VirtualFree

The VirtualFree function releases or decommits (or both) a region of pages within the virtual address space of the calling process.

BOOL VirtualFree(
  LPVOID lpAddress,  // address of region of committed pages
  DWORD dwSize,      // size of region
  DWORD dwFreeType   // type of free operation
);
 

Parameters

lpAddress

Pointer to the base address of the region of pages to be freed. If the dwFreeType parameter includes the MEM_RELEASE flag, this parameter must be the base address returned by the VirtualAlloc function when the region of pages was reserved.

dwSize

Specifies the size, in bytes, of the region to be freed. If the dwFreeType parameter includes the MEM_RELEASE flag, this parameter must be zero. Otherwise, the region of affected pages includes all pages containing one or more bytes in the range from the lpAddress parameter to (lpAddress+dwSize). This means that a 2-byte range straddling a page boundary causes both pages to be freed.

dwFreeType

Specifies the type of free operation. One, but not both, of the following flags can be specified:

Flag	Meaning
MEM_DECOMMIT	Decommits the specified region of committed pages.
	An attempt to decommit an uncommitted page will not cause the function to fail. This means that a range of committed or uncommitted pages can be decommitted without having to worry about a failure.
MEM_RELEASE	Releases the specified region of reserved pages. If this flag is specified, the dwSize parameter must be zero, or the function fails.

Return Values

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

Remarks

VirtualFree can perform one of the following operations:

• Decommit a region of committed or uncommitted pages.
• Release a region of reserved pages.
• Decommit and release a region of committed or uncommitted pages.

To release a region of pages, the entire range of pages must be in the same state (all reserved or all committed) and the entire region originally reserved by the VirtualAlloc function must be released at the same time. If only part of the pages in the original reserved region are committed, you must first call VirtualFree to decommit the committed pages and then call VirtualFree again to release the entire block.

Pages that have been released are free pages available for subsequent allocation operations. Attempting to read from or write to a free page results in an access violation exception.

VirtualFree can decommit an uncommitted page; this means that a range of committed or uncommitted pages can be decommitted without having to worry about a failure. Decommitting a page releases its physical storage, either in memory or in the paging file on disk. If a page is decommitted but not released, its state changes to reserved, and it can be committed again by a subsequent call to VirtualAlloc. Attempting to read from or write to a reserved page results in an access violation exception.

The current state of the entire range of pages must be compatible with the type of free operation specified by the dwFreeType parameter. Otherwise, the function fails and no pages are released or decommitted.

QuickInfo

  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Requires version 1.0 or later.
  Header: Declared in winbase.h.
  Import Library: Use kernel32.lib.

See Also

Memory Management Overview, Memory Management Functions, GlobalAlloc, GlobalFree, VirtualAlloc, VirtualFreeVlm