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@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001,
@c   2002, 2003, 2004, 2005, 2006, 2007  Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
@setfilename ../info/processes
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@node Processes, Display, Abbrevs, Top
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@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
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called a ``process.''  Lisp programs can use this object to communicate
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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.

@defun processp object
This function returns @code{t} if @var{object} is a process,
@code{nil} otherwise.
@end defun

@menu
* Subprocess Creation::      Functions that start subprocesses.
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* Shell Arguments::          Quoting an argument to pass it to a shell.
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* 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.
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* Query Before Exit::        Whether to query if exiting will kill a process.
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* Transaction Queues::	     Transaction-based communication with subprocesses.
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* Network::                  Opening network connections.
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* Network Servers::          Network servers let Emacs accept net connections.
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* Datagrams::                UDP network connections.
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* Low-Level Network::        Lower-level but more general function
                               to create connections and servers.
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* Misc Network::             Additional relevant functions for network connections.
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* Byte Packing::             Using bindat to pack and unpack binary data.
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@end menu

@node Subprocess Creation
@section Functions that Create Subprocesses

  There are three functions that create a new subprocess in which to run
a program.  One of them, @code{start-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}).

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

@cindex execute program
@cindex @code{PATH} environment variable
@cindex @code{HOME} environment variable
  In all cases, the function's @var{program} argument specifies 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 @code{PATH}.  The standard file name
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constructs, @samp{~}, @samp{.}, and @samp{..}, are interpreted as
usual in @code{exec-path}, but environment variable substitutions
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(@samp{$HOME}, etc.) are not recognized; use
@code{substitute-in-file-name} to perform them (@pxref{File Name
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Expansion}).  @code{nil} in this list refers to
@code{default-directory}.
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  Executing a program can also try adding suffixes to the specified
name:

@defvar 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 defvar

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

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  Each of the subprocess-creating functions has a @var{buffer-or-name}
argument which specifies where the standard output from the program will
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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 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.
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@cindex program arguments
  All three of the subprocess-creating functions have a @code{&rest}
argument, @var{args}.  The @var{args} must all be strings, and they are
supplied to @var{program} as separate command line arguments.  Wildcard
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characters and other shell constructs have no special meanings in these
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strings, since the strings are passed directly to the specified program.
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  The subprocess gets its current directory from the value of
@code{default-directory} (@pxref{File Name Expansion}).

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

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

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@node Shell Arguments
@section Shell Arguments
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@cindex arguments for shell commands
@cindex shell command arguments
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  Lisp programs sometimes need to run a shell and give it a command
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that contains file names that were specified by the user.  These
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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
This function returns a string which 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
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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.
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@example
;; @r{This example shows the behavior on GNU and Unix systems.}
(shell-quote-argument "foo > bar")
     @result{} "foo\\ \\>\\ bar"
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;; @r{This example shows the behavior on MS-DOS and MS-Windows.}
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(shell-quote-argument "foo > bar")
     @result{} "\"foo > bar\""
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@end example

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

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

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

  After a @dfn{synchronous process} is created, Emacs waits for the
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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.
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  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
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@code{SIGKILL} and quits immediately (except on MS-DOS, where killing
other processes doesn't work).  @xref{Quitting}.
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  The synchronous subprocess functions return an indication of how the
process terminated.
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  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}.
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@defun call-process program &optional infile destination display &rest args
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This function calls @var{program} in a separate process and waits for
it to finish.

The standard input for the process comes from file @var{infile} if
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@var{infile} is not @code{nil}, and from the null device otherwise.
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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 string
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Insert the output in a buffer with that name, before point.
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@item @code{t}
Insert the output in the current buffer, before point.

@item @code{nil}
Discard the output.

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

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

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@item @code{(@var{real-destination} @var{error-destination})}
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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}.
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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.
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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.
@end table
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If @var{display} is non-@code{nil}, then @code{call-process} redisplays
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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
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non-@acronym{ASCII} characters are encountered.  There are fundamental
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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.
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The remaining arguments, @var{args}, are strings that specify command
line arguments for the program.

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.

