  The variable
@code{shell-file-name} specifies which shell to use.

The point of running a program through the shell, rather than directly
with @code{make-process} or @code{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 @code{shell-quote-argument} first,
so that any special shell characters do @emph{not} have their special
shell meanings.  @xref{Shell Arguments}.  Of course, when executing
commands based on user input you should also consider the security
implications.
@end defun

@defun start-file-process-shell-command name buffer-or-name command
This function is like @code{start-process-shell-command}, but uses
@code{start-file-process} internally.  Because of this, @var{command}
can also be executed on remote hosts, depending on @code{default-directory}.
@end defun

@defvar process-connection-type
This variable controls the type of device used to communicate with
asynchronous subprocesses.  If it is non-@code{nil}, then ptys are
used, when available.  Otherwise, pipes are used.

The value of @code{process-connection-type} takes effect when
@code{make-process} or @code{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
@code{make-process} is called with a non-@code{nil} value of the
@code{:stderr} parameter; in that case, Emacs will communicate with
the process using pipes.  It is also ignored if ptys are unavailable
(MS-Windows).

@smallexample
@group
(let ((process-connection-type nil))  ; @r{use a pipe}
  (start-process @dots{}))
@end group
@end smallexample

To determine whether a given subprocess actually got a pipe or a pty,
use the function @code{process-tty-name} (@pxref{Process
Information}).
@end defvar

@defvar process-error-pause-time
If a process sentinel/filter function has an error, Emacs will (by
default) pause Emacs for @code{process-error-pause-time} seconds after
displaying this error, so that users will see the error in question.
However, this can lead to situations where Emacs becomes unresponsive
(if there's a lot of these errors happening), so this can be disabled
by setting @code{process-error-pause-time} to 0.
@end defvar

@node Deleting Processes
@section Deleting Processes
@cindex deleting processes

  @dfn{Deleting a process} disconnects Emacs immediately from the
subprocess.  Processes are deleted automatically after they terminate,
but not necessarily right away.  You can delete a process explicitly
at any time.  If you explicitly delete a terminated process before it
is deleted automatically, no harm results.  Deleting a running
process sends a signal to terminate it (and its child processes, if
any), and calls the process sentinel.  @xref{Sentinels}.

  When a process is deleted, the process object itself continues to
exist as long as other Lisp objects point to it.  All the Lisp
primitives that work on process objects accept deleted processes, but
those that do I/O or send signals will report an error.  The process
mark continues to point to the same place as before, usually into a
buffer where output from the process was being inserted.

@defopt delete-exited-processes
This variable controls automatic deletion of processes that have
terminated (due to calling @code{exit} or to a signal).  If it is
@code{nil}, then they continue to exist until the user runs
@code{list-processes}.  Otherwise, they are deleted immediately after
they exit.
@end defopt

@defun delete-process &optional process
This function deletes a process, killing it with a @code{SIGKILL}
signal if the process was running a program.  The argument may be a
process, the name of a process, a buffer, or the name of a buffer.  (A
buffer or buffer-name stands for the process that
@code{get-buffer-process} returns, and a missing or @code{nil}
@var{process} means that the current buffer's process should be
killed.)  Calling @code{delete-process} on a running process
terminates it, updates the process status, and runs the sentinel
immediately.  If the process has already terminated, calling
@code{delete-process} has no effect on its status, or on the running
of its sentinel (which will happen sooner or later).

If the process object represents a network, serial, or pipe
connection, its status changes to @code{closed}; otherwise, it changes
to @code{signal}, unless the process already exited.  @xref{Process
Information, process-status}.

@smallexample
@group
(delete-process "*shell*")
     @result{} nil
@end group
@end smallexample
@end defun

@node Process Information
@section Process Information
@cindex process information

  Several functions return information about processes.

@deffn Command list-processes &optional query-only buffer
This command displays a listing of all living processes.  In addition,
it finally deletes any process whose status was @samp{Exited} or
@samp{Signaled}.  It returns @code{nil}.

The processes are shown in a buffer named @file{*Process List*}
(unless you specify otherwise using the optional argument @var{buffer}),
whose major mode is Process Menu mode.

If @var{query-only} is non-@code{nil}, it only lists processes
whose query flag is non-@code{nil}.  @xref{Query Before Exit}.
@end deffn

@defun process-list
This function returns a list of all processes that have not been deleted.

@smallexample
@group
(process-list)
     @result{} (#<process display-time> #<process shell>)
@end group
@end smallexample
@end defun

@defun num-processors &optional query
This function returns the number of processors, a positive integer.
Each usable thread execution unit counts as a processor.
By default, the count includes the number of available processors,
which you can override by setting the
@url{https://www.openmp.org/spec-html/5.1/openmpse59.html,
@env{OMP_NUM_THREADS} environment variable of OpenMP}.
If the optional argument @var{query} is @code{current},
this function ignores @env{OMP_NUM_THREADS};
if @var{query} is @code{all}, this function also counts processors
that are on the system but are not available to the current process.
@end defun

@defun get-process name
This function returns the process named @var{name} (a string), or
@code{nil} if there is none.  The argument @var{name} can also be a
process object, in which case it is returned.

@smallexample
@group
(get-process "shell")
     @result{} #<process shell>
@end group
@end smallexample
@end defun

@defun process-command process
This function returns the command that was executed to start
@var{process}.  This is a list of strings, the first string being the
program executed and the rest of the strings being the arguments that
were given to the program.  For a network, serial, or pipe connection,
this is either @code{nil}, which means the process is running or
@code{t} (process is stopped).

@smallexample
@group
(process-command (get-process "shell"))
     @result{} ("bash" "-i")
@end group
@end smallexample
@end defun

@defun process-contact process &optional key no-block
This function returns information about how a network, a serial, or a
pipe connection was set up.  When @var{key} is @code{nil}, it returns
@code{(@var{hostname} @var{service})} for a network connection,
@code{(@var{port} @var{speed})} for a serial connection, and @code{t}
for a pipe connection.  For an ordinary child process, this function
always returns @code{t} when called with a @code{nil} @var{key}.

If @var{key} is @code{t}, the value is the complete status information
for the connection, server, serial port, or pipe; that is, the list of
keywords and values specified in @code{make-network-process},
@code{make-serial-process}, or @code{make-pipe-process}, except that
some of the values represent the current status instead of what you
specified.

For a network process, the values include (see
@code{make-network-process} for a complete list):

@table @code
@item :buffer
The associated value is the process buffer.
@item :filter
The associated value is the process filter function.  @xref{Filter
Functions}.
@item :sentinel
The associated value is the process sentinel function.  @xref{Sentinels}.
@item :remote
In a connection, the address in internal format of the remote peer.
@item :local
The local address, in internal format.
@item :service
In a server, if you specified @code{t} for @var{service},
this value is the actual port number.
@end table

@code{:local} and @code{:remote} are included even if they were not
specified explicitly in @code{make-network-process}.

For a serial connection, see @code{make-serial-process} and
@code{serial-process-configure} for the list of keys.  For a pipe
connection, see @code{make-pipe-process} for the list of keys.

If @var{key} is a keyword, the function returns the value corresponding
to that keyword.

If @var{process} is a non-blocking network stream that hasn't been
fully set up yet, then this function will block until that has
happened.  If given the optional @var{no-block} parameter, this
function will return @code{nil} instead of blocking.
@end defun

@defun process-id process
This function returns the @acronym{PID} of @var{process}.  This is an
integral number that distinguishes the process @var{process} from all
other processes running on the same computer at the current time.  The
@acronym{PID} of a process is chosen by the operating system kernel
when the process is started and remains constant as long as the
process exists.  For network, serial, and pipe connections, this
function returns @code{nil}.
@end defun

@defun process-name process
This function returns the name of @var{process}, as a string.
@end defun

@defun process-status process-name
This function returns the status of @var{process-name} as a symbol.
The argument @var{process-name} must be a process, a buffer, or a
process name (a string).

The possible values for an actual subprocess are:

@table @code
@item run
for a process that is running.
@item stop
for a process that is stopped but continuable.
@item exit
for a process that has exited.
@item signal
for a process that has received a fatal signal.
@item open
for a network, serial, or pipe connection that is open.
@item closed
for a network, serial, or pipe connection that is closed.  Once a
connection is closed, you cannot reopen it, though you might be able
to open a new connection to the same place.
@item connect
for a non-blocking connection that is waiting to complete.
@item failed
for a non-blocking connection that has failed to complete.
@item listen
for a network server that is listening.
@item nil
if @var{process-name} is not the name of an existing process.
@end table

@smallexample
@group
(process-status (get-buffer "*shell*"))
     @result{} run
@end group
@end smallexample

For a network, serial, or pipe connection, @code{process-status}
returns one of the symbols @code{open}, @code{stop}, or @code{closed}.
The latter means that the other side closed the connection, or Emacs
did @code{delete-process}.  The value @code{stop} means that
@code{stop-process} was called on the connection.
@end defun

@defun process-live-p process
This function returns non-@code{nil} if @var{process} is alive.  A
process is considered alive if its status is @code{run}, @code{open},
@code{listen}, @code{connect} or @code{stop}.
@end defun

@defun process-type process
This function returns the symbol @code{network} for a network
connection or server, @code{serial} for a serial port connection,
@code{pipe} for a pipe connection, or @code{real} for a subprocess
created for running a program.
@end defun

@defun process-exit-status process
This function returns the exit status of @var{process} or the signal
number that killed it.  (Use the result of @code{process-status} to
determine which of those it is.)  If @var{process} has not yet
terminated, the value is 0.  For network, serial, and pipe connections
that are already closed, the value is either 0 or 256, depending on
whether the connection was closed normally or abnormally.
@end defun

@defun process-tty-name process &optional stream
This function returns the terminal name that @var{process} is using for
its communication with Emacs---or @code{nil} if it is using pipes
instead of a pty (see @code{process-connection-type} in
@ref{Asynchronous Processes}).  By default, this function returns the
terminal name if any of @var{process}'s standard streams use a
terminal.  If @var{stream} is one of @code{stdin}, @code{stdout}, or
@code{stderr}, this function returns the terminal name (or @code{nil},
as above) that @var{process} uses for that stream specifically.  You
can use this to determine whether a particular stream uses a pipe or a
pty.

If @var{process} represents a program running on a remote host, this
function returns the @emph{local} terminal name that communicates with
@var{process}; you can get the terminal name used by that program on
the remote host with the process property @code{remote-tty}.  If
@var{process} represents a network, serial, or pipe connection, this
function always returns @code{nil}.
@end defun

@defun process-coding-system process
@anchor{Coding systems for a subprocess}
This function returns a cons cell @code{(@var{decode} . @var{encode})},
describing the coding systems in use for decoding output from, and
encoding input to, @var{process} (@pxref{Coding Systems}).
@end defun

@defun set-process-coding-system process &optional decoding-system encoding-system
This function specifies the coding systems to use for subsequent output
from and input to @var{process}.  It will use @var{decoding-system} to
decode subprocess output, and @var{encoding-system} to encode subprocess
input.
@end defun

  Every process also has a property list that you can use to store
miscellaneous values associated with the process.

@defun process-get process propname
This function returns the value of the @var{propname} property
of @var{process}.
@end defun

@defun process-put process propname value
This function sets the value of the @var{propname} property
of @var{process} to @var{value}.
@end defun

@defun process-plist process
This function returns the process plist of @var{process}.
@end defun

@defun set-process-plist process plist
This function sets the process plist of @var{process} to @var{plist}.
@end defun

@node Input to Processes
@section Sending Input to Processes
@cindex process input

  Asynchronous subprocesses receive input when it is sent to them by
Emacs, which is done with the functions in this section.  You must
specify the process to send input to, and the input data to send.  If
the subprocess runs a program, the data appears on the standard input
of that program; for connections, the data is sent to the connected
device or program.

@c FIXME which?
  Some operating systems have limited space for buffered input in a
pty.  On these systems, Emacs sends an @acronym{EOF} periodically
amidst the other characters, to force them through.  For most
programs, these @acronym{EOF}s do no harm.

