1.50 1.51 1.52
MS-DOS
kbprg kbbuglist
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The information in this article applies to:
- The C Run-time, included with:
Microsoft Visual C++ for Windows, versions 1.50, 1.51 and 1.52
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SYMPTOMS
========
When using virtual memory allocation function _vmalloc() in the C runtime
library, the program may fail on _vmalloc() if all of the following
conditions are met :
- You use _vmalloc() to do block allocation instead of page allocation.
- You reuse the virtual memory without repeatedly calling _vheapinit() and
_vheapterm().
- You use _vmalloc() to allocate an array of memory blocks that has a
minimum of 1000 elements. The pseudo code for the algorithm could
resemble this:
_vheapinit(...);
// _vmalloc() fails on the second iteration when j=512.
for ( i=0; i<2; i++ )
{
for ( j=0; j<1000; j++ )
handle[j] = _vmalloc(...);
for ( j=0; j<1000; j++ )
_vfree( handle[j] );
}
_vheapterm();
The program allocates and frees the virtual memory blocks correctly for the
first iteration of the outer For loop, but fails on the second iteration of
the outer For loop when the inner For loop goes to iteration 512.
RESOLUTION
==========
To work around the problem, use one of these two suggestions:
- Use page allocation instead of block allocation. This could be done by
allocating a memory block with size greater than 2042 bytes. The virtual
memory manager adds six bytes for the block header to the size of the
allocation and rounds the result up to the next multiple of the virtual
page size (2048 bytes).
-or-
- Call _vheapinit() and _vheapterm() to reinitialize the virtual memory
manager when virtual memory needs to be reallocated. For example:
for ( i=0; i<2; i++ )
{
_vheapinit(...);
for ( n=0; n<1000; n++ )
handle[n] = _vmalloc(...);
for ( n=0; n<1000; n++ )
_vfree( handle[n] );
_vheapterm();
}
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: None
*/
#include <stdio.h>
#include <stdlib.h>
#include <vmemory.h>
#define NUM_ELEMENT 1000
#define SIZE 2042
_vmhnd_t vm_handle[NUM_ELEMENT];
int main()
{
int i;
unsigned long n;
unsigned hmem = NUM_ELEMENT;
char __far *pntr;
printf("\nTest allocating virtual memory");
// Initializing virtual memory managers.
if(_vheapinit(0, _VM_ALLDOS, _VM_ALLSWAP)==0)
{
printf("\nError initializing virtual memory manager\n");
exit(-1);
}
printf("\nVirtual memory manager initialization done");
// Allocating virtual memory.
for ( i=0; i<2; i++ )
{
printf("\n\nAllocating memory... ");
for( n=0; n<hmem; n++ )
{
vm_handle[n] = _vmalloc(SIZE);
if ( vm_handle[n]==_VM_NULL )
{
printf("Cannot allocate virtual memory. Entry = %d\n", n);
_vheapterm();
exit(-1);
}
}
printf("Done\n");
// Testing the virtual memory allocation.
printf("Testing... ");
for ( n=0; n<hmem; n++ )
{
pntr = (char __far *)_vload(vm_handle[n], _VM_DIRTY);
if( pntr==NULL )
{
printf("Error in testing %d", n);
exit(-1);
}
}
printf("Done\n");
// Deleting virtual memory allocated earlier.
for ( n=0; n<hmem; n++ )
_vfree(vm_handle[n]);
printf("Memory freed...Done\n");
}
_vheapterm();
return(1);
}
REFERENCES
==========
For more information on virtual memory block allocation and page
allocation, please see the following article in the Microsoft Knowledge
Base:
ARTICLE-ID: Q93211
TITLE : INF: _vmalloc() May Allocate Larger Memory Block Than Expected
Additional reference words: 1.50 1.51 buglist8.00
KBCategory: kbprg kbbuglist
KBSubCategory: VirtualMem