__RPC__in 定义在 rpcsal.h 中，它不包含在 C++Builder 6 中（它是在 C++Builder 2007 中添加的）。您可以根据需要在代码中手动定义__RPC__in（和其他相关定义），然后再包括mmdeviceapi.h，例如：
#ifndef __RPCSAL_H_VERSION__
#define __RPC_in
// other related defines, such as __RPC__out...
#endif
#include <mmdeviceapi.h>
__RPC__in 和其他相关符号只是帮助代码自我记录的预处理器宏，它们实际上对 C++ 编译器没有任何作用（但对 Microsoft 的 PreFAST 分析工具有用）。
这里是rpcsal.h的内容（去掉了PREFAST定义，否则内容太大，StackOverflow无法接受）：
#pragma option push -b -a8 -pc -A- /*P_O_Push*/
/****************************************************************\
* *
* rpcsal.h - markers for documenting the semantics of RPC APIs *
* *
* Version 1.0 *
* *
* Copyright (c) 2004 Microsoft Corporation. All rights reserved. *
* *
\****************************************************************/
// -------------------------------------------------------------------------------
// Introduction
//
// rpcsal.h provides a set of annotations to describe how RPC functions use their
// parameters - the assumptions it makes about them, adn the guarantees it makes
// upon finishing. These annotations are similar to those found in specstrings.h,
// but are designed to be used by the MIDL compiler when it generates annotations
// enabled header files.
//
// IDL authors do not need to annotate their functions declarations. The MIDL compiler
// will interpret the IDL directives and use one of the annotations contained
// in this header. This documentation is intended to help those trying to understand
// the MIDL-generated header files or those who maintain their own copies of these files.
//
// -------------------------------------------------------------------------------
// Differences between rpcsal.h and specstrings.h
//
// There are a few important differences between the annotations found in rpcsal.h and
// those in specstrings.h:
//
// 1. [in] parameters are not marked as read-only. They may be used for scratch space
// at the server and changes will not affect the client.
// 2. String versions of each macro alleviates the need for a special type definition
//
// -------------------------------------------------------------------------------
// Interpreting RPC Annotations
//
// These annotations are interpreted precisely in the same way as those in specstrings.h.
// Please refer to that header for information related to general usage in annotations.
//
// To construct an RPC annotation, concatenate the appropriate value from each category
// along with a leading __RPC_. A typical annotation looks like "__RPC__in_string".
//
// |----------------------------------------------------------------------------------|
// | RPC Annotations |
// |------------|------------|---------|--------|----------|----------|---------------|
// | Level | Usage | Size | Output | Optional | String | Parameters |
// |------------|------------|---------|--------|----------|----------|---------------|
// | <> | <> | <> | <> | <> | <> | <> |
// | _deref | _in | _ecount | _full | _opt | _string | (size) |
// | _deref_opt | _out | _bcount | _part | | | (size,length) |
// | | _inout | | | | | |
// | | | | | | | |
// |------------|------------|---------|--------|----------|----------|---------------|
//
// Level: Describes the buffer pointer's level of indirection from the parameter or
// return value 'p'.
//
// <> : p is the buffer pointer.
// _deref : *p is the buffer pointer. p must not be NULL.
// _deref_opt : *p may be the buffer pointer. p may be NULL, in which case the rest of
// the annotation is ignored.
//
// Usage: Describes how the function uses the buffer.
//
// <> : The buffer is not accessed. If used on the return value or with _deref, the
// function will provide the buffer, and it will be uninitialized at exit.
// Otherwise, the caller must provide the buffer. This should only be used
// for alloc and free functions.
// _in : The function will only read from the buffer. The caller must provide the
// buffer and initialize it. Cannot be used with _deref.
// _out : The function will only write to the buffer. If used on the return value or
// with _deref, the function will provide the buffer and initialize it.
// Otherwise, the caller must provide the buffer, and the function will
// initialize it.
// _inout : The function may freely read from and write to the buffer. The caller must
// provide the buffer and initialize it. If used with _deref, the buffer may
// be reallocated by the function.
//
// Size: Describes the total size of the buffer. This may be less than the space actually
// allocated for the buffer, in which case it describes the accessible amount.
//
// <> : No buffer size is given. If the type specifies the buffer size (such as
// with LPSTR and LPWSTR), that amount is used. Otherwise, the buffer is one
// element long. Must be used with _in, _out, or _inout.
// _ecount : The buffer size is an explicit element count.
// _bcount : The buffer size is an explicit byte count.
//
// Output: Describes how much of the buffer will be initialized by the function. For
// _inout buffers, this also describes how much is initialized at entry. Omit this
// category for _in buffers; they must be fully initialized by the caller.
//
// <> : The type specifies how much is initialized. For instance, a function initializing
// an LPWSTR must NULL-terminate the string.
// _full : The function initializes the entire buffer.
// _part : The function initializes part of the buffer, and explicitly indicates how much.
//
// Optional: Describes if the buffer itself is optional.
