ExAllocatePoolWithTag

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本文深入解析ExAllocatePoolWithTag函数，介绍其如何在Windows操作系统中高效地分配内存，并通过参数控制内存类型、大小及标记。阐述了函数返回值及其注意事项，同时说明了内存分配的策略和优化技巧。

ExAllocatePoolWithTag

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

PVOID
ExAllocatePoolWithTag(
IN POOL_TYPE PoolType ,
IN SIZE_T NumberOfBytes ,
IN ULONG Tag
);

Parameters

PoolType

Specifies the type of pool memory to allocate. Each allocation code path should use a unique pool tag to help debuggers and verifiers identify the code path. You can modify the PoolType value by using a bitwise OR 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 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. For a description of the available pool memory types, see POOL_TYPE .

NumberOfBytes

Specifies the number of bytes to allocate.

Tag

Specifies the pool tag for the allocated memory. Specify the pool tag as a character literal of up to four characters delimited by single quotation marks (for example, 'Tag1'). The string is usually specified in reverse order (for example, '1gaT'). The ASCII value of each character in the tag must be between 0 and 127. Every allocation code path should use a unique pool tag to ensure that debuggers and verifiers identify a distinct allocated block.

Return Value

ExAllocatePoolWithTag returns NULL if there is insufficient memory in the free pool to satisfy the request. Use of POOL_RAISE_IF_ALLOCATION_FAILURE is not recommended because it is costly. Any successful allocation that requests NumberOfBytes > PAGE_SIZE wastes all unused bytes on the last-allocated page. However, on Windows Vista and later, the unused bytes are no longer wasted.

Comments

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 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.

A successful allocation requesting NumberOfBytes < PAGE_SIZE of nonpaged pool gives the caller exactly the number of requested bytes of memory. Any successful allocation that requests NumberOfBytes > PAGE_SIZE wastes all unused bytes on the last-allocated page.

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 .

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.

Callers of ExAllocatePoolWithTag must be executing at IRQL <= DISPATCH_LEVEL. A caller executing at DISPATCH_LEVEL must specify a NonPaged Xxx 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.

Avoid calling with NumberOfBytes == 0. Doing so will result in pool header wastage.

Warning Memory that ExAllocatePoolWithTag 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).