_RxAllocatePoolWithTag function

_RxAllocatePoolWithTag allocates memory from a pool with a four-byte tag at the beginning of the block that can be used to help catch instances of memory trashing.


VOID* _RxAllocatePoolWithTag(
   ULONG Type,
   ULONG Size,
   ULONG Tag,
   PSZ   FileName,
   ULONG LineNumber


The type of the pool to be allocated. This parameter can be one of the following enumeration values for POOL_TYPE:

Nonpageable system memory that can be accessed from any IRQL. NonPagedPool memory is a scarce resource and drivers should allocate it only when necessary. The system can only allocate buffers larger than PAGE_SIZE from NonPagedPool in multiples of PAGE_SIZE. Requests for buffers larger than PAGE_SIZE, but not a PAGE_SIZE multiple, waste nonpageable memory.

Pageable system memory that can only be allocated and accessed at IRQL < DISPATCH_LEVEL.

The size of the memory block, in bytes, to be allocated.

The four-byte tag to be used to mark the allocated buffer. For a description of how to use tags, see ExAllocatePoolWithTag. The ASCII value of each character in the tag must be between 0 and 127.

A pointer to the source file name where the memory allocation occurred. This parameter is not currently used.

The line number in the source file where the memory allocation occurred. This parameter is not currently used.

Return value

RxAllocatePoolWithTag 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.


It is recommended that the RxAllocatePoolWithTag macro be called instead of using this routine directly. On retail builds, this macro is defined to call ExAllocatePoolWithTag. On checked builds, this macro is defined to call _RxAllocatePoolWithTag.

The _RxAllocatePoolWithTag routine calls ExAllocatePoolWithTagPriority with the priority (importance of the request) set to LowPoolPriority. The system may fail the request for LowPoolPriority when it runs low on resources. A driver should be prepared to recover from an allocation failure when using this routine.

When the system allocates a buffer from pool memory of PAGE_SIZE or greater, it aligns the buffer on a page boundary. Memory requests smaller than PAGE_SIZE are not necessarily aligned on page boundaries, but always fit within a single page, and are aligned on an 8-byte boundary. Any successful allocation that requests a block larger than PAGE_SIZE that is not a multiple of 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. The value of Tag is normally displayed in reverse order. For example, if a caller passes 'Fred' as a Tag, it would appear as 'derF' if memory is dumped or when tracking memory usage in the debugger.

Memory allocated with _RxAllocatePoolWithTag should be released by calling _RxFreePool.

Callers of _RxAllocatePoolWithTag must be executing at IRQL <= DISPATCH_LEVEL. A caller executing at DISPATCH_LEVEL must specify a NonPagedPool value for the Type parameter. A caller executing at IRQL <= APC_LEVEL can specify any POOL_TYPE value for the Type parameter.


Target platform



Ntrxdef.h (include Ntrxdef.h)


See Remarks section.

See also