BUG: Post Increment Operator Works Incorrectly for __huge Ptrs
Q125969
1.00 1.50 1.51 1.52
WINDOWS
kbtool kbbuglist
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The information in this article applies to:
- The Microsoft C/C++ compiler (CL.EXE), included with:
Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
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SYMPTOMS
=========
When the post increment operator ++ is applied to a huge pointer within an
argument list for a function call, the resulting pointer value will not be
normalized. This problem occurs only when you use the optimizing compiler
to compile the code. The pointer will not cross over the 64K data-segment
boundary. Instead, it wraps around and points to the beginning of the
current data segment. The sample code in this article demonstrates the
problem.
RESOLUTION
==========
When using the optimizing compiler, do not use the increment operator for
huge pointers inside function calls. Instead, increment the pointer after
the function call.
STATUS
======
Microsoft has confirmed this to be a bug in the Microsoft products listed
at the beginning of this article. We are researching this problem and will
post new information here in the Microsoft Knowledge Base as it becomes
available.
MORE INFORMATION
================
Sample Code to Reproduce Problem
--------------------------------
/* Compile options needed: /f- or /O2 or /Ox or any combinations
of compiler optimizations, except /Od..
*/
#include <stdio.h>
#include <malloc.h>
#define ARRAYSIZE 32769L /* (64K/2)+1 of integers */
void Change( int __huge *ptr ) { *ptr += 1; }
int main( void )
{
int __huge *ptr, __huge *save;
long i;
ptr = (int __huge *)_halloc(ARRAYSIZE, (size_t)sizeof( int ));
save = ptr;
/* Instead of adding one to the last element of the array (which
is in a new 64K segment), save wraps around to the beginning
of the array and increments the element in the first position
a second time.
*/
for (i = 0; i < ARRAYSIZE; i++)
Change( save++ );
printf( "ptr[0] == %d, incremented twice.\n",
ptr[0L] );
printf( "ptr[32767] == %d, incremented once.\n",
ptr[32767L] );
printf( "ptr[32768] == %d, not incremented.\n",
ptr[32768L] );
_hfree( ptr );
return 0;
}
Additional reference words: autoincrement gpf 1.00 1.50 8.00 8.00c
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KBSubcategory: CLIss