//
// <> : The pointer to the buffer must not be NULL.
// _opt : The pointer to the buffer might be NULL. It will be checked before being dereferenced.
//
// String: Describes if the buffer is NULL terminated
//
// <> : The buffer is not assumed to be NULL terminated
// _string : The buffer is assumed to be NULL terminated once it has been initialized
//
// Parameters: Gives explicit counts for the size and length of the buffer.
//
// <> : There is no explicit count. Use when neither _ecount nor _bcount is used.
// (size) : Only the buffer's total size is given. Use with _ecount or _bcount but not _part.
// (size,length) : The buffer's total size and initialized length are given. Use with _ecount_part
// and _bcount_part.
//
// Notes:
//
// 1. Specifying two buffer annotations on a single parameter results in unspecified behavior
// (e.g. __RPC__in_bcount(5) __RPC__out_bcount(6)
//
// 2. The size of the buffer and the amount that has been initialized are separate concepts.
// Specify the size using _ecount or _bcount. Specify the amount that is initialized using
// _full, _part, or _string. As a special case, a single element buffer does not need
// _ecount, _bcount, _full, or _part
//
// 3. The count may be less than the total size of the buffer in which case it describes the
// accessible portion.
//
// 4. "__RPC__opt" and "__RPC_deref" are not valid annotations.
//
// 5. The placement of _opt when using _deref is important:
// __RPC__deref_opt_... : Input may be NULL
// __RPC__deref_..._opt : Output may be NULL
// __RPC__deref_opt_..._opt : Both input and output may be NULL
//
#pragma once
#include <specstrings.h>
#ifndef __RPCSAL_H_VERSION__
#define __RPCSAL_H_VERSION__ ( 100 )
#endif // __RPCSAL_H_VERSION__
#ifdef __REQUIRED_RPCSAL_H_VERSION__
#if ( __RPCSAL_H_VERSION__ < __REQUIRED_RPCSAL_H_VERSION__ )
#error incorrect <rpcsal.h> version. Use the header that matches with the MIDL compiler.
#endif
#endif
#ifdef __cplusplus
extern "C" {
#endif // #ifdef __cplusplus
#if (_MSC_VER >= 1000) && !defined(__midl) && defined(_PREFAST_)
// PREFAST defines removed for brevity...
#else // not prefast
#define __RPC__range(min,max)
#define __RPC__in_range(min,max)
#define __RPC__in
#define __RPC__in_string
#define __RPC__in_opt_string
#define __RPC__in_ecount(size)
#define __RPC__in_ecount_full(size)
#define __RPC__in_ecount_full_string(size)
#define __RPC__in_ecount_part(size, length)
#define __RPC__in_ecount_full_opt(size)
#define __RPC__in_ecount_full_opt_string(size)
#define __RPC__inout_ecount_full_opt_string(size)
#define __RPC__in_ecount_part_opt(size, length)
#define __RPC__in_xcount(size)
#define __RPC__in_xcount_full(size)
#define __RPC__in_xcount_full_string(size)
#define __RPC__in_xcount_part(size, length)
#define __RPC__in_xcount_full_opt(size)
#define __RPC__in_xcount_full_opt_string(size)
#define __RPC__inout_xcount_full_opt_string(size)
#define __RPC__in_xcount_part_opt(size, length)
#define __RPC__deref_in
#define __RPC__deref_in_string
#define __RPC__deref_in_opt
#define __RPC__deref_in_opt_string
#define __RPC__deref_opt_in
#define __RPC__deref_opt_in_string
#define __RPC__deref_opt_in_opt
#define __RPC__deref_opt_in_opt_string
#define __RPC__deref_in_ecount(size)
#define __RPC__deref_in_ecount_part(size, length)
#define __RPC__deref_in_ecount_full(size)
#define __RPC__deref_in_ecount_full_opt(size)
#define __RPC__deref_in_ecount_full_string(size)
#define __RPC__deref_in_ecount_full_opt_string(size)
#define __RPC__deref_in_ecount_opt(size)
#define __RPC__deref_in_ecount_opt_string(size)
#define __RPC__deref_in_ecount_part_opt(size, length)
#define __RPC__deref_in_xcount(size)
#define __RPC__deref_in_xcount_part(size, length)
#define __RPC__deref_in_xcount_full(size)
#define __RPC__deref_in_xcount_full_opt(size)
#define __RPC__deref_in_xcount_full_string(size)
#define __RPC__deref_in_xcount_full_opt_string(size)
#define __RPC__deref_in_xcount_opt(size)
#define __RPC__deref_in_xcount_opt_string(size)
#define __RPC__deref_in_xcount_part_opt(size, length)
// [out]
#define __RPC__out
#define __RPC__out_ecount(size)
#define __RPC__out_ecount_part(size, length)
