RtlLongLongAdd - NtDoc

#ifndef _NTINTSAFE_H_INCLUDED_
#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP | WINAPI_PARTITION_SYSTEM | WINAPI_PARTITION_GAMES)
/////////////////////////////////////////////////////////////////////////
//
// signed operations
//
// Strongly consider using unsigned numbers.
//
// Signed numbers are often used where unsigned numbers should be used.
// For example file sizes and array indices should always be unsigned.
// (File sizes should be 64bit integers; array indices should be size_t.)
// Subtracting a larger positive signed number from a smaller positive
// signed number with RtlIntSub will succeed, producing a negative number,
// that then must not be used as an array index (but can occasionally be
// used as a pointer index.) Similarly for adding a larger magnitude
// negative number to a smaller magnitude positive number.
//
// intsafe.h does not protect you from such errors. It tells you if your
// integer operations overflowed, not if you are doing the right thing
// with your non-overflowed integers.
//
// Likewise you can overflow a buffer with a non-overflowed unsigned index.
//
#if defined(ENABLE_INTSAFE_SIGNED_FUNCTIONS)

//
// LONGLONG Addition
//
_Must_inspect_result_
__inline
NTSTATUS
RtlLongLongAdd(
    _In_ LONGLONG llAugend,
    _In_ LONGLONG llAddend,
    _Out_ _Deref_out_range_(==, llAugend + llAddend) LONGLONG* pllResult
    )
{
    NTSTATUS status;
    LONGLONG llResult = llAugend + llAddend;
    
    //
    // Adding positive to negative never overflows.
    // If you add two positive numbers, you expect a positive result.
    // If you add two negative numbers, you expect a negative result.
    // Overflow if inputs are the same sign and output is not that sign.
    //
    if (((llAugend < 0) == (llAddend < 0))  &&
        ((llAugend < 0) != (llResult < 0)))
    {
        *pllResult = LONGLONG_ERROR;
        status = STATUS_INTEGER_OVERFLOW;
    }
    else
    {
        *pllResult = llResult;
        status = STATUS_SUCCESS;
    }

    return status;
}

#endif
#endif
#endif
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