C6250
warning C6250: Calling <function> VirtualFree without the MEM_RELEASE flag may free memory but not address descriptors (VADs); results in address space leaks
This warning indicates that a call to VirtualFree without the MEM_RELEASE flag only decommits the pages, and does not release them. To decommit and release pages, use MEM_RELEASE flag in call to VirtualFree. If any pages in the region are committed, the function first decommits and then releases them. After this operation, the pages are in the free state. If you specify this flag, dwSize must be zero, and lpAddress must point to the base address returned by the VirtualAlloc function when the region was reserved. The function fails if either of these conditions is not met.
You can ignore this warning if your code later frees the address space by calling VirtualFree with the MEM_RELEASE flag.
Example
The following sample code generates this warning:
#include <windows.h>
#include <stdio.h>
#define PAGELIMIT 80
DWORD dwPages = 0; // count of pages
DWORD dwPageSize; // page size
VOID f( )
{
LPVOID lpvBase; // base address of the test memory
SYSTEM_INFO sSysInfo; // system information
GetSystemInfo(&sSysInfo);
dwPageSize = sSysInfo.dwPageSize;
// Reserve pages in the process's virtual address space
lpvBase = VirtualAlloc(
NULL, // system selects address
PAGELIMIT*dwPageSize, // size of allocation
MEM_RESERVE,
PAGE_NOACCESS);
//
// code to access memory
// ...
if (lpvBase != NULL)
{
if (VirtualFree( lpvBase, 0, MEM_DECOMMIT )) // decommit pages
{
puts ("MEM_DECOMMIT Succeeded");
}
else
{
puts("MEM_DECOMMIT failed");
}
}
else
{
puts("lpvBase == NULL");
}
}
To correct this warning, use the following sample code:
#include <windows.h>
#include <stdio.h>
#define PAGELIMIT 80
DWORD dwPages = 0; // count of pages
DWORD dwPageSize; // page size
VOID f( )
{
LPVOID lpvBase; // base address of the test memory
SYSTEM_INFO sSysInfo; // system information
GetSystemInfo(&sSysInfo);
dwPageSize = sSysInfo.dwPageSize;
// Reserve pages in the process's virtual address space
lpvBase = VirtualAlloc(
NULL, // system selects address
PAGELIMIT*dwPageSize, // size of allocation
MEM_RESERVE,
PAGE_NOACCESS);
//
// code to access memory
// ...
if (lpvBase != NULL)
{
if (VirtualFree(lpvBase, 0,MEM_RELEASE )) // decommit & release
{
// code ...
}
else
{
puts("MEM_RELEASE failed");
}
}
else
{
puts("lpvBase == Null ");
// code...
}
}