emacs/lispref/processes.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/processes
@node Processes, System Interface, Abbrevs, Top
@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.

@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.
* Synchronous Processes::    Details of using synchronous subprocesses.
* MS-DOS Subprocesses::      On MS-DOS, you must indicate text vs binary
                                for data sent to and from a subprocess.
* 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.
* Transaction Queues::	     Transaction-based communication with subprocesses.
* Network::                  Opening network connections.
@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}).

  Synchronous and asynchronous processes are explained in following
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
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}).

  Each of the subprocess-creating functions has a @var{buffer-or-name}
argument which specifies where the standard output from the program will
go.  If @var{buffer-or-name} is @code{nil}, that 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.

@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
characters and other shell constructs are not allowed in these strings,
since they are passed directly to the specified program.  

  @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.

  The subprocess gets its current directory from the value of
@code{default-directory} (@pxref{File Name Expansion}).

@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}.

@defvar exec-directory 
@pindex wakeup
The value of this variable is the name of a directory (a string) that
contains programs that come with GNU Emacs, that are intended for Emacs
to invoke.  The program @code{wakeup} is an example of such a program;
the @code{display-time} command uses it to get a reminder once per
minute.
@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

@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 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.  @xref{Quitting}.

  The synchronous subprocess functions returned @code{nil} in version
18.  In version 19, they return an indication of how the process
terminated.

@defun call-process program &optional infile destination display &rest args
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
@var{infile} is not @code{nil} and from @file{/dev/null} 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 string
Find or create a buffer with that name, then insert
the output in that buffer, 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 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.

@item (@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}.
The value @code{nil} means discard it, @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.
@end table

If @var{display} is non-@code{nil}, then @code{call-process} redisplays
the buffer as output is inserted.  Otherwise the function 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.

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)
     @result{} nil

---------- Buffer: foo ----------
/usr/user/lewis/manual
---------- Buffer: foo ----------
@end group

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

---------- Buffer: bar ----------
lewis:5LTsHm66CSWKg:398:21:Bil Lewis:/user/lewis:/bin/csh

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

The @code{insert-directory} function contains a good example of the use
of @code{call-process}:

@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 call-process-region start end program &optional delete destination display &rest args
This function sends the text between @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{buffer}
is @code{t}, to insert the output in the current buffer.

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.

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{} nil

---------- 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
(call-process-region 
 start end         
 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

@node MS-DOS Subprocesses
@section MS-DOS Subprocesses

  On MS-DOS, you must indicate whether the data going to and from
a synchronous subprocess are text or binary.  Text data requires
translation between the end-of-line convention used within Emacs
(a single newline character) and the convention used outside Emacs
(the two-character sequence, @sc{crlf}).

  The variable @code{binary-process-input} applies to input sent to the
subprocess, and @code{binary-process-output} applies to output received
from it.  A non-@code{nil} value means the data is non-text; @code{nil}
means the data is text, and calls for conversion.

@defvar binary-process-input
If this variable is @code{nil}, convert newlines to @sc{crlf} sequences in
the input to a synchronous subprocess.
@end defvar

@defvar binary-process-output
If this variable is @code{nil}, convert @sc{crlf} sequences to newlines in
the output from a synchronous subprocess.
@end defvar

  @xref{Files and MS-DOS}, for related information.

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

  After an @dfn{asynchronous process} is created, Emacs and the Lisp
program both continue running immediately.  The process may thereafter
run in parallel with Emacs, and the two may communicate with each other
using the functions described in following sections.  Here we describe
how to create an asynchronous process with @code{start-process}.

@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
already exists, then @var{name} is modified (by adding @samp{<1>}, etc.)
to be unique.  The buffer @var{buffer-or-name} is the buffer to
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
command.
@end defun

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

@sc{pty}s are usually preferable for processes visible to the user, as
in Shell mode, because they allow job control (@kbd{C-c}, @kbd{C-z},
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 addition, the
total number of @sc{pty}s is limited on many systems and it is good not
to waste them.