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

@smallexample
@group
(call-process "pwd" nil t)
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     @result{} 0
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---------- Buffer: foo ----------
/usr/user/lewis/manual
---------- Buffer: foo ----------
@end group

@group
(call-process "grep" nil "bar" nil "lewis" "/etc/passwd")
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     @result{} 0
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---------- Buffer: bar ----------
lewis:5LTsHm66CSWKg:398:21:Bil Lewis:/user/lewis:/bin/csh

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

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Here is a good example of the use of @code{call-process}, which used to
be found in the definition of @code{insert-directory}:
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@smallexample
@group
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(call-process insert-directory-program nil t nil @var{switches}
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              (if full-directory-p
                  (concat (file-name-as-directory file) ".")
                file))
@end group
@end smallexample
@end defun

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@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 handler based
on the value of the variable @code{default-directory}.  The current
working directory of the subprocess is @code{default-directory}.

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

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

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

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

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If @var{buffer} is a list of the form @code{(@var{real-destination}
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@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.
@end defun

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@defun call-process-region start end program &optional delete destination display &rest args
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This function sends the text from @var{start} to @var{end} as
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standard input to a process running @var{program}.  It deletes the text
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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.
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The arguments @var{destination} and @var{display} control what to do
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with the output from the subprocess, and whether to update the display
as it comes in.  For details, see the description of
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@code{call-process}, above.  If @var{destination} is the integer 0,
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@code{call-process-region} discards the output and returns @code{nil}
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immediately, without waiting for the subprocess to finish (this only
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works if asynchronous subprocesses are supported).
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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
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@var{destination} is @code{t}, this output is inserted in the current
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buffer.

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

@group
(call-process-region 1 6 "cat" nil t)
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     @result{} 0
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---------- Buffer: foo ----------
inputinput@point{}
---------- Buffer: foo ----------
@end group
@end smallexample

  The @code{shell-command-on-region} command uses
@code{call-process-region} like this:

@smallexample
@group
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(call-process-region
 start end
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 shell-file-name      ; @r{Name of program.}
 nil                  ; @r{Do not delete region.}
 buffer               ; @r{Send output to @code{buffer}.}
 nil                  ; @r{No redisplay during output.}
 "-c" command)        ; @r{Arguments for the shell.}
@end group
@end smallexample
@end defun

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@defun call-process-shell-command command &optional infile destination display &rest args
This function executes the shell command @var{command} synchronously
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in a separate process.  The final arguments @var{args} are additional
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arguments to add at the end of @var{command}.  The other arguments
are handled as in @code{call-process}.
@end defun

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@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

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

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  After an @dfn{asynchronous process} is created, Emacs and the subprocess
both continue running immediately.  The process thereafter runs
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in parallel with Emacs, and the two can communicate with each other
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using the functions described in the following sections.  However,
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communication is only partially asynchronous: Emacs sends data to the
process only when certain functions are called, and Emacs accepts data
from the process only when Emacs is waiting for input or for a time
delay.

  Here we describe how to create an asynchronous process.
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@defun start-process name buffer-or-name program &rest args
This function creates a new asynchronous subprocess and starts the
program @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; if a process with this name
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already exists, then @var{name} is modified (by appending @samp{<1>},
etc.) to be unique.  The buffer @var{buffer-or-name} is the buffer to
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associate with the process.

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

In the example below, the first process is started and runs (rather,
sleeps) for 100 seconds.  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" "/user/lewis/bin")
     @result{} #<process my-process<1>>

---------- Buffer: foo ----------
total 2
lrwxrwxrwx  1 lewis     14 Jul 22 10:12 gnuemacs --> /emacs
-rwxrwxrwx  1 lewis     19 Jul 30 21:02 lemon

Process my-process<1> finished

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

@defun start-process-shell-command name buffer-or-name command &rest command-args
This function is like @code{start-process} except that it uses a shell
to execute the specified command.  The argument @var{command} is a shell
command name, and @var{command-args} are the arguments for the shell
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command.  The variable @code{shell-file-name} specifies which shell to
use.
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The point of running a program through the shell, rather than directly
with @code{start-process}, is so that you can employ shell features such
as wildcards in the arguments.  It follows that if you include an
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arbitrary user-specified arguments in the command, you should quote it
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with @code{shell-quote-argument} first, so that any special shell
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characters do @emph{not} have their special shell meanings.  @xref{Shell
Arguments}.
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@end defun

@defvar process-connection-type
@cindex pipes
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@cindex @acronym{PTY}s
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This variable controls the type of device used to communicate with
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asynchronous subprocesses.  If it is non-@code{nil}, then @acronym{PTY}s are
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used, when available.  Otherwise, pipes are used.
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@acronym{PTY}s are usually preferable for processes visible to the user, as
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in Shell mode, because they allow job control (@kbd{C-c}, @kbd{C-z},
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etc.) to work between the process and its children, whereas pipes do
not.  For subprocesses used for internal purposes by programs, it is
often better to use a pipe, because they are more efficient.  In
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addition, the total number of @acronym{PTY}s is limited on many systems and
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it is good not to waste them.
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The value of @code{process-connection-type} takes effect when
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@code{start-process} is called.  So you can specify how to communicate
with one subprocess by binding the variable around the call to
@code{start-process}.

