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VirtualAlloc function

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.

Syntax


LPVOID WINAPI VirtualAlloc(
  _In_opt_  LPVOID lpAddress,
  _In_      SIZE_T dwSize,
  _In_      DWORD flAllocationType,
  _In_      DWORD flProtect
);

Parameters

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.

ValueMeaning
MEM_COMMIT
0x00001000

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.

MEM_RESERVE
0x00002000

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

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.

MEM_RESET
0x00080000

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
0x1000000

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.

ValueMeaning
MEM_LARGE_PAGES
0x20000000

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.

MEM_PHYSICAL
0x00400000

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.

MEM_TOP_DOWN
0x00100000

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

MEM_WRITE_WATCH
0x00200000

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.

Remarks

Each page has an associated page state. The VirtualAlloc function can perform the following operations:

  • Commit a region of reserved pages
  • Reserve a region of free pages
  • Simultaneously reserve and commit a region of free pages

VirtualAlloc cannot reserve a reserved page. It can commit a page that is already committed. This means you can commit a range of pages, regardless of whether they have already been committed, and the function will not fail.

You can use VirtualAlloc to reserve a block of pages and then make additional calls to VirtualAlloc to commit individual pages from the reserved block. This enables a process to reserve a range of its virtual address space without consuming physical storage until it is needed.

If the lpAddress parameter is not NULL, the function uses the lpAddress and dwSize parameters to compute the region of pages to be allocated. The current state of the entire range of pages must be compatible with the type of allocation specified by the flAllocationType parameter. Otherwise, the function fails and none of the pages are allocated. This compatibility requirement does not preclude committing an already committed page, as mentioned previously.

To execute dynamically generated code, use VirtualAlloc to allocate memory and the VirtualProtect function to grant PAGE_EXECUTE access.

The VirtualAlloc function can be used to reserve an Address Windowing Extensions (AWE) region of memory within the virtual address space of a specified process. This region of memory can then be used to map physical pages into and out of virtual memory as required by the application. The MEM_PHYSICAL and MEM_RESERVE values must be set in the AllocationType parameter. The MEM_COMMIT value must not be set. The page protection must be set to PAGE_READWRITE.

The VirtualFree function can decommit a committed page, releasing the page's storage, or it can simultaneously decommit and release a committed page. It can also release a reserved page, making it a free page.

When creating a region that will be executable, the calling program bears responsibility for ensuring cache coherency via an appropriate call to FlushInstructionCache once the code has been set in place. Otherwise attempts to execute code out of the newly executable region may produce unpredictable results.

Examples

For an example, see Reserving and Committing Memory.

Requirements

Minimum supported client

Windows XP [desktop apps only]

Minimum supported server

Windows Server 2003 [desktop apps only]

Header

WinBase.h (include Windows.h)

Library

Kernel32.lib

DLL

Kernel32.dll

See also

Memory Management Functions
Virtual Memory Functions
VirtualAllocEx
VirtualFree
VirtualLock
VirtualProtect
VirtualQuery

 

 

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