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ExInitializeLookasideListEx routine

The ExInitializeLookasideListEx routine initializes a lookaside list.


NTSTATUS ExInitializeLookasideListEx(
  _Out_     PLOOKASIDE_LIST_EX    Lookaside,
  _In_opt_  PALLOCATE_FUNCTION_EX Allocate,
  _In_opt_  PFREE_FUNCTION_EX     Free,
  _In_      POOL_TYPE             PoolType,
  _In_      ULONG                 Flags,
  _In_      SIZE_T                Size,
  _In_      ULONG                 Tag,
  _In_      USHORT                Depth


Lookaside [out]

A pointer to the LOOKASIDE_LIST_EX structure to initialize. On return, this structure describes an empty lookaside list. The caller must use nonpaged system space for this structure, regardless of whether the entries in the lookaside list are allocated from paged or nonpaged memory. On 64-bit platforms, this structure must be 16-byte aligned.

Allocate [in, optional]

A pointer to a caller-supplied LookasideListAllocateEx routine that allocates a new lookaside-list entry. The ExAllocateFromLookasideListEx routine calls this LookasideListAllocateEx routine if the lookaside list is empty (contains no entries). This parameter is optional and can be specified as NULL if a custom allocation routine is not required. If this parameter is NULL, calls to ExAllocateFromPagedLookasideList automatically allocate the paged or nonpaged storage (as determined by the PoolType parameter) for the new entries.

Free [in, optional]

A pointer to a caller-supplied LookasideListFreeEx routine that frees a previously allocated lookaside-list entry. The ExFreeToPagedLookasideList routine calls this LookasideListFreeEx routine if the lookaside list is full (that is, the list already contains the maximum number of entries, as determined by the operating system). This parameter is optional and can be specified as NULL if a custom deallocation routine is not required. If this parameter is NULL, calls to ExFreeToPagedLookasideList automatically free the storage for the specified entries.

PoolType [in]

Specifies the pool type of the entries in the lookaside list. Set this parameter to a valid POOL_TYPE enumeration value.

Flags [in]

Specifies an optional flag value to modify the default behavior of the LookasideListAllocateEx routine. Set this parameter to zero or to one of the following EX_LOOKASIDE_LIST_EX_FLAGS_XXX flag bits.

Flag bitMeaning


If the allocation fails, raise an exception.


If the allocation fails, return NULL instead of raising an exception. This flag is intended for use with an allocation routine, such as ExAllocatePoolWithQuotaTag, that charges quotas for pool usage.


These two flag bits are mutually exclusive.

If Allocate is NULL, set Flags to either zero or EX_LOOKASIDE_LIST_EX_FLAGS_RAISE_ON_FAIL, but not to EX_LOOKASIDE_LIST_EX_FLAGS_FAIL_NO_RAISE. Otherwise, the behavior of the default allocation routine is undefined.

If Flags = EX_LOOKASIDE_LIST_EX_FLAGS_RAISE_ON_FAIL, the PoolType parameter value is bitwise ORed with the POOL_RAISE_IF_ALLOCATION_FAILURE flag bit to form the PoolType parameter value that is passed to the LookasideListAllocateEx routine. The LookasideListAllocateEx routine can pass this PoolType value, without modification, to the ExAllocatePoolWithTag routine. For more information about the POOL_RAISE_IF_ALLOCATION_FAILURE flag, see ExAllocatePoolWithTag.

If Flags = EX_LOOKASIDE_LIST_EX_FLAGS_FAIL_NO_RAISE, the PoolType parameter value is bitwise ORed with the POOL_QUOTA_FAIL_INSTEAD_OF_RAISE flag bit to form the PoolType parameter value that is passed to the LookasideListAllocateEx routine. The LookasideListAllocateEx routine can pass this PoolType value, without modification, to the ExAllocatePoolWithQuotaTag routine. For more information about the POOL_QUOTA_FAIL_INSTEAD_OF_RAISE flag, see ExAllocatePoolWithQuotaTag.

Size [in]

Specifies the size, in bytes, of each entry in the lookaside list.

Tag [in]

Specifies the four-byte pool tag to use to mark the allocated storage for lookaside-list entries. For more information about pool tags, see the description of the Tag parameter in ExAllocatePoolWithTag.

Depth [in]

Reserved. Always set this parameter to zero.

Return value

ExInitializeLookasideListEx returns STATUS_SUCCESS if the call is successful. Possible return values include the following error code:

Return codeDescription

The PoolType parameter value is not valid.


The Flags parameter value is not valid.



A driver must call this routine to initialize a lookaside list before the driver can begin to use the list. A lookaside list is a pool of fixed-size buffers that the driver can manage locally to reduce the number of calls to system allocation routines and, thereby, to improve performance. The buffers are stored as entries in the lookaside list. All entries in the list are of the same, uniform size, which is specified by the Size parameter.

After ExInitializeLookasideListEx returns, the lookaside list is initialized but contains no entries. When a client calls the ExAllocateFromLookasideListEx routine to request an entry, this routine determines that the lookaside list is empty and calls the driver-supplied LookasideListAllocateEx routine to dynamically allocate storage for a new entry. Additional entries might be allocated in response to similar requests from clients. Later, when clients call the ExFreeToLookasideListEx to release these entries, this routine inserts the entries into the lookaside list. If the number of entries in the list reaches a limit that is determined by the operating system, ExFreeToLookasideListEx ceases to add further entries to the list and instead passes these entries to the driver-supplied LookasideListFreeEx routine to be freed.

