[PATCH] Unbuffer stdout and stderr on windows

[Yao Qi]

Thanks for the review, Joel.

On 08/22/2013 10:17 PM, Joel Brobecker wrote:
>> +#ifdef __MINGW32__
>> >+  /* Ensure stderr is unbuffered.  */
>> >+  setvbuf (stderr, NULL, _IONBF, BUFSIZ);
>> >+#endif
> Can you also document the reason why you make stderr unbuffered here?
> If we capture most of what was discussed, it'll help anyone trying
> to understand the code later on...
> 

Sure, comment is added.

>> >    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
>             decided

Fixed.

>> >+     when something is written to stderr.  */
>> >+  update_stderr_ui_file (gdb_stderr);
>> >+#endif
> Just a question: Would it be possible to emulate line buffering
> as well? In my experience, the size of the buffer is fairly big,
> and causes the output to appear only at fairly large intervals,
> and in fairly large blobs.
> 

I am not sure.  The goal of this patch is to keep the order of outputs
from stdout and stderr correct, so I don't think much on emulating
line buffering for stdout.  IMO, it can be addressed by a separated
patch.

> 
>> >+
>> >    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)
> Please, document every new function, no matter how trivial. That way,
> there is no discussion about whether the function should be documented
> or not.

OK, documented in the updated patch.

-- 
Yao (齐尧)

gdb:

2013-08-23  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    |   20 ++++++++++++++++++++
 gdb/ui-file.c |   32 ++++++++++++++++++++++++++++++++
 gdb/ui-file.h |    3 +++
 3 files changed, 55 insertions(+), 0 deletions(-)

diff --git a/gdb/main.c b/gdb/main.c
index 1c240e4..332df9e 100644
--- a/gdb/main.c
+++ b/gdb/main.c
@@ -375,8 +375,28 @@ captured_main (void *data)
   saved_command_line[0] = '\0';
   instream = stdin;
 
+#ifdef __MINGW32__
+  /* Ensure stderr is unbuffered.  A Cygwin pty or pipe is implemented
+     as a Windows pipe, and Windows buffers on pipes.  */
+  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 decided 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..4b26b78 100644
--- a/gdb/ui-file.c
+++ b/gdb/ui-file.c
@@ -654,6 +654,38 @@ stdio_file_fseek (struct ui_file *file, long offset, int whence)
   return fseek (stdio->file, offset, whence);
 }
 
+/* This is the implementation of ui_file method to_write for stderr.
+   gdb_stdout is flushed before writing to gdb_stderr.  */
+
+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);
+}
+
+/* This is the implementation of ui_file method to_fputs for stderr.
+   gdb_stdout is flushed before writing to gdb_stderr.  */
+
+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