FLT_PARAMETERS for IRP_MJ_MDL_READ union
The following FLT_PARAMETERS union member is used when FLT_IO_PARAMETER_BLOCK.MajorFunction is IRP_MJ_MDL_READ.
Syntax
typedef union _FLT_PARAMETERS {
... ;
struct {
LARGE_INTEGER FileOffset;
ULONG POINTER_ALIGNMENT Length;
ULONG POINTER_ALIGNMENT Key;
PMDL *MdlChain;
} MdlRead;
... ;
} FLT_PARAMETERS, *PFLT_PARAMETERS;
Members
MdlRead: Structure containing the following members.
FileOffset: Starting byte within the cached file.
Length: Length, in bytes, of the data to be read from the cached file.
Key: Key value associated with a byte-range lock on the target file. If the range to be read overlaps or is a subrange of an exclusively locked range within the file, this parameter must be the key for that exclusive lock. The exclusive lock must be held by the parent process of the calling thread; otherwise, this parameter is ignored.
MdlChain: Pointer to a variable that receives a pointer to a chain of one or more memory descriptor lists (MDL) that describe the pages containing the data to be read.
Remarks
IRP_MJ_MDL_READ is a fast I/O operation. It does the same thing as IRP_MJ_READ + IRP_MN_MDL except for the following difference:
- The IRP-based operation sets up caching on the file if it isn’t already cached before doing the MDL work.
- The Fast IO operation fails if the file isn’t already cached.
The FLT_PARAMETERS structure for IRP_MJ_MDL_READ operations contains the parameters for a fast I/O MdlRead operation. The operation is represented by a (FLT_CALLBACK_DATA) structure, with the operation's parameters in the FLT_IO_PARAMETER_BLOCK structure that Iopb points to.
If a fast I/O IRP_MJ_MDL_READ request fails, the issuer of the I/O determines how to reissue the request. For instance, the request could be reissued as an IRP-based operation using IRP_MJ_READ + IRP_MN_MDL.
Requirements
| Requirement type | Requirement |
|---|---|
| Header | Fltkernel.h (include Fltkernel.h) |