  Subprocess input is normally encoded using a coding system before the
subprocess receives it, much like text written into a file.  You can use
@code{set-process-coding-system} to specify which coding system to use
(@pxref{Process Information}).  Otherwise, the coding system comes from
@code{coding-system-for-write}, if that is non-@code{nil}; or else from
the defaulting mechanism (@pxref{Default Coding Systems}).

  Sometimes the system is unable to accept input for that process,
because the input buffer is full.  When this happens, the send functions
wait a short while, accepting output from subprocesses, and then try
again.  This gives the subprocess a chance to read more of its pending
input and make space in the buffer.  It also allows filters (including
the one currently running), sentinels and timers to run---so take
account of that in writing your code.

  In these functions, the @var{process} argument can be a process or
the name of a process, or a buffer or buffer name (which stands
for a process via @code{get-buffer-process}).  @code{nil} means
the current buffer's process.

@defun process-send-string process string
This function sends @var{process} the contents of @var{string} as
standard input.  It returns @code{nil}.  For example, to make a
Shell buffer list files:

@smallexample
@group
(process-send-string "shell<1>" "ls\n")
     @result{} nil
@end group
@end smallexample
@end defun

@defun process-send-region process start end
This function sends the text in the region defined by @var{start} and
@var{end} as standard input to @var{process}.

An error is signaled unless both @var{start} and @var{end} are
integers or markers that indicate positions in the current buffer.  (It
is unimportant which number is larger.)
@end defun

@defun process-send-eof &optional process
This function makes @var{process} see an end-of-file in its
input.  The @acronym{EOF} comes after any text already sent to it.
The function returns @var{process}.

@smallexample
@group
(process-send-eof "shell")
     @result{} "shell"
@end group
@end smallexample
@end defun

@defun process-running-child-p &optional process
This function will tell you whether a @var{process}, which must not be
a connection but a real subprocess, has given control of its terminal
to a child process of its own.  If this is true, the function returns
the numeric ID of the foreground process group of @var{process}; it
returns @code{nil} if Emacs can be certain that this is not so.  The
value is @code{t} if Emacs cannot tell whether this is true.  This
function signals an error if @var{process} is a network, serial, or
pipe connection, or if the subprocess is not active.
@end defun

@node Signals to Processes
@section Sending Signals to Processes
@cindex process signals
@cindex sending signals
@cindex signals

  @dfn{Sending a signal} to a subprocess is a way of interrupting its
activities.  There are several different signals, each with its own
meaning.  The set of signals and their names is defined by the operating
system.  For example, the signal @code{SIGINT} means that the user has
typed @kbd{C-c}, or that some analogous thing has happened.

  Each signal has a standard effect on the subprocess.  Most signals
kill the subprocess, but some stop (or resume) execution instead.  Most
signals can optionally be handled by programs; if the program handles
the signal, then we can say nothing in general about its effects.

  You can send signals explicitly by calling the functions in this
section.  Emacs also sends signals automatically at certain times:
killing a buffer sends a @code{SIGHUP} signal to all its associated
processes; killing Emacs sends a @code{SIGHUP} signal to all remaining
processes.  (@code{SIGHUP} is a signal that usually indicates that the
user ``hung up the phone'', i.e., disconnected.)

  Each of the signal-sending functions takes two optional arguments:
@var{process} and @var{current-group}.

  The argument @var{process} must be either a process, a process
name, a buffer, a buffer name, or @code{nil}.  A buffer or buffer name
stands for a process through @code{get-buffer-process}.  @code{nil}
stands for the process associated with the current buffer.  Except
with @code{stop-process} and @code{continue-process}, an error is
signaled if @var{process} does not identify an active process, or if
it represents a network, serial, or pipe connection.

  The argument @var{current-group} is a flag that makes a difference
when you are running a job-control shell as an Emacs subprocess.  If it
is non-@code{nil}, then the signal is sent to the current process-group
of the terminal that Emacs uses to communicate with the subprocess.  If
the process is a job-control shell, this means the shell's current
subjob.  If @var{current-group} is @code{nil}, the signal is
sent to the process group of the immediate subprocess of Emacs.  If
the subprocess is a job-control shell, this is the shell itself.  If
@var{current-group} is @code{lambda}, the signal is sent to the
process-group that owns the terminal, but only if it is not the shell
itself.

  The flag @var{current-group} has no effect when a pipe is used to
communicate with the subprocess, because the operating system does not
support the distinction in the case of pipes.  For the same reason,
job-control shells won't work when a pipe is used.  See
@code{process-connection-type} in @ref{Asynchronous Processes}.

@defun interrupt-process &optional process current-group
This function interrupts the process @var{process} by sending the
signal @code{SIGINT}.  Outside of Emacs, typing the interrupt
character (normally @kbd{C-c} on some systems, and @key{DEL} on
others) sends this signal.  When the argument @var{current-group} is
non-@code{nil}, you can think of this function as typing @kbd{C-c}
on the terminal by which Emacs talks to the subprocess.
@end defun

@deffn Command kill-process &optional process current-group
This command kills the process @var{process} by sending the
signal @code{SIGKILL}.  This signal kills the subprocess immediately,
and cannot be handled by the subprocess.  Interactively, it'll prompt
the user for a process name, defaulting to the process (if any) in the
current buffer.
@end deffn

@defun quit-process &optional process current-group
This function sends the signal @code{SIGQUIT} to the process
@var{process}.  This signal is the one sent by the quit
character (usually @kbd{C-\}) when you are not inside
Emacs.
@end defun

@defun stop-process &optional process current-group
This function stops the specified @var{process}.  If it is a real
subprocess running a program, it sends the signal @code{SIGTSTP} to
that subprocess.  If @var{process} represents a network, serial, or
pipe connection, this function inhibits handling of the incoming data
from the connection; for a network server, this means not accepting
new connections.  Use @code{continue-process} to resume normal
execution.

Outside of Emacs, on systems with job control, the stop character
(usually @kbd{C-z}) normally sends the @code{SIGTSTP} signal to a
subprocess.  When @var{current-group} is non-@code{nil}, you can think
of this function as typing @kbd{C-z} on the terminal Emacs uses to
communicate with the subprocess.
@end defun

@defun continue-process &optional process current-group
This function resumes execution of the process @var{process}.  If it
is a real subprocess running a program, it sends the signal
@code{SIGCONT} to that subprocess; this presumes that @var{process}
was stopped previously.  If @var{process} represents a network,
serial, or pipe connection, this function resumes handling of the
incoming data from the connection.  For serial connections, data that
arrived during the time the process was stopped might be lost.
@end defun

@deffn Command signal-process process signal &optional remote
This function sends a signal to process @var{process}.  The argument
@var{signal} specifies which signal to send; it should be an integer,
or a symbol whose name is a signal.

The @var{process} argument can be a system process @acronym{ID} (an
integer); that allows you to send signals to processes that are not
children of Emacs.  @xref{System Processes}.

If @var{process} is a process object which contains the property
@code{remote-pid}, or @var{process} is a number and @var{remote} is a
remote file name, @var{process} is interpreted as process on the
respective remote host, which will be the process to signal.
@end deffn

Sometimes, it is necessary to send a signal to a non-local
asynchronous process.  This is possible by writing an own
@code{interrupt-process} or @code{signal-process} implementation.
This function must be added then to @code{interrupt-process-functions}
or @code{signal-process-functions}, respectively.

@defvar interrupt-process-functions
This variable is a list of functions to be called for
@code{interrupt-process}.  The arguments of the functions are the same
as for @code{interrupt-process}.  These functions are called in the
order of the list, until one of them returns non-@code{nil}.  The
default function, which shall always be the last in this list, is
@code{internal-default-interrupt-process}.

This is the mechanism, how Tramp implements @code{interrupt-process}.
@end defvar

@defvar signal-process-functions
This variable is a list of functions to be called for
@code{signal-process}.  The arguments of the functions are the same as
for @code{signal-process}.  These functions are called in the order of
the list, until one of them returns non-@code{nil}.  The default
function, which shall always be the last in this list, is
@code{internal-default-signal-process}.

This is the mechanism, how Tramp implements @code{signal-process}.
@end defvar

@node Output from Processes
@section Receiving Output from Processes
@cindex process output
@cindex output from processes

  The output that an asynchronous subprocess writes to its standard
output stream is passed to a function called the @dfn{filter
function}.  The default filter function simply inserts the output into
a buffer, which is called the associated buffer of the process
(@pxref{Process Buffers}).  If the process has no buffer then the
default filter discards the output.

  If the subprocess writes to its standard error stream, by default
the error output is also passed to the process filter function.
Alternatively, you could use the @code{:stderr} parameter with a
non-@code{nil} value in a call to @code{make-process}
(@pxref{Asynchronous Processes, make-process}) to make the destination
of the error output separate from the standard output.

  When a subprocess terminates, Emacs reads any pending output,
then stops reading output from that subprocess.  Therefore, if the
subprocess has children that are still live and still producing
output, Emacs won't receive that output.

  Output from a subprocess can arrive only while Emacs is waiting: when
reading terminal input (see the function @code{waiting-for-user-input-p}),
in @code{sit-for} and @code{sleep-for} (@pxref{Waiting}), in
@code{accept-process-output} (@pxref{Accepting Output}), and in
functions which send data to processes (@pxref{Input to Processes}).
This minimizes the problem of timing errors that usually plague parallel
programming.  For example, you can safely create a process and only
then specify its buffer or filter function; no output can arrive
before you finish, if the code in between does not call any primitive
that waits.

@defvar process-adaptive-read-buffering
On some systems, when Emacs reads the output from a subprocess, the
output data is read in very small blocks, potentially resulting in
very poor performance.  This behavior can be remedied to some extent
by setting the variable @code{process-adaptive-read-buffering} to a
non-@code{nil} value (the default), as it will automatically delay reading
from such processes, thus allowing them to produce more output before
Emacs tries to read it.
@end defvar

@menu
* Process Buffers::         By default, output is put in a buffer.
* Filter Functions::        Filter functions accept output from the process.
* Decoding Output::         Filters can get unibyte or multibyte strings.
* Accepting Output::        How to wait until process output arrives.
* Processes and Threads::   How processes and threads interact.
@end menu

@node Process Buffers
@subsection Process Buffers

  A process can (and usually does) have an @dfn{associated buffer},
which is an ordinary Emacs buffer that is used for two purposes: storing
the output from the process, and deciding when to kill the process.  You
can also use the buffer to identify a process to operate on, since in
normal practice only one process is associated with any given buffer.
Many applications of processes also use the buffer for editing input to
be sent to the process, but this is not built into Emacs Lisp.

  By default, process output is inserted in the associated buffer.
(You can change this by defining a custom filter function,
@pxref{Filter Functions}.)  The position to insert the output is
determined by the @code{process-mark}, which is then updated to point
to the end of the text just inserted.  Usually, but not always, the
@code{process-mark} is at the end of the buffer.

@findex process-kill-buffer-query-function
  Killing the associated buffer of a process also kills the process.
Emacs asks for confirmation first, if the process's
@code{process-query-on-exit-flag} is non-@code{nil} (@pxref{Query
Before Exit}).  This confirmation is done by the function
@code{process-kill-buffer-query-function}, which is run from
@code{kill-buffer-query-functions} (@pxref{Killing Buffers}).

@defun process-buffer process
This function returns the associated buffer of the specified
@var{process}.

@smallexample
@group
(process-buffer (get-process "shell"))
     @result{} #<buffer *shell*>
@end group
@end smallexample
@end defun

@defun process-mark process
This function returns the process marker for @var{process}, which is the
marker that says where to insert output from the process.

If @var{process} does not have a buffer, @code{process-mark} returns a
marker that points nowhere.

The default filter function uses this marker to decide where to
insert process output, and updates it to point after the inserted text.
That is why successive batches of output are inserted consecutively.

Custom filter functions normally should use this marker in the same fashion.
For an example of a filter function that uses @code{process-mark},
@pxref{Process Filter Example}.

