Obey coding-system-for-write when writing stdout/stderr in batch

* src/print.c (printchar_to_stream):
* src/xdisp.c (message_to_stderr): If coding-system-for-write has
a non-nil value, use it to encode output in preference to
locale-coding-system.  See the discussions in
http://lists.gnu.org/archive/html/emacs-devel/2016-01/msg00048.html
for the details.

* doc/lispref/os.texi (Terminal Output): Document how to send
non-ASCII text via 'send-string-to-terminal'.
(Batch Mode): Document how text written to standard streams is
encoded.  Fix inaccuracy regarding which output streams are used
by output functions in batch mode.

2 files changed, 21 insertions, 2 deletions

diff --git a/src/print.c b/src/print.c
index 975675014d9..269d8f250e2 100644
--- a/src/print.c
+++ b/src/print.c
@@ -200,6 +200,13 @@ printchar_to_stream (unsigned int ch, FILE *stream)
 {
   Lisp_Object dv IF_LINT (= Qnil);
   ptrdiff_t i = 0, n = 1;
+  Lisp_Object coding_system = Vlocale_coding_system;
+  bool encode_p = false;
+
+  if (!NILP (Vcoding_system_for_write))
+    coding_system = Vcoding_system_for_write;
+  if (!NILP (coding_system))
+    encode_p = true;
 
   if (CHAR_VALID_P (ch) && DISP_TABLE_P (Vstandard_display_table))
     {
@@ -228,8 +235,11 @@ printchar_to_stream (unsigned int ch, FILE *stream)
           unsigned char mbstr[MAX_MULTIBYTE_LENGTH];
           int len = CHAR_STRING (ch, mbstr);
           Lisp_Object encoded_ch =
-            ENCODE_SYSTEM (make_multibyte_string ((char *) mbstr, 1, len));
+            make_multibyte_string ((char *) mbstr, 1, len);
 
+          if (encode_p)
+            encoded_ch = code_convert_string_norecord (encoded_ch,
+                                                       coding_system, true);
           fwrite (SSDATA (encoded_ch), 1, SBYTES (encoded_ch), stream);
 #ifdef WINDOWSNT
           if (print_output_debug_flag && stream == stderr)
diff --git a/src/xdisp.c b/src/xdisp.c
index b18bfd0d49d..ee748bd8680 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -10206,7 +10206,16 @@ message_to_stderr (Lisp_Object m)
     }
   if (STRINGP (m))
     {
-      Lisp_Object s = ENCODE_SYSTEM (m);
+      Lisp_Object coding_system = Vlocale_coding_system;
+      Lisp_Object s;
+
+      if (!NILP (Vcoding_system_for_write))
+        coding_system = Vcoding_system_for_write;
+      if (!NILP (coding_system))
+        s = code_convert_string_norecord (m, coding_system, true);
+      else
+        s = m;
+
       fwrite (SDATA (s), SBYTES (s), 1, stderr);
     }
   if (!cursor_in_echo_area)