@smallexample
@group
(let ((process-connection-type nil))  ; @r{Use a pipe.}
  (start-process @dots{}))
@end group
@end smallexample
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To determine whether a given subprocess actually got a pipe or a
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@acronym{PTY}, use the function @code{process-tty-name} (@pxref{Process
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Information}).
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@end defvar

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

  @dfn{Deleting a process} disconnects Emacs immediately from the
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subprocess.  Processes are deleted automatically after they terminate,
but not necessarily right away.  You can delete a process explicitly
at any time.  If you delete a terminated process explicitly before it
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is deleted automatically, no harm results.  Deleting a running
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process sends a signal to terminate it (and its child processes if
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any), and calls the process sentinel if it has one.  @xref{Sentinels}.
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  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.
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@defopt delete-exited-processes
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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.
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@end defopt
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@defun delete-process process
This function deletes a process, killing it with a @code{SIGKILL}
signal.  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.)  Calling
@code{delete-process} on a running process terminates it, updates the
process status, and runs the sentinel (if any) 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).
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@smallexample
@group
(delete-process "*shell*")
     @result{} nil
@end group
@end smallexample
@end defun

@node Process Information
@section Process Information

  Several functions return information about processes.
@code{list-processes} is provided for interactive use.

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@deffn Command list-processes &optional query-only
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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}.
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If @var{query-only} is non-@code{nil} then it lists only processes
whose query flag is non-@code{nil}.  @xref{Query Before Exit}.
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@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 get-process name
This function returns the process named @var{name}, or @code{nil} if
there is none.  An error is signaled if @var{name} is not a string.

@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.

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

@defun process-id process
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This function returns the @acronym{PID} of @var{process}.  This is an
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integer that distinguishes the process @var{process} from all other
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processes running on the same computer at the current time.  The
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@acronym{PID} of a process is chosen by the operating system kernel when the
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process is started and remains constant as long as the process exists.
@end defun

@defun process-name process
This function returns the name of @var{process}.
@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, a
process name (string) or a buffer name (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 connection that is open.
@item closed
for a network 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.
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@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.
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@item nil
if @var{process-name} is not the name of an existing process.
@end table

@smallexample
@group
(process-status "shell")
     @result{} run
@end group
@group
(process-status (get-buffer "*shell*"))
     @result{} run
@end group
@group
x
     @result{} #<process xx<1>>
(process-status x)
     @result{} exit
@end group
@end smallexample

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

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@defun process-tty-name process
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 terminal (see @code{process-connection-type} in
@ref{Asynchronous Processes}).
@end defun

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@defun process-coding-system process
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@anchor{Coding systems for a subprocess}
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This function returns a cons cell describing the coding systems in use
for decoding output from @var{process} and for encoding input to
@var{process} (@pxref{Coding Systems}).  The value has this form:

@example
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(@var{coding-system-for-decoding} . @var{coding-system-for-encoding})
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@end example
@end defun

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@defun set-process-coding-system process &optional decoding-system encoding-system
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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.
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@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}.
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@end defun

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@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.  The
data appears on the ``standard input'' of the subprocess.

  Some operating systems have limited space for buffered input in a
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@acronym{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.
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  Subprocess input is normally encoded using a coding system before the
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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}).
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  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, sentinels
and timers to run---so take account of that in writing your code.

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  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.  If it is @code{nil}, the current buffer's process is used.
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  The function returns @code{nil}.

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


@group
---------- Buffer: *shell* ----------
...
introduction.texi               syntax-tables.texi~
introduction.texi~              text.texi
introduction.txt                text.texi~
...
---------- Buffer: *shell* ----------
@end group
@end smallexample
@end defun

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@defun process-send-region process start end
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This function sends the text in the region defined by @var{start} and
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@var{end} as standard input to @var{process}.
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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.)
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@end defun
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@defun process-send-eof &optional process
This function makes @var{process} see an end-of-file in its
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input.  The @acronym{EOF} comes after any text already sent to it.
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The function returns @var{process}.
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@smallexample
@group
(process-send-eof "shell")
     @result{} "shell"
@end group
@end smallexample
@end defun

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@defun process-running-child-p process
This function will tell you whether a subprocess has given control of
its terminal to its own child process.  The value is @code{t} if this is
true, or if Emacs cannot tell; it is @code{nil} if Emacs can be certain
that this is not so.
@end defun

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@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.)

