Reserves or commits a region of memory within the virtual address space of a specified process. The function initializes the memory it allocates to zero, unless MEM_RESET is used.

Syntax

LPVOID WINAPI VirtualAllocEx(
  __in      HANDLE hProcess,
  __in_opt  LPVOID lpAddress,
  __in      SIZE_T dwSize,
  __in      DWORD flAllocationType,
  __in      DWORD flProtect
);

Parameters

hProcess [in]

The handle to a process. The function allocates memory within the virtual address space of this process.

The handle must have the PROCESS_VM_OPERATION access right. For more information, see Process Security and Access Rights.

lpAddress [in, optional]

The pointer that specifies a desired starting address for the region of pages that you want to allocate.

If you are reserving memory, the function rounds this address down to the nearest multiple of the allocation granularity.

If you are committing memory that is already reserved, the function rounds this address down to the nearest page boundary. To determine the size of a page and the allocation granularity on the host computer, use the GetSystemInfo function.

If lpAddress is NULL, the function determines where to allocate the region.

dwSize [in]

The size of the region of memory to allocate, in bytes.

If lpAddress is NULL, the function rounds dwSize up to the next page boundary.

If lpAddress is not NULL, the function allocates all pages that contain one or more bytes in the range from lpAddress to lpAddress+dwSize. This means, for example, that a 2-byte range that straddles a page boundary causes the function to allocate both pages.

flAllocationType [in]

The type of memory allocation. This parameter must contain one of the following values.

Value	Meaning
MEM_COMMIT 0x1000	Allocates physical storage in memory or in the paging file on disk for the specified reserved memory pages. The function initializes the memory to zero. To reserve and commit pages in one step, call VirtualAllocEx with MEM_COMMIT | MEM_RESERVE. The function fails if you attempt to commit a page that has not 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 0x2000	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 commit reserved pages by calling VirtualAllocEx again with MEM_COMMIT. To reserve and commit pages in one step, call VirtualAllocEx with MEM_COMMIT |MEM_RESERVE. Other memory allocation functions, such as malloc and LocalAlloc, cannot use reserved memory until it has been released.
MEM_RESET 0x80000	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 zeroes. If you want the range to contain zeroes, decommit the memory and then recommit it. When you use MEM_RESET, the VirtualAllocEx function ignores the value of fProtect. However, you must still set fProtect to a valid protection value, such as PAGE_NOACCESS. VirtualAllocEx 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.

This parameter can also specify the following values as indicated.

Value	Meaning
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. Windows XP/2000:   This flag is not supported.
MEM_PHYSICAL 0x400000	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 0x100000	Allocates memory at the highest possible address.

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.

Protection attributes specified when protecting a page cannot conflict with those specified when allocating a page.

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 VirtualAllocEx 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

VirtualAllocEx 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 VirtualAllocEx to reserve a block of pages and then make additional calls to VirtualAllocEx 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 is allocated. This compatibility requirement does not preclude committing an already committed page; see the preceding list.

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

The VirtualAllocEx 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 VirtualFreeEx 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.

Requirements

See Also

Memory Management Functions
Virtual Memory Functions
VirtualFreeEx
VirtualLock
VirtualProtect
VirtualQuery

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Build date: 6/12/2008