VirtualAlloc2FromApp function (memoryapi.h)
Reserves, commits, or changes the state of a region of pages in the virtual address space of the calling process. Memory allocated by this function is automatically initialized to zero.
Using this function, you can: for new allocations, specify a range of virtual address space and a power-of-2 alignment restriction; specify an arbitrary number of extended parameters; specify a preferred NUMA node for the physical memory as an extended parameter; and specify a placeholder operation (specifically, replacement).
To specify the NUMA node, see the ExtendedParameters parameter.
Syntax
PVOID VirtualAlloc2FromApp(
[in, optional] HANDLE Process,
[in, optional] PVOID BaseAddress,
[in] SIZE_T Size,
[in] ULONG AllocationType,
[in] ULONG PageProtection,
[in, out, optional] MEM_EXTENDED_PARAMETER *ExtendedParameters,
[in] ULONG ParameterCount
);
Parameters
[in, optional] Process
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.
[in, optional] BaseAddress
The pointer that specifies a desired starting address for the region of pages that you want to allocate.
If BaseAddress is NULL, the function determines where to allocate the region.
If BaseAddress is not NULL, then any provided MEM_ADDRESS_REQUIREMENTS structure must consist of all zeroes, and the base address must be a multiple of the system allocation granularity. To determine the allocation granularity, use the GetSystemInfo function.
[in] Size
The size of the region of memory to allocate, in bytes.
The size must always be a multiple of the page size.
If BaseAddress is not NULL, the function allocates all pages that contain one or more bytes in the range from BaseAddress to BaseAddress+Size. This means, for example, that a 2-byte range that straddles a page boundary causes the function to allocate both pages.
[in] AllocationType
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
Virtual2AllocFromApp with
Attempting to commit a specific address range by specifying MEM_COMMIT without MEM_RESERVE and a non-NULL BaseAddress 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 disk.
You can commit reserved pages in subsequent calls to the Virtual2AllocFromApp function. To reserve and commit pages in one step, call Virtual2AllocFromApp 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. |
|
Replaces a placeholder with a normal private allocation. Only data/pf-backed section views are supported (no images, physical memory, etc.). When you replace a placeholder, BaseAddress and Size must exactly match those of the placeholder,
and any provided MEM_ADDRESS_REQUIREMENTS structure must consist of all zeroes.
After you replace a placeholder with a private allocation, to free that allocation back to a placeholder, see the dwFreeType parameter of VirtualFree and VirtualFreeEx. A placeholder is a type of reserved memory region. |
|
To create a placeholder, call
VirtualAlloc2 with
MEM_RESERVE | MEM_RESERVE_PLACEHOLDER and PageProtection set to PAGE_NOACCESS. To free/split/coalesce a placeholder, see the dwFreeType parameter of VirtualFree and VirtualFreeEx.
A placeholder is a type of reserved memory region. |
|
Indicates that data in the memory range specified by BaseAddress and
Size 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 Virtual2AllocFromApp function ignores the value of Protection. However, you must still set Protection to a valid protection value, such as PAGE_NOACCESS. Virtual2AllocFromApp 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 BaseAddress and Size
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 Virtual2AllocFromApp function ignores the value of Protection. However, you must still set Protection to a valid protection value, such as PAGE_NOACCESS. |
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. If you specify this value, you must also specify MEM_RESERVE and MEM_COMMIT. |
|
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. |
[in] PageProtection
The memory protection for the region of pages to be allocated. If the pages are being committed, you can specify one of the memory protection constants. The following constants generate an error:
- PAGE_EXECUTE
- PAGE_EXECUTE_READ
- PAGE_EXECUTE_READWRITE
- PAGE_EXECUTE_WRITECOPY
[in, out, optional] ExtendedParameters
An optional pointer to one or more extended parameters of type MEM_EXTENDED_PARAMETER. Each of those extended parameter values can itself have a Type field of either MemExtendedParameterAddressRequirements or MemExtendedParameterNumaNode. If no MemExtendedParameterNumaNode extended parameter is provided, then the behavior is the same as for the VirtualAlloc/MapViewOfFile functions (that is, the preferred NUMA node for the physical pages is determined based on the ideal processor of the thread that first accesses the memory).
[in] ParameterCount
The number of extended parameters pointed to by ExtendedParameters.
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
This API helps support high-performance games, and server applications, which have particular requirements around managing their virtual address space. For example, mapping memory on top of a previously reserved region; this is useful for implementing an automatically wrapping ring buffer. And allocating memory with specific alignment; for example, to enable your application to commit large/huge page-mapped regions on demand.
You can call Virtual2AllocFromApp from Windows Store apps with just-in-time (JIT) capabilities to use JIT functionality. The app must include the codeGeneration capability in the app manifest file to use JIT capabilities.
Each page has an associated page state. The Virtual2AllocFromApp 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
You can use Virtual2AllocFromApp to reserve a block of pages and then make additional calls to Virtual2AllocFromApp 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 BaseAddress parameter is not NULL, the function uses the BaseAddress and Size 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 AllocationType 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.
Virtual2AllocFromApp does not allow the creation of executable pages.
The Virtual2AllocFromApp 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 code examples, see Virtual2Alloc.
Requirements
| Requirement | Value |
|---|---|
| Minimum supported client | Windows 10 [desktop apps only] |
| Minimum supported server | Windows Server 2016 [desktop apps only] |
| Target Platform | Windows |
| Header | memoryapi.h (include Windows.h) |
| Library | WindowsApp.lib |
| DLL | Kernel32.dll |
See also
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