When the user is expected to enter input in the process buffer for
transmission to the process, the process marker separates the new input
from previous output.
@end defun

@defun set-process-buffer process buffer
This function sets the buffer associated with @var{process} to
@var{buffer}.  If @var{buffer} is @code{nil}, the process becomes
associated with no buffer; if non-@code{nil}, the process mark will be
set to point to the end of @var{buffer}.
@end defun

@defun get-buffer-process buffer-or-name
This function returns a nondeleted process associated with the buffer
specified by @var{buffer-or-name}.  If there are several processes
associated with it, this function chooses one (currently, the one most
recently created, but don't count on that).  Deletion of a process
(see @code{delete-process}) makes it ineligible for this function to
return.

It is usually a bad idea to have more than one process associated with
the same buffer.

@smallexample
@group
(get-buffer-process "*shell*")
     @result{} #<process shell>
@end group
@end smallexample

Killing the process's buffer deletes the process, which kills the
subprocess with a @code{SIGHUP} signal (@pxref{Signals to Processes}).
@end defun

If the process's buffer is displayed in a window, your Lisp program
may wish to tell the process the dimensions of that window, so that
the process could adapt its output to those dimensions, much as it
adapts to the screen dimensions.  The following functions allow
communicating this kind of information to processes; however, not all
systems support the underlying functionality, so it is best to provide
fallbacks, e.g., via command-line arguments or environment variables.

@defun set-process-window-size process height width
Tell @var{process} that its logical window size has dimensions
@var{width} by @var{height}, in character units.  If this function
succeeds in communicating this information to the process, it returns
@code{t}; otherwise it returns @code{nil}.
@end defun

When windows that display buffers associated with process change their
dimensions, the affected processes should be told about these changes.
By default, when the window configuration changes, Emacs will
automatically call @code{set-process-window-size} on behalf of every
process whose buffer is displayed in a window, passing it the smallest
dimensions of all the windows displaying the process's buffer.  This
works via @code{window-configuration-change-hook} (@pxref{Window
Hooks}), which is told to invoke the function that is the value of
the variable @code{window-adjust-process-window-size-function} for
each process whose buffer is displayed in at least one window.  You
can customize this behavior by setting the value of that variable.

@defopt window-adjust-process-window-size-function
The value of this variable should be a function of two arguments: a
process and the list of windows displaying the process's buffer.  When
the function is called, the process's buffer is the current buffer.
The function should return a cons cell @w{@code{(@var{width}
. @var{height})}} that describes the dimensions of the logical process
window to be passed via a call to @code{set-process-window-size}.  The
function can also return @code{nil}, in which case Emacs will not call
@code{set-process-window-size} for this process.

Emacs supplies two predefined values for this variable:
@code{window-adjust-process-window-size-smallest}, which returns the
smallest of all the dimensions of the windows that display a process's
buffer; and @code{window-adjust-process-window-size-largest}, which
returns the largest dimensions.  For more complex strategies, write
your own function.

This variable can be buffer-local.
@end defopt

If the process has the @code{adjust-window-size-function} property
(@pxref{Process Information}), its value overrides the global and
buffer-local values of
@code{window-adjust-process-window-size-function}.

@node Filter Functions
@subsection Process Filter Functions
@cindex filter function
@cindex process filter

@cindex default filter function of a process
  A process @dfn{filter function} is a function that receives the
standard output from the associated process.  @emph{All} output from
that process is passed to the filter.  The default filter simply
outputs directly to the process buffer.

  By default, the error output from the process, if any, is also
passed to the filter function, unless the destination for the standard
error stream of the process was separated from the standard output
when the process was created.  Emacs will only call the filter
function during certain function calls.  @xref{Output from Processes}.
Note that if any of those functions are called by the filter, the
filter may be called recursively.

  A filter function must accept two arguments: the associated process
and a string, which is output just received from it.  The function is
then free to do whatever it chooses with the output.

@c Note this text is duplicated in the sentinels section.
  Quitting is normally inhibited within a filter function---otherwise,
the effect of typing @kbd{C-g} at command level or to quit a user
command would be unpredictable.  If you want to permit quitting inside
a filter function, bind @code{inhibit-quit} to @code{nil}.  In most
cases, the right way to do this is with the macro
@code{with-local-quit}.  @xref{Quitting}.

  If an error happens during execution of a filter function, it is
caught automatically, so that it doesn't stop the execution of whatever
program was running when the filter function was started.  However, if
@code{debug-on-error} is non-@code{nil}, errors are not caught.
This makes it possible to use the Lisp debugger to debug filter
functions.  @xref{Debugger}.

  Many filter functions sometimes (or always) insert the output in the
process's buffer, mimicking the actions of the default filter.
Such filter functions need to make sure that they save the
current buffer, select the correct buffer (if different) before
inserting output, and then restore the original buffer.
They should also check whether the buffer is still alive, update the
process marker, and in some cases update the value of point.  Here is
how to do these things:

@anchor{Process Filter Example}
@smallexample
@group
(defun ordinary-insertion-filter (proc string)
  (when (buffer-live-p (process-buffer proc))
    (with-current-buffer (process-buffer proc)
      (let ((moving (= (point) (process-mark proc))))
@end group
@group
        (save-excursion
          ;; @r{Insert the text, advancing the process marker.}
          (goto-char (process-mark proc))
          (insert string)
          (set-marker (process-mark proc) (point)))
        (if moving (goto-char (process-mark proc)))))))
@end group
@end smallexample

  To make the filter force the process buffer to be visible whenever new
text arrives, you could insert a line like the following just before the
@code{with-current-buffer} construct:

@smallexample
(display-buffer (process-buffer proc))
@end smallexample

  To force point to the end of the new output, no matter where it was
previously, eliminate the variable @code{moving} from the example and
call @code{goto-char} unconditionally.  Note that this doesn't
necessarily move the window point.  The default filter actually uses
@code{insert-before-markers} which moves all markers, including the
window point.  This may move unrelated markers, so it's generally
better to move the window point explicitly, or set its insertion type
to @code{t} (@pxref{Window Point}).

@ignore
  In earlier Emacs versions, every filter function that did regular
expression searching or matching had to explicitly save and restore the
match data.  Now Emacs does this automatically for filter functions;
they never need to do it explicitly.
@end ignore
  Note that Emacs automatically saves and restores the match data
while executing filter functions.  @xref{Match Data}.

  The output to the filter may come in chunks of any size.  A program
that produces the same output twice in a row may send it as one batch of
200 characters one time, and five batches of 40 characters the next.  If
the filter looks for certain text strings in the subprocess output, make
sure to handle the case where one of these strings is split across two
or more batches of output; one way to do this is to insert the
received text into a temporary buffer, which can then be searched.

@defun set-process-filter process filter
This function gives @var{process} the filter function @var{filter}.
If @var{filter} is @code{nil}, it gives the process the default
filter, which inserts the process output into the process buffer.  If
@var{filter} is @code{t}, Emacs stops accepting output from the
process, unless it's a network server process that listens for
incoming connections.
@end defun

@defun process-filter process
This function returns the filter function of @var{process}.
@end defun

In case the process's output needs to be passed to several filters, you can
use @code{add-function} to combine an existing filter with a new one.
@xref{Advising Functions}.

  Here is an example of the use of a filter function:

@smallexample
@group
(defun keep-output (process output)
   (setq kept (cons output kept)))
     @result{} keep-output
@end group
@group
(setq kept nil)
     @result{} nil
@end group
@group
(set-process-filter (get-process "shell") 'keep-output)
     @result{} keep-output
@end group
@group
(process-send-string "shell" "ls ~/other\n")
     @result{} nil
kept
     @result{} ("lewis@@slug:$ "
@end group
@group
"FINAL-W87-SHORT.MSS    backup.otl              kolstad.mss~
address.txt             backup.psf              kolstad.psf
backup.bib~             david.mss               resume-Dec-86.mss~
backup.err              david.psf               resume-Dec.psf
backup.mss              dland                   syllabus.mss
"
"#backups.mss#          backup.mss~             kolstad.mss
")
@end group
@end smallexample

@ignore   @c The code in this example doesn't show the right way to do things.
Here is another, more realistic example, which demonstrates how to use
the process mark to do insertion in the same fashion as the default filter:

@smallexample
@group
;; @r{Insert input in the buffer specified by @code{my-shell-buffer}}
;;   @r{and make sure that buffer is shown in some window.}
(defun my-process-filter (proc str)
  (let ((cur (selected-window))
        (pop-up-windows t))
    (pop-to-buffer my-shell-buffer)
@end group
@group
    (goto-char (point-max))
    (insert str)
    (set-marker (process-mark proc) (point-max))
    (select-window cur)))
@end group
@end smallexample
@end ignore

@node Decoding Output
@subsection Decoding Process Output
@cindex decode process output

  When Emacs writes process output directly into a multibyte buffer,
it decodes the output according to the process output coding system.
If the coding system is @code{raw-text} or @code{no-conversion}, Emacs
converts the unibyte output to multibyte using
@code{string-to-multibyte}, and inserts the resulting multibyte text.

  You can use @code{set-process-coding-system} to specify which coding
system to use (@pxref{Process Information}).  Otherwise, the coding
system comes from @code{coding-system-for-read}, if that is
non-@code{nil}; or else from the defaulting mechanism (@pxref{Default
Coding Systems}).  If the text output by a process contains null
bytes, Emacs by default uses @code{no-conversion} for it; see
@ref{Lisp and Coding Systems, inhibit-null-byte-detection}, for how to
control this behavior.

  @strong{Warning:} Coding systems such as @code{undecided}, which
determine the coding system from the data, do not work entirely
reliably with asynchronous subprocess output.  This is because Emacs
has to process asynchronous subprocess output in batches, as it
arrives.  Emacs must try to detect the proper coding system from one
batch at a time, and this does not always work.  Therefore, if at all
possible, specify a coding system that determines both the character
code conversion and the end of line conversion---that is, one like
@code{latin-1-unix}, rather than @code{undecided} or @code{latin-1}.

@c Let's keep the index entries that were there for
@c set-process-filter-multibyte and process-filter-multibyte-p,
@cindex filter multibyte flag, of process
@cindex process filter multibyte flag
  When Emacs calls a process filter function, it provides the process
output as a multibyte string or as a unibyte string according to the
process's filter coding system.  Emacs
decodes the output according to the process output coding system,
which usually produces a multibyte string, except for coding systems
such as @code{binary} and @code{raw-text}.

@node Accepting Output
@subsection Accepting Output from Processes
@cindex accept input from processes

  Output from asynchronous subprocesses normally arrives only while
Emacs is waiting for some sort of external event, such as elapsed time
or terminal input.  Occasionally it is useful in a Lisp program to
explicitly permit output to arrive at a specific point, or even to wait
until output arrives from a process.

@defun accept-process-output &optional process seconds millisec just-this-one
This function allows Emacs to read pending output from processes.  The
output is given to their filter functions.  If @var{process} is
non-@code{nil} then this function does not return until some output
has been received from @var{process} or @var{process} has closed the
connection.

The arguments @var{seconds} and @var{millisec} let you specify timeout
periods.  The former specifies a period measured in seconds and the
latter specifies one measured in milliseconds.  The two time periods
thus specified are added together, and @code{accept-process-output}
returns after that much time, even if there is no
subprocess output.

The argument @var{millisec} is obsolete (and should not be used),
because @var{seconds} can be floating point to specify
waiting a fractional number of seconds.  If @var{seconds} is 0, the
function accepts whatever output is pending but does not wait.

If @var{process} is a process, and the argument @var{just-this-one} is
non-@code{nil}, only output from that process is handled, suspending output
from other processes until some output has been received from that
process or the timeout expires.  If @var{just-this-one} is an integer,
also inhibit running timers.  This feature is generally not
recommended, but may be necessary for specific applications, such as
speech synthesis.

