Improve documentation of 'process-connection-type'

* doc/lispref/processes.texi (Asynchronous Processes): Clarify
better when it is advisable to use pipes for communicating
with subprocesses.  (Bug#33050)

diff --git a/doc/lispref/processes.texi b/doc/lispref/processes.texi
index 34426f339c6f5395a1314883ebc7b8a8d5ce6e04..0868912b14530e0ac102241a9aceecac734daf6c 100644 (file)
--- a/doc/lispref/processes.texi
+++ b/doc/lispref/processes.texi
@@ -590,8 +590,8 @@ communication is only partially asynchronous: Emacs sends data to the
 process only when certain functions are called, and Emacs accepts data
 from the process only while waiting for input or for a time delay.
 
-@cindex pty
-@cindex pipe
+@cindex pty, when to use for subprocess communications
+@cindex pipe, when to use for subprocess communications
   An asynchronous process is controlled either via a @dfn{pty}
 (pseudo-terminal) or a @dfn{pipe}.  The choice of pty or pipe is made
 when creating the process, by default based on the value of the
@@ -601,11 +601,13 @@ Shell mode, because they allow for job control (@kbd{C-c}, @kbd{C-z},
 etc.)@: between the process and its children, and because interactive
 programs treat ptys as terminal devices, whereas pipes don't support
 these features.  However, for subprocesses used by Lisp programs for
-internal purposes, it is often better to use a pipe, because pipes are
+internal purposes (i.e., with no user interaction), where significant
+amounts of data need to be exchanged between the subprocess and the
+Lisp program, it is often better to use a pipe, because pipes are
 more efficient, and because they are immune to stray character
 injections that ptys introduce for large (around 500 byte) messages.
-Also, the total number of ptys is limited on many systems and it is
-good not to waste them.
+Also, the total number of ptys is limited on many systems, and it is
+good not to waste them unnecessarily.
 
 @defun make-process &rest args
 This function is the basic low-level primitive for starting
@@ -658,7 +660,9 @@ pipe, or @code{nil} to use the default derived from the value of the
 @code{process-connection-type} variable.  This parameter and the value
 of @code{process-connection-type} are ignored if a non-@code{nil}
 value is specified for the @code{:stderr} parameter; in that case, the
-type will always be @code{pipe}.
+type will always be @code{pipe}.  On systems where ptys are not
+available (MS-Windows), this parameter is likewise ignored, and pipes
+are used unconditionally.
 
 @item :noquery @var{query-flag}
 Initialize the process query flag to @var{query-flag}.
@@ -863,7 +867,8 @@ around the call to these functions.
 Note that the value of this variable is ignored when
 @code{make-process} is called with a non-@code{nil} value of the
 @code{:stderr} parameter; in that case, Emacs will communicate with
-the process using pipes.
+the process using pipes.  It is also ignored if ptys are unavailable
+(MS-Windows).
 
 @smallexample
 @group