[PATCH] Unbuffer stdout and stderr on windows

[Yao Qi]

On 08/16/2013 11:10 PM, Pedro Alves wrote:
> - First, make stderr always unbuffered.  That's what you get on
> most platforms anyway.
> - Then, somewhere along fputs_unfiltered or some central output routine,
> keep track of which was the last to be used between stdout and stderr.
> If outputting to stderr, flush stdout first.  Don't need to do the
> opposite, since stderr will always be unbuffered.

This patch below generally implements what we discussed here...

In this patch, we firstly set stderr unbuffered.  Then, we overwrite
gdb_stderr's methods 'to_write' and 'to_fputs', so that gdb_stdout
is flushed when something is written to gdb_stderr.

Regression tested on mingw32 native gdb with a local patch to set
stdout/stderr to binary mode.

-- 
Yao (齐尧)

gdb:

2013-08-22  Yao Qi  <yao@codesourcery.com>

	* main.c (captured_main) [__MINGW32__]: Set stderr unbuffered.
	and all update_stderr_ui_file.
	* ui-file.c (stderr_file_write): New function.
	(stderr_file_fputs): New function.
	(update_stderr_ui_file): New function.
	* ui-file.h (update_stderr_ui_file): Declare.
---
 gdb/main.c    |   19 +++++++++++++++++++
 gdb/ui-file.c |   26 ++++++++++++++++++++++++++
 gdb/ui-file.h |    3 +++
 3 files changed, 48 insertions(+), 0 deletions(-)

diff --git a/gdb/main.c b/gdb/main.c
index 1c240e4..8e00c27 100644
--- a/gdb/main.c
+++ b/gdb/main.c
@@ -375,8 +375,27 @@ captured_main (void *data)
   saved_command_line[0] = '\0';
   instream = stdin;
 
+#ifdef __MINGW32__
+  /* Ensure stderr is unbuffered.  */
+  setvbuf (stderr, NULL, _IONBF, BUFSIZ);
+#endif
+
   gdb_stdout = stdio_fileopen (stdout);
   gdb_stderr = stdio_fileopen (stderr);
+#ifdef __MINGW32__
+  /* There is no real line-buffering on Windows, see
+     http://msdn.microsoft.com/en-us/library/86cebhfs%28v=vs.71%29.aspx
+     so the stdout is either fully-buffered or non-buffered.  We can't
+     make stdout non-buffered, because of two concerns,
+     1.  non-buffering hurts performance,
+     2.  non-buffering may change GDB's behavior when it is interacting
+     with front-end, such as Emacs.
+
+     We decide to leave stdout as fully buffered, but flush it first
+     when something is written to stderr.  */
+  update_stderr_ui_file (gdb_stderr);
+#endif
+
   gdb_stdlog = gdb_stderr;	/* for moment */
   gdb_stdtarg = gdb_stderr;	/* for moment */
   gdb_stdin = stdio_fileopen (stdin);
diff --git a/gdb/ui-file.c b/gdb/ui-file.c
index cf5a86d..649d892 100644
--- a/gdb/ui-file.c
+++ b/gdb/ui-file.c
@@ -654,6 +654,32 @@ stdio_file_fseek (struct ui_file *file, long offset, int whence)
   return fseek (stdio->file, offset, whence);
 }
 
+static void
+stderr_file_write (struct ui_file *file, const char *buf, long length_buf)
+{
+  gdb_flush (gdb_stdout);
+  stdio_file_write (file, buf, length_buf);
+}
+
+static void
+stderr_file_fputs (const char *linebuffer, struct ui_file *file)
+{
+  gdb_flush (gdb_stdout);
+  stdio_file_fputs (linebuffer, file);
+}
+
+/* Overwrite UI_FILE's methods 'to_write' and 'to_fputs' by function
+   in which gdb_stdout is flushed.  */
+
+void
+update_stderr_ui_file (struct ui_file *ui_file)
+{
+  /* Method 'to_write_async_safe' is not overwritten, because it is
+     only used on linux host.  */
+  set_ui_file_write (ui_file, stderr_file_write);
+  set_ui_file_fputs (ui_file, stderr_file_fputs);
+}
+
 /* Like fdopen().  Create a ui_file from a previously opened FILE.  */
 
 struct ui_file *
diff --git a/gdb/ui-file.h b/gdb/ui-file.h
index 9fef68c..6d96c0b 100644
--- a/gdb/ui-file.h
+++ b/gdb/ui-file.h
@@ -129,6 +129,9 @@ extern struct ui_file *mem_fileopen (void);
 /* Open/create a STDIO based UI_FILE using the already open FILE.  */
 extern struct ui_file *stdio_fileopen (FILE *file);
 
+extern void update_stderr_ui_file (struct ui_file *ui_file);
+
+
 /* Open NAME returning an STDIO based UI_FILE.  */
 extern struct ui_file *gdb_fopen (const char *name, const char *mode);
 
-- 
1.7.7.6