The function @code{accept-process-output} returns non-@code{nil} if it
got output from @var{process}, or from any process if @var{process} is
@code{nil}; this can occur even after a process has exited if the
corresponding connection contains buffered data.  The function returns
@code{nil} if the timeout expired or the connection was closed before output
arrived.
@end defun

If a connection from a process contains buffered data,
@code{accept-process-output} can return non-@code{nil} even after the
process has exited.  Therefore, although the following loop:

@example
;; This loop contains a bug.
(while (process-live-p process)
  (accept-process-output process))
@end example

@noindent
will often read all output from @var{process}, it has a race condition
and can miss some output if @code{process-live-p} returns @code{nil}
while the connection still contains data.  Better is to write the loop
like this:

@example
(while (accept-process-output process))
@end example

If you have passed a non-@code{nil} @var{stderr} to
@code{make-process}, it will have a standard error process.
@xref{Asynchronous Processes}.  In that case, waiting for process
output from the main process doesn't wait for output from the standard
error process.  To make sure you have received both all of standard
output and all of standard error from a process, use the following
code:

@example
(while (accept-process-output process))
(while (accept-process-output stderr-process))
@end example

If you passed a buffer to the @var{stderr} argument of
@code{make-process}, you still have to wait for the standard error
process, like so:

@example
(let* ((stdout (generate-new-buffer "stdout"))
       (stderr (generate-new-buffer "stderr"))
       (process (make-process :name "test"
                              :command '("my-program")
                              :buffer stdout
                              :stderr stderr))
       (stderr-process (get-buffer-process stderr)))
  (unless (and process stderr-process)
    (error "Process unexpectedly nil"))
  (while (accept-process-output process))
  (while (accept-process-output stderr-process)))
@end example

@noindent
Only when both @code{accept-process-output} forms return @code{nil},
you can be sure that the process has exited and Emacs has read all its
output.

Reading pending standard error from a process running on a remote host
is not possible this way.

@node Processes and Threads
@subsection Processes and Threads
@cindex processes, threads

  Because threads were a relatively late addition to Emacs Lisp, and
due to the way dynamic binding was sometimes used in conjunction with
@code{accept-process-output}, by default a process is locked to the
thread that created it.  When a process is locked to a thread, output
from the process can only be accepted by that thread.

  A Lisp program can specify to which thread a process is to be
locked, or instruct Emacs to unlock a process, in which case its
output can be processed by any thread.  Only a single thread will wait
for output from a given process at one time---once one thread begins
waiting for output, the process is temporarily locked until
@code{accept-process-output} or @code{sit-for} returns.

  If the thread exits, all the processes locked to it are unlocked.

@defun process-thread process
Return the thread to which @var{process} is locked.  If @var{process}
is unlocked, return @code{nil}.
@end defun

@defun set-process-thread process thread
Set the locking thread of @var{process} to @var{thread}.  @var{thread}
may be @code{nil}, in which case the process is unlocked.
@end defun

@node Sentinels
@section Sentinels: Detecting Process Status Changes
@cindex process sentinel
@cindex sentinel (of process)

  A @dfn{process sentinel} is a function that is called whenever the
associated process changes status for any reason, including signals
(whether sent by Emacs or caused by the process's own actions) that
terminate, stop, or continue the process.  The process sentinel is
also called if the process exits.  The sentinel receives two
arguments: the process for which the event occurred, and a string
describing the type of event.

@cindex default sentinel function of a process
  If no sentinel function was specified for a process, it will use the
default sentinel function, which inserts a message in the process's
buffer with the process name and the string describing the event.

  The string describing the event looks like one of the following (but
this is not an exhaustive list of event strings):

@itemize @bullet
@item
@code{"finished\n"}.

@item
@code{"deleted\n"}.

@item
@code{"exited abnormally with code @var{exitcode} (core dumped)\n"}.
The ``core dumped'' part is optional, and only appears if the process
dumped core.

@item
@code{"failed with code @var{fail-code}\n"}.

@item
@code{"@var{signal-description} (core dumped)\n"}.  The
@var{signal-description} is a system-dependent textual description of
a signal, e.g., @code{"killed"} for @code{SIGKILL}.  The ``core
dumped'' part is optional, and only appears if the process dumped
core.

@item
@code{"open from @var{host-name}\n"}.

@item
@code{"open\n"}.

@item
@code{"run\n"}.

@item
@code{"connection broken by remote peer\n"}.
@end itemize

  A sentinel runs only while Emacs is waiting (e.g., for terminal
input, or for time to elapse, or for process output).  This avoids the
timing errors that could result from running sentinels at random places in
the middle of other Lisp programs.  A program can wait, so that
sentinels will run, by calling @code{sit-for} or @code{sleep-for}
(@pxref{Waiting}), or @code{accept-process-output} (@pxref{Accepting
Output}).  Emacs also allows sentinels to run when the command loop is
reading input.  @code{delete-process} calls the sentinel when it
terminates a running process.

  Emacs does not keep a queue of multiple reasons to call the sentinel
of one process; it records just the current status and the fact that
there has been a change.  Therefore two changes in status, coming in
quick succession, can call the sentinel just once.  However, process
termination will always run the sentinel exactly once.  This is
because the process status can't change again after termination.

  Emacs explicitly checks for output from the process before running
the process sentinel.  Once the sentinel runs due to process
termination, no further output can arrive from the process.

  A sentinel that writes the output into the buffer of the process
should check whether the buffer is still alive.  If it tries to insert
into a dead buffer, it will get an error.  If the buffer is dead,
@code{(buffer-name (process-buffer @var{process}))} returns @code{nil}.

@c Note this text is duplicated in the filter functions section.
  Quitting is normally inhibited within a sentinel---otherwise, the
effect of typing @kbd{C-g} at command level or to quit a user command
would be unpredictable.  If you want to permit quitting inside a
sentinel, bind @code{inhibit-quit} to @code{nil}.  In most cases, the
right way to do this is with the macro @code{with-local-quit}.
@xref{Quitting}.

  If an error happens during execution of a sentinel, it is caught
automatically, so that it doesn't stop the execution of whatever
programs was running when the sentinel was started.  However, if
@code{debug-on-error} is non-@code{nil},  errors are not caught.
This makes it possible to use the Lisp debugger to debug the
sentinel.  @xref{Debugger}.

  While a sentinel is running, the process sentinel is temporarily
set to @code{nil} so that the sentinel won't run recursively.
For this reason it is not possible for a sentinel to specify
a new sentinel.

@ignore
  In earlier Emacs versions, every sentinel that did regular expression
searching or matching had to explicitly save and restore the match data.
Now Emacs does this automatically for sentinels; they never need to do
it explicitly.
@end ignore
 Note that Emacs automatically saves and restores the match data
while executing sentinels.  @xref{Match Data}.

@defun set-process-sentinel process sentinel
This function associates @var{sentinel} with @var{process}.  If
@var{sentinel} is @code{nil}, then the process will have the default
sentinel, which inserts a message in the process's buffer when the
process status changes.

Changes in process sentinels take effect immediately---if the sentinel
is slated to be run but has not been called yet, and you specify a new
sentinel, the eventual call to the sentinel will use the new one.

@smallexample
@group
(defun msg-me (process event)
   (princ
     (format "Process: %s had the event '%s'" process event)))
(set-process-sentinel (get-process "shell") 'msg-me)
     @result{} msg-me
@end group
@group
(kill-process (get-process "shell"))
     @print{} Process: #<process shell> had the event 'killed'
     @result{} #<process shell>
@end group
@end smallexample
@end defun

@defun process-sentinel process
This function returns the sentinel of @var{process}.
@end defun

In case a process status changes need to be passed to several sentinels, you
can use @code{add-function} to combine an existing sentinel with a new one.
@xref{Advising Functions}.

@defun waiting-for-user-input-p
While a sentinel or filter function is running, this function returns
non-@code{nil} if Emacs was waiting for keyboard input from the user at
the time the sentinel or filter function was called, or @code{nil} if it
was not.
@end defun

@node Query Before Exit
@section Querying Before Exit

  When Emacs exits, it terminates all its subprocesses.  For
subprocesses that run a program, it sends them the @code{SIGHUP}
signal; connections are simply closed.  Because subprocesses may be
doing valuable work, Emacs normally asks the user to confirm that it
is ok to terminate them.  Each process has a query flag, which, if
non-@code{nil}, says that Emacs should ask for confirmation before
exiting and thus killing that process.  The default for the query flag
is @code{t}, meaning @emph{do} query.

@defun process-query-on-exit-flag process
This returns the query flag of @var{process}.
@end defun

@defun set-process-query-on-exit-flag process flag
This function sets the query flag of @var{process} to @var{flag}.  It
returns @var{flag}.

Here is an example of using @code{set-process-query-on-exit-flag} on a
shell process to avoid querying:

@smallexample
@group
(set-process-query-on-exit-flag (get-process "shell") nil)
     @result{} nil
@end group
@end smallexample
@end defun

@defopt confirm-kill-processes
If this user option is set to @code{t} (the default), then Emacs asks
for confirmation before killing processes on exit.  If it is
@code{nil}, Emacs kills processes without confirmation, i.e., the
query flag of all processes is ignored.
@end defopt

@node System Processes
@section Accessing Other Processes
@cindex system processes

  In addition to accessing and manipulating processes that are
subprocesses of the current Emacs session, Emacs Lisp programs can
also access other processes.  We call these @dfn{system processes}, to
distinguish them from Emacs subprocesses.

  Emacs provides several primitives for accessing system processes.
Not all platforms support these primitives; on those which don't,
these primitives return @code{nil}.

@defun list-system-processes
This function returns a list of all the processes running on the
system.  Each process is identified by its @acronym{PID}, a numerical
process ID that is assigned by the OS and distinguishes the process
from all the other processes running on the same machine at the same
time.

If @code{default-directory} points to a remote host, processes of that
host are returned.
@end defun

@defun process-attributes pid
This function returns an alist of attributes for the process specified
by its process ID @var{pid}.  Each association in the alist is of the
form @code{(@var{key} . @var{value})}, where @var{key} designates the
attribute and @var{value} is the value of that attribute.  The various
attribute @var{key}s that this function can return are listed below.
Not all platforms support all of these attributes; if an attribute is
not supported, its association will not appear in the returned alist.

If @code{default-directory} points to a remote host, @var{pid} is
regarded as process of that host.

@table @code
@item euid
The effective user ID of the user who invoked the process.  The
corresponding @var{value} is a number.  If the process was invoked by
the same user who runs the current Emacs session, the value is
identical to what @code{user-uid} returns (@pxref{User
Identification}).

@item user
User name corresponding to the process's effective user ID, a string.

@item egid
The group ID of the effective user ID, a number.

@item group
Group name corresponding to the effective user's group ID, a string.

@item comm
The name of the command that runs in the process.  This is a string
that usually specifies the name of the executable file of the process,
without the leading directories.  However, some special system
processes can report strings that do not correspond to an executable
file of a program.

@item state
The state code of the process.  This is a short string that encodes
the scheduling state of the process.  Here's a list of the most
frequently seen codes:

@table @code
@item "D"
uninterruptible sleep (usually I/O)
@item "R"
running
@item "S"
interruptible sleep (waiting for some event)
@item "T"
stopped, e.g., by a job control signal
@item "Z"
zombie: a process that terminated, but was not reaped by its parent
@end table

@noindent
For the full list of the possible states, see the manual page of the
@command{ps} command.

@item ppid
The process ID of the parent process, a number.

@item pgrp
The process group ID of the process, a number.

@item sess
The session ID of the process.  This is a number that is the process
ID of the process's @dfn{session leader}.

@item ttname
A string that is the name of the process's controlling terminal.  On
Unix and GNU systems, this is normally the file name of the
corresponding terminal device, such as @file{/dev/pts65}.

@item tpgid
The numerical process group ID of the foreground process group that
uses the process's terminal.

@item minflt
The number of minor page faults caused by the process since its
beginning.  (Minor page faults are those that don't involve reading
from disk.)

@item majflt
The number of major page faults caused by the process since its
beginning.  (Major page faults require a disk to be read, and are thus
more expensive than minor page faults.)

@item cminflt
@itemx cmajflt
Like @code{minflt} and @code{majflt}, but include the number of page
faults for all the child processes of the given process.

@item utime
Time spent by the process in the user context, for running the
application's code.  The corresponding @var{value} is a Lisp
timestamp (@pxref{Time of Day}).

@item stime
Time spent by the process in the system (kernel) context, for
processing system calls.  The corresponding @var{value} is a Lisp
timestamp.

@item time
The sum of @code{utime} and @code{stime}.  The corresponding
@var{value} is a Lisp timestamp.

