ExAllocatePool function (wdm.h)

Article
01/13/2023

Warning

ExAllocatePool is obsolete and has been deprecated in Windows 10, version 2004. It has been replaced by ExAllocatePool2. For more information, see Updating deprecated ExAllocatePool calls to ExAllocatePool2 and ExAllocatePool3.

When developing drivers for version of Windows prior to Windows 10, version 2004, use ExAllocatePoolZero.

ExAllocatePool allocates pool memory of the specified type and returns a pointer to the allocated block.

Syntax

PVOID ExAllocatePool(
  [in] __drv_strictTypeMatch(__drv_typeExpr)POOL_TYPE PoolType,
  [in] SIZE_T                                         NumberOfBytes
);

Parameters

[in] PoolType

Specifies the type of pool memory to allocate. For a description of the available pool memory types, see POOL_TYPE.

You can modify PoolType by using a bitwise OR with the POOL_COLD_ALLOCATION flag as a hint to the kernel to allocate the memory from pages that are likely to be paged out quickly. To reduce the amount of resident pool memory as much as possible, you should not reference these allocations frequently. The POOL_COLD_ALLOCATION flag is only advisory and is available for Windows XP and later versions of the Windows operating system.

[in] NumberOfBytes

Specifies the number of bytes to allocate.

Return value

ExAllocatePool returns NULL if there is insufficient memory in the free pool to satisfy the request. Otherwise the routine returns a pointer to the allocated memory.

Remarks

This routine is used for the general pool allocation of memory.

If NumberOfBytes is PAGE_SIZE or greater, a page-aligned buffer is allocated. Memory allocations of PAGE_SIZE or less 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.

A successful allocation requesting NumberOfBytes < PAGE_SIZE of nonpaged pool gives the caller exactly the number of requested bytes of memory. If an allocation request for NumberOfBytes > PAGE_SIZE succeeds and NumberOfBytes is not an exact multiple of PAGE_SIZE, the last page in the allocation contains bytes that are not part of the caller's allocation. If possible, the pool allocator uses these bytes. To avoid corrupting data that belongs to other kernel-mode components, drivers must access only storage addresses that they have explicitly allocated.

If ExAllocatePool returns NULL, the caller should return the NTSTATUS value STATUS_INSUFFICIENT_RESOURCES or should delay processing to another point in time.

Callers of ExAllocatePool must be executing at IRQL <= DISPATCH_LEVEL. A caller executing at DISPATCH_LEVEL must specify a NonPagedXxx value for PoolType. A caller executing at IRQL <= APC_LEVEL can specify any POOL_TYPE value, but the IRQL and environment must also be considered for determining the page type.

Do not set NumberOfBytes = 0. Avoid zero-length allocations because they waste pool header space and, in many cases, indicate a potential validation issue in the calling code. For this reason, Driver Verifier flags such allocations as possible errors.

The system automatically sets certain standard event objects when the amount of pool (paged or nonpaged) is high or low. Drivers can wait for these events to tune their pool usage. For more information, see Standard Event Objects.

Memory that ExAllocatePool allocates is uninitialized. A kernel-mode driver must first zero this memory if it is going to make it visible to user-mode software (to avoid leaking potentially privileged contents).

Requirements

Requirement	Value
Minimum supported client	Obsolete. This routine is exported only for existing binaries. Use ExAllocatePoolWithTag instead.
Target Platform	Universal
Header	wdm.h (include Wdm.h, Ntddk.h, Ntifs.h, Classpnp.h, Smcnt.h)
Library	NtosKrnl.lib
DLL	NtosKrnl.exe
IRQL	IRQL <= DISPATCH_LEVEL (see Remarks section)
DDI compliance rules	CheckDeviceObjectFlags(wdm), HwStorPortProhibitedDDIs(storport), IrqlExAllocatePool(wdm), PowerDownAllocate(wdm), PowerUpFail(wdm), SpNoWait(storport), StorPortStartIo(storport), UnsafeAllocatePool(kmdf), UnsafeAllocatePool(wdm)