If the driver does not supply LookasideListAllocateEx and LookasideListFreeEx routines, the ExAllocateFromLookasideListEx and ExFreeToLookasideListEx routines use default allocation and deallocation routines instead.

There is no benefit to providing LookasideListAllocateEx and LookasideListFreeEx routines that do nothing but call ExAllocatePoolWithTag and ExFreePool. The same effect can be achieved with better performance by simply setting the Allocate and Free parameters to NULL.

Before a driver unloads, it must explicitly free any lookaside lists it created. Failure to do so is a serious programming error. Call the ExDeleteLookasideListEx routine to free a lookaside list. This routine frees the storage for any remaining entries in the specified lookaside list and then removes the list from the system-wide set of active lookaside lists.

The operating system keeps track of all lookaside lists that are currently in use. As both the amount of available nonpaged memory and the demand for lookaside list entries vary over time, the operating system dynamically adjusts its limits for the maximum number of entries in each nonpaged lookaside list.

In Windows 2000 and later versions of Windows, a lookaside list that contains paged or nonpaged entries can be described by a PAGED_LOOKASIDE_LIST or NPAGED_LOOKASIDE_LIST structure, respectively.

For more information about lookaside lists, see Using Lookaside Lists.

Callers of ExInitializeLookasideListEx can be running at IRQL <= DISPATCH_LEVEL, but are typically running at IRQL = PASSIVE_LEVEL.


The driver-supplied LookasideListAllocateEx and LookasideListFreeEx routines both receive Lookaside parameters that point to the LOOKASIDE_LIST_EX structure that describes the lookaside list. The routines can use this parameter to access private data that the driver has associated with the lookaside list. For example, the driver might allocate an instance of the following structure to collect private data for each lookaside list that it creates:

typedef struct
  ULONG NumberOfAllocations;  // number of entries allocated
  ULONG NumberOfFrees;        // number of entries freed
  LOOKASIDE_LIST_EX LookasideField;

The driver can initialize a lookaside list as shown in the following code example:

#define ENTRY_SIZE  256
#define MY_POOL_TAG  'tsLL'	  
MyContext = ExAllocatePoolWithTag(NonPagedPool, sizeof(MY_PRIVATE_DATA), MY_POOL_TAG);
if (MyContext)
    MyContext.NumberOfAllocations = 0;
    MyContext.NumberOfFrees = 0;

    status = ExInitializeLookasideListEx(

The following code example shows how the LookasideListAllocateEx routine can use its Lookaside parameter to access the private data that is associated with the lookaside list:

    __in POOL_TYPE  PoolType,
    __in SIZE_T  NumberOfBytes,
    __in ULONG  Tag,
    __inout PLOOKASIDE_LIST_EX  Lookaside)
    MY_PRIVATE_DATA *MyContext;
    PVOID NewEntry;

    MyContext = CONTAINING_RECORD(Lookaside, MY_PRIVATE_DATA, LookasideField);

    NewEntry = ExAllocatePoolWithTag(PoolType, NumberOfBytes, Tag);

    if (NewEntry)
        MyContext->NumberOfAllocations = InterlockedIncrement(MyContext->NumberOfAllocations);

    return NewEntry;

The CONTAINING_RECORD macro is defined in the Ntdef.h header file. The LookAsideListFreeEx routine can similarly use its Lookaside parameter to access private data.

After the MyLookasideListAllocateEx routine in this example returns, ExAllocateFromLookasideListEx inserts the buffer pointed to by the NewEntry variable into the lookaside list. To make this insertion operation thread-safe, ExAllocateFromLookasideListEx synchronizes its access of the lookaside list with other list insertion and removal operations that might be performed by other threads. Similarly, when ExFreeFromLookasideListEx removes a buffer from the lookaside list, it synchronizes its access to the list.

ExAllocateFromLookasideListEx and ExFreeFromLookasideListEx do not synchronize their calls to driver-supplied LookasideListAllocateEx and LookasideListFreeEx routines. Thus, if the MyLookasideListAllocateEx and MyLookasideListFreeEx routines in the preceding code examples must be thread-safe, the driver must provide the necessary synchronization.

The example routine, MyLookasideListAllocateEx, synchronizes its access of the MyContext->NumberOfAllocations variable with other threads that might increment and decrement this variable. To provide this synchronization, MyLookasideListAllocateEx calls the InterlockedIncrement routine to atomically increment this variable. Similarly, the MyLookasideListFreeEx routine (not shown) can call the InterlockedDecrement routine to atomically decrement this variable.

However, if the sole purpose of the MyContext->NumberOfAllocations variable in the preceding code example is simply to gather statistics on lookaside list allocations, atomic increments and decrements are hardly necessary. In this case, the remote possibility of a missed increment or decrement should not be a concern.

For more information about thread safety for lookaside lists, see Using Lookaside Lists.


Target platform



Available starting with Windows Vista.


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




<= DISPATCH_LEVEL (see Remarks section)

See also




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