The value @code{process-connection-type} is used when
@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

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

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

  @dfn{Deleting a process} disconnects Emacs immediately from the
subprocess, and removes it from the list of active processes.  It sends
a signal to the subprocess to make the subprocess terminate, but this is
not guaranteed to happen immediately.  The process object itself
continues to exist as long as other Lisp objects point to it.

  You can delete a process explicitly at any time.  Processes are
deleted automatically after they terminate, but not necessarily right
away.  If you delete a terminated process explicitly before it is
deleted automatically, no harm results.

@defvar 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 defvar

@defun delete-process name
This function deletes the process associated with @var{name}, killing it
with a @code{SIGHUP} signal.  The argument @var{name} may be a process,
the name of a process, a buffer, or the name of a buffer.

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

@defun process-kill-without-query process
This function declares that Emacs need not query the user if
@var{process} is still running when Emacs is exited.  The process will
be deleted silently.  The value is @code{t}.

@smallexample
@group
(process-kill-without-query (get-process "shell"))
     @result{} t
@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.

@deffn Command list-processes
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}.
@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
This function returns the @sc{pid} of @var{process}.  This is an
integer that distinguishes the process @var{process} from all other
processes running on the same computer at the current time.  The
@sc{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.
@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.
@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}.

In earlier Emacs versions (prior to version 19), the status of a network
connection was @code{run} if open, and @code{exit} if closed.
@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

@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

@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
@sc{pty}.  On these systems, Emacs sends an @sc{eof} periodically amidst
the other characters, to force them through.  For most programs,
these @sc{eof}s do no harm.

@defun process-send-string process-name string
This function sends @var{process-name} the contents of @var{string} as
standard input.  The argument @var{process-name} must be a process or
the name of a process.  If it is @code{nil}, the current buffer's
process is used.

  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

@deffn Command process-send-region process-name start end
This function sends the text in the region defined by @var{start} and
@var{end} as standard input to @var{process-name}, which is a process or
a process name.  (If it is @code{nil}, the current buffer's process is
used.)

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 deffn

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

  If @var{process-name} is not supplied, or if it is @code{nil}, then
this function sends the @sc{eof} to the current buffer's process.  An
error is signaled if the current buffer has no process.

  The function returns @var{process-name}.

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

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

  The argument @var{process-name} must be either a process, the name of
one, or @code{nil}.  If it is @code{nil}, the process defaults to the
process associated with the current buffer.  An error is signaled if
@var{process-name} does not identify a process.

  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 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}.

@defun interrupt-process &optional process-name current-group
This function interrupts the process @var{process-name} by sending the
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

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

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

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

On systems with job control, the ``stop character'' (usually @kbd{C-z})
sends this signal (outside of Emacs).  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-name current-group
This function resumes execution of the process @var{process} by sending
it the signal @code{SIGCONT}.  This presumes that @var{process-name} was
stopped previously.
@end defun

@c Emacs 19 feature
@defun signal-process pid signal
This function sends a signal to process @var{pid}, which need not be
a child of Emacs.  The argument @var{signal} specifies which signal
to send; it should be an integer.
@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
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.

@menu
* Process Buffers::       If no filter, output is put in a buffer.
* Filter Functions::      Filter functions accept output from the process.
* Accepting Output::      Explicitly permitting subprocess output.
                            Waiting for subprocess output.
@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
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.

@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
transmission to the process, the process marker is useful for
distinguishing 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.
@end defun

@defun get-buffer-process buffer-or-name
This function returns the process associated with @var{buffer-or-name}.
If there are several processes associated with it, then one is chosen.
(Presently, the one chosen is the one most recently created.)  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

@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,
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.

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

  A filter function 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 filters at random places in
the middle of other Lisp programs.  You may explicitly cause Emacs to
wait, so that filter functions will run, by calling @code{sit-for} or
@code{sleep-for} (@pxref{Waiting}), or @code{accept-process-output}
(@pxref{Accepting Output}).  Emacs is also waiting when the command loop
is reading input.

