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ExAllocatePoolWithTagPriority routine

The ExAllocatePoolWithTagPriority routine allocates pool memory of the specified type.

Syntax


PVOID ExAllocatePoolWithTagPriority(
  _In_  POOL_TYPE        PoolType,
  _In_  SIZE_T           NumberOfBytes,
  _In_  ULONG            Tag,
  _In_  EX_POOL_PRIORITY Priority
);

Parameters

PoolType [in]

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

You can modify the PoolType value by bitwise-ORing this value with the POOL_RAISE_IF_ALLOCATION_FAILURE flag. This flag causes an exception to be raised if the request cannot be satisfied.

Similarly, you can modify the PoolType value by bitwise-ORing this value 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 starting with Windows XP.

NumberOfBytes [in]

The number of bytes to allocate.

Tag [in]

The pool tag to use for the allocated memory. For more information, see the Tag parameter of ExAllocatePoolWithTag.

Priority [in]

The priority of this request. Set this parameter to one of the following EX_POOL_PRIORITY enumeration values.

Priority valueDescription

LowPoolPriority

Specifies that the system may fail the request when it runs low on resources. Driver allocations that can recover from an allocation failure use this priority.

NormalPoolPriority

Specifies that the system may fail the request when it runs very low on resources. Most drivers should use this value.

HighPoolPriority

Specifies that the system must not fail the request, unless it is completely out of resources. Drivers only use this value when it is critically important for the request to succeed.

 

The EX_POOL_PRIORITY enumeration defines XxxSpecialPoolOverrun and XxxSpecialPoolUnderrun variants to specify how memory should be allocated when Driver Verifier (or special pool) is enabled. If the driver specifies XxxSpecialPoolUnderrun, when the memory manager allocates memory from special pool, it allocates it at the start of a physical page. If the driver specifies XxxSpecialPoolOverrun, the memory manager allocates it at the end of a physical page.

Return value

ExAllocatePoolWithTagPriority returns NULL if there is insufficient memory in the free pool to satisfy the request unless POOL_RAISE_IF_ALLOCATION_FAILURE is specified. Otherwise, the routine returns a pointer to the allocated memory.

Remarks

Callers of ExAllocatePoolWithTagPriority 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.

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.

Note  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.

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

Note  Memory that ExAllocatePoolWithTagPriority 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

Target platform

Universal

Version

Available starting with Windows 2000.

Header

Wdm.h (include Wdm.h, Ntddk.h, or Ntifs.h)

Library

Ntoskrnl.lib

IRQL

<= DISPATCH_LEVEL (see Remarks section)

DDI compliance rules

IrqlExAllocatePool, IrqlExFree2, HwStorPortProhibitedDDIs, SpNoWait, StorPortStartIo

See also

ExAllocatePoolWithTag
ExFreePool
POOL_TYPE

 

 

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