Strange ASSERT failure.

Joern Dahl-Stamnes -- Jorn.Dahl-Stamnes@fysel.ntnu.no
Friday, February 21, 1997

Environment: NT 3.51, VC++ 1.52

I have this strange problem in a program.

In the CMyView::OnDraw() function I have this code:

{
	static char*	pSymbols = "BRSWL";
	int		i, xpos, ypos;

	// ...
	// i is within range!
	pDC->TextOut (xpos,ypos,&pSymbols[i],1);
	// ...
}

As long as !pDC->IsPrinting() it work without problem in both debug and
in release mode. But when I, in debug mode, do a Print Preview or a
Print, I get an Assert failed in CPreviewDC::ExtTextOut(). The line
with the ASSERT statement says:

	ASSERT(AfxIsValidAddress(lpszString, nCount));

If I look at the lpszString is say "BRSWL" and nCount say 1. This is
correct!

Now, if I change my OnDraw() to:

{
	static char*	pSymbols = "BRSWL";
	int		i, xpos, ypos;
	char		buffer[8];

	// ...
	// i is within range!
	buffer[0] = pSymbols[i];
	pDC->TextOut (xpos,ypos,buffer,1);
	// ..
}

It work in Print Preview and in Print too! Why??????
-- 
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| Joern Yngve Dahl-Stamnes, Norwegian University of Science and Technology  |
| e-mail: Jorn.Dahl-Stamnes@fysel.ntnu.no                                   | 
| phone : +73 59 44 12, fax: +73 59 14 41                                   |
| Surfing the net? Try http://www.fysel.ntnu.no/dahls/dahls.html            |
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---

Jim Lawson Williams -- jimlw@mail.ccur.com.au
Sunday, February 23, 1997

At 10:38 21-02-97 +0100, Joern Dahl-Stamnes wrote:
>Environment: NT 3.51, VC++ 1.52
>
>I have this strange problem in a program.
>
>In the CMyView::OnDraw() function I have this code:
>
>{
>	static char*	pSymbols = "BRSWL";
>	int		i, xpos, ypos;
>
>	// ...
>	// i is within range!
>	pDC->TextOut (xpos,ypos,&pSymbols[i],1);
>	// ...
>}
>
G'day!
>From "Books Online", re CDC::TextOut()  --

virtual BOOL TextOut( int x, int y, LPCTSTR lpszString, int nCount );
  
BOOL TextOut( int x, int y, const CString& str );

"char*" and "CString" are not synonymous.

Regards,
Jim LW




>From the BBC's "Barchester Chronicles":

    "I know that ultimately we are not supposed to understand.
    But I also know that we must try."

       -- the Reverend Septimus Harding, 
          tax-consultant, crypt-analyst, clog-dancer, C++ programmer

---

David Lowndes -- David.Lowndes@bj.co.uk
Monday, February 24, 1997

[Mini-digest: 2 responses]

>But when I, in debug mode, do a Print Preview or a
Print, I get an Assert failed in CPreviewDC::ExtTextOut(). The line
with the ASSERT statement says:

	ASSERT(AfxIsValidAddress(lpszString, nCount));
<

This is because the ASSERT in several of the MFC wrappers for TextOut &
associated routines was incorrect! It's only recently been addressed and
some are still outstanding (see below). It's testing the string as
having read-write access, where it should only test for read-access.
Where the compiler can optimise the string and place it in a const
segment you will get this ASSERT occurring. The ASSERT should have been:

	ASSERT( AfxIsValidAddress( lpszString, nCount, FALSE ) );

Mike B - I think you'll find the problem still exists in:

CMetaFileDC::ExtTextOut
CPreviewDC::TabbedTextOut

>Dave Lowndes
-----From: kenn@owl.co.uk (Ken Nicolson)

On Fri, 21 Feb 1997 10:38:49 +0100 (MET), you wrote:

>Environment: NT 3.51, VC++ 1.52
>
>I have this strange problem in a program.
>
>In the CMyView::OnDraw() function I have this code:
>
>{
>	static char*	pSymbols =3D "BRSWL";
>	int		i, xpos, ypos;
>
>	// ...
>	// i is within range!
>	pDC->TextOut (xpos,ypos,&pSymbols[i],1);
>	// ...
>}
>
>As long as !pDC->IsPrinting() it work without problem in both debug and
>in release mode. But when I, in debug mode, do a Print Preview or a
>Print, I get an Assert failed in CPreviewDC::ExtTextOut(). The line
>with the ASSERT statement says:
>
>	ASSERT(AfxIsValidAddress(lpszString, nCount));
>
>If I look at the lpszString is say "BRSWL" and nCount say 1. This is
>correct!

This looks like a bug in VC 1.52 and CPreviewDC - it should be the
test:

    ASSERT(AfxIsValidAddress(lpszString, nCount, FALSE));

as the final parameter, defaulting to TRUE, is to test if the pointer
is writable too. As you've got a static char *, the compiler probably
puts the "BRSWL" string into a read-only segment, so it is not
writable. The prototype for TextOut is:

    BOOL TextOut( int x, int y, LPCTSTR lpszString, int nCount );

so the string can be const, therefore the MFC should not fail with an
unwritable string.

Try declaring your string like:

   char pSymbols[] =3D "BRSWL";

as this will place the string in writable memory, or you would better
fixing the ASSERT() in the MFC code, as other TextOut()s like:

   pDC->TextOut (xpos,ypos,"literal string",14);

will spew too.

HTH

Ken
--=20
Ken Nicolson, Panasonic OWL: kenn@owl.co.uk http://www.owl.co.uk
       At Home: ken@ride.demon.co.uk http://www.ride.demon.co.uk
#include 

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