  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}.
@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}, the error-catching is turned
off.  This makes it possible to use the Lisp debugger to debug the
filter function.  @xref{Debugger}.

  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
semipermanently, these filter functions must use @code{unwind-protect}
to make sure to restore the previous 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:

@smallexample
@group
(defun ordinary-insertion-filter (proc string)
  (let ((old-buffer (current-buffer)))
    (unwind-protect
        (let (moving)
          (set-buffer (process-buffer proc))
          (setq moving (= (point) (process-mark proc)))
@end group
@group
          (save-excursion
            ;; @r{Insert the text, moving the process-marker.}
            (goto-char (process-mark proc))
            (insert string)
            (set-marker (process-mark proc) (point)))
          (if moving (goto-char (process-mark proc))))
      (set-buffer old-buffer))))
@end group
@end smallexample

@noindent
The reason to use an explicit @code{unwind-protect} rather than letting
@code{save-excursion} restore the current buffer is so as to preserve
the change in point made by @code{goto-char}.

  To make the filter force the process buffer to be visible whenever new
text arrives, insert the following line just before the
@code{unwind-protect}:

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

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

  In earlier Emacs versions, every filter function that did regexp
searching or matching had to explicitly save and restore the match data.
Now Emacs does this automatically; filter functions never need to do it
explicitly.  @xref{Match Data}.

  A filter function 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}.

  The output to the function 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.

@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)
    (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 Accepting Output
@subsection Accepting Output 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
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
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 whether or not there has been any
subprocess output.

The argument @var{seconds} need not be an integer.  If it is a floating
point number, this function waits for a fractional number of seconds.
Some systems support only a whole number of seconds; on these systems,
@var{seconds} is rounded down.  If the system doesn't support waiting
fractions of a second, you get an error if you specify nonzero
@var{millisec}.

Not all operating systems support waiting periods other than multiples
of a second; on those that do not, you get an error if you specify
nonzero @var{millisec}.

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
@cindex sentinel

  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.

  The string describing the event looks like one of the following:

@itemize @bullet
@item 
@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

  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}
(@pxref{Waiting}), or @code{accept-process-output} (@pxref{Accepting
Output}).  Emacs is also waiting when the command loop is reading input.

  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}.  @xref{Quitting}.

  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}.

  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}, the error-catching is turned
off.  This makes it possible to use the Lisp debugger to debug the
sentinel.  @xref{Debugger}.

  In earlier Emacs versions, every sentinel that did regexp searching or
matching had to explicitly save and restore the match data.  Now Emacs
does this automatically; sentinels never need to do it explicitly.
@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 no sentinel.
The default behavior when there is no sentinel is to insert a message in
the process's buffer when the process status changes.

@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}, or @code{nil} if it
has none.
@end defun

@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, @code{nil} if it
was not.
@end defun

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

You can use a @dfn{transaction queue} for more convenient communication
with subprocesses 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
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 the
entire answer, but nothing less; that's how @code{tq-enqueue} determines
where the answer ends.

The return value of @code{tq-enqueue} itself is not meaningful.
@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

  Emacs Lisp programs can open TCP network connections 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, so 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 process at the other end; that process must decide
what to do about closure of the connection.

  You can distinguish process objects representing network connections
from those representing subprocesses with the @code{process-status}
function.  It always returns either @code{open} or @code{closed} for a
network connection, and it never returns either of those values for a
real subprocess.  @xref{Process Information}.

@defun open-network-stream name buffer-or-name host service
This function opens a TCP connection for a service to a host.  It
returns a process object to represent 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-or-name} argument is the buffer to associate with the
connection.  Output from the connection is inserted in the buffer,
unless you specify a filter function to handle the output.  If
@var{buffer-or-name} 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).
@end defun