CcMapData function (ntifs.h)

Artikel
10/21/2021

The CcMapData routine maps a specified byte range of a cached file to a buffer in memory.

Syntax

BOOLEAN CcMapData(
  [in]  PFILE_OBJECT   FileObject,
  [in]  PLARGE_INTEGER FileOffset,
  [in]  ULONG          Length,
  [in]  ULONG          Flags,
  [out] PVOID          *Bcb,
  [out] PVOID          *Buffer
);

Parameters

[in] FileObject

Pointer to a file object for the file whose data is to be mapped for read access.

[in] FileOffset

Pointer to a variable that specifies the starting byte offset within the cached file where the desired data resides.

[in] Length

Length of desired data in bytes.

[in] Flags

Bitmask of flags specifying how the mapping operation is to be performed. This is a bitwise OR combination of one or more of the following values:

Value	Meaning
MAP_WAIT	The caller can be put into a wait state until the data has been mapped.
MAP_NO_READ	Only pages that are already resident in memory are to be mapped.

Note In Windows 2000and earlier, this parameter was a BOOLEAN value named Wait:

Wait

Set to TRUE if the caller can be put into a wait state until the data has been mapped, FALSE otherwise.

[out] Bcb

On the first call this returns a pointer to a buffer control block (BCB) structure. This pointer must be supplied as input on all subsequent calls, for this buffer.

[out] Buffer

Pointer to a buffer containing the mapped data.

Return value

CcMapData returns TRUE if the data for the cached file was mapped successfully, FALSE otherwise.

Remarks

CcMapData maps data in a cached file for read access. Note that after CcMapData is called, the data is mapped; but it is not pinned. This distinction is important. Data that is mapped but not pinned cannot safely be modified. To pin the data, use CcPinMappedData, CcPinRead, or CcPreparePinWrite.

Every successful call to CcMapData must be matched by a subsequent call to CcUnpinData.

CcMapData cannot map data across view boundaries in the cache manager. The cache manager manages files in the system in 256 KB-aligned views. (The cache manager's view size is specified by the system-defined constant VACB_MAPPING_GRANULARITY, which is set to 256 KB in ntifs.h.) Mapped regions cannot span more than one 256 KB view. Therefore, the largest region that can be mapped is 256 KB, beginning at a 256 KB-aligned offset in the file.

Mapping a byte range in a cached file does not ensure that the pages remain resident in memory. As long as the pages are mapped, the byte range is guaranteed to stay mapped into the system cache virtual address space, but the memory manager can page out the physical pages as the system's memory demand requires.

If the MAP_WAIT flag is set (or Wait is TRUE), CcMapData is guaranteed to complete the mapping request and return TRUE. If the required pages of the cached file are already resident in memory, the data is mapped immediately and no blocking will occur. If any needed pages are not resident, the caller is put in a wait state until all required pages have been made resident and the data can be mapped. If the MAP_WAIT flag is not set (or Wait is FALSE) and the data cannot be mapped immediately, CcMapData returns FALSE.

The pointer returned in Buffer is valid until CcUnpinData is called. If CcPinMappedData is called while this pointer is still valid, the pointer remains valid after the call to CcPinMappedData (but only until CcUnpinData is called).

If any failure occurs, CcMapData raises a status exception for that particular failure. For example, if a pool allocation failure occurs, CcMapData raises a STATUS_INSUFFICIENT_RESOURCES exception; if an I/O error occurs, CcMapData raises the status exception of the I/O error. Therefore, to gain control if a failure occurs, the driver should wrap the call to CcMapData in a try-except or try-finally statement.

To cache a file, use CcInitializeCacheMap.

Requirements

Requirement	Value
Target Platform	Universal
Header	ntifs.h (include Ntifs.h)
Library	NtosKrnl.lib
DLL	NtosKrnl.exe
IRQL	< DISPATCH_LEVEL

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