  Each of the signal-sending functions takes two optional arguments:
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@var{process} and @var{current-group}.
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  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.  An error
is signaled if @var{process} does not identify a process.
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  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
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of the terminal that Emacs uses to communicate with the subprocess.  If
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the process is a job-control shell, this means the shell's current
subjob.  If it 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.

  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}.

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@defun interrupt-process &optional process current-group
This function interrupts the process @var{process} by sending the
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signal @code{SIGINT}.  Outside of Emacs, typing the ``interrupt
character'' (normally @kbd{C-c} on some systems, and @code{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

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@defun kill-process &optional process current-group
This function kills the process @var{process} by sending the
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signal @code{SIGKILL}.  This signal kills the subprocess immediately,
and cannot be handled by the subprocess.
@end defun

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

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@defun stop-process &optional process current-group
This function stops the process @var{process} by sending the
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signal @code{SIGTSTP}.  Use @code{continue-process} to resume its
execution.

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Outside of Emacs, on systems with job control, the ``stop character''
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(usually @kbd{C-z}) normally sends this signal.  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.
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@end defun

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@defun continue-process &optional process current-group
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This function resumes execution of the process @var{process} by sending
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it the signal @code{SIGCONT}.  This presumes that @var{process} was
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stopped previously.
@end defun

@c Emacs 19 feature
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@defun signal-process process signal
This function sends a signal to process @var{process}.  The argument
@var{signal} specifies which signal to send; it should be an integer.

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The @var{process} argument can be a system process @acronym{ID}; that
allows you to send signals to processes that are not children of
Emacs.
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@end defun

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

  There are two ways to receive the output that a subprocess writes to
its standard output stream.  The output can be inserted in a buffer,
which is called the associated buffer of the process, or a function
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called the @dfn{filter function} can be called to act on the output.  If
the process has no buffer and no filter function, its output is
discarded.
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  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.

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  Output from a subprocess can arrive only while Emacs is waiting: when
reading terminal input, in @code{sit-for} and @code{sleep-for}
(@pxref{Waiting}), and in @code{accept-process-output} (@pxref{Accepting
Output}).  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.

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

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  It is impossible to separate the standard output and standard error
streams of the subprocess, because Emacs normally spawns the subprocess
inside a pseudo-TTY, and a pseudo-TTY has only one output channel.  If
you want to keep the output to those streams separate, you should
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redirect one of them to a file---for example, by using an appropriate
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shell command.

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@menu
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* Process Buffers::         If no filter, output is put in a buffer.
* Filter Functions::        Filter functions accept output from the process.
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* Decoding Output::         Filters can get unibyte or multibyte strings.
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* Accepting Output::        How to wait until process output arrives.
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@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.

  Unless the process has a filter function (@pxref{Filter Functions}),
its output is inserted in the associated buffer.  The position to insert
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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.
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@defun process-buffer process
This function returns the associated buffer of the process
@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.

Insertion of process output in a buffer uses this marker to decide where
to insert, and updates it to point after the inserted text.  That is why
successive batches of output are inserted consecutively.

Filter functions normally should use this marker in the same fashion
as is done by direct insertion of output in the buffer.  A good
example of a filter function that uses @code{process-mark} is found at
the end of the following section.

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

@defun get-buffer-process buffer-or-name
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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.
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@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

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

  A process @dfn{filter function} is a function that receives the
standard output from the associated process.  If a process has a filter,
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then @emph{all} output from that process is passed to the filter.  The
process buffer is used directly for output from the process only when
there is no filter.
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  The filter function can only be called when Emacs is waiting for
something, because process output arrives only at such times.  Emacs
waits when reading terminal input, in @code{sit-for} and
@code{sleep-for} (@pxref{Waiting}), and in @code{accept-process-output}
(@pxref{Accepting Output}).

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  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.
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  Quitting is normally inhibited within a filter function---otherwise,
the effect of typing @kbd{C-g} at command level or to quit a user
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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}.
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  If an error happens during execution of a filter function, it is
caught automatically, so that it doesn't stop the execution of whatever
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program was running when the filter function was started.  However, if
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@code{debug-on-error} is non-@code{nil}, the error-catching is turned
off.  This makes it possible to use the Lisp debugger to debug the
filter function.  @xref{Debugger}.

