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ExAllocatePool2 function (wdm.h)

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The ExAllocatePool2 routine allocates pool memory of the specified type and returns a pointer to the allocated block.

Syntax

DECLSPEC_RESTRICT PVOID ExAllocatePool2(
  POOL_FLAGS Flags,
  SIZE_T     NumberOfBytes,
  ULONG      Tag
);

Parameters

Flags

A ULONG64-typed value specifying the type of pool memory along with required and optional attributes. Multiple flag values can be combined using bit-wise OR. See POOL_FLAGS for possible values.

NumberOfBytes

Specifies a non-zero number of bytes to allocate.

Tag

The pool tag to use for the allocated memory. Specify the pool tag as a non-zero character literal of one to four characters delimited by single quotation marks (for example, Tag1). The string is usually specified in reverse order (for example, 1gaT). Each ASCII character in the tag must be a value in the range 0x20 (space) to 0x7E (tilde). Each allocation code path should use a unique pool tag to help debuggers and verifiers identify the code path.

Return value

ExAllocatePool2 returns a pointer to the allocated memory.

The following conditions cause the function to return NULL by default. If POOL_FLAG_RAISE_ON_FAILURE is specified, the function raises an exception.

  • Insufficient memory
  • Tag is set to 0 or invalid POOL_FLAGS are specified

Remarks

If you are building a driver that targets versions of Windows prior to Windows 10, version 2004, use ExAllocatePoolZero, ExAllocatePoolUninitialized, ExAllocatePoolQuotaZero, or ExAllocatePoolQuotaUninitialized.

This routine has the following differences from the earlier allocation routines (ExAllocatePoolWithTag, ExAllocatePoolWithQuotaTag, ExAllocatePoolWithTagPriority):

  1. Memory is zero initialized unless POOL_FLAG_UNINITIALIZED is specified.

  2. Return behavior in the event of unsuccessful allocation. ExAllocatePoolWithQuotaTag raises an exception by default.

  3. Tags with a value of 0 are invalid.

When replacing ExAllocatePoolWithQuotaTag with ExAllocatePool2, you must specify the POOL_FLAG_USE_QUOTA flag. For more info about pool flags, see POOL_FLAGS.

If NumberOfBytes is PAGE_SIZE or greater, a page-aligned buffer is allocated. Memory allocations of PAGE_SIZE or less are allocated within a page and do not cross page boundaries. Memory allocations of less than PAGE_SIZE are not necessarily page-aligned but are aligned to 8-byte boundaries in 32-bit systems and to 16-byte boundaries in 64-bit systems.

Drivers may only use up to the NumberOfBytes they have explicitly allocated. Accessing memory outside of this range may corrupt the pool and cause the system to crash.

The system associates the pool tag with the allocated memory. Programming tools, such as WinDbg, can display the pool tag associated with each allocated buffer. Gflags, a tool included in Debugging Tools for Windows, turns on a system feature that requests allocation from special pool for a particular pool tag. Poolmon, which is included in the WDK, tracks memory by pool tag.

The value of Tag is stored, and sometimes displayed, in reverse (little-endian) order. For example, if a caller passes Fred as a Tag, it appears as derF in a pool dump and in pool usage tracking in the debugger, and as 0x64657246 in the registry and in tool displays.

The allocated buffer can be freed with either ExFreePool or ExFreePoolWithTag.

Callers of ExAllocatePool2 must be running at IRQL <= DISPATCH_LEVEL. A caller executing at DISPATCH_LEVEL must specify POOL_FLAG_NON_PAGED or POOL_FLAG_NON_PAGED_EXECUTABLE for Flags. A caller executing at IRQL <= APC_LEVEL can specify POOL_FLAG_PAGED, but if the memory will be accessed from code running at DISPATCH_LEVEL, you must still allocate non-paged memory.

In a non-uniform memory access (NUMA) multiprocessor architecture, ExAllocatePool2 attempts to allocate memory that is local to the processor that is calling ExAllocatePool2. If no local memory is available, ExAllocatePool2 allocates the closest available memory.

Memory that ExAllocatePool2 allocates is zero initialized. Kernel-mode drivers should not opt-out of zeroing for allocations that will be copied to an untrusted location (user-mode, over the network, etc.) to avoid disclosing sensitive information.

Requirements

Requirement Value
Minimum supported client Windows 10, version 2004
Target Platform Universal
Header wdm.h (include Wdm.h, Ntddk.h, Ntifs.h)
Library NtosKrnl.lib
DLL NtosKrnl.exe
IRQL IRQL <= DISPATCH_LEVEL (see Remarks section)
DDI compliance rules HwStorPortProhibitedDDIs, SpNoWait, StorPortStartIo

See also

ExAllocatePool3