@item cutime
@itemx cstime
@itemx ctime
Like @code{utime}, @code{stime}, and @code{time}, but include the
times of all the child processes of the given process.

@item pri
The numerical priority of the process.

@item nice
The @dfn{nice value} of the process, a number.  (Processes with smaller
nice values get scheduled more favorably.)

@item thcount
The number of threads in the process.

@item start
The time when the process was started, as a Lisp timestamp.

@item etime
The time elapsed since the process started, as a Lisp timestamp.

@item vsize
The virtual memory size of the process, measured in kilobytes.

@item rss
The size of the process's @dfn{resident set}, the number of kilobytes
occupied by the process in the machine's physical memory.

@item pcpu
The percentage of the CPU time used by the process since it started.
The corresponding @var{value} is a nonnegative floating-point number.
Although in theory the number can exceed 100 on a multicore platform,
it is usually less than 100 because Emacs is typically single-threaded.

@item pmem
The percentage of the total physical memory installed on the machine
used by the process's resident set.  The value is a floating-point
number between 0 and 100.

@item args
The command-line with which the process was invoked.  This is a string
in which individual command-line arguments are separated by blanks;
whitespace characters that are embedded in the arguments are quoted as
appropriate for the system's shell: escaped by backslash characters on
GNU and Unix, and enclosed in double quote characters on Windows.
Thus, this command-line string can be directly used in primitives such
as @code{shell-command}.
@end table

@end defun


@node Transaction Queues
@section Transaction Queues
@cindex transaction queue

@c That's not very informative.  What is a transaction, and when might
@c I want to use one?
You can use a @dfn{transaction queue} to communicate with a subprocess
using transactions.  First use @code{tq-create} to create a transaction
queue communicating with a specified process.  Then you can call
@code{tq-enqueue} to send a transaction.

@defun tq-create process
This function creates and returns a transaction queue communicating with
@var{process}.  The argument @var{process} should be a subprocess
capable of sending and receiving streams of bytes.  It may be a child
process, or it may be a TCP connection to a server, possibly on another
machine.
@end defun

@defun tq-enqueue queue question regexp closure fn &optional delay-question
This function sends a transaction to queue @var{queue}.  Specifying the
queue has the effect of specifying the subprocess to talk to.

The argument @var{question} is the outgoing message that starts the
transaction.  The argument @var{fn} is the function to call when the
corresponding answer comes back; it is called with two arguments:
@var{closure}, and the answer received.

The argument @var{regexp} is a regular expression that should match
text at the end of the entire answer, but nothing before; that's how
@code{tq-enqueue} determines where the answer ends.

If the argument @var{delay-question} is non-@code{nil}, delay sending
this question until the process has finished replying to any previous
questions.  This produces more reliable results with some processes.
@ignore

@c Let's not mention it then.
The return value of @code{tq-enqueue} itself is not meaningful.
@end ignore
@end defun

@defun tq-close queue
Shut down transaction queue @var{queue}, waiting for all pending transactions
to complete, and then terminate the connection or child process.
@end defun

Transaction queues are implemented by means of a filter function.
@xref{Filter Functions}.

@node Network
@section Network Connections
@cindex network connection
@cindex TCP
@cindex UDP

  Emacs Lisp programs can open stream (TCP) and datagram (UDP) network
connections (@pxref{Datagrams}) to other processes on the same machine
or other machines.
A network connection is handled by Lisp much like a subprocess, and is
represented by a process object.  However, the process you are
communicating with is not a child of the Emacs process, has no
process @acronym{ID}, and you can't kill it or send it signals.  All you
can do is send and receive data.  @code{delete-process} closes the
connection, but does not kill the program at the other end; that
program must decide what to do about closure of the connection.

  Lisp programs can listen for connections by creating network
servers.  A network server is also represented by a kind of process
object, but unlike a network connection, the network server never
transfers data itself.  When it receives a connection request, it
creates a new network connection to represent the connection just
made.  (The network connection inherits certain information, including
the process plist, from the server.)  The network server then goes
back to listening for more connection requests.

  Network connections and servers are created by calling
@code{make-network-process} with an argument list consisting of
keyword/argument pairs, for example @code{:server t} to create a
server process, or @code{:type 'datagram} to create a datagram
connection.  @xref{Low-Level Network}, for details.  You can also use
the @code{open-network-stream} function described below.

  To distinguish the different types of processes, the
@code{process-type} function returns the symbol @code{network} for a
network connection or server, @code{serial} for a serial port
connection, @code{pipe} for a pipe connection, or @code{real} for a
real subprocess.

  The @code{process-status} function returns @code{open},
@code{closed}, @code{connect}, @code{stop}, or @code{failed} for
network connections.  For a network server, the status is always
@code{listen}.  Except for @code{stop}, none of those values is
possible for a real subprocess.  @xref{Process Information}.

  You can stop and resume operation of a network process by calling
@code{stop-process} and @code{continue-process}.  For a server
process, being stopped means not accepting new connections.  (Up to 5
connection requests will be queued for when you resume the server; you
can increase this limit, unless it is imposed by the operating
system---see the @code{:server} keyword of @code{make-network-process},
@ref{Network Processes}.)  For a network stream connection, being
stopped means not processing input (any arriving input waits until you
resume the connection).  For a datagram connection, some number of
packets may be queued but input may be lost.  You can use the function
@code{process-command} to determine whether a network connection or
server is stopped; a non-@code{nil} value means yes.

@cindex network connection, encrypted
@cindex encrypted network connections
@cindex @acronym{TLS} network connections
@cindex @acronym{STARTTLS} network connections
Emacs can create encrypted network connections, using the built-in
support for the GnuTLS Transport Layer Security Library; see
@uref{https://www.gnu.org/software/gnutls/, the GnuTLS project page}.
If your Emacs was compiled with GnuTLS support, the function
@code{gnutls-available-p} is defined and returns non-@code{nil}.  For
more details, @pxref{Top,, Overview, emacs-gnutls, The Emacs-GnuTLS manual}.
The @code{open-network-stream} function can transparently handle the
details of creating encrypted connections for you, using whatever
support is available.

@defun open-network-stream name buffer host service &rest parameters
This function opens a TCP connection, with optional encryption, and
returns a process object that represents the connection.

The @var{name} argument specifies the name for the process object.  It
is modified as necessary to make it unique.

The @var{buffer} argument is the buffer to associate with the
connection.  Output from the connection is inserted in the buffer,
unless you specify your own filter function to handle the output.  If
@var{buffer} is @code{nil}, it means that the connection is not
associated with any buffer.

The arguments @var{host} and @var{service} specify where to connect to;
@var{host} is the host name (a string), and @var{service} is the name of
a defined network service (a string) or a port number (an integer like
@code{80} or an integer string like @code{"80"}).

The remaining arguments @var{parameters} are keyword/argument pairs
that are mainly relevant to encrypted connections:

@table @code

@item :nowait @var{boolean}
If non-@code{nil}, try to make an asynchronous connection.

@item :noquery @var{query-flag}
Initialize the process query flag to @var{query-flag}.
@xref{Query Before Exit}.

@item :coding @var{coding}
Use this to set the coding systems used by the network process, in
preference to binding @code{coding-system-for-read} or
@code{coding-system-for-write}.  @xref{Network Processes}, for
details.

@item :type @var{type}
The type of connection.  Options are:

@table @code
@item plain
An ordinary, unencrypted connection.
@item tls
@itemx ssl
A @acronym{TLS} (Transport Layer Security) connection.
@item nil
@itemx network
Start with a plain connection, and if parameters @samp{:success}
and @samp{:capability-command} are supplied, try to upgrade to an encrypted
connection via @acronym{STARTTLS}.  If that fails, retain the
unencrypted connection.
@item starttls
As for @code{nil}, but if @acronym{STARTTLS} fails drop the connection.
@item shell
A shell connection.
@end table

@item :always-query-capabilities @var{boolean}
If non-@code{nil}, always ask for the server's capabilities, even when
doing a @samp{plain} connection.

@item :capability-command @var{capability-command}
Command to query the host capabilities.  This can either be a string
(which will then be sent verbatim to the server), or a function
(called with a single parameter; the "greeting" from the server when
connecting), and should return a string.

@item :end-of-command @var{regexp}
@itemx :end-of-capability @var{regexp}
Regular expression matching the end of a command, or the end of the
command @var{capability-command}.  The latter defaults to the former.

@item :starttls-function @var{function}
Function of one argument (the response to @var{capability-command}),
which returns either @code{nil}, or the command to activate @acronym{STARTTLS}
if supported.

@item :success @var{regexp}
Regular expression matching a successful @acronym{STARTTLS} negotiation.

@item :use-starttls-if-possible @var{boolean}
If non-@code{nil}, do opportunistic @acronym{STARTTLS} upgrades even if Emacs
doesn't have built-in @acronym{TLS} support.

@item :warn-unless-encrypted @var{boolean}
If non-@code{nil}, and @code{:return-value} is also non-@code{nil},
Emacs will warn if the connection isn't encrypted.  This is useful for
protocols like @acronym{IMAP} and the like, where most users would
expect the network traffic to be encrypted.

@vindex network-stream-use-client-certificates
@item :client-certificate @var{list-or-t}
Either a list of the form @code{(@var{key-file} @var{cert-file})},
naming the certificate key file and certificate file itself, or
@code{t}, meaning to query @code{auth-source} for this information
(@pxref{Help for users,,auth-source, auth, Emacs auth-source
Library}).  Only used for @acronym{TLS} or @acronym{STARTTLS}.  To
enable automatic queries of @code{auth-source} when
@code{:client-certificate} is not specified customize
@code{network-stream-use-client-certificates} to t.

@item :return-list @var{cons-or-nil}
The return value of this function.  If omitted or @code{nil}, return a
process object.  Otherwise, a cons of the form @code{(@var{process-object}
. @var{plist})}, where @var{plist} has keywords:

@table @code
@item :greeting @var{string-or-nil}
If non-@code{nil}, the greeting string returned by the host.
@item :capabilities @var{string-or-nil}
If non-@code{nil}, the host's capability string.
@item :type @var{symbol}
The connection type: @samp{plain} or @samp{tls}.
@end table

@item :shell-command @var{string-or-nil}
If the connection @code{type} is @code{shell}, this parameter will be
interpreted as a format-spec string that will be executed to make the
connection.  The specs available are @samp{%s} for the host name and
@samp{%p} for the port number.  For instance, if you want to first ssh
to @samp{gateway} before making a plain connection, then this
parameter could be something like @samp{ssh gateway nc %s %p}.

@end table

@end defun


@node Network Servers
@section Network Servers
@cindex network servers

  You create a server by calling @code{make-network-process}
(@pxref{Network Processes}) with @code{:server t}.  The server will
listen for connection requests from clients.  When it accepts a client
connection request, that creates a new network connection, itself a
process object, with the following parameters:

@itemize @bullet
@item
The connection's process name is constructed by concatenating the
server process's @var{name} with a client identification string.  The
@c FIXME?  What about IPv6?  Say briefly what the difference is?
client identification string for an IPv4 connection looks like
@samp{<@var{a}.@var{b}.@var{c}.@var{d}:@var{p}>}, which represents an
address and port number.  Otherwise, it is a
unique number in brackets, as in @samp{<@var{nnn}>}.  The number
is unique for each connection in the Emacs session.

@item
If the server has a non-default filter, the connection process does
not get a separate process buffer; otherwise, Emacs creates a new
buffer for the purpose.  The buffer name is the server's buffer name
or process name, concatenated with the client identification string.

The server's process buffer value is never used directly, but the log
function can retrieve it and use it to log connections by inserting
text there.

@item
The communication type and the process filter and sentinel are
inherited from those of the server.  The server never directly
uses its filter and sentinel; their sole purpose is to initialize
connections made to the server.

@item
The connection's process contact information is set according to the client's
addressing information (typically an IP address and a port number).
This information is associated with the @code{process-contact}
keywords @code{:host}, @code{:service}, @code{:remote}.

@item
The connection's local address is set up according to the port
number used for the connection.