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  Many filter functions sometimes or always insert the text in the
process's buffer, mimicking the actions of Emacs when there is no
filter.  Such filter functions need to use @code{set-buffer} in order to
be sure to insert in that buffer.  To avoid setting the current buffer
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semipermanently, these filter functions must save and restore the
current buffer.  They should also update the process marker, and in some
cases update the value of point.  Here is how to do these things:
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@smallexample
@group
(defun ordinary-insertion-filter (proc string)
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  (with-current-buffer (process-buffer proc)
    (let ((moving (= (point) (process-mark proc))))
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@end group
@group
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      (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))))))
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@end group
@end smallexample

@noindent
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The reason to use @code{with-current-buffer}, rather than using
@code{save-excursion} to save and restore the current buffer, is so as
to preserve the change in point made by the second call to
@code{goto-char}.
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  To make the filter force the process buffer to be visible whenever new
text arrives, insert the following line just before the
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@code{with-current-buffer} construct:
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@smallexample
(display-buffer (process-buffer proc))
@end smallexample

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  To force point to the end of the new output, no matter where it was
previously, eliminate the variable @code{moving} and call
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@code{goto-char} unconditionally.

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  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.  @xref{Match Data}.
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  A filter function that writes the output into the buffer of the
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process should check whether the buffer is still alive.  If it tries to
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insert into a dead buffer, it will get an error.  The expression
@code{(buffer-name (process-buffer @var{process}))} returns @code{nil}
if the buffer is dead.
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  The output to the function may come in chunks of any size.  A program
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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.
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@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 no filter.
@end defun

@defun process-filter process
This function returns the filter function of @var{process}, or @code{nil}
if it has none.
@end defun

  Here is an example of 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[8] % "
@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 is done when
there is no filter function:

@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)
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  (let ((cur (selected-window))
        (pop-up-windows t))
    (pop-to-buffer my-shell-buffer)
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@end group
@group
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    (goto-char (point-max))
    (insert str)
    (set-marker (process-mark proc) (point-max))
    (select-window cur)))
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@end group
@end smallexample
@end ignore

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@node Decoding Output
@subsection Decoding Process Output
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@cindex decode process output
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  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
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@code{string-to-multibyte}, and inserts the resulting multibyte text.
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  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}).

  @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}.

@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 multibyte flag.  If the flag is non-@code{nil}, Emacs
decodes the output according to the process output coding system to
produce a multibyte string, and passes that to the process.  If the
flag is @code{nil}, Emacs puts the output into a unibyte string, with
no decoding, and passes that.

  When you create a process, the filter multibyte flag takes its
initial value from @code{default-enable-multibyte-characters}.  If you
want to change the flag later on, use
@code{set-process-filter-multibyte}.

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

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

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

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@defun accept-process-output &optional process seconds microsec just-this-one
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This function allows Emacs to read pending output from processes.  The
output is inserted in the associated buffers or 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}.

@c Emacs 19 feature
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The arguments @var{seconds} and @var{microsec} let you specify timeout
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periods.  The former specifies a period measured in seconds and the
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latter specifies one measured in microseconds.  The two time periods
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thus specified are added together, and @code{accept-process-output}
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returns after that much time, whether or not there has been any
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subprocess output.

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The argument @var{microsec} is semi-obsolete nowadays because
@var{seconds} can be a floating point number to specify waiting a
fractional number of seconds.  If @var{seconds} is 0, the function
accepts whatever output is pending but does not wait.
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@c Emacs 22.1 feature
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If @var{process} is a process, and the argument @var{just-this-one} is
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non-@code{nil}, only output from that process is handled, suspending output
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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.

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The function @code{accept-process-output} returns non-@code{nil} if it
did get some output, or @code{nil} if the timeout expired before output
arrived.
@end defun

@node Sentinels
@section Sentinels: Detecting Process Status Changes
@cindex process sentinel
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@cindex sentinel (of process)
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  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
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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.
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  The string describing the event looks like one of the following:

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

@item
@code{"exited abnormally with code @var{exitcode}\n"}.

@item
@code{"@var{name-of-signal}\n"}.

@item
@code{"@var{name-of-signal} (core dumped)\n"}.
@end itemize

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  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 them 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}
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(@pxref{Waiting}), or @code{accept-process-output} (@pxref{Accepting
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Output}).  Emacs also allows sentinels to run when the command loop is
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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.