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Reserves or commits a region of pages in the virtual address space of the calling process. Memory allocated by this function is automatically initialized to zero, unless MEM_RESET is specified.

To allocate memory in the address space of another process, use the VirtualAllocEx function.


  _In_opt_  LPVOID lpAddress,
  _In_      SIZE_T dwSize,
  _In_      DWORD flAllocationType,
  _In_      DWORD flProtect


lpAddress [in, optional]
The starting address of the region to allocate. If the memory is being reserved, the specified address is rounded down to the nearest multiple of the allocation granularity. If the memory is already reserved and is being committed, the address is rounded down to the next page boundary. To determine the size of a page and the allocation granularity on the host computer, use the GetSystemInfo function. If this parameter is NULL, the system determines where to allocate the region.
dwSize [in]
The size of the region, in bytes. If the lpAddress parameter is NULL, this value is rounded up to the next page boundary. Otherwise, the allocated pages include all pages containing one or more bytes in the range from lpAddress to lpAddress+dwSize. This means that a 2-byte range straddling a page boundary causes both pages to be included in the allocated region.
flAllocationType [in]
The type of memory allocation. This parameter must contain one of the following values.
Value Meaning

Allocates memory charges (from the overall size of memory and the paging files on disk) for the specified reserved memory pages. The function also guarantees that when the caller later initially accesses the memory, the contents will be zero. Actual physical pages are not allocated unless/until the virtual addresses are actually accessed.

To reserve and commit pages in one step, call VirtualAlloc with MEM_COMMIT | MEM_RESERVE.

Attempting to commit a specific address range by specifying MEM_COMMIT without MEM_RESERVE and a non-NULL lpAddress fails unless the entire range has already been reserved. The resulting error code is ERROR_INVALID_ADDRESS.

An attempt to commit a page that is already committed does not cause the function to fail. This means that you can commit pages without first determining the current commitment state of each page.


Reserves a range of the process's virtual address space without allocating any actual physical storage in memory or in the paging file on

You can commit reserved pages in subsequent calls to the VirtualAlloc function. To reserve and commit pages in one step, call VirtualAlloc with MEM_COMMIT | MEM_RESERVE.

Other memory allocation functions, such as malloc and LocalAlloc, cannot use a reserved range of memory until it is released.


Indicates that data in the memory range specified by lpAddress and dwSize is no longer of interest. The pages should not be read from or written to the paging file. However, the memory block will be used again later, so it should not be decommitted. This value cannot be used with any other value.

Using this value does not guarantee that the range operated on with MEM_RESET will contain zeros. If you want the range to contain zeros, decommit the memory and then recommit it.

When you specify MEM_RESET, the VirtualAlloc function ignores the value of flProtect. However, you must still set flProtect to a valid protection value, such as PAGE_NOACCESS.

VirtualAlloc returns an error if you use MEM_RESET and the range of memory is mapped to a file. A shared view is only acceptable if it is mapped to a paging file.


MEM_RESET_UNDO should only be called on an address range to which MEM_RESET was successfully applied earlier. It indicates that the data in the specified memory range specified by lpAddress and dwSize is of interest to the caller and attempts to reverse the effects of MEM_RESET. If the function succeeds, that means all data in the specified address range is intact. If the function fails, at least some of the data in the address range has been replaced with zeroes.

This value cannot be used with any other value. If MEM_RESET_UNDO is called on an address range which was not MEM_RESET earlier, the behavior is undefined. When you specify MEM_RESET, the VirtualAlloc function ignores the value of flProtect. However, you must still set flProtect to a valid protection value, such as PAGE_NOACCESS.

Windows Server 2008 R2, Windows 7, Windows Server 2008, Windows Vista, Windows Server 2003, and Windows XP: The MEM_RESET_UNDO flag is not supported until Windows 8 and Windows Server 2012.

This parameter can also specify the following values as indicated.
Value Meaning

Allocates memory using large page support.

The size and alignment must be a multiple of the large-page minimum. To obtain this value, use the GetLargePageMinimum function.


Reserves an address range that can be used to map Address Windowing Extensions (AWE) pages.

This value must be used with MEM_RESERVE and no other values.

Allocates memory at the highest possible address. This can be slower than regular allocations, especially when there are many allocations.

Causes the system to track pages that are written to in the allocated region. If you specify this value, you must also specify MEM_RESERVE.

To retrieve the addresses of the pages that have been written to since the region was allocated or the write-tracking state was reset, call the GetWriteWatch function. To reset the write-tracking state, call GetWriteWatch or ResetWriteWatch. The write-tracking feature remains enabled for the memory region until the region is freed.

flProtect [in]
The memory protection for the region of pages to be allocated. If the pages are being committed, you can specify any one of the memory protection constants.

Return value

If the function succeeds, the return value is the base address of the allocated region of pages.

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