Re: [ros-dev] [ros-diffs] [mjmartin] 40557: - Add volatile to variables in OutputDebugStringA. Fix #1 of ? for 3424.

[Aleksey Bragin]

This is a PSEH specific "feature". GCC may think that the variable is  
not used when it's declared or used(?) and optimize it away. To  
prevent this a volatile is added.

At least that's how I remember KJK explained that, and iirc there was  
a ML thread here some time ago about same thing, when Dmitry  
Gorbachev added same thing to the ntoskrnl.

WBR,
Aleksey Bragin.


On Apr 21, 2009, at 9:24 PM, Michael Martin wrote:

>  
>  IIRC, at one point, someone had said something along the lines that  
>  under certain criteria variables used inside SEH2 blocks had to be  
>  declared with volatile.
>  Not sure if this had been fixed but either way it does looks like  
>  an SEH problem.
>
>  martin
>
> >   Date: Sat, 18 Apr 2009 12:36:31 +0200
> >  From: timo.kreuzer(a)web.de
> >  To: ros-dev(a)reactos.org
> >  Subject: Re: [ros-dev] [ros-diffs] [mjmartin] 40557: - Add   
>  volatile to variables
> in OutputDebugStringA. Fix #1 of ? for 3424.
> >  
> >  Can you explain, why the variables need to be volatile? This   
>  looks like
> >   a SEH problem.
> >
> >  mjmartin(a)svn.reactos.org schrieb:
> >  > Author: mjmartin
> >  > Date: Fri Apr 17 13:59:03 2009
> >  > New Revision: 40557
> >  >
> >  > URL: http://svn.reactos.org/svn/reactos?rev=40557&view=rev
> >  > Log:
> >  > - Add volatile to variables in OutputDebugStringA. Fix #1 of ?   
>  for 3424.
> >   >
> >  > Modified:
> >  > trunk/reactos/dll/win32/kernel32/debug/output.c
> >  >
> >  > Modified: trunk/reactos/dll/win32/kernel32/debug/output.c
> >  > URL: http://svn.reactos.org/svn/reactos/trunk/reactos/dll/win32/  
> kernel32/debug/output.c?rev=40557&r1=40556&r2=40557&view=diff
> >   >   
> ====================================================================== 
>  ========
> >   > ---
> > trunk/reactos/dll/win32/kernel32/debug/output.c   
>  [iso-8859-1] (original)
> >   > +++
> > trunk/reactos/dll/win32/kernel32/debug/output.c   
>  [iso-8859-1] Fri Apr 17 13:59:03
> 2009
> >   > @@ -219,9 +219,9 @@
> >  > static BOOL s_bDBMonMutexTriedOpen = FALSE;
> >  >
> >  > /* local copy of the mutex handle */
> >  > -	 HANDLE hDBMonMutex = s_hDBMonMutex;
> >  > +	 volatile HANDLE hDBMonMutex = s_hDBMonMutex;
> >  > /* handle to the Section of the shared buffer */
> >  > -	 HANDLE hDBMonBuffer = NULL;
> >  > +	 volatile HANDLE hDBMonBuffer = NULL;
> >  >
> >  > /* pointer to the mapped view of the shared buffer. It consist   
>  of the
> current
> >   > process id followed by the message string
> > */
> >  > @@ -229,11 +229,11 @@
> >  >
> >  > /* event: signaled by the debug message monitor when   
>  OutputDebugString can
> write
> >   > to the shared buffer */
> >  > -	 HANDLE hDBMonBufferReady = NULL;
> >  > +	 volatile HANDLE hDBMonBufferReady = NULL;
> >  >
> >  > /* event: to be signaled by OutputDebugString when it's done   
>  writing to
> the
> > >   shared
> > > buffer */
> > >  -	 HANDLE hDBMonDataReady = NULL;
> > >  +	 volatile HANDLE hDBMonDataReady = NULL;
> > >
> > >  /* mutex not opened, and no previous attempts to open/create it */
> > >  if(hDBMonMutex == NULL && !s_bDBMonMutexTriedOpen)
> > >  @@ -295,10 +295,10 @@
> > >  _SEH2_TRY
> > >  {
> > >  /* size of the current output block */
> > >  -	 SIZE_T nRoundLen;
> > >  +	 volatile SIZE_T nRoundLen;
> > >
> > >  /* size of the remainder of the string */
> > >  -	 SIZE_T nOutputStringLen;
> > >  +	 volatile SIZE_T nOutputStringLen;
> > >
> > >  /* output the whole string */
> > >  nOutputStringLen = strlen(_OutputString);
> > >  @@ -339,11 +339,12 @@
> > >  else
> > >  {
> > >  /* output in blocks of 512 characters */
> > >  -	 CHAR a_cBuffer[512];
> > >  +	 volatile PCHAR a_cBuffer;
> > >  +	 a_cBuffer = (CHAR*)HeapAlloc(GetProcessHeap(), 0, 512);
> > >
> > >  /* write a maximum of 511 bytes */
> > >  -	 if(nOutputStringLen > (sizeof(a_cBuffer) - 2))
> > >  -	 nRoundLen = sizeof(a_cBuffer) - 2;
> > >  +	 if(nOutputStringLen > 510)
> > >  +	 nRoundLen = 510;
> > >  else
> > >  nRoundLen = nOutputStringLen;
> > >
> > >  @@ -355,6 +356,8 @@
> > >
> > >  /* send the current block to the kernel debugger */
> > >  DbgPrint("%s", a_cBuffer);
> > >  +
> > >  +	 HeapFree(GetProcessHeap(), 0, a_cBuffer);
> > >  }
> > >
> > >  /* move to the next block */
> > >
> > >
> > >  
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