@item
The client process's plist is initialized from the server's plist.
@end itemize

@node Datagrams
@section Datagrams
@cindex datagrams

  A @dfn{datagram} connection communicates with individual packets rather
than streams of data.  Each call to @code{process-send} sends one
datagram packet (@pxref{Input to Processes}), and each datagram
received results in one call to the filter function.

  The datagram connection doesn't have to talk with the same remote
peer all the time.  It has a @dfn{remote peer address} which specifies
where to send datagrams to.  Each time an incoming datagram is passed
to the filter function, the peer address is set to the address that
datagram came from; that way, if the filter function sends a datagram,
it will go back to that place.  You can specify the remote peer
address when you create the datagram connection using the
@code{:remote} keyword.  You can change it later on by calling
@code{set-process-datagram-address}.

@defun process-datagram-address process
If @var{process} is a datagram connection or server, this function
returns its remote peer address.
@end defun

@defun set-process-datagram-address process address
If @var{process} is a datagram connection or server, this function
sets its remote peer address to @var{address}.
@end defun

@node Low-Level Network
@section Low-Level Network Access

  You can also create network connections by operating at a lower
level than that of @code{open-network-stream}, using
@code{make-network-process}.

@menu
* Proc: Network Processes.   Using @code{make-network-process}.
* Options: Network Options.  Further control over network connections.
* Features: Network Feature Testing.
                             Determining which network features work on
                               the machine you are using.
@end menu

@node Network Processes
@subsection @code{make-network-process}

   The basic function for creating network connections and network
servers is @code{make-network-process}.  It can do either of those
jobs, depending on the arguments you give it.

@defun make-network-process &rest args
This function creates a network connection or server and returns the
process object that represents it.  The arguments @var{args} are a
list of keyword/argument pairs.  Omitting a keyword is always
equivalent to specifying it with value @code{nil}, except for
@code{:coding}, @code{:filter-multibyte}, and @code{:reuseaddr}.  Here
are the meaningful keywords (those corresponding to network options
are listed in the following section):

@table @asis
@item :name @var{name}
Use the string @var{name} as the process name.  It is modified if
necessary to make it unique.

@item :type @var{type}
Specify the communication type.  A value of @code{nil} specifies a
stream connection (the default); @code{datagram} specifies a datagram
connection; @code{seqpacket} specifies a sequenced packet stream
connection.  Both connections and servers can be of these types.

@item :server @var{server-flag}
If @var{server-flag} is non-@code{nil}, create a server.  Otherwise,
create a connection.  For a stream type server, @var{server-flag} may
be an integer, which then specifies the length of the queue of pending
connections to the server.  The default queue length is 5.

@item :host @var{host}
Specify the host to connect to.  @var{host} should be a host name or
Internet address, as a string, or the symbol @code{local} to specify
the local host.  If you specify @var{host} for a server, it must
specify a valid address for the local host, and only clients
connecting to that address will be accepted.  When using @code{local},
by default IPv4 will be used, specify a @var{family} of @code{ipv6} to
override this.  To listen on all interfaces, specify an address of
@samp{"0.0.0.0"} for IPv4 or @samp{"::"} for IPv6.  Note that on some
operating systems, listening on @samp{"::"} will also listen on IPv4,
so attempting to then listen separately on IPv4 will result in
@code{EADDRINUSE} errors (@samp{"Address already in use"}).

@item :service @var{service}
@var{service} specifies a port number to connect to; or, for a server,
the port number to listen on.  It should be a service name like
@samp{"https"} that translates to a port number, or an integer like @samp{443}
or an integer string like @samp{"443"} that specifies the port number
directly.  For a server, it can also be @code{t}, which means to let
the system select an unused port number.

@item :family @var{family}
@var{family} specifies the address (and protocol) family for
communication.  @code{nil} means determine the proper address family
automatically for the given @var{host} and @var{service}.
@code{local} specifies a Unix socket, in which case @var{host} is
ignored.  @code{ipv4} and @code{ipv6} specify to use IPv4 and IPv6,
respectively.

@item :use-external-socket @var{use-external-socket}
If @var{use-external-socket} is non-@code{nil} use any sockets passed
to Emacs on invocation instead of allocating one.  This is used by the
Emacs server code to allow on-demand socket activation.  If Emacs
wasn't passed a socket, this option is silently ignored.

@item :local @var{local-address}
For a server process, @var{local-address} is the address to listen on.
It overrides @var{family}, @var{host} and @var{service}, so you
might as well not specify them.

@item :remote @var{remote-address}
For a connection, @var{remote-address} is the address to connect to.
It overrides @var{family}, @var{host} and @var{service}, so you
might as well not specify them.

For a datagram server, @var{remote-address} specifies the initial
setting of the remote datagram address.

The format of @var{local-address} or @var{remote-address} depends on
the address family:

@itemize -
@item
An IPv4 address is represented as a five-element vector of four 8-bit
integers and one 16-bit integer
@code{[@var{a} @var{b} @var{c} @var{d} @var{p}]} corresponding to
numeric IPv4 address @var{a}.@var{b}.@var{c}.@var{d} and port number
@var{p}.

@item
An IPv6 address is represented as a nine-element vector of 16-bit
integers @code{[@var{a} @var{b} @var{c} @var{d} @var{e} @var{f}
@var{g} @var{h} @var{p}]} corresponding to numeric IPv6 address
@var{a}:@var{b}:@var{c}:@var{d}:@var{e}:@var{f}:@var{g}:@var{h} and
port number @var{p}.

@item
A local address is represented as a string, which specifies the address
in the local address space.

@item
An unsupported-family address is represented by a cons
@code{(@var{f} . @var{av})}, where @var{f} is the family number and
@var{av} is a vector specifying the socket address using one element
per address data byte.  Do not rely on this format in portable code,
as it may depend on implementation defined constants, data sizes, and
data structure alignment.
@end itemize

@item :nowait @var{bool}
If @var{bool} is non-@code{nil} for a stream connection, return
without waiting for the connection to complete.  When the connection
succeeds or fails, Emacs will call the sentinel function, with a
second argument matching @code{"open"} (if successful) or
@code{"failed"}.  The default is to block, so that
@code{make-network-process} does not return until the connection has
succeeded or failed.

If you're setting up an asynchronous TLS connection, you have to also
provide the @code{:tls-parameters} parameter (see below).

Depending on the capabilities of Emacs, how asynchronous
@code{:nowait} is may vary.  The three elements that may (or may not)
be done asynchronously are domain name resolution, socket setup, and
(for TLS connections) TLS negotiation.

Many functions that interact with process objects, (for instance,
@code{process-datagram-address}) rely on them at least having a socket
before they can return a useful value.  These functions will block
until the socket has achieved the desired status.  The recommended way
of interacting with asynchronous sockets is to place a sentinel on the
process, and not try to interact with it before it has changed status
to @samp{"run"}.  That way, none of these functions will block.

@item :tls-parameters
When opening a TLS connection, this should be where the first element
is the TLS type (which should either be @code{gnutls-x509pki} or
@code{gnutls-anon}, and the remaining elements should form a keyword
list acceptable for @code{gnutls-boot}.  (This keyword list can be
obtained from the @code{gnutls-boot-parameters} function.)  The TLS
connection will then be negotiated after completing the connection to
the host.

@item :stop @var{stopped}
If @var{stopped} is non-@code{nil}, start the network connection or
server in the stopped state.

@item :buffer @var{buffer}
Use @var{buffer} as the process buffer.

@item :coding @var{coding}
Use @var{coding} as the coding system for this process.  To specify
different coding systems for decoding data from the connection and for
encoding data sent to it, specify @code{(@var{decoding} .
@var{encoding})} for @var{coding}.

If you don't specify this keyword at all, the default
is to determine the coding systems from the data.

@item :noquery @var{query-flag}
Initialize the process query flag to @var{query-flag}.
@xref{Query Before Exit}.

@item :filter @var{filter}
Initialize the process filter to @var{filter}.

@item :filter-multibyte @var{multibyte}
If @var{multibyte} is non-@code{nil}, strings given to the process
filter are multibyte, otherwise they are unibyte.  The default is @code{t}.

@item :sentinel @var{sentinel}
Initialize the process sentinel to @var{sentinel}.

@item :log @var{log}
Initialize the log function of a server process to @var{log}.  The log
function is called each time the server accepts a network connection
from a client.  The arguments passed to the log function are
@var{server}, @var{connection}, and @var{message}; where @var{server}
is the server process, @var{connection} is the new process for the
connection, and @var{message} is a string describing what has
happened.

@item :plist @var{plist}
Initialize the process plist to @var{plist}.
@end table

The original argument list, modified with the actual connection
information, is available via the @code{process-contact} function.
@end defun

@node Network Options
@subsection Network Options

  The following network options can be specified when you create a
network process.  Except for @code{:reuseaddr}, you can also set or
modify these options later, using @code{set-network-process-option}.

  For a server process, the options specified with
@code{make-network-process} are not inherited by the client
connections, so you will need to set the necessary options for each
child connection as it is created.

@table @asis
@item :bindtodevice @var{device-name}
If @var{device-name} is a non-empty string identifying a network
interface name (see @code{network-interface-list}), only handle
packets received on that interface.  If @var{device-name} is @code{nil}
(the default), handle packets received on any interface.

Using this option may require special privileges on some systems.

@item :broadcast @var{broadcast-flag}
If @var{broadcast-flag} is non-@code{nil} for a datagram process, the
process will receive datagram packet sent to a broadcast address, and
be able to send packets to a broadcast address.  This is ignored for a stream
connection.

@item :dontroute @var{dontroute-flag}
If @var{dontroute-flag} is non-@code{nil}, the process can only send
to hosts on the same network as the local host.

@item :keepalive @var{keepalive-flag}
If @var{keepalive-flag} is non-@code{nil} for a stream connection,
enable exchange of low-level keep-alive messages.

@item :linger @var{linger-arg}
If @var{linger-arg} is non-@code{nil}, wait for successful
transmission of all queued packets on the connection before it is
deleted (see @code{delete-process}).  If @var{linger-arg} is an
integer, it specifies the maximum time in seconds to wait for queued
packets to be sent before closing the connection.  The default is
@code{nil}, which means to discard unsent queued packets when the
process is deleted.

@c FIXME  Where out-of-band data is ...?
@item :oobinline @var{oobinline-flag}
If @var{oobinline-flag} is non-@code{nil} for a stream connection,
receive out-of-band data in the normal data stream.  Otherwise, ignore
out-of-band data.

@item :priority @var{priority}
Set the priority for packets sent on this connection to the integer
@var{priority}.  The interpretation of this number is protocol
specific; such as setting the TOS (type of service) field on IP
packets sent on this connection.  It may also have system dependent
effects, such as selecting a specific output queue on the network
interface.

@item :reuseaddr @var{reuseaddr-flag}
If @var{reuseaddr-flag} is non-@code{nil} (the default) for a stream
server process, allow this server to reuse a specific port number (see
@code{:service}), unless another process on this host is already
listening on that port.  If @var{reuseaddr-flag} is @code{nil}, there
may be a period of time after the last use of that port (by any
process on the host) where it is not possible to make a new server on
that port.
@end table

@defun set-network-process-option process option value &optional no-error
This function sets or modifies a network option for network process
@var{process}.  The accepted options and values are as for
@code{make-network-process}.  If @var{no-error} is non-@code{nil},
this function returns @code{nil} instead of signaling an error if
@var{option} is not a supported option.  If the function successfully
completes, it returns @code{t}.

The current setting of an option is available via the
@code{process-contact} function.
@end defun

@node Network Feature Testing
@subsection Testing Availability of Network Features

  To test for the availability of a given network feature, use
@code{featurep} like this:

@example
(featurep 'make-network-process '(@var{keyword} @var{value}))
@end example

@noindent
The result of this form is @code{t} if it works to specify
@var{keyword} with value @var{value} in @code{make-network-process}.
Here are some of the @var{keyword}---@var{value} pairs you can test in
this way.