#define __RPC__out_ecount_full(size)
#define __RPC__out_ecount_full_string(size)
#define __RPC__out_xcount(size)
#define __RPC__out_xcount_part(size, length)
#define __RPC__out_xcount_full(size)
#define __RPC__out_xcount_full_string(size)
// [in,out]
#define __RPC__inout
#define __RPC__inout_string
#define __RPC__opt_inout
#define __RPC__inout_ecount(size)
#define __RPC__inout_ecount_part(size, length)
#define __RPC__inout_ecount_full(size)
#define __RPC__inout_ecount_full_string(size)
#define __RPC__inout_xcount(size)
#define __RPC__inout_xcount_part(size, length)
#define __RPC__inout_xcount_full(size)
#define __RPC__inout_xcount_full_string(size)
// [in,unique]
#define __RPC__in_opt
#define __RPC__in_ecount_opt(size)
#define __RPC__in_xcount_opt(size)
// [in,out,unique]
#define __RPC__inout_opt
#define __RPC__inout_opt_string
#define __RPC__inout_ecount_opt(size)
#define __RPC__inout_ecount_part_opt(size, length)
#define __RPC__inout_ecount_full_opt(size)
#define __RPC__inout_ecount_full_string(size)
#define __RPC__inout_xcount_opt(size)
#define __RPC__inout_xcount_part_opt(size, length)
#define __RPC__inout_xcount_full_opt(size)
#define __RPC__inout_xcount_full_string(size)
// [out] **
#define __RPC__deref_out
#define __RPC__deref_out_string
#define __RPC__deref_out_opt
#define __RPC__deref_out_opt_string
#define __RPC__deref_out_ecount(size)
#define __RPC__deref_out_ecount_part(size, length)
#define __RPC__deref_out_ecount_full(size)
#define __RPC__deref_out_ecount_full_string(size)
#define __RPC__deref_out_xcount(size)
#define __RPC__deref_out_xcount_part(size, length)
#define __RPC__deref_out_xcount_full(size)
#define __RPC__deref_out_xcount_full_string(size)
// [in,out] **, second pointer decoration.
#define __RPC__deref_inout
#define __RPC__deref_inout_string
#define __RPC__deref_inout_opt
#define __RPC__deref_inout_opt_string
#define __RPC__deref_inout_ecount_full(size)
#define __RPC__deref_inout_ecount_full_string(size)
#define __RPC__deref_inout_ecount_opt(size)
#define __RPC__deref_inout_ecount_part_opt(size, length)
#define __RPC__deref_inout_ecount_full_opt(size)
#define __RPC__deref_inout_ecount_full_opt_string(size)
#define __RPC__deref_inout_xcount_full(size)
#define __RPC__deref_inout_xcount_full_string(size)
#define __RPC__deref_inout_xcount_opt(size)
#define __RPC__deref_inout_xcount_part_opt(size, length)
#define __RPC__deref_inout_xcount_full_opt(size)
#define __RPC__deref_inout_xcount_full_opt_string(size)
// #define __RPC_out_opt out_opt is not allowed in rpc
// [in,out,unique]
#define __RPC__deref_opt_inout
#define __RPC__deref_opt_inout_string
#define __RPC__deref_opt_inout_ecount(size)
#define __RPC__deref_opt_inout_ecount_part(size, length)
#define __RPC__deref_opt_inout_ecount_full(size)
#define __RPC__deref_opt_inout_ecount_full_string(size)
#define __RPC__deref_opt_inout_xcount(size)
#define __RPC__deref_opt_inout_xcount_part(size, length)
#define __RPC__deref_opt_inout_xcount_full(size)
#define __RPC__deref_opt_inout_xcount_full_string(size)
#define __RPC__deref_out_ecount_opt(size)
#define __RPC__deref_out_ecount_part_opt(size, length)
#define __RPC__deref_out_ecount_full_opt(size)
#define __RPC__deref_out_ecount_full_opt_string(size)
#define __RPC__deref_out_xcount_opt(size)
#define __RPC__deref_out_xcount_part_opt(size, length)
#define __RPC__deref_out_xcount_full_opt(size)
#define __RPC__deref_out_xcount_full_opt_string(size)
#define __RPC__deref_opt_inout_opt
#define __RPC__deref_opt_inout_opt_string
#define __RPC__deref_opt_inout_ecount_opt(size)
#define __RPC__deref_opt_inout_ecount_part_opt(size, length)
#define __RPC__deref_opt_inout_ecount_full_opt(size)
#define __RPC__deref_opt_inout_ecount_full_opt_string(size)
#define __RPC__deref_opt_inout_xcount_opt(size)
#define __RPC__deref_opt_inout_xcount_part_opt(size, length)
#define __RPC__deref_opt_inout_xcount_full_opt(size)
#define __RPC__deref_opt_inout_xcount_full_opt_string(size)
#define __RPC_full_pointer
#define __RPC_unique_pointer
#define __RPC_ref_pointer
#define __RPC_string
#endif
#ifdef __cplusplus
}
#endif
#pragma option pop /*P_O_Pop*/
那么你可以这样做：
#include "rpcsal.h"
#include <mmdeviceapi.h>