Allocating Audio buffer[Windows driver]

I want to use the following IMiniportWaveRTStream::AllocateAudioBuffer method which is defined in portcls.h.

NTSTATUS AllocateAudioBuffer(
  [in]   ULONG               RequestedSize,
  [out]  PMDL                *AudioBufferMdl,
  [out]  ULONG               *ActualSize,
  [out]  ULONG               *OffsetFromFirstPage,
  [out]  MEMORY_CACHING_TYPE *CacheType
);

But i want to use it in a C file. I found a structure which achieves the same but with one extra parameter.

NTSTATUS AllocateAudioBuffer(
  INTERFACE THIS,
  [in]   ULONG               RequestedSize,
  [out]  PMDL                *AudioBufferMdl,
  [out]  ULONG               *ActualSize,
  [out]  ULONG               *OffsetFromFirstPage,
  [out]  MEMORY_CACHING_TYPE *CacheType
);

The interface strucure is defined as

typedef struct _INTERFACE {
    USHORT Size;
    USHORT Version;
    PVOID Context;
    PINTERFACE_REFERENCE InterfaceReference;
    PINTERFACE_DEREFERENCE InterfaceDereference;
    // interface specific entries go here
} INTERFACE, *PINTERFACE;

What does this INTERFACE refers too? And how to obtain a reference to it.

Reference: http://ift.tt/1DmMw7R