@table @code
@item (:nowait t)
Non-@code{nil} if non-blocking connect is supported.
@item (:type datagram)
Non-@code{nil} if datagrams are supported.
@item (:family local)
Non-@code{nil} if local (a.k.a.@: ``UNIX domain'') sockets are supported.
@item (:family ipv6)
Non-@code{nil} if IPv6 is supported.
@item (:service t)
Non-@code{nil} if the system can select the port for a server.
@end table

  To test for the availability of a given network option, use
@code{featurep} like this:

@example
(featurep 'make-network-process '@var{keyword})
@end example

@noindent
The accepted @var{keyword} values are @code{:bindtodevice}, etc.
For the complete list, @pxref{Network Options}.  This form returns
non-@code{nil} if that particular network option is supported by
@code{make-network-process} (or @code{set-network-process-option}).

@node Misc Network
@section Misc Network Facilities

  These additional functions are useful for creating and operating
on network connections.  Note that they are supported only on some
systems.

@defun network-interface-list &optional full family
This function returns a list describing the network interfaces of the
machine you are using.  The value is an alist whose elements have the
form @code{(@var{ifname} . @var{address})}.  @var{ifname} is a string
naming the interface, @var{address} has the same form as the
@var{local-address} and @var{remote-address} arguments to
@code{make-network-process}, i.e. a vector of integers.  By default
both IPv4 and IPv6 addresses are returned if possible.

Optional argument @var{full} non-@code{nil} means to instead return a
list of one or more elements of the form @w{@code{(@var{ifname}
@var{addr} @var{bcast} @var{netmask})}}.  @var{ifname} is a non-unique
string naming the interface.  @var{addr}, @var{bcast}, and
@var{netmask} are vectors of integers detailing the IP address,
broadcast address, and network mask.

Optional argument @var{family} specified as symbol @code{ipv4} or
@code{ipv6} restricts the returned information to IPv4 and IPv6
addresses respectively, independently of the value of @var{full}.
Specifying @code{ipv6} when IPv6 support is not available will result
in an error being signaled.

Some examples:

@example
(network-interface-list) @result{}
(("vmnet8" .
  [172 16 76 1 0])
 ("vmnet1" .
  [172 16 206 1 0])
 ("lo0" .
  [65152 0 0 0 0 0 0 1 0])
 ("lo0" .
  [0 0 0 0 0 0 0 1 0])
 ("lo0" .
  [127 0 0 1 0]))
@end example

@example
(network-interface-list t) @result{}
(("vmnet8"
  [172 16 76 1 0]
  [172 16 76 255 0]
  [255 255 255 0 0])
 ("vmnet1"
  [172 16 206 1 0]
  [172 16 206 255 0]
  [255 255 255 0 0])
 ("lo0"
  [65152 0 0 0 0 0 0 1 0]
  [65152 0 0 0 65535 65535 65535 65535 0]
  [65535 65535 65535 65535 0 0 0 0 0])
 ("lo0"
  [0 0 0 0 0 0 0 1 0]
  [0 0 0 0 0 0 0 1 0]
  [65535 65535 65535 65535 65535 65535 65535 65535 0])
 ("lo0"
  [127 0 0 1 0]
  [127 255 255 255 0]
  [255 0 0 0 0]))
@end example
@end defun

@defun network-interface-info ifname
This function returns information about the network interface named
@var{ifname}.  The value is a list of the form
@code{(@var{addr} @var{bcast} @var{netmask} @var{hwaddr} @var{flags})}.

@table @var
@item addr
The Internet protocol address.
@item bcast
The broadcast address.
@item netmask
The network mask.
@item hwaddr
The layer 2 address (Ethernet MAC address, for instance).
@item flags
The current flags of the interface.
@end table

Note that this function returns only IPv4 information.
@end defun

@defun format-network-address address &optional omit-port
This function converts the Lisp representation of a network address to
a string.

A five-element vector @code{[@var{a} @var{b} @var{c} @var{d} @var{p}]}
represents an IPv4 address @var{a}.@var{b}.@var{c}.@var{d} and port
number @var{p}.  @code{format-network-address} converts that to the
string @code{"@var{a}.@var{b}.@var{c}.@var{d}:@var{p}"}.

A nine-element vector @code{[@var{a} @var{b} @var{c} @var{d} @var{e}
@var{f} @var{g} @var{h} @var{p}]} represents an IPv6 address along
with a port number.  @code{format-network-address} converts that to
the string
@code{"[@var{a}:@var{b}:@var{c}:@var{d}:@var{e}:@var{f}:@var{g}:@var{h}]:@var{p}"}.

If the vector does not include the port number, @var{p}, or if
@var{omit-port} is non-@code{nil}, the result does not include the
@code{:@var{p}} suffix.
@end defun

@defun network-lookup-address-info name &optional family hints
This function perform hostname lookups on @var{name}, which is
expected to be an ASCII-only string, otherwise it signals an error.  Call
@code{puny-encode-domain} on @var{name} first if you wish to lookup
internationalized hostnames.

If successful, this function returns a list of Lisp representations of network
addresses (@pxref{Network Processes}, for a description of the
format), otherwise return @code{nil}.  In the latter case, it also logs
an error message hopefully explaining what went wrong.

By default, this function attempts both IPv4 and IPv6 lookups.  The
optional argument @var{family} controls this behavior, specifying the
symbol @code{ipv4} or @code{ipv6} restricts lookups to IPv4 and IPv6
respectively.

If optional argument @var{hints} is @code{numeric}, the function
treats the @var{name} as a numerical IP address (and does not perform DNS
lookups).  This can be used to check whether a string is a valid
numerical representation of an IP address, or to convert a numerical
string to its canonical representation. e.g.@:

@example
(network-lookup-address-info "127.1" 'ipv4 'numeric)
    @result{} ([127 0 0 1 0])

(network-lookup-address-info "::1" nil 'numeric)
    @result{} ([0 0 0 0 0 0 0 1 0])
@end example

Be warned that there are some surprising valid forms,
especially for IPv4, e.g @samp{0xe3010203} and @samp{0343.1.2.3} are both
valid, as are @samp{0} and @samp{1} (but they are invalid for IPv6).
@end defun

@node Serial Ports
@section Communicating with Serial Ports
@cindex @file{/dev/tty}
@cindex @file{COM1}
@cindex serial connections

  Emacs can communicate with serial ports.  For interactive use,
@kbd{M-x serial-term} opens a terminal window.  In a Lisp program,
@code{make-serial-process} creates a process object.

  The serial port can be configured at run-time, without having to
close and re-open it.  The function @code{serial-process-configure}
lets you change the speed, bytesize, and other parameters.  In a
terminal window created by @code{serial-term}, you can click on the
mode line for configuration.

  A serial connection is represented by a process object, which can be
used in a similar way to a subprocess or network process.  You can send and
receive data, and configure the serial port.  A serial process object
has no process ID, however, and you can't send signals to it, and the
status codes are different from other types of processes.
@code{delete-process} on the process object or @code{kill-buffer} on
the process buffer close the connection, but this does not affect the
device connected to the serial port.

  The function @code{process-type} returns the symbol @code{serial}
for a process object representing a serial port connection.

  Serial ports are available on GNU/Linux, Unix, and MS Windows systems.

@deffn Command serial-term port speed &optional line-mode
Start a terminal-emulator for a serial port in a new buffer.
@var{port} is the name of the serial port to connect to.  For
example, this could be @file{/dev/ttyS0} on Unix.  On MS Windows, this
could be @file{COM1}, or @file{\\.\COM10} (double the backslashes in
Lisp strings).

@c FIXME is 9600 still the most common value, or is it 115200 now?
@c (Same value, 9600, appears below as well.)
@var{speed} is the speed of the serial port in bits per second.  9600
is a common value.  The buffer is in Term mode; see @ref{Term Mode,,,
emacs, The GNU Emacs Manual}, for the commands to use in that buffer.
You can change the speed and the configuration in the mode line menu.
If @var{line-mode} is non-@code{nil}, @code{term-line-mode} is used;
otherwise @code{term-raw-mode} is used.
@end deffn

@defun make-serial-process &rest args
This function creates a process and a buffer.  Arguments are specified
as keyword/argument pairs.  Here's the list of the meaningful
keywords, with the first two (@var{port} and @var{speed}) being mandatory:

@table @code
@item :port @var{port}
This is the name of the serial port.  On Unix and GNU systems, this is
a file name such as @file{/dev/ttyS0}.  On Windows, this could be
@file{COM1}, or @file{\\.\COM10} for ports higher than @file{COM9}
(double the backslashes in Lisp strings).

@item :speed @var{speed}
The speed of the serial port in bits per second.  This function calls
@code{serial-process-configure} to handle the speed; see the
following documentation of that function for more details.

@item :name @var{name}
The name of the process.  If @var{name} is not given, @var{port} will
serve as the process name as well.

@item :buffer @var{buffer}
The buffer to associate with the process.  The value can be either a
buffer or a string that names a buffer.  Process output goes at the
end of that buffer, unless you specify an output stream or filter
function to handle the output.  If @var{buffer} is not given, the
process buffer's name is taken from the value of the @code{:name}
keyword.

@item :coding @var{coding}
If @var{coding} is a symbol, it specifies the coding system used for
both reading and writing for this process.  If @var{coding} is a cons
@code{(@var{decoding} . @var{encoding})}, @var{decoding} is used for
reading, and @var{encoding} is used for writing.  If not specified,
the default is to determine the coding systems from the data itself.

@item :noquery @var{query-flag}
Initialize the process query flag to @var{query-flag}.  @xref{Query
Before Exit}.  The flags defaults to @code{nil} if unspecified.

@item :stop @var{bool}
Start process in the stopped state if @var{bool} is
non-@code{nil}.  In the stopped state, a serial process does not
accept incoming data, but you can send outgoing data.  The stopped
state is cleared by @code{continue-process} and set by
@code{stop-process}.

@item :filter @var{filter}
Install @var{filter} as the process filter.

@item :sentinel @var{sentinel}
Install @var{sentinel} as the process sentinel.

@item :plist @var{plist}
Install @var{plist} as the initial plist of the process.

@item :bytesize
@itemx :parity
@itemx :stopbits
@itemx :flowcontrol
These are handled by @code{serial-process-configure}, which is called
by @code{make-serial-process}.
@end table

The original argument list, possibly modified by later configuration,
is available via the function @code{process-contact}.

Here is an example:

@example
(make-serial-process :port "/dev/ttyS0" :speed 9600)
@end example
@end defun

@defun serial-process-configure &rest args
@cindex baud, in serial connections
@cindex bytesize, in serial connections
@cindex parity, in serial connections
@cindex stopbits, in serial connections
@cindex flowcontrol, in serial connections

This function configures a serial port connection.  Arguments are
specified as keyword/argument pairs.  Attributes that are not given
are re-initialized from the process's current configuration (available
via the function @code{process-contact}), or set to reasonable default
values.  The following arguments are defined:

@table @code
@item :process @var{process}
@itemx :name @var{name}
@itemx :buffer @var{buffer}
@itemx :port @var{port}
Any of these arguments can be given to identify the process that is to
be configured.  If none of these arguments is given, the current
buffer's process is used.

@item :speed @var{speed}
The speed of the serial port in bits per second, a.k.a.@: @dfn{baud
rate}.  The value can be any number, but most serial ports work only
at a few defined values between 1200 and 115200, with 9600 being the
most common value.  If @var{speed} is @code{nil}, the function ignores
all other arguments and does not configure the port.  This may be
useful for special serial ports such as Bluetooth-to-serial converters,
which can only be configured through @samp{AT} commands sent through the
connection.  The value of @code{nil} for @var{speed} is valid only for
connections that were already opened by a previous call to
@code{make-serial-process} or @code{serial-term}.

@item :bytesize @var{bytesize}
The number of bits per byte, which can be 7 or 8.  If @var{bytesize}
is not given or @code{nil}, it defaults to 8.

@item :parity @var{parity}
The value can be @code{nil} (don't use parity), the symbol
@code{odd} (use odd parity), or the symbol @code{even} (use even
parity).  If @var{parity} is not given, it defaults to no parity.

@item :stopbits @var{stopbits}
The number of stopbits used to terminate a transmission
of each byte.  @var{stopbits} can be 1 or 2.  If @var{stopbits} is not
given or @code{nil}, it defaults to 1.

