Re: [PATCH] Unbuffer stdout and stderr on windows

Yao Qi
2013-08-28T07:23:39Z

On 08/28/2013 04:39 AM, Pedro Alves wrote:
> Better say that "it is not used on Windows hosts". But I think
> it'd be better even to say:
>
> /* Method 'to_write_async_safe' is not overwritten, because
> there's no way to flushing a stream in an async-safe manner.
> Fortunately, it doesn't really matter, because:
> - that method is only used for printing internal debug output
> from signal handlers.
> - Windows hosts don't have a concept of async-safeness. Signal
> handlers run in a separate thread, so they can call
> the regular non-async-safe output routines freely. */

OK.

[...]
> hide that in ui-file.c itself. So e.g., we'd have a new stderr_fileopen
> method, and then both places that create gdb_stderr would use it, like:
>
> -gdb_stderr = stdio_fileopen (stderr);
> +gdb_stderr = stderr_fileopen ();
>
> We could also that whole set of Windows-specific comments there
> too.

Right. Function stderr_fileopen is added in the updated patch.
What do you think?

--
Yao (é½å°§)

gdb:

2013-08-28 Yao Qi <yao@codesourcery.com>
Pedro Alves <palves@redhat.com>

* event-top.c (gdb_setup_readline): Call stderr_fileopen
instead of stdio_fileopen.
* main.c (captured_main) [__MINGW32__]: Set stderr unbuffered.
Call stderr_fileopen instead of stdio_fileopen.
* ui-file.c (stderr_file_write): New function.
(stderr_file_fputs): New function.
(stderr_fileopen): New function.
* ui-file.h (stderr_fileopen): Declare.
---
gdb/event-top.c | 2 +-
gdb/main.c | 9 ++++++++-
gdb/ui-file.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
gdb/ui-file.h | 3 +++
4 files changed, 66 insertions(+), 2 deletions(-)

diff --git a/gdb/event-top.c b/gdb/event-top.c
index f00ab7d..f1d55b3 100644
--- a/gdb/event-top.c
+++ b/gdb/event-top.c
@@ -955,7 +955,7 @@ gdb_setup_readline (void)
time. */
if (!batch_silent)
gdb_stdout = stdio_fileopen (stdout);
- gdb_stderr = stdio_fileopen (stderr);
+ gdb_stderr = stderr_fileopen ();
gdb_stdlog = gdb_stderr; /* for moment */
gdb_stdtarg = gdb_stderr; /* for moment */
gdb_stdtargerr = gdb_stderr; /* for moment */
diff --git a/gdb/main.c b/gdb/main.c
index 1c240e4..11f4b03 100644
--- a/gdb/main.c
+++ b/gdb/main.c
@@ -375,8 +375,15 @@ 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);
+ gdb_stderr = stderr_fileopen ();
+
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..d29b8da 100644
--- a/gdb/ui-file.c
+++ b/gdb/ui-file.c
@@ -654,6 +654,60 @@ 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);
+}
+
+/* Create a ui_file from stderr. */
+
+struct ui_file *
+stderr_fileopen (void)
+{
+ struct ui_file *ui_file = 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. */
+
+ /* Method 'to_write_async_safe' is not overwritten, because there's
+ no way to flushing a stream in an async-safe manner.
+ Fortunately, it doesn't really matter, because:
+ - that method is only used for printing internal debug output
+ from signal handlers.
+ - Windows hosts don't have a concept of async-safeness. Signal
+ handlers run in a separate thread, so they can call
+ the regular non-async-safe output routines freely. */
+ set_ui_file_write (ui_file, stderr_file_write);
+ set_ui_file_fputs (ui_file, stderr_file_fputs);
+#endif
+
+ return ui_file;
+}
+
/* 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..dca800f 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 struct ui_file *stderr_fileopen (void);
+
+
/* Open NAME returning an STDIO based UI_FILE. */
extern struct ui_file *gdb_fopen (const char *name, const char *mode);

--
1.7.7.6