// fltuserstructures.h

typedef struct _FILTER_MESSAGE_HEADER {
  ULONG     ReplyLength;
  ULONGLONG MessageId;
} FILTER_MESSAGE_HEADER, *PFILTER_MESSAGE_HEADER;
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Windows Driver Kit DDI reference (ns-fltuserstructures-_filter_message_header)

_FILTER_MESSAGE_HEADER structure

Description

The FILTER_MESSAGE_HEADER structure contains message header information.

Members

`ReplyLength`

On output from FilterGetMessage, this field receives the length, in bytes, of the expected reply, including the FILTER_REPLY_HEADER header. Set to zero if no reply is expected.

`MessageId`

On output from FilterGetMessage, this field receives the unique identifier (ID) for the message sent by the kernel-mode driver. If the application replies to the message, it must set this ID in the MessageId field of the FILTER_REPLY_HEADER header in the reply.

Remarks

To receive messages from a kernel-mode minifilter, a user-mode application typically defines a custom message structure. This structure typically consists of this header structure, followed by an application-defined structure to hold the actual message data.

FILTER_MESSAGE_HEADER - NtDoc