@item :flowcontrol @var{flowcontrol}
The type of flow control to use for this connection, which is either
@code{nil} (don't use flow control), the symbol @code{hw} (use RTS/CTS
hardware flow control), or the symbol @code{sw} (use XON/XOFF software
flow control).  If @var{flowcontrol} is not given, it defaults to no
flow control.
@end table

Internally, @code{make-serial-process} calls
@code{serial-process-configure} for the initial configuration of the
serial port.
@end defun

@node Byte Packing
@section Packing and Unpacking Byte Arrays
@cindex byte packing and unpacking

  This section describes how to pack and unpack arrays of bytes,
usually for binary network protocols.  These functions convert byte arrays
to alists, and vice versa.  The byte array can be represented as a
@c FIXME?  No multibyte?
unibyte string or as a vector of integers, while the alist associates
symbols either with fixed-size objects or with recursive sub-alists.
To use the functions referred to in this section, load the
@code{bindat} library.
@c It doesn't have any autoloads.

@cindex serializing
@cindex deserializing
@cindex packing
@cindex unpacking
  Conversion from byte arrays to nested alists is also known as
@dfn{deserializing} or @dfn{unpacking}, while going in the opposite
direction is also known as @dfn{serializing} or @dfn{packing}.

@menu
* Bindat Types::            Describing data layout.
* Bindat Functions::        Doing the unpacking and packing.
* Bindat Computed Types::   Advanced data layout specifications.
@end menu

@node Bindat Types
@subsection Describing Data Layout
@cindex bindat types

@cindex data layout specification
@cindex bindat type expression
@cindex base type, in bindat specification
@cindex composite type, in bindat specification
  To control unpacking and packing, you write a @dfn{data layout
specification}, also called a @dfn{Bindat type expression}.  This can
be a @dfn{base type} or a @dfn{composite type} made of several fields,
where the specification controls the length of each field to be
processed, and how to pack or unpack it.  We normally keep bindat type
values in variables whose names end in @code{-bindat-spec}; that kind
of name is automatically recognized as risky (@pxref{File Local
Variables}).

@defmac bindat-type &rest type
Creates a Bindat type @emph{value} object according to the Bindat type
@emph{expression} @var{type}.
@end defmac

@cindex endianness, in bindat specification
@cindex big endian, in bindat specification
@cindex little endian, in bindat specification
@cindex network byte ordering, in Bindat specification
  A field's @dfn{type} describes the size (in bytes) of the object
that the field represents and, in the case of multibyte fields, how
the bytes are ordered within the field.  The two possible orderings
are @dfn{big endian} (also known as ``network byte ordering'') and
@dfn{little endian}.  For instance, the number @code{#x23cd} (decimal
9165) in big endian would be the two bytes @code{#x23} @code{#xcd};
and in little endian, @code{#xcd} @code{#x23}.  Here are the possible
type values:

@table @code
@item u8
@itemx byte
Unsigned byte, with length 1.

@item uint @var{bitlen} &optional @var{le}
Unsigned integer in network byte order (big-endian), with @var{bitlen} bits.
@var{bitlen} has to be a multiple of 8.
If @var{le} is non-@code{nil}, then use little-endian byte order.

@item sint @var{bitlen} @var{le}
Signed integer in network byte order (big-endian), with @var{bitlen} bits.
@var{bitlen} has to be a multiple of 8.
If @var{le} is non-@code{nil}, then use little-endian byte order.

@item str @var{len}
Unibyte string (@pxref{Text Representations}) of length @var{len} bytes.
When packing, the first @var{len} bytes of the input string are copied
to the packed output.  If the input string is shorter than @var{len},
the remaining bytes will be null (zero) unless a pre-allocated string
was provided to @code{bindat-pack}, in which case the remaining bytes
are left unmodified.  If the input string is multibyte with only ASCII
and @code{eight-bit} characters, it is converted to unibyte before it
is packed; other multibyte strings signal an error.  When unpacking,
any null bytes in the packed input string will appear in the unpacked
output.

@item strz &optional @var{len}
If @var{len} is not provided, this is a variable-length
null-terminated unibyte string (@pxref{Text Representations}).  When
packing into @code{strz}, the entire input string is copied to the
packed output followed by a null (zero) byte.  (If pre-allocated
string is provided for packing into @code{strz}, that pre-allocated
string should have enough space for the additional null byte appended
to the output string contents, @pxref{Bindat Functions}).  The length
of the packed output is the length of the input string plus one (for
the null terminator).  The input string must not contain any null
bytes.  If the input string is multibyte with only ASCII and
@code{eight-bit} characters, it is converted to unibyte before it is
packed; other multibyte strings signal an error.  When unpacking a
@code{strz}, the resulting output string will contain all bytes up to
(but excluding) the null byte that terminated the input string.

If @var{len} is provided, @code{strz} behaves the same as @code{str},
but with a couple of differences:

@itemize @bullet
@item
When packing, a null terminator is written after the packed input
string if the number of characters in the input string is less than
@var{len}.

@item
When unpacking, the first null byte encountered in the packed string
is interpreted as the terminating byte, and it and all subsequent
bytes are excluded from the result of the unpacking.
@end itemize

@quotation Caution
The packed output will not be null-terminated unless the input string
is shorter than @var{len} bytes or it contains a null byte within the
first @var{len} bytes.
@end quotation

@item vec @var{len} [@var{type}]
Vector of @var{len} elements.  The type of the elements is given by
@var{type}, defaulting to bytes.  The @var{type} can be any Bindat
type expression.

@item repeat @var{len} [@var{type}]
Like @code{vec}, but it unpacks to and packs from lists, whereas
@code{vec} unpacks to vectors.

@item bits @var{len}
List of bits that are set to 1 in @var{len} bytes.  The bytes are
taken in big-endian order, and the bits are numbered starting with
@code{8 * @var{len} @minus{} 1} and ending with zero.  For example:
@code{bits 2} unpacks @code{#x28} @code{#x1c} to @w{@code{(2 3 4 11 13)}}
and @code{#x1c} @code{#x28} to @w{@code{(3 5 10 11 12)}}.

@item fill @var{len}
@var{len} bytes used as a mere filler.  In packing, these bytes are
left unchanged, which normally means they remain zero.
When unpacking, this just returns nil.

@item align @var{len}
Same as @code{fill} except the number of bytes is that needed to skip
to the next multiple of @var{len} bytes.

@item type @var{exp}
This lets you refer to a type indirectly: @var{exp} is a Lisp
expression which should return a Bindat type @emph{value}.

@item unit @var{exp}
This is a trivial type which uses up 0 bits of space.  @var{exp}
describes the value returned when we try to ``unpack'' such a field.

@item struct @var{fields}...
Composite type made of several fields.  Every field is of the form
@code{(@var{name} @var{type})} where @var{type} can be any Bindat
type expression.  @var{name} can be @code{_} when the field's value
does not deserve to be named, as is often the case for @code{align}
and @code{fill} fields.
When the context makes it clear that this is a Bindat type expression,
the symbol @code{struct} can be omitted.
@end table

In the types above, @var{len} and @var{bitlen} are given as an integer
specifying the number of bytes (or bits) in the field.  When the
length of a field is not fixed, it typically depends on the value of
preceding fields.  For this reason, the length @var{len} does not have
to be a constant but can be any Lisp expression and it can refer to
the value of previous fields via their name.

For example, the specification of a data layout where a leading byte gives
the size of a subsequent vector of 16 bit integers could be:
@example
(bindat-type
  (len      u8)
  (payload  vec (1+ len) uint 16))
@end example

@node Bindat Functions
@subsection Functions to Unpack and Pack Bytes
@cindex bindat functions

  In the following documentation, @var{type} refers to a Bindat type
value as returned from @code{bindat-type}, @var{raw} to a byte
array, and @var{struct} to an alist representing unpacked field data.

@defun bindat-unpack type raw &optional idx
This function unpacks data from the unibyte string or byte
array @var{raw}
according to @var{type}.  Normally, this starts unpacking at the
beginning of the byte array, but if @var{idx} is non-@code{nil}, it
specifies a zero-based starting position to use instead.

The value is an alist or nested alist in which each element describes
one unpacked field.
@end defun

@defun bindat-get-field struct &rest name
This function selects a field's data from the nested alist
@var{struct}.  Usually @var{struct} was returned by
@code{bindat-unpack}.  If @var{name} corresponds to just one argument,
that means to extract a top-level field value.  Multiple @var{name}
arguments specify repeated lookup of sub-structures.  An integer
@var{name} acts as an array index.

For example, @w{@code{(bindat-get-field @var{struct} a b 2 c)}} means
to find field @code{c} in the third element of subfield @code{b} of
field @code{a}.  (This corresponds to @code{@var{struct}.a.b[2].c} in
the C programming language syntax.)
@end defun

  Although packing and unpacking operations change the organization of
data (in memory), they preserve the data's @dfn{total length}, which is
the sum of all the fields' lengths, in bytes.  This value is not
generally inherent in either the specification or alist alone; instead,
both pieces of information contribute to its calculation.  Likewise, the
length of a string or array being unpacked may be longer than the data's
total length as described by the specification.

@defun bindat-length type struct
This function returns the total length of the data in @var{struct},
according to @var{type}.
@end defun

@defun bindat-pack type struct &optional raw idx
This function returns a byte array packed according to @var{type} from
the data in the alist @var{struct}.  It normally creates and fills a
new byte array starting at the beginning.  However, if @var{raw}
is non-@code{nil}, it specifies a pre-allocated unibyte string or vector to
pack into.  If @var{idx} is non-@code{nil}, it specifies the starting
offset for packing into @var{raw}.

When pre-allocating, you should make sure @code{(length @var{raw})}
meets or exceeds the total length to avoid an out-of-range error.
@end defun

@defun bindat-ip-to-string ip
Convert the Internet address vector @var{ip} to a string in the usual
dotted notation.
@c FIXME?  Does it do IPv6?

@example
(bindat-ip-to-string [127 0 0 1])
     @result{} "127.0.0.1"
@end example
@end defun

@node Bindat Computed Types
@subsection Advanced data layout specifications
@cindex bindat computed types

Bindat type expressions are not limited to the types described
earlier.  They can also be arbitrary Lisp forms returning Bindat
type expressions.  For example, the type below describes data which
can either contain a 24-bit error code or a vector of bytes:

@example
(bindat-type
  (len      u8)
  (payload  . (if (zerop len) (uint 24) (vec (1- len)))))
@end example

@cindex bindat packing and unpacking into arbitrary types
Furthermore, while composite types are normally unpacked to (and
packed from) association lists, this can be changed via the use of
the following special keyword arguments:

@table @code
@item :unpack-val @var{exp}
When the list of fields ends with this keyword argument, then the value
returned when unpacking is the value of @var{exp} instead of the
standard alist.  @var{exp} can refer to all the previous fields by
their name.

@item :pack-val @var{exp}
If a field's type is followed by this keyword argument, then the value
packed into this field is returned by @var{exp} instead of being
extracted from the alist.

@item :pack-var @var{name}
If the list of fields is preceded by this keyword argument, then all
the subsequent @code{:pack-val} arguments can refer to the overall
value to pack into this composite type via the variable named
@var{name}.
@end table

For example, one could describe a 16-bit signed integer as follows:

@example
(defconst sint16-bindat-spec
  (let* ((max (ash 1 15))
         (wrap (+ max max)))
    (bindat-type :pack-var v
                 (n uint 16 :pack-val (if (< v 0) (+ v wrap) v))
                 :unpack-val (if (>= n max) (- n wrap) n))))
@end example

Which would then behave as follows:
@example
(bindat-pack sint16-bindat-spec -8)
     @result{} "\377\370"

(bindat-unpack sint16-bindat-spec "\300\100")
     @result{} -16320
@end example

@cindex define new bindat type forms
@cindex bindat, define new type forms
Finally, you can define new Bindat type forms to use in Bindat type
expressions with @code{bindat-defmacro}:

@defmac bindat-defmacro name args &rest body
Define a new Bindat type expression named @var{name} and taking
arguments @var{args}.  Its behavior follows that of @code{defmacro},
which the important difference that the new forms can only be used
within Bindat type expressions.
@end defmac