Useful Macros for Pointer Verification

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Introduction

Most know the useful macros provided by various libraries named ASSERT, VERIFY (and similar) which just validates the provided argument. If the validation evaluates to false, then a debug notification is raised.

In many cases, this is sufficient for simple verifications, but validating pointers is not the strength of this method. Look at this example:

void MyFunction(LPSOMESTRUCT pData)
{
  ASSERT(pData != NULL);
  // ... more code
}

When you pass a NULL pointer to this function, it will correctly detect it, but what if you pass 0xcdcdcdcd? It's not NULL and it's most probably not a valid address either. ASSERT will not catch it and your application will throw an exception.

More Macros

Here, a more advanced solution is required. One possible solution is the use of the functions provided by the Windows API: IsBadReadPtr(), IsBadWritePtr(), IsBadStringPtr(). These functions take a memory location and a size as arguments and verify that the calling process really has read and/or write access to the location. It might be that the memory at the location is only partially accessible from your process, or that the memory is read or write only. These functions also detect these situations.

I've wrapped these functions into handy macros which you can use similar to the ASSERT and VERIFY macros.

#ifdef _DEBUG

#define VERIFY_ISWRITEPOINTER(a) \
       { if(::IsBadWritePtr(a, sizeof(LPDWORD))) \
		{ ::OutputDebugString(_T("Parameter ") _T(#a) \
		_T(" is not a valid write pointer\r\n"));}}
#define VERIFY_ISREADPOINTER(a) \
		{ if(::IsBadReadPtr(a, sizeof(LPDWORD)))\
		{ ::OutputDebugString(_T("Parameter ") _T(#a) \
		_T(" is not a valid read pointer\r\n"));}}

#define VERIFY_ISWRITEDATA(a, l)\
		{ if(::IsBadWritePtr(a, l)) \
		{ ::OutputDebugString(_T("Parameter ") _T(#a) \
		_T(" is not a valid write area\r\n"));}}
#define VERIFY_ISREADDATA(a, l)\
		{ if(::IsBadReadPtr(a, l))  \
		{ ::OutputDebugString(_T("Parameter ") _T(#a) \
		_T(" is not a valid read area\r\n"));}}

#define ASSERT_ISWRITEPOINTER(a)\
		{ if(::IsBadWritePtr(a, sizeof(LPDWORD))) \
		{ ::OutputDebugString(_T("Parameter ") _T(#a) \
		_T(" is not a valid write pointer\r\n")); ASSERT(false);}}
#define ASSERT_ISREADPOINTER(a)\
		{ if(::IsBadReadPtr(a, sizeof(LPDWORD)))  \
		{ ::OutputDebugString(_T("Parameter ") _T(#a) \
		_T(" is not a valid read pointer\r\n")); ASSERT(false);}}

#define ASSERT_ISWRITEDATA(a, l)\
		{ if(::IsBadWritePtr(a, l)) \
		{ ::OutputDebugString(_T("Parameter ") _T(#a) \
		_T(" is not a valid write area\r\n")); ASSERT(false);}}
#define ASSERT_ISREADDATA(a, l)		{ if(::IsBadReadPtr(a, l))  \
		{ ::OutputDebugString(_T("Parameter ") _T(#a)\
		_T(" is not a valid read area\r\n")); ASSERT(false);}}

#else

#define VERIFY_ISWRITEPOINTER(a)
#define VERIFY_ISREADPOINTER(a)	

#define VERIFY_ISWRITEDATA(a, l)
#define VERIFY_ISREADDATA(a, l)	

#define ASSERT_ISWRITEPOINTER(a)
#define ASSERT_ISREADPOINTER(a)

#define ASSERT_ISWRITEDATA(a, l)
#define ASSERT_ISREADDATA(a, l)

#endif

Our sample from before can be changed to this:

void MyFunction(LPSOMESTRUCT pData)
{
  ASSERT_ISREADDATA(pData, sizeof(SOMESTRUCT));
  // ... more code
}

Now it will correctly assert when you pass the address 0xcdcdcdcd or any other location from which the function cannot read at least sizeof(SOMESTRUCT) bytes and the debug output will show "Parameter pData is not a valid read area".

I have found this to be a valuable tool when you write functions which take in or out pointers. Many problems related to bad pointers can easily be cured by using these validation macros.

Compatibility

This is compatible with any Windows version without restriction. It can be used with any Visual C++ version and all eVC versions. Anyway, using the macros is your responsibility. :)

License

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